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post #1 of 86 (permalink) Old 15-09-2012, 11:58 Thread Starter
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MGF Header Tank and Leak Test Method

Regular folks here know that my MGF Freestyle has had a small but persistent leak ( i just top up ) and on at least one occasion has boiled up through the header tank cap when sitting stationary a long time, like after a run and sitting then in stationary traffic..

i cannot see yet, in a year, any evidence of where exactly the leak is ( or as i think now - was )

so as part of the investigation i decided to see what the pressure cap was doing under pressure, and put the system under normal pressure without running the engine...

and i made this -

i went down to my local bicycle store, and for a contribution to their christmas fund... they gave me two old tubes with good schrader valves in... and so.. with a bit of adaptaion.. using a bit of hose with a hole in it.. i was able to centre that shrader valve still with its tube attached - it slid over the pipe with the hole in it.. and then i could pump up the coolant system with the header tank cap on.. using a hand/ foot pump, to see what happened.

the first time told me immediately the header tank cap was duff... i couldnt get any pressure.. and the cap just hissed air..

so i changed the cap for another from my Trophy... and was able to get pressure... but because the gauge on the hand / foot pump isnt graduated well - all i could say is it worked.. i can put pressure in, and the cap will hold some before it vents..
i need a decent bicycle digital pressure gauge now... with a more detailed scale.. since 1 bar is only 14.69 psi..

but the other thing this will do is allow me to put a non venting cap on.. and get the pressure to stay at 1 bar.. longer... so then i can look for leaks...

i ran the engine for maybe 10 minutes on idle to get it warm.. and the valve didnt leak and pressing it, vented fluid under pressure.. i am very happy now.. that i know what some of my underpressure boiling was caused by, that i had no idea of.. until i made this simple bit of kit.

Last edited by Incony; 15-09-2012 at 12:13.
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post #2 of 86 (permalink) Old 15-09-2012, 12:10
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And what were your conclusions/findings from said experiment?
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post #3 of 86 (permalink) Old 15-09-2012, 12:24 Thread Starter
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Originally Posted by MisterBateson View Post
And what were your conclusions/findings from said experiment?

if you read the post... i had a duff cap... i couldnt get pressure... so the water was boiling early, at normal atmospheric pressure...and then venting out of the duff cap.. so that is where my leak was i think... right now.

i still dont know the exact pressure the replacement cap is venting at.. until i get a better gauge.. even my digital tyre gauge, cant read that low pressure of 1 bar..

i havent got a non venting cap yet to try a constant pressure.. since the foot pump too will just leak air pressure when one stops pumping... and the vent cap also once triggered - bleeds pressuire until it re seats...

but certainly now i can use that valve to see what the pressure in the system is - once i get a gauge to do that...

i will post pics..
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post #4 of 86 (permalink) Old 15-09-2012, 12:59
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Good mod, as the cap is usually first thing to go its a good way of fault finding.
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post #5 of 86 (permalink) Old 15-09-2012, 15:38 Thread Starter
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small level pressure gauges arent easily off the shelf things.. but the old style pencil tyre pressure indicators go up in psi and kg so.. by blocking off the duff cap centre vent i was able to use it as a non vented cap... and put some pressure in the system that would stay... and that worked well i could take a few hand pumps and feel the pressure increase.., then pop off the pump and put the pencil gauge on i could happily get it read over 10 psi... one can actually feel that pressure when one uses the foot pump.. pushing against ones hand..


being mr dangerous..i started the car.. with the sealed cap on and looked to see what the pressure was... and this is a warm engine.. from start up, soon the temp gauge was reading normal.. and i took a reading... 1 psi.. huh? i know i can get 10 plus psi using the pump.. so i turned off the engine... and took another reading.. it had gone up slightly.. 2psi...

so think those low readings are because ive not got the car driving... under load etc..

i just expected the pressure to rise quickly.. and it doesnt....?

i am going to source an accurate dial gauge ... but i tried the pencil gauge then on my tyres and happily read off 36 psi.. and its full scale is fifty..

i also figure this kinda method would quickly see an exhaust gas leak into the fluid from a dropped liner... exhaust gas will quickly push up the pressure like the foot pump can..

i need to make the piece of kit more sturdy.. using some copper pipe, then i would be happy to take the car like that - for my daily work journey of 44 miles and see what the pressure got up to.. if i can use a dial gauge..

It also tells me that the normal header caps even when working, and in good condition.. dont hold pressure for anytime.. they bleed it.. since the non vented cap i made.. didnt bleed pressure near as much as the working vented one..

Last edited by Incony; 15-09-2012 at 15:48.
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post #6 of 86 (permalink) Old 15-09-2012, 16:02
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If you can measure the temperature of the coolant accurately, steam tables will tell what the saturation pressure for that temperature is.

1013 mb = 100 degrees
1100 mb = 102.3
1200 = 104.8
1300 = 107.1
1400 = 109.3
1500 = 111.4
1600 = 113.3
1700 = 115.2

up to 2000 mb (2 bar) which gives 120.2.
As the values are linear, you can interpolate the interval to get any intermediate pressures.

The above may help as its usually easier to measure small temperature differences than pressure differences, using either a mercury or infra red thermometer.
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post #7 of 86 (permalink) Old 15-09-2012, 16:27 Thread Starter
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AHH... TY...

In my local BnQ one can buy millibar scales... i guess for central heating checks... they ar in the plumbing section.. but i dont think the full scale of those is enough.. which is a pity if they are the means..

i can measure temperature easily... my voltmeter has a temperature reader, with a decent thermistor type wand,

its how i was able to check the operation of my home made temperature switch kit, i bought from maplin.. since i could set it using the meter temp reading.
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post #8 of 86 (permalink) Old 15-09-2012, 17:21 Thread Starter
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this is the kinda thing i want...

0 to 2 bar for 11.. so i would see the vent pressure mid scale.

and i now have the gauge and the valve fitted in a section of copper tube... i will post more info../

and here now is a more recent update - march 2017

ok.. being Mr Dangerous again, i shoehorned the pressure gauge into my MGF Freestyle jiggle pipe return in the header tank line.

started the engine, checked for leaks... gauge didnt move at first the pressure takes time to build up.. but as soon as i revved the engine, the pressure started to climb.

1. the temp gauge climbing to normal.. idle revs,

2 the pressure gauge reading at that time.

it climbed to that point quite fast.. and was very rev dependent.. it didnt drop below, but at every idle point it settled a little higher up.. and over idle revs.. say up to 2000 rpm i could see 8psi


revving higher to 4000 rpm i could see the gauge reading 13 plus psi.. near the 1 bar limit.. i couldnt take a pic of that and rev at the same time.. but it shows that the system pressure is pump force dependent.. as the pump turns faster, the pressure goes up .

at each time i let go of the revs and it idled, the gauge stabilised at about 7psi..

at idle.. engine warm.. no fans on..

at that point i had seen enough for this shoehorn example... i know now that the pump can push 13 plus psi into the system.. at normal operating conditions...

when i turned the engine off, the pressure dropped from 7 to 5 psi.. and stabilised.. sitting like that as the engine cooled... very gradually falling slowly.

i will post more... when i have some better hose connections and fittings..

i am pleased to see the gauge working... its a very useful bit of kit... i shall explore it further now i know it works.

of particular interest to me.. is now i understand what the normal operating idle pressure should be in a good sound system with no leaks... is a pressure switch to change a led from red to green... i.e. if there is a leak or low pressure below say 5 psi at idle something is wrong... the system cant get up to pressure... likewise it would also tell me of high pressure.. ( from a cylinder leak into the system.. )

much more useful than a coolant level switch.. since the system will change pressure before the fluid leaks low, and one would see abnormal pressure quicker... long before the vent cap vents at 1 bar.. just under 15psi. ie.. if 7 psi at idle is about normal, then 12 psi at idle isnt

it took 50 minutes today as posted this reply for the gauge to reach 2 psi... from stopping the engine..

Last edited by Incony; 15-03-2017 at 07:57.
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post #9 of 86 (permalink) Old 15-03-2017, 08:13 Thread Starter
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ive been looking for ideas for a cheap pressure switch, able to work with wet air, like the coolant is, and think an oil pressure switch like is fitted to most cars, and switches at or near 5 psi.. might work..

else the solution is more expensive and means buying something more dedicated with a change over switch and a settable pressure...

i have been considering wether i could use an ecu input.. to trigger a warning... and not have to make a dedicated light and wiring from the switch at the header tank, to the dash.. my F has one unused dash light in the centre column.. i could fit a tricolour led there..

i also want to create some rev verses pressure scale, showing the gauge reading verses revs... and see if its linear or algorithmic..

when i rev the engine the gauge movement is like watching a rev counter - its thats fast..

i suspect it should be fairly linear, unless the ambient pressure changes very suddenly and dramatically... like a thunderstorm or cloudburst or i suddenly climb up a very steep mountain...

the important thing is to find the stable window at idle revs over various ambient temperatures and pressures... and i think that's going to be close to my shoehorn test - around 0.5 bar... 7 psi. i live and drive most near sea level here in norfolk.

with the summer heat yet to come.. ive not had the engine bay fan turn on much this year... and Sue and Johns excellent table of steam saturation pressure/temp... is going to help me . at the moment i am not factoring temperature into the model... and it will be interesting if not vital, to get some accurate coolant temp readings for the rev/ pressure scale.

from that, i can get a couple of switches put into the jiggle /header tank pipe.. and be able to see normal /abnormal operation of the coolant system..

if the header tank cap , starts to fail open.. idle pressure will fall... if there is a leak, idle pressure will fall.. if the system is overpressured the pressure will rise.. maybe even stay abnormally high, and why i want two switches.. one for low pressure and one for high...

ive also considered making the gauge fit in the cab... using the heater pipe as its pressure feed and then i can just give its scale some green and red marks... but thats quite a complicated thing to do. i dont want to have to rebleed a perfectly working system this mo..

i can buy the switches and leds and put them in much easier.

Last edited by Incony; 15-03-2017 at 09:01.
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post #10 of 86 (permalink) Old 15-03-2017, 10:38 Thread Starter
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These are 20 each, adjustable from 0.2 to 2.5 bar.. higher than needed... and configurable on purchase to be NO or NC contacts.. and a reasonable accuracy window- at least good enough for what i want..

and being in the uk, helps too.

i am going to email them and see what they think.

so.. lets suppose i have a red zone... below 0.5 bar... (50kPa) a green zone between 50kPa and 70kPa and a red zone above 70 kPa

i have three things i want to see.

one switch turns on, i.e. the pressure is above 50kPa while its off i get a red indicator on. it turn to green, when its on.

if its off.. i get a red indicator only

if the pressure goes above 70kPa the high red zone switch turns on.. if it does, it needs to turn off the green indicator if its on...

that will show the red indicator... and i could have a yellow indicator showing the high switch is on.

i.e red is pressure fault, green is all ok and red and yellow is high pressure.

i know i can get high pressure the pump can do that...

when i know the exact range of the pump, maybe i can adjust the high point to suit better... maybe 80 or 90 kPa..

so it would normally be, red low... then green then red and yellow... it would normally go from red to green.. but overpressure green turns off and red and yellow comes on

thats easily done with two relays or a semiconductor. and somewhere to put two leds... red and a bicolor green / yellow... or even three leds if i can find space.?

Last edited by Incony; 15-03-2017 at 11:08.
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post #11 of 86 (permalink) Old 15-03-2017, 12:34 Thread Starter
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my first basic cct diagram.

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post #12 of 86 (permalink) Old 17-03-2017, 20:56 Thread Starter
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i have sent the suppliers of the pressure sense switches an email...

i have had a reply.. their initial concern is the max temp the switches need to operate in... so ive replied to that and await a response.

because the extreme temp..( 103 degrees C ) is a failure mode... i actually dont care if the switches fail when its reached... i want to know the pressure before that happens.. not when its happening.
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post #13 of 86 (permalink) Old 17-03-2017, 21:48
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The K series will go up to 110'c.
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post #14 of 86 (permalink) Old 18-03-2017, 22:23 Thread Starter
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temperature is pressure dependent, talking cars...

else... the coolant would boil at 100 degrees C

it doesnt because the pressure stops that...


temperature is pressure dependent..

and what do i want to know? the pressure.

is it above 0.8 bar? is it the vent cap pressure?

in effect - what the temperature is, isnt my direct concern... its what the pressure is...

if its near ambient.. then the coolant boils at ambient.. if it above that,, the coolant doesnt boil.. if its at the vent cap limit.. the vent cap opens and limits the pressure...

either way.. the pressure determines when the coolant will boil... not what the maximum temp can be. since unless the vent cap doesnt open.. it cant exceed 1.1 bar or the set point of the vent cap.

i want to know the pressure before it reaches the vent cap limit...

the vent cap doesnt care about temperature... its marked in bar... and bar is pressure. not temperature.

i already have a temp sensor... not precise.. but it tells me when the temp is normal or not...

i dont have a pressure sensor yet, that tells me when the pressure is low or high... or normal..

that is what i want. ie.. if the temp and pressure are normal, that is what i want to see..

Last edited by Incony; 18-03-2017 at 22:44.
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post #15 of 86 (permalink) Old 19-03-2017, 18:01
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Originally Posted by Incony View Post
temperature is pressure dependent, talking cars...

else... the coolant would boil at 100 degrees C

it doesnt because the pressure stops that...
Unfortunately this statement is completely wrong!

Increasing the pressure increases the boiling temperature of the water! It does not lower the temperature.

At sea level (1 bar pressure) water boils at 100 deg C.

If you go up a mountain water will boil at a lower temperature, as the atmospheric pressure is lower.

If the pressure is increased to 2 bar, then water will boil at 120 deg C

At 3 bar pressure water will boil when it reaches 135 deg C

This is why the cooling system is pressurized, not to keep the water cooler, but to raise the boiling temperature higher. The hotter the water, the more energy it is holding, and moving to the radiator. If you had an efficient cooling system, that kept the water temperature below 100 deg C, then you would not need to pressurize it (unless you drove up a mountain :-) ).

Also you are missing a valuable point about pressure.

If a radiator cap is marked at 1.1 bar, then it is actually 2.1bar absolute pressure (1.1 bar plus 1.0 bar atmospheric pressure).

This means the boiling temperature of the TF coolant is about 120 deg C.

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Last edited by g0rsq; 19-03-2017 at 18:11.
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post #16 of 86 (permalink) Old 19-03-2017, 21:08 Thread Starter
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if i am missing a point and the temperature of the coolant in your mgf gets to 120 degrees C.. and the pressure exceeds 1.1 bar..

show me..

i know from the gauge this mo.. what the normal operating pressure is..

i have shown it - i could make a video.. do you need that?

do you want me to to show the pressure temp scale in my mgf..? do you think in normal operating conditions the temp will exceed 103 degrees C and 1.1 bar pressure? ok... i can go there. can you?

lets go there..

my bet is.. before the temperature reaches 110 degrees C, i will know what the pressure in the system is...

if i know that.. then.. what can i do?

if the expansion cap hasn't already told me?

i need to regulate the pressure.. its the only reason the coolant can reach 110 degrees C... if it has..

in effect i must have kept the pressure high to stop the coolant boiling.. so it can be 110 degrees C

what i needed to do.. was stop the pressure reaching the extreme pressure.. then the temperature could not be that high..

i cant imagine anyone driving an F with the pressure in the system high enough to allow the coolant to reach 10 degrees above ambient boiling point.


you do..? ok.. i can go there.. show me how you determine that..

do you know the pressure in the system when the temperature of the coolant is 110 degrees C?

"When a temperature of 102C is reached, the ECM switches the fan on via a relay. The fan switches off at 96C."

so if the fan is on.. the ecu thinks the temperature is 102 degrees C

how does it know that? it uses a temp sensor..

so how do you know that the temp reaches 110 degrees C... because the fans are on.. or are you measuring the temp?

if so.. tell us how you do that..


to accurately measure the temp of the coolant i would need a certified temp measuring device,, coupled to the coolant at the point i wanted to measure... ?

and therefore how does one who knows the temperature in the system can reach 110 degrees C,. do it?

Last edited by Incony; 19-03-2017 at 21:53.
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post #17 of 86 (permalink) Old 20-03-2017, 23:56
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I find your experimentation interesting, and read with interest, but if I can point out a misconception you have, then this should help you understand your results.

Please research "Gauge" Pressure V "Absolute" Pressure.

To help:-

your gauge is indicating "Gauge" Pressure, as it starts off at 0bar when reading atmospheric pressure.

However atmospheric pressure is 1bar, so when your gauge reads 0bar it is actual reading 1bar absolute pressure. To convert gauge pressure to absolute pressure you need to add 1bar to every reading.

At atmospheric pressure (1bar absolute, or 0bar gauge) water boils at 100 degC

At 0bar absolute pressure (or a perfect vacuum) water actually boils at 0degC

Water can exist at any temperature you like if the pressure is high enough to prevent boiling.

If the pressure is 2bar absolute (or 1bar Gauge) then the water can be at any temperature up to 120degC.

Temperature is not dependent on pressure, nor is pressure dependent upon temperature.

When you revved your engine with the pressure gauge attached, the pressure rose and fell rapidly. Did the temperature also rise and fall rapidly?...NO.

When you pressure test your cooling system with a valve and pump, did the coolant temperature also rise?...NO

The pressure just sets the maximum limit that the water can exist before it boils.

To accurately assess if the engine is about to overheat (water about to turn to steam) you will need to measure the pressure of the coolant, atmospheric pressure, and the coolant temperature. none of these are constants.

"When a temperature of 102C is reached, the ECM switches the fan on via a relay. The fan switches off at 96C."

so if the fan is on.. the ecu thinks the temperature is 102 degrees C
If the fan is running the temperature is anything over 96degC (the temperature must have reached 102 degC first though). However the ECU knows exactly what the temperature is, as it is measuring it!

i cant imagine anyone driving an F with the pressure in the system high enough to allow the coolant to reach 10 degrees above ambient boiling point.
The pressure required to boil water at 110degC is 1.38bar absolute pressure (0.38bar Gauge pressure). Your experiment shows this pressure easily reached at a fast idle!

and therefore how does one who knows the temperature in the system can reach 110 degrees C,. do it?
110 degC is the "general" maximum coolant temperature. Anything above this is cause for concern. The cooling system will allow temperatures up to 120degC as at this temperature (and a pressure limit of 1.1bar) the coolant will boil. At this temperature the cooling system will fail!

PS, all the information is based upon a coolant made up of pure water. Adding anti-freeze also increases the boiling point of water by up to 10 degC, so you can get higher temperatures for a given pressure before the coolant boils.
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post #18 of 86 (permalink) Old 22-03-2017, 13:38 Thread Starter
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ok.. i understand...

ty.. and you are correct...

but, for all intents and purposes , where i live.. the ambient pressure will never reach of any substantial consequence to me..

so my 0 bar reading means that to me.. not that is 0,8 bar 0r 1.2 bar...

"Near Earth’s surface the pressure decreases with height at a rate of about 3.5 millibars for every 30 metres (100 feet)"

"Beacon Hill is the highest point in the English county of Norfolk. The hill is located 0.75 miles (1.2 km) south of the village of West Runton on the North Norfolk coast. At its summit the hill is 103 metres (338 ft) above sea level."

12 millibars is 0.012 Bar... or 1.2 kPa or just over one hundredth of my gauge scale.. 0 to 1 Bar..

i would need 100 millibars to move my scale above or stay below zero by one tenth. thats like diving down in air at atmospheric pressure 2800 feet? or climbing 2800 feet?

so even in very bad weather, or very good weather i dont think the .01 bar differences will greatly affect what i want to see... very low or very high readings at the wrong time.. it is far less than one tenth of a Bar... its around 100th of a bar, and pressure records show that... 1053 to @ 950 hPa...

so i am discounting atmospheric pressure completely.. i dont discount temperature and humidity, and i do have some idea from the ecu if it concerned about high temperature. the fans come on..

but i have a problem in sourcing the right sensors this mo..

"Good Morning -

Thank you for the email.

The pressure switch is primarily designed to be used in hydraulic systems where the working temperatures are lower than those experienced in an engine application.

On higher temperature applications we have to use a syphon system in order to protect the switch from the higher temperatures.

Therefore we don’t recommend the switch for application.

Best Regards

Jonathan May
Technical Sales Engineer

which is disappointing but not the end of considering other types...

oil pressure sensors are common and work typically 3 to 5 Bar switching point... assuming that ambient pressure is considered zero. its a consideration, they operate above 120 degrees C easily.

the higher pressure sensing isnt so easy to source yet.. but i am looking..

the other way to do it would be to use proximity sensors which operate at very small detecting distance... to see the gauge needle on my gauge. though its easy to source such detectors, making the electrical circuit to give me useful info, isnt easy.. since i would need several detectors, at the points i am interested in...

and i could plumb in the gauge permanently and see it too.. but that means changing the heater pipe connection to include pipe work for the gauge..

so if i can find pressure switches to do the job instead - its an easy solution..

anyone know of oil pressure sensors that switch above 0.8 Bar?

AND.. in reality i dont think this mo, that the pressure of the pump, even when the coolant is cold.. exceeds the pressure of the vent cap... else it would be venting all the time, pushing coolant out - and drawing in external air when the pressure drops... just like an unpressurised coolant system rad cap does.. remember those on cars? i used to have a pipe on mine into a bottle.. and when the engine cooled it drew the coolant back in... saved me topping up the rad all the time..

the F doesnt do that... unless the vent cap opens because of internal " vacuum" or internal high pressure.

i still think that knowing the pressure status is very valuable... even if i display it just with three leds.. and using two pressure sensing switches..

until i can source two switches my plan is to semi permanently plumb in the gauge to the header tank... and get some reasonable idea of the scale of the system in operation. Summer comes, and it will be interesting to see what happens on very hot days.. and the late summer thunderstorms.

Last edited by Incony; 22-03-2017 at 14:44.
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post #19 of 86 (permalink) Old 22-03-2017, 19:57 Thread Starter
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these would work.. but i would need something like a 555 semiconductor to be the level trip and turn the leds on and off.. the downside is the price..

Compact Rugged Pressure Transmitters

something as simple as this to drive the leds.. maybe a few mods to get the switching point adjustable.

Voltage Controlled Switch using the 555

thinking about that, i would only need one sensor then... because two 555`s can determine the switching. in effect one in reverse of the other.. and therefore i also get the midpoint switch if i use two relays.

still, i need to semi permanently fix the gauge in to see what the actual scale settings will be... it will also let me set up the 555`s properly..

and i think the kind of accuracy i need is well within the limits of the devices - a couple of kPa. is all i need to warn me.. below 5 say and above 8... ? and once i know what the car actually gives across a range of use in different weathers i can probably tune it more accurately.. it might go above seven often, for example but not often above 9... even with the coolant cold and the engine flat out.. it might go below 5 sometimes but never below 3 etc etc.. and that in good sealed system with a fully operational cap..

i dont know for certain yet.. but i really think its worth finding out. even when my head gasket failed it wasnt the temperature that told me... it was the leak... the cap leaked ( so i didnt get proper pressure) , and the head gasket failed over time and i lost fluid.. so i also couldn't get pressure ... i never saw the temp gauge go higher than half way in all that time.. a year.. but i bet i would have seen the pressure changes if i could have ... and understood the consequences better.?

Last edited by Incony; 22-03-2017 at 20:34.
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