Tricking Motronic to increase idle speed

[JHZR2]

My 91 318i has a good idle, I suppose, and it never stalls. The engine has no shakes or harmonics at idle, it feels smooth.

But it idles low, like 550 or so. And, when the AC kicks on, it spikes up to about 800, then slows, and does a similar thing when the AC clicks off. It is just kind of annoying, and I think that if the idle was a bit faster, it wouldnt happen this way.

Ive checked everything. ICV is from 06, O2 sensor is from 04, all rubber connections are recent and tight. Tests for leaks came back wth nothing. ICV is working, as disconnecting it on a cold engine will cause it to stall. Also, no matter how I adjust the stop for the first stage throttle butterfly, and how I adjust the set screw in the ICV, they balance each other out after one shutdown and startup of the engine, so it is alwas idling the same low speed.

So there must be a bad connection, one that has high resistance or somehow doesnt provide a good signal. Dont know where it is, and Ive cleaned and looked after all I can.

So is there another way to trick the ECU to force it to idle a bit faster? Any recommendations?

Thanks!

 

[HTSS_TR]

My '94 LS400 has opposite problem, the idle is too high at around 850-900 RPM when in D and 1100-1200 RPM when in N or P, the spec is around 650 RPM. I changed the ICV few months ago and it fixed the high idle for few days only.

 

[Colt45ws]

Its probably set to il
dle that low. My CV has the LX programming and also idles at 550 hot.

 

[Trav]

I don't think there is anyway to :trick: the ECM other than to put a reprogrammed chip in it. Turner in MA can probably do a chip for this.
Did you check the volts/ohms of the ICV? I don't have the specs in front of me but i can find out.

 

[Papa Bear]

Originally Posted By: JHZR2
My 91 318i has a good idle, I suppose, and it never stalls. The engine has no shakes or harmonics at idle, it feels smooth.

But it idles low, like 550 or so. And, when the AC kicks on, it spikes up to about 800, then slows, and does a similar thing when the AC clicks off. It is just kind of annoying, and I think that if the idle was a bit faster, it wouldnt happen this way.

Ive checked everything. ICV is from 06, O2 sensor is from 04, all rubber connections are recent and tight. Tests for leaks came back wth nothing. ICV is working, as disconnecting it on a cold engine will cause it to stall. Also, no matter how I adjust the stop for the first stage throttle butterfly, and how I adjust the set screw in the ICV, they balance each other out after one shutdown and startup of the engine, so it is alwas idling the same low speed.

So there must be a bad connection, one that has high resistance or somehow doesnt provide a good signal. Dont know where it is, and Ive cleaned and looked after all I can.

So is there another way to trick the ECU to force it to idle a bit faster? Any recommendations?

Thanks!



How long has it done this? Why wait 21 yrs to "trick" it?

 

[eljefino]

Just for grins did you put another tach on it? Maybe your gauge is falling out of calibration.

Sounds like it anticipates the AC load and the AC takes slightly longer to kick in than the preprogrammed amount of time. Could attack it from that angle.

 

[JHZR2]

Originally Posted By: Papa Bear


How long has it done this? Why wait 21 yrs to "trick" it?


LOL, it has always hiccuped when the AC clicks on, but the idle has gotten lower over time, I believe.

 

[JHZR2]

Originally Posted By: Trav

Did you check the volts/ohms of the ICV? I don't have the specs in front of me but i can find out.


Not sure how I'd check the volts of the ICV, as it has a special connector on it and no good way to get a probe in there. Ohms should be doable.

 

[Trav]

Just Ohms on this one and a 12v function test.
Many times voltage testing of sensors if needed is done at the ECM connector.

You can also check the TPS.

 

[mechtech2]

General advice -
Some cars have switch on the Power Steering that causes the idle to go up when under pressure, so the engine does not bog when making slow or stopped turns of the wheel.
You can often 'jump' the switch to get the higher idle all the time.
But what is the car spec'd to idle at? Is it supposed to be at 500? That sounds low.
If so, you need repairs of 'who knows what' at this point.

 

[Scott_Tucker]

Dinan or Turner should be able to make you a chip if you want to artificially bump the idle up.