M42 runs poorly after partial cool down.

[Mark 42]

After a complete warmup, if I let the car cool for about 45 minutes,
when I start it up it hesitates and sometimes even dies when I accelerate.

It does it for only about a minute, and only if it cools for 30 to 90 minutes
after it was warmed up before.

So far we have replaced sensors and a leaky combination hose, which has
changed the way it behaves (it recovers a bit faster), but it is still doing it.

Also, if I turn it off and back on, that seems to clear the condition too.

We're at a point where the next thing to replace is probably the ECU,
but it has already cost a lot of hassle and money, with only a little
improvement, so I really would like to find someone who had the same
problem and figured out exactly what the cause was. It's getting
frustrating to change parts and not resolve the problem.

Anyone got a "silver bullet" for this one?

[oldmann]

CK coolant temp sensor, there are 2, both under intake manifold, front 1 feeds info to ECU. Search "m42 temp sensor" for sensor values read with a ohm meter.
Read your post again, you replaced sensors, try CKing the intake temp sensor, also need to CK fuel pressure to the rail, fuel pump ck valve may be bleeding off pressure after shutdown.
Also vapor canister needs looked at, the hose's and valve operation, may be hanging open, or when it opens there may be a cracked hose, in or out.

[roguetoaster]

What hoses and sensors have you replaced? I am thinking it might be a bad TPS or IACV if it's fine when it's up and running.

[mcskibadee1]

I have almost the exact same issue http://www.r3vlimited.com/board/showthread.php?t=285738

If you figure it out please post here!

[Mark 42]

I forgot to mention the "Mash the gas pedal" trick (I found that one too).

This morning it did start doing it after I stopped to fill the tank - maybe 5 or 10 minutes at most - which surprised me.

I have replaced the air cleaner, fuel filter and spark plugs.
The sensors and the combination hose were replaced by a mechanic, who is
also getting quite frustrated with this problem, I think.
The mechanic even went so far as to take parts off of his own E30
and swap them onto my car to avoid buying more new parts to use for diagnosis.

If anyone finds the cause & resolution for this with Google or otherwise, please post it here!

[ballarde30]

I had the same problem with mine and ended up checking or replacing all the sensors I could think of. I gave up and randomly came across this a few months ago and haven't had a problem since.

http://www.e30tech.com/forum/showthread.php?t=105848

[Mark 42]

I've seen the wire it refers to. Mine is plugged in - I even took it apart and
put it back together to make sure it was making contact.

I will try it with the wire disconnected, maybe the P.O. saw it apart and
plugged it in thinking it had just fallen apart. It shouldn't hurt to try.

[bmwman91]

Yeah, if you have a car with an O2 sensor, you do not want that wire connected. That was for the LLCO output of the AFM which was sort of an "alternative" for the O2 sensor, but it causes all sorts of hell when you connect it on a car that has an O2 sensor.

[Mark 42]

I disconnected it this morning (Gray with black stripe) - will see what results.
If it works, I'll post a photo for future ref - probably worth a sticky if it does work.

[mr ilia]

updates?

[Mark 42]

updates?

I only got one data point - and it was after 1-1/2 hour, so may have been too long of a cool down to be valid.

Tonight I'll do TKD with my kids, and that is a 45 minute event (it's the
activity that highlighted the problem in the first place - and since
it's a couple of days per week, it's why I didn't decide to just live with the problem).

I will definitely report back when I have enough tests to know for sure.

[mcskibadee1]

Damn, i will have to try this when I get home.

[Mark 42]

Damn, i will have to try this when I get home.

Please report back.

I'm holding off until I get a few more "test runs" (30 to 60 minute
cool downs from fully warmed up). It will take me about a week
to get enough trial runs to feel confident that I can say whether
it cured the problem or not. Of course, one failure would also be
conclusive.

[mcskibadee1]

Update: So I was telling my brother who was the previous owner of this car about this fix that was posted and he has this blank stare on his face. Apparently when we replaced his windshield wiper motor he noticed that those plugs were disconnected and connected them back together. I just unplugged them and so far so good. This issue would literally happen every time except mornings and I have started it twice while warm and it is fine.

[Mark 42]

You would think they would have put protective caps over the plugs or
a tag which reads "Leave unconnected for vehicles with Oxygen Sensor
and Catalytic Convertor".

Mine was reading an O2 sensor fault - which the mechanic cleared,
and then it returned, so we replaced the O2 sensor. I wonder if the
wire being connected was what caused the O2 Sensor fault indication.
I can't really fault the mechanic - it's just not possible to know every
detail of every car, even when you only service certain brands.
I've let them know about the wire being a possible cause, and will
follow up with the results - but sure wish they'd found out from someone
else before I brought my car. Sometimes that's just the way things go. :(