M20B20 higher and rough idle

[petdej]

Please, I need help about this problem:
On my car (engine M20B20 with Motronic 1.3 - ECU 172) there is problem with idle. Idle is higher and quite rough (about 1000 when warm, when cold a little higher). LPG conversion is done long time ago. Idle is the almost same on petrol and on LPG (same problem is present on both fuels). So, I already done thorough check this parts with new one or with good one from working car:
ECU (with another one 172 and 381) - same
ICV (cleaning and two spare parts) - nothing
CTS (new one and used one) - nothing
AFS (with one from working car) - nothing
Another throttle body with adjusted throttle plate to specs with new TPS - still nothing.
Changed intake manifold gaskets, throttle body gasket, valve cover gasket and under valve cover "rubber caps", oil cap, injectors O-rings, O-rings at oil dipstick.
At last, checked complete wiring and harness from ECU to engine and sensors...
Smoke test is OK (no vacuum leaks).
I am totally confused.
Any ideas ?
Thanks in advance...

[Brianmoooore]

Remove the ICV, and either block both hoses, or block one port of the ICV (finger of a rubber glove over one port) and refit it.
What is the idle like then? Still high, or you need to apply throttle to keep the engine running?

[petdej]

Update:
With blocked one port of the ICV, engine is struggling to idle (about 550-600 rpm approx.), very rough. It seems like it is barely idling.

[Brianmoooore]

That seems to indicate that the extra air the engine needs to idle higher than it should is getting in via the ICV. rather than a leak or a maladjusted throttle stop.
Unfortunately, you seem to have tried everything that could possibly be wrong with the ICV control.
Time for some lateral thinking. You say the engine is idling high, presumably by looking at the tacho. Is it possible that you have a coding chip for a four cylinder engine fitted in your instrument cluster?

[petdej]

Thank you for your prompt response. I doubt that ICV somehow lets in too much air, but that it is not a problem in ICV itself. That's why the situation confused me, because I checked or tried everything else that came to my mind. It is impossible that the code plug-chip in the instrument panel is wrong, it is an original, factory board that worked well. There is a possibility that the tachometer itself is problematic, I will try another tachometer, there are two more instrument panels of the same type. But, checked earlier with a precision test tachometer for 6 cylinders and the number of revolutions was similar to the number of revolutions on the tachometer in the car. I'll check again though. As soon as I check, let you know.
Best Regards.

[Blanca]

Tried a set of spark plugs? Just a thought. Could be running on 5 1/2 cyl.

[petdej]

Of course... Brand new spark plugs, NGK ZGR5A with adjusted gap. New distributor cap, rotor, cables...

[Blanca]

Have you tried the old 'pull a plug' trick? Pull each plug cap off, if there is no difference on one then that's the cylinder to worry about, if no change, do same with injector leads. at least you will eliminate two possibilities. or find the fault, lucky you.

[petdej]

New update:
I tried with two instrument clusters that were removed from the vehicle with the M20B20 as well.
Idle, unfortunately is almost same, about 1K RPM.
I'm out of luck and out of ideas.
Any new ideas, please, anybody ?

[Blanca]

Having tried the obvious (and not so obvious) whatever is left, however unlikely, must be the answer.

Think sideways and picture the engine in your mind, what IS working OK, and tick off one by one what you know to be good, what's left is suppositious and to be investigated.. If your not sure 100% something is 100% good, look again until it is 100%.

Do not hope all over the place guessing, you'll go nuts, if not already.

[coopman]

Does anyone have a recording of a "perfect" M20B20 engine idling?

(I've recently cleaned my ICV, throttle body and changed the big rubber boot - the idle is in the right range and doesn't sound particularly rough, but I'm not sure it's perfect either.)

[petdej]

I don't know why, but I have a feeling that the problem is around the throttle body. This is currently the only possible place where "excess" air can somehow enter the intake, since the throttle body gasket is good, new. No vacuum leaks, tested many times with a smoke machine. Maybe too much air is entering through the throttle body, no matter how everything is adjusted properly. Is it possible that the throttle plate is deformed or worn out? Although I've tried two throttle housings, that's the only thing that comes to mind right now. Really, a big mystery that drove me crazy. If I take off the boot on the intake and leave only the ICV for intake air, and completely close the throttle with a plastic bag, could that be a test for the throttle housing? Eventually disconnect AFM during "test" ? Maybe this is stupid to ask, but I have no more ideas.

[Lemon98]

Does anyone have a recording of a "perfect" M20B20 engine idling?

(I've recently cleaned my ICV, throttle body and changed the big rubber boot - the idle is in the right range and doesn't sound particularly rough, but I'm not sure it's perfect either.)

I can get a video of mine for you later today. As far as I'm aware everything is working pretty well, it starts everyday and the idle is good smooth and increases and decreases as I'd expect with engine temp.

It is a bit sluggish to rev up immediately from idle if I give it too much throttle and it does have a 'mild' chip but the engine is stock bar an air filter. Mine is also a 172 ECU 1990 year


Also when you mentioned the throttle body is it possible that it has a 2.5 throttle body on it instead with an unadjusted tune?

[Brianmoooore]

M20 is a straight six engine. This is the 'perfect' configuration for a IC engine, with a power stroke every 120 degrees of the crank, so the idle should be extremely smooth.

[Blanca]

My 'tick over' is a constant 'buzz' from the exhaust at 900 RPM, no hunting, no stuttering. I changed the valve cover to throttle body pipe a few weeks ago, that helped smooth it out Also strip and clean ICV., that alloy oxidises a lot.
Not sure if your engine has the air adjuster thingy on the right side of the inlet manifold (over valve cover) but fine tuning that helped too in conjunction with a colortune..

[Brianmoooore]

It seems that the idle speed has been confirmed as definitely excessive, and that the excess air that the engine MUST have to do this is entering via the ICV.
Conclusion: The ICV is being opened slightly to far by the ECU, but for reasons that haven't been determined, in spite of virtually everything relevant being checked or replaced.
Has a replacement ECU been tried? Have you checked the existing one for signs of tampering - any possibility of it having been 'chipped' at some time?

[petdej]

Thank you for reply, Brianmoooore.

As I stated in a first post, there is two spare ECUs (non-tampered, with original EPROMs, checked with EPROM programmer) in my spare parts (172 and 381). With any ECU, idle is higher. Even tried different original EPROMs, different versions of ROM dumps for ECU 172 and 381 (I have EPROM programmer, so I tried even that !). No difference. Real headache.

[petdej]

At last... SOLVED !

It seems amazing, but all three tachometers I have are faulty !
I asked a friend who has the same car, the older generation, with the L-jetronic system to try out his instrument cluster. YES! To be sure, I measured again with one precision test and calibrated tachometer for 6 cylinders. Again ... about 750 RPM! Damn lying tachometers ! Thanks everyone for the help. I will remember this lesson well.

[Brianmoooore]

There's another high idle thread just started. Will it be the same??