325is warm start rough idle

[RoDo]

I have an 88 325is that I've only had a few weeks. It came with a warm start idle issue that my tinkering has only made worse.

Issue:
When starting the car after it is up to operating temperature it will idle very rough, even dying sometimes. It only does this for about 30 seconds or so before shaping up and idling smooth as it should. It runs great thereafter. The typical scenario is I drive somewhere, run into a store for a bit and come back out and start the car. If I shut the car off and start it again immediately or within a few minutes it does not exhibit the issue. Also from a stone cold start it idles perfect.

What I have done since I got the car:

• changed oil & filter
• replaced air filter
• replaced spark plugs
• adjusted valves
• run 1.5 bottles fuel injector cleaner through system (other .5 is still in the tank)
• replaced plug wires
• replaced coil
• replaced distributor cap & rotor
• reset ECU/DME by disconnecting negative battery terminal (this actually made the problem significantly worse)
• replaced fuel filter
• adjusted valves (again to doublecheck my work since I am new to E30s)
• cleaned idle control valve
• cleaned throttle body

My focus at this point is the coolant temp sensor. My operating theory is that the car cools enough to fall into the middle temp range on the sensor and it then is giving a bad reading, possibly making the car run rich as if it is cold. Then after the warm coolant gets to flowing the sensor gives a good reading. I have tested resistance per the Bentley but being a noobie with an Ohm meter I am not sure what I'm seeing. The car is up to operating temp and my multimeter reads anywhere from .262 to .267. I can turn the dial on my multimeter to move the decimal point but it doesn't change the value. Looking at the table in the Bentley this may indicate that the coolant temp sensor is testing out good for the high temp range and that my decimal point is off on the multimeter. Or it may not, I don't fully understand what I am seeing. I haven't yet had the chance to test the coolant temp sensor when cold to see what I get then.

Other areas of focus seem to be the idle control valve & the throttle position switch. Neither of which I have tested yet.

Am I on the right track? This issue is getting really annoying! Thanks in advance for your help.

[jlevie]

It could be a bad temp sensor, but it could also be leaking injectors or even an intake leak.

[RoDo]

It could be a bad temp sensor, but it could also be leaking injectors or even an intake leak.

Thanks for the suggestion. If I had an intake leak wouldn't the issue be constant and not just at startup?

Can you say more about a leaking injector? What are the effects of that?

[jlevie]

An intake leak has the greatest at startup because the DME is running in open loop mode then. Once the engine is at temperature and it has been running for a short time the DME shifts to closed loop mode and can compensate for an intake leak (if the O2 sensor is good).

When the engine is shut off the check valve in the fuel pump and the FPR will hold pressure in the fuel rail. A dirty or worn injector can drip fuel into the manifold until the pressure drops, resulting in a very rich condition on a restart. It is pretty easy to check for this by pulling the injectors and rail, then jumpering the fuel pump relay and checking to see if any of the injectors are dripping fuel.

[RoDo]

An intake leak has the greatest at startup because the DME is running in open loop mode then. Once the engine is at temperature and it has been running for a short time the DME shifts to closed loop mode and can compensate for an intake leak (if the O2 sensor is good).

When the engine is shut off the check valve in the fuel pump and the FPR will hold pressure in the fuel rail. A dirty or worn injector can drip fuel into the manifold until the pressure drops, resulting in a very rich condition on a restart. It is pretty easy to check for this by pulling the injectors and rail, then jumpering the fuel pump relay and checking to see if any of the injectors are dripping fuel.

Thanks for the explanation Jim. I have the car in the garage in the early stages of teardown to replace the timing belt (and complementary stuff while I am in there). I will get back to troubleshooting this particular issue next week and update with what I find.

[zedfez]

If there is a temp sensor issue, I would think that the cold start injector could be spraying even on warm starts. That would be really easy to test, just get the car warmed up, turn in off, disconnect the wire to the injector and try starting it up again.

[RoDo]

If there is a temp sensor issue, I would think that the cold start injector could be spraying even on warm starts. That would be really easy to test, just get the car warmed up, turn in off, disconnect the wire to the injector and try starting it up again.

Do all E30s have a cold start injector? I don't have my Bentley here at work and for some reason I thought my 88 didn't have one? If it does have one that's a great idea.

Also have an update, I figured out how to read my ohm meter and I have tested the coolant temp sensor. Seems to be returning values that are in line with the Bentley so I'm much less suspicious of that guy now...

[NC325iC]

only the early cars have a cold start injector