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Strange misfire.

Posted by Ferdinand 
November 16, 2014 10:12PM
My 1990 325i had a mysterious engine misfire. When first started in the mornings it would sometimes stumble badly, barely idle, missing on several cylinders. It seemed to clear up after the engine warmed up, but then would start stumbling again when stuck in stop-and-go rush hour traffic. Even when running at highway speed, sometimes it would suddenly lose power and the check-engine light would start flashing. The flash codes merely tells me it's running rich. Ya, obviously, I know that. But it doesn't tell me why it's running too rich.

I couldn't figure it out so I took it in to see Rudy. He kept it for a couple of days trying to diagnose the problem. He said all the various sensors checked out okay and he suspected the main computer might be at fault. When I told him a previous owner had installed an aftermarket chip in the computer he said, "Aha!" That's got to be the problem.

So I found an inexpensive used OEM computer and we swapped that in. Bingo. That cured it...


... for a couple of months. Then it started misfiring again. :cry:

It got worse, to the point where I really didn't trust it anymore. When I tried starting it one morning, I could barely coax it into idling while it fogged out the entire neighbourhood with black smoke. Not good.

I was mega-bummed. This might be the end. I was depressed and didn't touch the car for nearly two months, driving my wife's car instead (FWD blech), while trying to decide what to do with my old BMW. I can't even sell it like this, if it's not running. So finally I decided I'd take it back to Rudy and get him to fix it, no matter how much it's going to cost.

Of course, after sitting untouched and neglected for two months, when I tried to start it the battery was completely dead. So I first had to recharge the battery. Fingers crossed that it would even run :pray:, I turned the key, and presto it idles perfectly fine!

It has completely fixed itself. It runs again! It's like a new car. Amazing.

I love my car again and drive it every day. Happy, happy, happy. But confused.
November 16, 2014 10:55PM
The problem could be a transistor switch. One failure mode of a transistor is that it can seem to work perfectly until it gets warmed up. Then it does not turn on the switch like it is supposed to. It can sort of heal up with being left unpowered for a while. Not sure which one it might be but there is one that turns on and off the current to the coil. Since the black smoke is fuel unburned, It still could be the computer or the crank position sensor which is a Hall Effect transistor. There is also an outside chance it could be the coil itself but those usually fail inoperative when hot and then when cooled, recover normal operation until hot again.

Bob in Everett
November 17, 2014 12:53AM
What are the three last digits in the part number of your engine computer? I think the 17X is a lot better than the 15X. The latter often fails pretty much like you describe here. I think it is possible to fix them by replacing the big output transistors for the ignition, but I don't remember exactly how or where.
November 17, 2014 06:33AM
Oh, the joy of running old cars!

I gather you have two Engine management computers, both with same symptoms, they work for a while then start misfiring.
Try swapping back just for the sake of science.

Check the simple stuff for yourself first, vacuum leaks, plugs and leads, etc. Fuel hoses too, and fuel pressure regulator. Check and clean all electrical connectors.
Make sure Rudy is "sensitive" to seasoned cars trouble, Many competent mechanics are not well trained to diagnose older cars anymore.

Good luck, keep us posted!
:mmbeer:
November 17, 2014 08:28PM
Quote
Ove Kvam
What are the three last digits in the part number of your engine computer? I think the 17X is a lot better than the 15X. The latter often fails pretty much like you describe here. I think it is possible to fix them by replacing the big output transistors for the ignition, but I don't remember exactly how or where.
I don't have the number handy at the moment.

I really don't want to touch :stickpoke: the computer again now because it's working perfectly at the moment.
November 17, 2014 08:36PM
Quote
Jose Pinto
I gather you have two Engine management computers, both with same symptoms, they work for a while then start misfiring.
Try swapping back just for the sake of science.
We threw the original one away because it had an aftermarket chip in it. The new one (a used OEM computer) is working perfectly at the moment.

Quote
Jose Pinto
Check the simple stuff for yourself first, vacuum leaks, plugs and leads, etc. Fuel hoses too, and fuel pressure regulator. Check and clean all electrical connectors.
Rudy initially suspected the fuel pressure regulator. It was a relatively inexpensive thing to replace, along with the dried out fuel lines. Unfortunately that made no difference.

It was frustrating because the misfire only surfaced intermittently. Whenever I had time and ambition to do diagnostics, it would be running fine and I couldn't find anything wrong. It even took Rudy several days before he gave up and recommended replacing the computer. And that fixed it, for a while.

When it started misfiring again I got depressed and left the car sitting neglected in my driveway for a couple of months.

After recharging the battery to get it started, the car has now run perfectly every day for the last three weeks. It seems to be completely cured. Bizarre.

Quote
Jose Pinto
Make sure Rudy is "sensitive" to seasoned cars trouble, Many competent mechanics are not well trained to diagnose older cars anymore.
Rudy specializes in old BMWs. He's great.
November 17, 2014 09:38PM
I think the crank position sensor is a suspect. Mine checked good by checking the resistance, but I got rid of my "no start" problem by replacing them (two in my '87 325). Maybe the missing is due to the sensor failing intermittently.

John
rkj
November 19, 2014 10:54PM
Quote
Ferdinand
Quote
Jose Pinto
I gather you have two Engine management computers, both with same symptoms, they work for a while then start misfiring.
Try swapping back just for the sake of science.
We threw the original one away because it had an aftermarket chip in it. The new one (a used OEM computer) is working perfectly at the moment.

Quote
Jose Pinto
Check the simple stuff for yourself first, vacuum leaks, plugs and leads, etc. Fuel hoses too, and fuel pressure regulator. Check and clean all electrical connectors.
Rudy initially suspected the fuel pressure regulator. It was a relatively inexpensive thing to replace, along with the dried out fuel lines. Unfortunately that made no difference.

It was frustrating because the misfire only surfaced intermittently. Whenever I had time and ambition to do diagnostics, it would be running fine and I couldn't find anything wrong. It even took Rudy several days before he gave up and recommended replacing the computer. And that fixed it, for a while.

When it started misfiring again I got depressed and left the car sitting neglected in my driveway for a couple of months.

After recharging the battery to get it started, the car has now run perfectly every day for the last three weeks. It seems to be completely cured. Bizarre.

Quote
Jose Pinto
Make sure Rudy is "sensitive" to seasoned cars trouble, Many competent mechanics are not well trained to diagnose older cars anymore.
Rudy specializes in old BMWs. He's great.

I've got something going on with my 88 325is too, very close to what Ferdy is going through. Since I've been driving my new car a lot the E30 sits more than usual. I was going to start with the fuse box and that round juction under the intake manifold. I'm sure they both need a good cleaning by now. Some fresh gas is in the cards too... with some seafoam and dry gas.

Rick
January 20, 2015 05:55PM
It is my experience that these Bosch Motronic systems are bullet proof, they don't fail.
Every issue I ever get with my car, the first words,...."bloody fuel injection system"
Only to poke around and find the Motronic free of blame, always something else, I would put in a new set of plug leads, just for sh*ts and giggles.
There is a thread here somewhere on hard starting that I posted, drove me insane, until I replaced the plug leads, not a minutes trouble since.

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E30'S AREN'T BUILT, THEY'RE CAUGHT IN THE WILD!!!



When in doubt, use full throttle,
it may not improve the situation, but it will end the suspence.
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