E30 Help! TPS? AFM? ECU? I don't even know!

#1

Here’s what I’m drivin’: 1988 BMW 325is (e30)

The problem:

The car cranks over fine, idles slightly over 1000 and returns to 700-800. The idle is smooth and nice and displays all kinds of e30 goodness. Within a minute, the idle becomes erratic, dips down and back up (never over 1000), and then eventually dies. No amount of throttle will change it - it’s like the engine isn’t even recognizing my desperate foot dumping on the pedal.

I’ve tried driving it with no avail. It drives fine for a few seconds, and then quickly the acceleration becomes haggard, rpms die down regardless of where my foot is on the pedal, and idle dips up and down until the eventual death of the car. Turn it back over and it starts fine! Idles fine for less than a minute. Then the monster comes back.

Here is what i’ve done to it:

replaced the o2 sensor, crankshaft position sensor, fuel filter, fuel pump, fuel pump relay, fuel pressure regulator, air filter, spark plugs and cleaned the AFM with AFM spray. The distributor cap and spark plug wires are from Bavarian Autosport and look fairly new.

I’ve tried testing the continuity of the throttle position switch using the Bentley manual as my guide. On the 2 and 18 tongs, I get continuity when the throttle is completely closed, but as SOON as I move it even to the slightest degree, I lose continuity. On tongs 3 and 18, I have continuity when the throttle is WIDE open, but nothing else. Is this correct?

On the AFM, the flap moves smoothly with equal resistance. I don’t know what else I can do! Please help! This little car is my only means of transportation and walking to work is getting REAL old. Any suggestions will be greatly appreciated.

Thanks

#2

Usually this kind of problem is a vacuum leak. They can be diabolical. One way to chase them down is to make a smoke generator. Google DIY smoke generator. I used to think you could chase them down by spraying a bit of butane of propane around, but altho you’ll read on the Web that it works, I couldn’t get it to work.

Your TPS sounds ok.

An AFM problem is possible. Testing an AFM is not infantry simple. The Bentley test procedure is wrong. If you Google 944 AFM you’ll find a site where a 944 guys talks about Bosche engine management a lot. His name is Wilk or something like that. His site has good info on testing an AFM. Might be better to just swap the suspect AFM out with another one tho. If your AFM spring was ever dicked with, testing won’t find the problem and the engine may never run correctly.

It wouldn’t hurt to put a fuel pressure gauge on just to rule FP out.

Most likely it’s a vacuum leak. Don’t be too hesitant to take the car to a good indy BMW shop. At some point a tricky problem isn’t worth the aggravation, especially when an experienced guy with lots of resources might be able to ID and fix the problem in a jiffy.

#3

Just ordered a new vacuum hose set and intake boot. Whether or not this is the concrete problem, ruling out the potential vacuum leak in the future will give me a little peace of mind. Thanks for the info on the AFM. I’ve been looking at buying a new one, but they’re just so damn expensive!

Also, just ran tests on the Idle control valve - according to Bentley, it’s acting well within spec. I’ll test the fuel pressure again just for good measure. It’s supposed to sit about 40psi or 3 bar, correct?

And mannnn, if there was a half decent BMW mechanic around, I would have already consulted with him/her. Being on the MS Gulf Coast, my options are limited when it comes to specialty mechanics and shops. Closest bet would be in New Orleans in which that city is all kinds of crazy and would rather not try to find my guru over there (still great for boozin’ and music, though).

Thanks for the help! I’ll continue with the AFM diagnostics via the 944 guy.

#4

Buying a vac hose set isn’t the right way to approach a vacuum leak. It requires tenacity and testing. I once didn’t beat a vac leak idle problem until I swapped engines. There’s lots of possible vac leak locations that have nothing to do with a vac hose.

IIRC correctly, the idle control valve only plays when engine is cold. Your symptoms don’t fit since you didn’t specify different cold/hot behavior.

Quit blindly buying things and replacing them. That’s the wrong approach. Until you are able to positively rule out a vac leak, assume that’s the problem. Focus on figuring out how to test for vac leak. That means making a smoke generator or taking it to a shop that has one.

A smoke generator is really just a metal can with a couple air hoses and a burned up kerosene soaked rag inside. You just have to figure out a way to provide low pressure air 2-3psi to the can so you can then put the smoke into the intake tract somewhere.