Chimaera Hunting and sometimes cutting out at certain temps

[Crxpaul]

Hi

I've had a look online and have tried a few things but can't seem to diagnose or fix an issue I'm having.

I've got a '97 Chimaera 500 - The car starts and runs fine, but when the engine is warming up and reaches around the 85 degrees mark on the temp gauge it will suddenly start hunting at idle up and down about 4-5 times. Sometimes the revs only fluctuate by a few hundred rpm and then settle back down into a normal idle but other times it is worse and the engine will either cut out or I'll have to quickly give it a blip of throttle a few times to keep it from cutting out until the idle smooths itself out again. This only ever happens at a very specific temperature (to the point where I can tell when it's about to happen by looking at the temp gauge) and once it has done this for a few seconds it runs fine again. The fans seem to be set to both kick on at about 90 degrees which usually happens a minute or two after the idle problem so I don't think it is related to this. Also once it has passed this temperature the car will run fine, and if I'm driving ratehr than at idle when the temp gauge goes past this point there is no noticeable difference in revs etc.

I've tried swapping out a few parts to see if it helps but none made any difference:
-Ignition amplifier
-Stepper motor
-I've tried my best to check for possible air leaks using brake cleaner and have resealed the intake plenum etc to try to rule out any possible leaks

I've just had the car hooked up to RoverGauge but I don't really know what I'm looking at if I'm honest. Everything looks fairly normal until the car reaches this temperature, then I suddenly got 100% lambda fuel trim (more fuel) on both odd and even for a few seconds while the car started hunting, then afterwards the fuel trim went back to 0% on the even bank but remained at 100% on the odd bank for about another 10 seconds - After this, both banks seemed to settle back down and remained on 0% again.

Any help or advise would be massively appreciated!! I can upload pics or vids maybe of the rovergause results if anyone is able to decifer this information?

Thanks!

[sixor8]

Erratic idle speed is often down to a faulty stepper motor. It receives the signal from the ECU and controls how much air is bled in to maintain idle.

[Crxpaul]

Thanks for the reply, I've already swapped the stepper motor out for a new one and it made no difference, also I'm getting fault codes for both labdas

[blaze_away]

As you have Rover Gauge already running I can help with analysing the data for you. From what you've said there is a possibility that you've not got things set up perfectly well.

I'll need you to run this process and record the data exactly as specified. Send me the logfile created and I'll then go through it for you.

[Belle427]

I would concentrate on the ignition side of things as a misfire can cause this.
Let frank above analyse some data for you.

[Crxpaul]

Thanks for the replies, I'll give it another go with rovergauge and will send the file over to you, really appreciate the help on this! It doesn't feel like a misfire but I might be wrong, I'll get that file saved and loaded on as soon as I can and hopefully will have a better idea then.

Looking at the guide posted above it says it is better to start the procedure with the engine cold so I will run through this tomorrow as the car has already been ran up to temperature today.

Edited by Crxpaul on Monday 14th July 16:25

[pmc_3]

When I had duff lambda readings it was the MAF sensor playing up. Might be worth seeing if you can borrow one to test.

[Crxpaul]

Thanks, I'll look into that 👌🏻

[Andy70]

mine had a split in the stepper motor to plenum hose, that was causing hunting, worth a look?

[blitzracing]

The shift of lambda to adding fuel 100 percent can be a misfire as an unburnt mixture looks like a lean mixture to a lambda probe. The fact it is on both banks means it is pre the distributor, ie coil, amp, dizzy or trigger, but the RoverGauge logging may show this up as unstable RPM outside what the engine is actually doing. It could also be an airleak with thermal expansion, but again needs to be common to both banks. You could try your spray around the plenum and see if you can alter the short term trim. It could also be a drop in fuel pressure from the pump, but far less likely at idle when fuel demand is low.

[Crxpaul]

Blazeaway very kindly analysed my rovergauge readings and diagnosed a likely air leak, I've investigated this with a smoke machine and found there to be a small leak between plenum base and intake manifold, another small leak on one of the plenum bolts on the opposite side to the base leak (the bolt had worked itself slightly loose), and also what seems like a comparatively large leak from the throttle spindle seal on the side opposite the throttle position sensor... I've ordered a couple of new spindle seals (the only ones I could find were for a landrover v8, so hopefully these will fit). Once the seals arrive I'll get everything air tight and re-run the rovergauge and see how it looks. I've also been told I may need to reset my base idle, but I'm going to fix the air leaks first in case this was having an effect on this. I will keep you all updated as I go along. At least I've now found a known problem, so if nothing else I'm one step closer to getting everything running the way it should.

Again, massive thanks to blazeaway for his assistance so far!

Edited by Crxpaul on Wednesday 16th July 20:38