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Engine was running rough, but now just rich. Can't find a problem....

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Posted (edited)

Hi folks,

I'm getting kinda demoralized here. TL;DR Car is running too rich after some rough running that seems to have gone away. No idea what the cause is. Car is modded R33 GTSt, FCON Pro, MAP sensor, stock (untouched) block (see also my other post ).

Few weeks ago did a long journey, mostly motorway cruising, but some small bumpy roads at the end. (It's possible something got wet or shaken loose...). Car was fine. Next morning fired up and immediately running rough, kinda chugging at idle (never done this before). It steadily improved with some light throttle. I couldn't see any problems - but I was away from home and needed to drive. I bimbled up the road and it had some hesitation but settled down. Driving started with occasional hesitation but pretty much cleared up (but not entirely). All of this at low revs but there wasn't a problem with increasing revs. I made the long journey home again with almost no issues (hesitation only a couple of times at steady / low revs). The only other obvious symptom was that it is running rich, like ~6%. For steady cruising, the AFRs on my wideband are about 13.8-14.1 and should be about 14.8.

Without finding anything obvious I suspected ignition / coil problem (being over 20 years old!). All spark plugs looked fine (maybe a bit black but they're not old, HKS brand.). So I did the R8 coil conversion (plus new ignition harness) but no change in rich mixture. However, any hesitation seems to have stopped. In fact, the car seems fine except for still running ~6% rich. Why is it rich?! I have checked a few things:

Fuel pressure is good (from cockpit pressure gauge). The engine temperature sensor seems fine (resistance and voltages check out), I have cleaned and replaced the engine earthing and I cannot find any loose connections or air leaks. I also replaced all the top breather pipes. I have tested the coldstart valve by blocking it and it seems to behave correctly (i.e. no effect once warmed up).

Now, not sure best plan of attack. There could be an air leak (guess I need a full leak down test for this?) or electrical problems with the MAP sensor or ECU but I don't know how to easily test this - plus it's running fine. (I will remove the FCON entirely and go back to sock ECU for a road test, not done yet, although maybe not very helpful since mapping will be different).

What else can I test without buying spare parts or a costly and complicated trip to the garage?



Edited by BennyT
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1 hour ago, BennyT said:

There could be an air leak

Won't be this. Under cruise, that is a vacuum leak, leading to lean operation, not rich. Only goes rich on boost, when you lose the air outwards.

The presence of an FCON is not a trivial thing here. I would take it to the most suitable mechanic, put it on the dyno and see WTF is going on. Internet diagnosis of this sort is not going to lead to anything except 10 suggestions for things it won't be followed by as surprise announcement from you that you found a family of frogs living in the cat converter and once they were moved on the problem went away.

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For the record, I think I've identified the problem - indeed an ECU problem. It looks like the ECU was still in A/F closed loop mode when I had some ignition problems (likely due to bad coil pack(s) and / or bad loom and earthing). As a result the fuel correction map seems to have over compensated and thrown the fueling out (probably got screwed up by the misfiring, maybe even bad AFR readings). Having deleted the correction map, AFRs have returned to "normal" (actually they're now too lean but I will remap in due course). No garage trip required just yet :)

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