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mgf
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Could anyone let us know what controls the idle revs at 900 when ticking over. Sometimes it idles at 2000 revs and it even happened at the MOT yesterday for the tester and he failed it on that alone. I told him it happens at times. So I went home for 3 hours and let the car cool down. Started her back up and idle went back to 900 so returned to the test centre he checked it and I got the Cert all ok. but I actually did nothing other than let the car cool down. The tester said he put his foot under the accelerator and it did'nt help so whats causing this??. BTY just fitted a new Alum 52mm trottle body.
I bought also a new CAT from Rimmers for £150 and last year the emissions CO2 was 0.29 and anything over 0.30 is a fail so just made it. With the new CAT the new reading was 0.00 CO2, It made a massive difference. Just the high revs :Dnow to sort.
 

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mg_tf
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I'd suspect an air leak on the intake side somewhere.

There is the IACV - Idle Air Control Valve. It is an active device that can select the amount of air going into the engine in the idle state - intake throttle valves are usually too crude to control idle speed on an engine without a separate idle air circuit. I once made the error of pulling to air tube off it whilst the engine was running and the revs went ballistic. It's in the engine bay, mounted on a bulkhead at the rear I think but don't quote me on that. I wanted to clean it inside - it is supposed to have a tendency to get blocked up. I think I needed a 'cranked' screwdriver to get the two retaining screws off. You could take it off and squirt brake cleaner or WD40 inside and clean it out to see if that helps .

Other than that there is the 'sticky throttle' syndrome. I have noticed (and others have said the same) that when at idle, if I press the accelerator pedal gently to try and get a few more revs, nothing happens and then it jumps up to 1200rpm or whatever.

I'd bet it was the IACV or an air leak elsewhere though.

One way to find air leaks is to wiggle the intake and any tubes coming off it to see if this opens/closes any split/hole in the manifold or tubes/ducting and see if the engine rpm changes. The other way is to spray something like an inflammable cold-start spray at the outside of the intake area to see if that alters the rpm (by getting sucked in and altering the air-fuel mixture). Careful with that last idea though ;0)
 

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MG TF
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When you fitted the new throttle body did you reset it? Another thing to check is a sticking throttle cable, very common.

(from another thread)
  1. Start engine and warm engine to its normal operating temperature (essential, as when cold, the engine requires some extra air/fuel - which on the MGF is managed by the idle air control valve: best not confuse the issue!)
  2. Now turn off the ignition.
  3. Switch on the ignition to position II without starting the engine (all the pre-start warning lights ought to be illuminated)
  4. Depress the throttle pedal to the carpet and fully release inside a period of 30 seconds or so.
  5. Wait for about 15 seconds...
  6. Switch off the ignition
 

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TF135
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Here's a pic of the IACV - the yellow bit is the oil filler.
4 x hex screws hold it on, then undo the elec connector and slide it off the hose.
Oily vapour gums it up after a while. There's a plunger inside that sticks but the later TF parts are sealed and you can't get at it.
I gather there's a (stepper?) motor that's meant to move this plunger to restrict/adjust air on start up then weaken off as it warms. You can hear this when you switch on, sometimes.
I soaked mine overnight in paraffin and shook it violently in a large jar as well. I wasn't really convinced it un-gummed it tho'...I ended up replacing mine and keeping the original as a spare; they're not too expensive.
Tbh I expect chris T's reset procedure is more likely to solve the problem...
135950
 

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You have a similar problem to me. Check this thread which is on-going.....https://www.the-t-bar.com/forum/pscan-uk-diagnostic-tool-forum/97138-how-does-pscan-compare-to-t4?start=12. Even with Pscan, it can be very difficult to diagnose the source of the problem.

Edit: You may find there is a simple solution. In my instance no error messages; IACV, Cam sensor, Map sensor,coils and leads replaced; checked hoses for leaks. The next thing I will do is clean the throttle body.
 

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On my car, the 2 pipes that go into the throttle body on the tight of the picture here are interchanged. But the workshop manual would indicate mine is right but in this picture it is wrong. ?????
vvc.jpg
 

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When you renew the throttle body or the IACV valve, it MUST BE SETUP corrrectly on T4. There is no other way to set it up other than on a T4.
 

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Those with Pscan would disagree with you....... especially those with vs 42b......If you are correct, god help us all when the T4 hardware finally gives up the ghost.
 

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TF135
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"On my car, the 2 pipes that go into the throttle body on the tight of the picture here are interchanged. But the workshop manual would indicate mine is right but in this picture it is wrong. ?????"

Hi Julian. No I don't think so. I think we're both correct; I took my pic standing by the driver's seat and looking backwards over the engine (ie behind yours, looking out of the pic). I thought Hobby might want to see what he might be taking apart or replacing.
And pete, I replaced mine w/out thinking or knowing that it needed to be set up. The seller didn't offer any advice, either it seems to work ok, I think...
 

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You are right plus there is a white mark on the hose that goes from the top pipe on the throttle body to the IACV to avoid any mixups (as per the photo above). Cleaned the throttle body which was pretty clean plus checked the clip attaching the air hose from the filter to throttle body. It was the original which is already pre-tensioned (ie not a jubilee clip where you can over tightened). Like the original poster, I am stumped as to what is causing the periodic high idling.
 

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TF135
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Oily MAF sensor? can be cleaned up with solvent. Vaguely remember I renewed an O ring on mine that I thought could leak.
Throttle body and IACV cleaned out. There's a sensor on the tb that might be 'iffy'.
Other things; sticky throttle cable? Oil + ecu temp sensors both inform the ecu; does the oil temp gauge read ok? Checked engine temp via OBD? Exhaust manifold / inlet manifold air leak?
Just a few thoughts...
 

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Oily MAF sensor? can be cleaned up with solvent. Vaguely remember I renewed an O ring on mine that I thought could leak.
Throttle body and IACV cleaned out. There's a sensor on the tb that might be 'iffy'.
Other things; sticky throttle cable? Oil + ecu temp sensors both inform the ecu; does the oil temp gauge read ok? Checked engine temp via OBD? Exhaust manifold / inlet manifold air leak?
Just a few thoughts...
Done all this and all the rest. I did find that the hose to the throttle from the IACV was back to front¦ it was rubbing up against the throttle position sensor until I turned the hose around but I doubt this will be the fix.

I have gone back to basics - I have adjusted the throttle idle valve to ensure that the physical throttle degree is ca. 4 degrees off the vertical. Next week I will reset the TPS so it knows that the throttle is at 4 degrees at idle. I will then test to see what happens on the idle RPM and stepper.

Looking at the PScan live data, when the idle rev increase to ca. 1500, the number of steps rises from 21 to 60 odd - I am therefore assuming that this is where the additional air is coming from, hence increasing the idle rpm.

It could simply be that I am not giving the engine enough time to adapt.
 

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TF135
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Yes agreed, throttle reset is the first priority - then if it continues suspect the stepper motor (I think it's in the IACV?) , hence replace IACV and if this doesn't work replace the TPS.
You're welcome to try my spare IACV (I haven't got a spare TPS); I think it's still around; it might just provide a solution w/out unnecessarily throwing cash away...
ps didn't know you could physically adjust TPS :sneaky: need more info on that!
 

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Yes agreed, throttle reset is the first priority - then if it continues suspect the stepper motor (I think it's in the IACV?) , hence replace IACV and if this doesn't work replace the TPS.
You're welcome to try my spare IACV (I haven't got a spare TPS); I think it's still around; it might just provide a solution w/out unnecessarily throwing cash away...
ps didn't know you could physically adjust TPS :sneaky: need more info on that!
TPS and IACV both new....I have reset the TPS and idling nicely at 1200 cold. Next week I will go on a run and see what happens re the Pscan data.
 

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TF135
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Yep 1200 cold is spot on, imo. I reckon it'll play ball now - tho' as per Chris T, re-set is most appropriate at 'normal warm' (but I'd depress the throttle pedal ~5x in that 30s time interval)
I presume you bit the bullet and replaced both at the same time, so we won't know which was the offender :confused:
(Anyway my money's on the IACV...) Pls let us know the good news when it's warmed up.
Should you need PScan verification, I wonder? Why not just take it on a 10 - 15ml round- trip to get it warm then do the re-set :cool:
 

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Yep 1200 cold is spot on, imo. I reckon it'll play ball now - tho' as per Chris T, re-set is most appropriate at 'normal warm' (but I'd depress the throttle pedal ~5x in that 30s time interval)
I presume you bit the bullet and replaced both at the same time, so we won't know which was the offender :confused:
(Anyway my money's on the IACV...) Pls let us know the good news when it's warmed up.
Should you need PScan verification, I wonder? Why not just take it on a 10 - 15ml round- trip to get it warm then do the re-set :cool:
The components were replaced weeks ago. They made no difference to the symptoms I have. Hobby, who started this thread, appears to have similar issues to me (as have many before me).
 

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Went for a run today. All perfect after several years of frustration including replacing many sensors and stopping me from using and enjoying the car. Idle steady at circa 875 rpm during and at the end of the run instead of finishing up at ca. 1500 rpm.

I think that the problem was caused by one of the following:

1. The throttle was not at 4 degrees off the vertical so the TPS was being reset at 4 degrees when the throttle was not physically at 4 degrees. Solution: Went back to basics and checked that physically the throttle was as close to 4 degrees at idle as possible using a piece of marked-up piece of paper. When the engine was hot, the stepper was showing 0 steps so I adjusted the idle stop screw so that the stepper was at 25 steps - this was 1/2 turn of the idle screw from where I had set it after the checking the throttle opening physically. I then reset the TPS on the basis the throttle was still close to 4 degrees off the vertical at idle.

2.The hose going from the throttle body to the IACV. It was the wrong way around and was perhaps putting 'pressure' as the car become hot, perhaps not by much but enough on the TPS (which I thought was poor design) to create my problem. Solution: I turned the hose around so no fouling of the TPS. I suspect the hose had been turned around by me during one experimentation using a silicon hose a couple of years ago and this was the start of my problems. Hint: On the hose, one end is a marked line in white on the hose - this attaches to the IACV.

Thanks for all the suggestions. Always appreciated. My take away is that the combination of Pscan and getting back to basics is a must to solve some problems on this car.
 

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TF135
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Good news (y)
That's got me thinking; mine's around 950rpm idle.
I thought all those adjustments were 'sealed off'. o_O
I presume the 4deg is the angle of the brass, circular throttle plate, idle stop screw is the black plasticky bit on the throttle body + that you adjusted the idle stop screw until PScan read 25 steps?
Will I be able to replicate your adjustments to get to he more normal 875-ish or do you reckon it's close enough?
(I've got a 'chinese' TF + PScan, but I haven't loaded it yet - still sorting out a laptop...)
 

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Spot on. You should get close as long as all your sensors are working and no vacuum leaks (vacuum pressure for me is 34 at an idle of ca. 875rpm per Pscan). Just relying on Pscan step count is not enough. You need to start with the basic check of ensuring the 4 degrees re the throttle plate and then use that as your starting point.

I will be interested in seeing what happens...

Three pointers

1. The black cap is an anti-tamper cover. Ease it off with a screwdriver
2. Before adjusting the idle screw, mark up the position of the screw in case you want to take it back to default.
3. After adjusting the idle screw, do the TPS reset. It assumes that the throttle üplate angle is still ca 4 degrees and is telling the TPS of that 'fact'.

As an aside, what is your current stepper reading at idle and hot engine?

Before I adjusted the idle screw (is it was set by the factory), it was circa 60 steps at hot idle - it should be between 20 and 40.......
 

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TF135
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Thanks for that. I'll definitely follow your advice. (Gotta fix up a lap top + learn my way around PScan first tho'...)
Also, adjust throttle plate to 4deg via cable or is there another way?
Sorry, not thinking right - that's using the idle screw isn't it?
 
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