Motronic doesn't like and frankly...

[Morgenster]

Picked up after a different thread concerning a 'shogun' reset:


if your idle goes downhill after running like a dream, your O2 sensor or MAF sensor needs replacing.

Sure about that? The thing is: I did it again after changing oil/plugs/filters/valve cover gaskets and cleaning the throttle body, ICV and vacuum hoses last weekend.
That was a week ago and it's gradually gotten worse again over the course of 5 drives and 150km.
So now I don't think there is a vacuum leak at all, because the symptom would still be there right from the reset, right? Or not?
If the MAF is bad then why doesn't the engine run really crappy? Now the only symptom I get is delayed response from my ICV and a low idle that gets a little nutty after applying throttle, sometimes stalling the engine on coming back to idle.
I'm in no position to read my engine codes nor is my mechanic (oddly enough). I don't think this would give conclusive answers anyway.
When I was doing all of the above I noticed the intake portion of the MAF was dirty but the wire seemed to be squeaky clean so I didn't touch it.
Sorry to make it hard to diagnose like this but I'm not in a financial position to start swapping out all sorts of sensors to find the problem.
I feel like I'm getting closer to hunting this bitch of a problem down unless my assumption about resets is wrong.
Any help/diagnostic trick/input is welcome and I would be thankful.

PS: doing the above-mentioned work on the engine was hell for a first timer and total newbie but oh so rewarding and I have you guys to thank for all this.

[shrike071]

You're "in no position" to even do a stomp-test and look up a few numbers? The car may tell you exactly what is wrong with it...

[Alexlind123]

It might have something to do with the LPG.

[BillionPa]

your O2 sensors have more than 100K km on em?

[Morgenster]

Thanks for the ideas guys.
Only US-spec cars have a stomptest function. Mine doesn't.
The LPG system should not be a part of the problem because it hasn't been operational for the whole week so the car's been running on benzin only.
As for the O2 sensor: The idle actually gets better after warmup. It's cold starts that are giving me this. I suspect a bad O2 sensor would give me trouble after warm-up. I don't know it's age though.
Am I right to think that a vacuum leak can't be the source of the problem?

[genphreak]

Spot on, O2 has no effect during warm up. I'd check the temperature sensors first, and check that you ahve all 6 injectors firing properly when cold, let alone the spark plugs. Unless the plugs are bad I would be surprised if it was. ONly thing that goes wrong are the wires (at a ridiculously senior age)...

Checking the temp sensors is easy to do with an ohmmeter. Unless it's vacuum of course... best to use a butane torch to find the leak there. (but not on the exhaust side (not that's any need, unless you want to blacken your face!!).

[Morgenster]

Spot on, O2 has no effect during warm up. I'd check the temperature sensors first, and check that you ahve all 6 injectors firing properly when cold, let alone the spark plugs. Unless the plugs are bad I would be surprised if it was. ONly thing that goes wrong are the wires (at a ridiculously senior age)...

Checking the temp sensors is easy to do with an ohmmeter. Unless it's vacuum of course... best to use a butane torch to find the leak there. (but not on the exhaust side (not that's any need, unless you want to blacken your face!!).

I was thinking along that line (temp sensors). Cold loop sensors can be TPS, MAF and temp sensors right? Or am I missing something.
I already tried finding a vacuum leak by spraying intake cleaner all over the intake parts. No dice. Is butane better?
Spark plugs are brand new. valve cover gasket is also and doesn't appear to be leaking since I changed it last weekend though I only checked the outside. The problem shouldn't be injectors, ICV or plugs since everything runs A-OK right after a reset. When I reset, the idle goes way up on start (2000 rpm) and then immediately down to 1200 > 1000 until warm and then it's a nice responsive 750 rpm.
The TPS read between 1900 and 5000+ ohms when I tested it with no hiccups.
So I guess all that remains is testing the MAF and temp sensors.
I'm still a little confused about their function and location.
I know the M50 has two. one is nearer to front than the other and I can't remember which oe does what.
Does one feed the cluster gauge and the other the DME? and why separately?

[Morgenster]

http://bmwfans.info/original/E34/Lim/520i-M50/ECE/L/M/1991/04/mg-11/ill-11_1281/#15

Is it number 8 and 6 and which does what?

[genphreak]

I was thinking along that line (temp sensors). Cold loop sensors can be TPS, MAF and temp sensors right? Or am I missing something.
I already tried finding a vacuum leak by spraying intake cleaner all over the intake parts. No dice. Is butane better?
Spark plugs are brand new. valve cover gasket is also and doesn't appear to be leaking since I changed it last weekend though I only checked the outside. The problem shouldn't be injectors, ICV or plugs since everything runs A-OK right after a reset. When I reset, the idle goes way up on start (2000 rpm) and then immediately down to 1200 > 1000 until warm and then it's a nice responsive 750 rpm.
The TPS read between 1900 and 5000+ ohms when I tested it with no hiccups.
So I guess all that remains is testing the MAF and temp sensors.
I'm still a little confused about their function and location.
I know the M50 has two. one is nearer to front than the other and I can't remember which oe does what.
Does one feed the cluster gauge and the other the DME? and why separately?They commonly go. Yes, the head mounted coolant sensors are for the gauge and the DME seperately. The MAF one will probably be OK.

That TPS reading... should your TPS not be open cirucuit at idle?
Mine clicks open and closed (M30 has a very different TPS) but what does the manual say it should be?

Let us know how the temp sensor tests work out. :) Nick

[BillionPa]

there is also the air intake temp sensor after the throttle body.

[Morgenster]

Thanks for the input guys.
This weekend I'm going to test every sensor I can get my hands on. I'll be borrowing a multimeter from work.

As for the TPS ohm-readings: TPS on M50 is a pot-sensor and should read 1k ohms at fully closed to 4k ohms fully open according to Bentley, but they don't specify any tolerances. I'll be testing it again anyway.

Now that the weather is improving the symptoms seem to get better again.

[BillionPa]

+- 5% is your "safe bet" tolerance, if the multimiter has been adjusted properly.

[Morgenster]

Just posting to follow up so people can refer to this:
My TPS resistance at fully closed is somewhere around 1,7k ohms.
I adjusted the little screw that blocks closure of the throttle butterfly (top left of throttle body) and it took some time to adjust but it worked.
I also found out that the margin on the TPS is 1-2k ohm for idle through a specialised software my mechanic showed me.
Cold starts are fine now.

[Barney Paull-Edwards]

It may help in future to be able to find out which plug/lead is u/s.Buy a snap-on or similar heat detector(looks like a plastic starting pistol),they are about $60 here(UK) but the best diagnostic tool ever.Just point at manifold and it will show the duff one,also good for all sorts of other troubles like wiring potential faults/sword defects etc.I have a twin (8) plug headed BM motor and it`s saved me a fortune in plugs and leads.