Car [near] stalls when coming to a stop with AC

[tn535i]

Odd, the microswitch for idle control when activated usually makes an audible click. You should use your dmm in ohms setting and look for resistance on the switch between pins 1 & 2 and see if it ever goes to near 0 when rotated closed (disconnected from the harness). Go ahead and check WOT throttle also which should show near 0 between 2 & 3 from about 80% on.

Also odd. IF you short 1 to 2 it should initiate idle control in the DME and it should go to computer controlled idle speed. IT might be worthwhile to measure voltage at pin #1 on the harness while you do this to make sure it goes to 0 volts. With 0 volts the DME it should try to control idle.

Pins 1&2 or 2&3 (harness) should return 5 volts with the TPS unplugged and that sounds fine. DME is looking for TPS to tell it something.

The ICV measurements are puzzling me. Was this with TPS plugged in or not? Regarless it is difficult to get a true and fair measure of the ICV voltage as it's a time based device, power on x% / off y% and it does this on each side alternately trying to open and close the ICV. I think ?? pin 1 on the ICV the DME grounds to open, pin2 is 12v from the main relay to both solenoids and pin 3 the DME uses to close (by grounding again). I believe voltage across pins may go to 0 when the DME is using or grounding that half, suggest the DME is trying to fully close your ICV. Have you checked the resistances across the pins on the ICV, unplugged of course, to check the two opposing solenoids for similar continuity? If one of those is bad you may get full battery voltage all the time like you report on 1&2.

Disclaimer: I could have the open and close operations all backwards. I think BMW refers to it as extend and retract.

So it's possible both your TPS is not grounding to give and idle signal AND there is a second problem with the ICV malfunctioning and only moving in one direction. My understanding is that when the TPS is giving no signal the ICV should be in a 50/50 state, humming and about midway.

So check resistance between 1&2 and 2&3 in the ICV it could be that one of the two solenoids is bad. You could also put 12v across each half and 'see' that alternates between closed and open. If those things happen like they are supposed to then I guess it's possible one driver in the DME is not functioning. OR it's possible I have no idea what the aftermarket chip does with idle control in neutral position when the TPS is open.

This is very puzzling which is often the case when there are two basic problems as I suspect. But not yet time to throw expensive parts at it until more testing.

Some Data: Measured my ICV across 1-2 & 2-3
before starting ~4 & 3.5
cold ~6 & 3
unplug tps ~5 & 4 volts, not 50:50 but closer to balanced
I'll check it again warm later if it would help.
Again I believe more voltage means less power on that half of the solenoid from DME.

[topher800]

To follow up, I replaced the TPS and it all works and acts as you describe. I tuned it following your directions. So far so good. It was clear as I was installing the new one, that the old one was bunk (e.g. new one rotated to a specific position before I put the screws in, old one wouldn't hold a position. and new one would click when I plugged it in).

So only after years of intense experimentation, rigorous scrutiny, literature searches and peer reviews do I come to these conclusions

Thanks for all the help and you are granted a PhD in E28 idle adjustment.

[roadstar]

I am sneaking in on this thread from my own "535i, intermittent stalling and low idle". I thought my problem was different but maybe it isn't.

I was out measuring my ICV and TPS. All measured with cold engine, will get the car out to adjust throttle body and TPS tomorrow.

ICV:

The coils of the ICV measured 18,8 + 20,2 ohm.
The connector measured 12V on 1+2 and 0 on 2+3

There was no difference with TPS plugged in or not, shouldn't the signal from the TPS alter these values. Or does it when engine is warm? I also tried with TPS plugged in and moving throttle to engage switch.

TPS:

The TPS measured correctly, continuity between 1+2 at no throttle and at 2+3 at full throttle.

The connector of the TPS measured 5V between the mid connector and either of the outer ones.

I am mostly puzzled by the readings of the ICV connector, shouldn't things happen when TPS is worked?

[Mike W.]

I am sneaking in on this thread from my own "535i, intermittent stalling and low idle". I thought my problem was different but maybe it isn't.

I was out measuring my ICV and TPS. All measured with cold engine, will get the car out to adjust throttle body and TPS tomorrow.

ICV:

The coils of the ICV measured 18,8 + 20,2 ohm.
The connector measured 12V on 1+2 and 0 on 2+3

There was no difference with TPS plugged in or not, shouldn't the signal from the TPS alter these values. Or does it when engine is warm? I also tried with TPS plugged in and moving throttle to engage switch.

TPS:

The TPS measured correctly, continuity between 1+2 at no throttle and at 2+3 at full throttle.

The connector of the TPS measured 5V between the mid connector and either of the outer ones.

I am mostly puzzled by the readings of the ICV connector, shouldn't things happen when TPS is worked?

My experiences don't seem to match those of others. When my TPS was out of adjustment, idle was just fine. No change in idle speed or smoothness. However I did have a dip in idle speed and sometimes dying coming up to a stop, especially with the A/C on. It does have a coasting fuel cutoff, as it should, but it seemed to need a little advance notice it needed to idle, hence the idle switch part of the TPS.

[roadstar]

I have been doing new measurements and tests.

O2 sensor at idle 0.115-0-0.180V. At some throttle (maybe 3000rpm) it rises to around 0.8V.

I unplugged TPS and adjusted throttle plate so rpm was around 1000. When I plugged TPS back in, after adjusting it, rpm dropped to 400. I then removed the ICV and it is halfway open as it should be. I also applied voltage to it and the opening closed/opened at the different connections. I left it open and plugged it back in and with ignition on it moved to halfway open. The thing I am unsure about is if it shouldn't react to opening/closing the TPS?

The fuel pressure was 35 psi at idle and when having the engine running on the driveway it shortly stalled a couple of times. On these occasions the pressure dropped shortly below 30. The link below is a video of monitoring the fuel pressure. Since pressure is low and I have a new fuel pump on the shelf, I guess I will replace the old one just to rule that out.

https://youtu.be/ZM1o0wHEUoU

And thanks guys for helping me out with this annoying matter! The car is in very good condition and it would be great to have it running as it looks.

Henrik.

[roadstar]

One more thing. I measured the Coolant temp sensor as well at 3800 before startup and 340 when warm. The engine died when removing the sensor at idle. I guess that means it is working.

[roadstar]

I havre replaced the main fuel pump but the problem persists. Fuel pressure at idle is a bit higher at 38 psi and drops to 34-35 when giving a bit of throttle. Idle is still very low.