runs great cold, and hot, but BADLY in between.

[njansenv]

Hey all. After wanting to buy one for YEARS, I finally picked up my first E30. It's an '87 325 (eta), and it runs pretty well...most of the time. It runs great when the engine's cold, or once it's completely warmed up, but is very rough when it's 'warm'. The problem seems to be sensor/ECU related, because it's almost like a switch is thrown when the problem clears up.

Any ideas? I did search....

The long term plan for the car is to get caught up on any maintenance stuff, then swap an i head and intake, Megasquirt it, and boost it. I already love this car.....
The car is being used primarily by my girlfriend for short city trips, and the
problem doesn't clear up until just before shes done with it. :)

Thanks,
Nathan

[jlevie]

The first thing I'd do would be to check for intake leaks and/or a sticking ICV. Then do a valve adjustment, replace the plugs, wires, rotor, cap, and O2 sensor. If it still isn't running quite right after that I'd suspect the engine temp sensor or the AFM. A bad DME is possible, but you'd usually have other more noticeable symptoms.

[redhatpat]

...I'd suspect the engine temp sensor ...

That's what I thought too. The Bentley has a simple test for that sensor. You do have a Bentley manual, don't you? ;)

[njansenv]

That's what I thought too. The Bentley has a simple test for that sensor. You do have a Bentley manual, don't you? ;)

Bentley manual?

Doh.
I'm still waiting for it.....

Valves are adjusted, no vacuum leaks (new ICV, hoses, valve cover gasket)

There is an exhaust leak pre-O2 sensor. Could that do it?

[jlevie]

I don't think a small exhaust leak would cause this. A gross leak might.

no vacuum leaks (new ICV, hoses, valve cover gasket)

Do you mean every hose associated with the intake? What about the dipstick o-rings, oil filler cap, brake booster, and evap control system? Any of those can be intake leaks.

How old is the O2 sensor? You can usually figure on it being usable for no more than 100k.

Does the engine come up to normal temperature in a reasonable time? A bad thermostat or a seized fan clutch will cause lower than normal engine temps.

[njansenv]

I believe that all the possible vacuum related leaks are fixed, but I'll try again. Interestingly, I just pulled the plug on one of the temp sensors while it was acting up...it cleared right up. (but it won't really start warm with the sensor unplugged: massively rich)

If the O2 sensor is bad, does the DME eventually 'sense it' on a run cycle and then run open loop? (So: open loop cold, closed loop with bad O2=bad running, back to open loop after 'some period of time')

I'm a little hesitant to just start buying parts for it....part of me is considering going to Megasquirt since it's something I'm (way) more familiar with. This would be like killing a fly with a mallet though... :)

Thanks again all.
Nathan

[jlevie]

believe that all the possible vacuum related leaks are fixed, but I'll try again. Interestingly, I just pulled the plug on one of the temp sensors while it was acting up...it cleared right up. (but it won't really start warm with the sensor unplugged: massively rich)

As I recall, the DME's temp sensor is an NTC type. So pulling the connector makes the DME think the engine is cold and thus requiring a richer mixture. If it runs better with the sensor disconnected it would sound like the engine is running lean when at temp (intake leak, bad AFM, or low fuel pressure).

If the O2 sensor is bad, does the DME eventually 'sense it' on a run cycle and then run open loop?

If the sensor has failed or is considerably off, the DME will detect that or run out of fuel trim and turn on the Check Engine Light. But well before either of those occur the sensor will have aged enough to upset the A/F ration calculation. Pre-Cat O2 sensors have a limited life and typically need to be changed no less often than every 100k. If this one is of that age or older, replace it.