'89 535 - Rough idle when warmed up

[gcraiga]

I recently had some work done on my 5-speed '89 535i with 129K miles because it had a hunting idle when cold and the check engine light had also recently come on. Also, when idling at stop lights with the car completely warmed up, the engine would occaisionally (once every few days) drop out like it was going to stall and then recover with a small rev.

My mechanic said the check engine light was on because of a fulty O2 sensor and that the hunting idle and the 'almost stall' was from a bad idle control valve. He replaced both and sent me off...

With the new idle control valve, the cold idle problems are gone, but now the car seems to have a rougher idle when running warmed up and it also seems to 'almost stall' more often... In fact, it completely stalled once while at a stoplight.

Before I bring it back, does anyone have any thoughts as to what could be causing this idle problem. All of the problems seem to be happening only when the engine is running warmed up (temp at 1/2 mark). Note that the engine never gets hotter than a bit above the 1/2 mark... So I don't think overheating is an issue.

Thanks in advance.

[Jeff N.]

...that the car may be running rich when warmed up. I would consider replacing the engine temperature sensor to make sure that the ECU is getting the right information on engine temp. Note there are two temp sensors on the car, one for the gauge and one for the computer. You want the later. I recall it's the one with the blue plastic top. About a $30 part if you mailorder from BMA and a DIY swap.

The other cause might be a vacuum leak. When cold, the car runs rich and if it's leaking somewhere, the impact might be minimal. However, as the car warms up, the mixture leans out (based on the sensor noted above) and with a leak, the car could be running lean. As you just had some topend work done, I'd carefully check all the intake clamps around the intake boot where the ICV connects. Also check the valve cover to intake boot hose. It cracks or splits when old. There should be a small 3 mm line running to the hose as well; comes from the bottom of the intake manifold about where the 3.5 marking is.

Check for leaks first and then maybe order up a new engine temp sensor. That would be what I would do.

Cheers!

Jeff



I recently had some work done on my 5-speed '89 535i with 129K miles because it had a hunting idle when cold and the check engine light had also recently come on. Also, when idling at stop lights with the car completely warmed up, the engine would occaisionally (once every few days) drop out like it was going to stall and then recover with a small rev.

My mechanic said the check engine light was on because of a fulty O2 sensor and that the hunting idle and the 'almost stall' was from a bad idle control valve. He replaced both and sent me off...

With the new idle control valve, the cold idle problems are gone, but now the car seems to have a rougher idle when running warmed up and it also seems to 'almost stall' more often... In fact, it completely stalled once while at a stoplight.

Before I bring it back, does anyone have any thoughts as to what could be causing this idle problem. All of the problems seem to be happening only when the engine is running warmed up (temp at 1/2 mark). Note that the engine never gets hotter than a bit above the 1/2 mark... So I don't think overheating is an issue.

Thanks in advance.

[mholbrook]

Ditto on the vacuum leak. With 129k you should replace those few vacuum lines that are there. Only about 2 or 3. Plus you need to be sure the rubber plenum between the AFM and throttle body are not cracked. Not expensive to replace and if you see a hint of a crack, replace it. Not expensive.