Idling Fault

[Wantanoilwell]

My Chimaera 400 (year 2000) has had an intermittent idling fault for many months now and I am not making much progress in curing it . Basically, it will start to ‘hunt’ when idling and often stalls at junctions. The problem only occurs when the engine is fully up to temperature and seems to occur at random, misbehaving one minute and then running absolutely fine the next .
Jason at Scole Engineering thought that the problem may have been a poor earth to the ECU and has now wired a new earth straight onto the battery (he has ruled out any issues with the stepper motor by the way). He also sent the ECU away to be tested and this came back with a clean bill of health. The replacement earth seemed to cure the problem for a couple of days, but it has since returned. Before the new earth was connected, whenever the idling fault occurred the engine would also ‘snatch’ during gear changes . This has now improved but is still manifests itself to a much lesser degree when the idling fault happens.
Another symptom which seems to be linked to the fault is that whenever the car is started from cold after the idling fault has occurred during the previous trip, it will run rough for the first minute or so, as if it is running really rich. It will then settle down and run normally for the next 20 minutes. After that, the fault may then start to appear.
There were two fault codes logged on the ECU when Jason checked it , namely lambda sensor and fuel pressure. However, the lambda sensors are both putting out the correct voltage and I am puzzled as to why fuel pressure would be an issue because the car accelerates normally even when the idling fault is present.
Anyway, the reason for the post is twofold.
1) Does anyone have an idea as to the possible cause?
2) Jason thinks the air mass meter may be at fault. Unfortunately, his spare has disappeared. Does anyone have a serviceable spare that I could borrow for a week or so to rule this in/out? I am quite happy to leave a deposit if required.
I live in Suffolk by the way

Thanks in advance,

Richard

[Lighterfuel]

Interesting; my 4.0HC has similar symptoms......but I've not got round to cleaning the stepper motor yet. I'm really hoping it's just a lazy stepper......will hopefully get a chance to clean her up tomorrow.

[Andy chimaera]

mine used to hunt a lot, i replaced air filter and coil, got it tuned cos all of the settings were a little bit out and it ticks over spot on every time now, is there anything else dirty or could do with replacing, plugs, leads etc? i think the problem with mine was that the last owner had a 'normal' mechanic looking after it and never had any gear to look after a tvr properly and i think he was just randomly turning screws and things to try to make it run better, but never replaced the things that needed replacing, so when i bought it, i changed everything, leads, plugs, coil dizzy cap and arm etc, took it to dulford, they plugged it in to their gubbins ( see i know my terminology lol) and said everything was a few degrees out, or a few percent wrong and set it up properly and it has ticked over properly ever since

[maintel]

Try changing the fuel pump relay. Its a multipole relay, albeit it looks like a normal one. Poor contacts caused mone to run erratically & overfuel to the extent that I could smell the excess when driving. Went like a rocket though!

[FlipFlopGriff]

Had something similar and swapped the ECU. It had been damp (or wet) in the past and the aluminium casing corroded so lots of white powdery deposits inside. Problem cured and approx 25% quicker now.
If you can borrow an ECU, worth a try.
FFG

[Wantanoilwell]

Thanks for the replies. Scole Eng did send the ECU away to be checked and it's been given the all clear. That being said, I suppose the only way to check for certain is to borrow one.

I've ordered a coil, rotor arm and dizzy cap and will change those at the weekend just to rule these out.

The fuel pump relay sounds like a possible cause as one of the fault codes logged in the ECU was low fuel pressure.

I'm pretty sure that it's something simple.....it's just finding out what!

[pincher]

Is the 'hunting' anything like the second vid in this thread?

http://pistonheads.com/gassing/topic.asp?h=0&f...

[blitzracing]

You need a whole series of things to be spot on to get a stable idle. If its not a sticky stepper, then it can be a faulty speedo calibration unit telling the ECU the car is moving when its not. This will cause a ramdomly higher idle than expected, but you need to be able to check the speed signal on the ECU (Pin 37 from memory). Its a pulsed signal that should dissapear below 3 mph or so if its working. Also the mixture needs to be spot on, if for any reason its running rich the car will tend to stall as it the engine gets warmer. A misfire can cause a misreading from the fault code reader, as unburnt fuel looks like a lean mixture, so it could be sensed as low fuel pressure, or a failing lambda (the output drops to 0 volts when the mixture is lean or the lamda is faulty). As for the AFM, they tend to make the engine overfuell horribly when they go wrong, as the output voltage goes up, so the ECU puts in more fuel. You need to take out a plug from each side and check the colours to start with. Black and sooty means to much fuel, very light brown or white means not enough. If one side if different to the other its likely you have an ignition issue as the ECU can fuel trim each side of the V8 independently, so it will richen up one side only for a failing plug on that side.


I normally have a web page about it all, but Ive not paid my bills to the web hosting company, so its off air until I do!

[Wantanoilwell]

The hunting is very similar to the youtube vid, except that mine rev's between 400 and 1000 rpm. Sometimes it will actually stall, other times the revs manage to recover after each dip.

I'll add a plug check on each bank to this weekend's planned activities.

I think that it does seem to run rich when this happens, as the exhausts emit a puff of black smoke each time that the revs recover. Maybe the plugs will confirm this.

Thanks for your input guys. This is all valuable information for me to work through!

[DHUDD]

Hi Rich

Have a look at the relays under the passenger floor! My Stalling problem was solved because the one relay that operates the lambdas and fuel injection was not working correctly! Ebay £15! Make sure you get the correct bosch type relay!

Regards
Dave