And another overheating issue - MG-Rover.org Forums
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post #1 of 10 (permalink) Old 19-04-2017, 15:56 Thread Starter
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And another overheating issue

Good afternoon everyone, I need the brains of the MG heroes here on the forum. The amount of issues this MG TF 135 sprint has given me is beginning to make my sanity look questionable. The issue I am having now is an overheating issue when idling. I first noticed it when I was on a long run on the motorway and when I come to a complete stop, after about 15 minutes of idling, the temp gauge started to rise. I plugged him in to my OBD2 reader and no fault codes. I am not experiencing any coolant lose, if anything I am experiencing coolant rise! I have attached a picture, this certainly isn't normal is it!? When he's cold he is sitting bang on the max line. I did a 500 mile round trip over the bank holiday with no problems... As long as he's moving and doesn't idle longer than 5 - 10 minutes, he behaves fine. The front fan is kicking in religiously at around 220f. At this point in time I suspect an airlock... Any help would be greatly appreciated! Many thanks
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post #2 of 10 (permalink) Old 19-04-2017, 16:21
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I would have thought idling for 15 minutes would cause a temperature increase if there is no flow through the radiator until the fan cuts in?
Is the problem the fan does not control the temperature after it cuts in initially...it should switch on and off as required to control the temperature.

If the fan can't bring the temperature down, the rad could be partially blocked internally, among other things.
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post #3 of 10 (permalink) Old 19-04-2017, 19:00 Thread Starter
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Thanks for your reply, funny you should say that I took it to an apparent mg specialist garage, which I shall not name, and said to them exactly that - I don't think my rad is good. They gave the classic 'it's yer 'edgasket mate', which was somewhat surprising as I used them for work in the past and they proved to be pretty good. The irony is these guys changed my underfloor coolant pipes about 4 months ago and didn't bleed the system from all three points. They only bled from two because apparently one is a 'pain in the arse' to get to.

By the way yes that's exactly it - the fan cuts in but doesn't seem to be able to bring the temperature down.
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post #4 of 10 (permalink) Old 20-04-2017, 09:40
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If the system has not been properly bled then that would be the first thing I would do, or get done.

As yet I have not done it myself but I am sure there are plenty of forum members who can give detailed advice on doing it.

There are other indicators of HGF like oil emulsification or deposits in the coolant tank, or persistent coolant loss. It is too easy for garages to blame all ills on gasket failure, although that is always a possibility.
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post #5 of 10 (permalink) Old 20-04-2017, 11:50
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Quote:
Originally Posted by MrKitty View Post
Thanks for your reply, funny you should say that I took it to an apparent mg specialist garage, which I shall not name, and said to them exactly that - I don't think my rad is good. They gave the classic 'it's yer 'edgasket mate', which was somewhat surprising as I used them for work in the past and they proved to be pretty good. The irony is these guys changed my underfloor coolant pipes about 4 months ago and didn't bleed the system from all three points. They only bled from two because apparently one is a 'pain in the arse' to get to.

By the way yes that's exactly it - the fan cuts in but doesn't seem to be able to bring the temperature down.
'
'Pain in the arse to get to'? Yes, one has to take off the engine cover to get to one of the three but such is life. For a MG specialist not to be bothered is a disgrace. What else cannot they be bothered doing properly...? Stop using them is my advice!
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post #6 of 10 (permalink) Old 20-04-2017, 12:15
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Have you confirmed the rad fan actually does cut in? I've had such an overheating problem with fans not working (could be fan, wiring or switch). Obviously, also check bleeding has been done properly as well.

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post #7 of 10 (permalink) Old 20-06-2017, 19:28 Thread Starter
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Just thought I'd let you guys know.... finally found the issue just in time with this wonderful weather, it was the coolant cap losing pressure!! No issues now
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post #8 of 10 (permalink) Old 22-06-2017, 21:34
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Gotta ask, why does that cause the coolant level to rise in the expansion bottle?
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post #9 of 10 (permalink) Old Yesterday, 03:13
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The coolant system needs to be held under pressure as it raises the boiling point of the coolant. The hotter the coolant the more it will expand and it has no pressure hold it in lower part of the tank.
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post #10 of 10 (permalink) Old Yesterday, 19:36
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Quote:
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Gotta ask, why does that cause the coolant level to rise in the expansion bottle?
Coolant expands when it gets hot, but not enough to fill the expansion tank and overflow!

However temperature of the coolant is the reason the coolant overflows when the cap fails to hold pressure!

Science bit:-

Water at atmospheric pressure boils at 100 degC
Water at 1 bar above atmospheric pressure (cooling system pressure) will boil at about 125 deg C

When water heats up it first gently releases dissolved gasses (nitrogen and oxygen), which causes the pressure rise in the coolant system. In a closed system, such as the F/TF cooling system, this building up of pressure provides equilibrium to prevent the coolant boiling. (In a sealed pressure vessel water will never boil!!!).
So long as the temperature of the water does not exceed the boiling point, for its pressure, then the coolant system stays in equilibrium and will not boil.

The F/TF cooling system keeps the coolant temperature at about 110degC and a maximum pressure of 1Bar over atmosphere, so as long as the system is pressurised to 1bar, and the temperature does not exceed 125degC it will not boil

However if the pressure is reduced to atmospheric, with a leaking cap for instance, its boiling point will be reduced from 125degC to 100degC but the actual coolant temperature is still about 110degC.

As the water now starts to boil it now rapidly releases large quantities of water vapor (steam) which is a lot less dense than water so expands in volume. This forms large bubbles in the coolant (like when a kettle boils) which displace the coolant out of the expansion bottle, casing it to overflow.

So excess temperature at reduced pressure, cause the water to boil, which displaces the water out of the expansion bottle.

You did ask
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