535i (1991) - strange hesitancy when pulling away

[M Malaviya]

Especially after the car has warmed up, if I switch off the engine, and start it again whilst the engine is still warm (say within 15 minutes), the car hesitates initially when I pull away, like its firing blanks. however, if I depress the accelerator more than normal, she's picks up fine. also, once on the move, she's fine. The car idles fine (550 revs) and is an automatic with a cat converter, done 83k miles. No faults show up on the display.

Any of you gurus know how to cure this problem, is it something that will get worse, or is it that the car is just being lazy in the hot weather? Any suggestions appreciated.

Thanks.

[Black 535i]

As well as the distributor cap. This cured my power losses and firing issues quite easily. You can get all the parts for about $150.00 U.S.

[Alexlind123]

550rpm seems like a really low idle. Mine idles at 810rpm.

[uls]

hey, this is a wild shot, cos i dont know much about cars, but it could be the fuel line, cos when i first drove my car it would get to the lights, then idle heavy, then when i went to go it wouldn't budge for a moment and then it would kick in... i'd get to the next set of lights and the car stalled.. after a while of trying to start the car it started but nothing happened when i put the foot flat to the floor after a moment it suddenly kicked in again....

when i pulled over and rang my mechanic i was quite shocked to find that the fuel distributor and the fuel line in the engine bay was cracked and fuel was spurting into my engine bay... in other words im lucky to be alive... just thought id tell you that cos it sounds like it may be a similar scenario... so beware!! :)

[M Malaviya]

Thanks for your comments; it turned out to be a bad lead (#4) that looked very porous at the plug end; only discovered it because I felt electrical pulses when I was checking all wires and tubes whilst the car was idling.

Oddly enough I had replaced all the leads and distributor about 18months ago.

[M Malaviya]

.......has always been around 550rpm......replaced ICV 6 months ago because it was roving from around 250 to 600 at idle.

[grave77]

normal M30 should be around 750 - 800 RPM, lower than that means something wrong, usually the M30 gets the hesitation from 2 faults, might be the AFM, or the bad O2 sensor. if you disconnected the O2 sensor and got rid of the hesitation especially when hot then it's he problem. if not then you have to check the AFM.

for sure check that you have good spark plugs and good fuel filter.

I agree with Alexlind here, I once forced the M30 engine to idle lower than 600 and it was really bad and rough!!

[M Malaviya]

Have had the car 5+ years, idle has always been the same revs - could be the rev counter reads wrong? The O2 sensor was changed about 3k miles ago. would'nt a bad O2 sensor show up as "check engine" fault?

Anyway, hesitancy seems to have disappeared - changed all the leads even though only #4 was suspect.

Appreciate all the feedbacks - thanks.

[genphreak]

The dash taco is not accurate at idle. You need to hook up a professional type to know. If you use a mechanic, ask to see it connected to their machine next time you go in so you can see how well she idles.

Idle is completely governed by the Idle Control Valve- the ECU opens and closes the ICV to obtain 750-800 like Grave said. If the ECU gets the wrong speed input (It is only a pulse red from the harmonic balancer so if it goes wrong you will know all about it- your engine's operation will screw up badly)

If your idle is wrong someone may have been playing with your AFM or throttle body and have upset their calibration settings- most people do this. It really screws things up. :) nick