M30 sometimes idling too fast when hot

[Robert K]

At times, my M30 engine in my 1991 535i does not want to return to the factory idle speed when coming to a stop. In the course of chasing down another problem, I've replaced all the vacuum lines, so I'm fairly certain that's not the problem. I've also put a little extra slack in the throttle and cruise control cables to make certain they're not holding the throttle open a little. I believe the DME is either sending the wrong signal to the idle control valve (ICV) or the valve is malfunctioning. Once I tap the accelerator pedal a time or two, the idle returns to normal. I've done the ICV electrical tests per Bentley and it supposedly checks out okay. Any ideas out there as to what else I need to look for?

Grace and peace,

Robert K
1991 535i

[Martin in Bellevue]

Howdy Rob, the old throttle position sensor got sticky on my 5er. A new one fixed the idle return behavior. It also helped to have wide open throttle known to the dme, with the new tps.

[Jeff N.]

Sounds like the throttle position sensor is not signaling "idle". I'd check the adjustment. I recall bentley's covers both testing and adjustment pretty well.

Jeff

[Jeff N.]

phbbt!

[Jr ///M5]

Robert,

During your recent work on the car, did you say that you had removed the Idle Control Valve and cleaned it with spray carb cleaner?

I just can't remember if you did or not. I know you mentioned a definate click on the throttle position sensor when it returned back to idle position and the ohm values were correct. Did you remove the afm and clean the throttle body where the throttle plates are? They could have a build up causing the plates to stick. Block open the throttle and clean the throttle plates and surrounding area with a rag and carb cleaner.

Jr

Jr

[Robert K]

Sorry about that. I forgot to mention that I just put a brand new Bosche throttle switch on. On my original, everything worked except for the WOT switch. And I have double and triple checked that it is tripping the closed throttle switch. That shouldn't be the problem. I had also cleaned the throttle body a couple of weeks ago. It should be fine. And as far as the AFM, it is fairly new. Probably less than two years old. However, I haven't taken it off to make certain that it's not hanging. I'll probably try to check it and the throttle body tomorrow after work.

I've still got a slight jerkiness at cruise, especially on downhill sections. Trying to find the problem is driving me nuts...and draining my bank account. So far, new plug wires, new TPS, new fuel pump, and some new vacuum hoses. Actually, I think I needed most of those parts, since most of them were original anyway. But, the problem still exists. If I don't find it soon....a blue with gray interior E39 528i is starting to sound pretty good.

Grace and peace,

Robert K
1991 535i

[Jeff N.]

I still think it's the TPS sticking...

BTW, when was the last time you checked the gap on the crank position sensor? 1mm I recall is spec. Is the CPS bolt tight?

Jeff

[Robert K]

Jeff,

Thanks for the tip. After work today, I decided to go out and check some of the things that were suggested. What was happening was this. If I opened the throttle and let it snap closed, it would flip the idle switch in the TPS. However, if I opened the throttle and slowly returned it to idle, it would not flip the idle switch. The switch was on the ragged edge of being correctly adjusted. So, I readjusted it so that it will definitely flip the idle switch even if it's slowly returned to idle. That makes a lot of sense now. Since the DME never thought the throttle was at idle, it wasn't real concerned about returning to the preset idle speed. I didn't start it up to check it out, but I'm sure this will take care of the fast idle problem. Thank you so much for the advice!

Grace and peace,

Robert K
1991 535i

[Jeff N.]

..the monkey typed Shakespear!