MyE28.com

The ole' 1987 535is (M30b34) is acting up. I have replaced just about everything I can, with spares, and no JOY. I am having a stall/hesitation while driving and/or ideling.
I replaced the dist, AFM, ICV, CTS, TPS, FP Relay, new main relay, cleaned all the fuses, checked the ECU for broken solder joints and re-soldered a few, checked both reference and speed sensor (good).
I am waiting for a new fuel filter, it's pretty old, and I am considering buying a new ECU, just because I don't know what else to do.
The car stalls or severely stumbles at idle. The MPG gauge shoots to the left as it misses and then, most of the time, recovers. I have done about everything I can find on MyE28 and am really out of ideas. Except as mentioned...the fuel filter.
Any advice is appreciated.

cwomike wrote:The car stalls or severely stumbles at idle. The MPG gauge shoots to the left as it misses and then, most of the time, recovers.

The MPG signal is based on the fuel injector control signal. The signal coming out of the ECU grounds the fuel injectors to actuate them, and it briefly isn't – the fuel injectors aren't getting told to open, so I doubt the fuel filter will fix anything, nor the fuel pump, fuel pump fuse, fuel pump relay, or fuel pressure regulator. The question to ask is, why would the ECU not pulse the injectors?

One possible answer is that the fuel injector signal from the ECU is dicey. This is not the usual 535i ECU problem, which is the ignition signal, but the solution is the similar: check the solder joints in the ECU associated with the fuel injector signal.

Another possibility is that the ECU thinks the engine is going a lot faster than it should, and cuts the fuel injectors to make it slow down. I don't know if the ECU software even has a rev limiter, but I do know that when idling, it wants to maintain a certain RPM (hard to believe with the M30, I know), and if the actual RPM is far enough above that it will actually stop supplying fuel to the engine.

You can see this in action if you engine brake while going down a hill. Watch your fuel econometer as you coast, then as you engine brake. You'll actually get better fuel economy when engine braking. That's because the ECU wants the engine to idle, but it's going 2000 rpm or so, so the ECU cuts the fuel completely.

I don't know why the ECU would think the engine is going too fast when it isn't, but it matches the symptoms.

Oh well, I was hoping for some flight maneuvers videos. Carry on. Oh, when I had this problem I also did a lot of troubleshooting. I finally replaced the ecu and that fixed it. It helps to have known good parts to help you in the troubleshooting.

I replaced the ECU and several other parts and the car continued to stall/stumble while idling or running at temp. When the dist cap was replaced, by the WIFE, she never checked the spark plug wires resistance. The wire set was only a couple of years old and I neglected to mention it. I was out of town in her car. She properly labelled the wires F-U-C-K-Y-O-U and returned them correctly. I checked them tonight and Whahla! the K wire to the coil measured >20M ohms. All other wires measured 6k with the ends installed as expected. I never thought a coil wire would be the cause of this issue. A replacement coil wire was about 2k and the car seems fine. Wife is not showing much enthusiasm, I hope she'll come around, she's great.
Thanks for all the recommendations guys. this site is irreplaceable.
Mike

So, solved?

That's a neat labeling idea... i think.

cwomike wrote:When the dist cap was replaced, by the WIFE, she never checked the spark plug wires resistance. I was out of town in her car. She properly labelled the wires F-U-C-K-Y-O-U and returned them correctly.

You're actually pointing out that your wife neglected to check the wire resistance? Really? Wow, this is clearly a case of "doesn't know what he has in front of him"...

She's a keeper. Don't piss her off.

But seriously, as I was reading this note, I was thinking: he's somewhat desperately and non-scientifically replacing expensive repair parts when he hasn't even thought of keeping up on the maintenance... this is backwards.

Spark plugs, dist cap, rotor, belts, air filter, fuel filter, valve adjustment (valve cover gasket and plugs), oxygen sensor.

Oil and oil filter change.

Then we can start talking about expensive parts, their age and likelihood of failure. I don't see any evidence that this ISN'T a loose or bad connection, intake leak, etc. (i.e. super cheap fix) yet!

Good luck!