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'88 beemer bucking bronco

I have an '88 BMW 735i that has been my nightmare (though she’s sure made me learn how to be a mechanic!)



She started with a blown head gasket (white smoke, foamy oil, and oil in the coolant reservoir). So I took off the head and had it checked and bingo, it had a warp between cylinders 2 and 3. So the head was machined and vacuum tested, and I put a new (thicker) gasket on. I changed the rotor, cap, wires, and plugs, and did a compression test, all with no problems. I also changed the thermostat and water pump and flushed the coolant for good measure, as I didn’t know what had caused the initial blowout.



Now she has a lot of carbon on the plugs, bucks like a bronco on acceleration, and still overheats. Moreover, she stalled out on a test drive today. While sitting on the side of the highway, she cranked again and again without starting…I think I had one last crank left in the battery when she finally turned over. So we limped home, but I’m stumped!



My research has suggested that it could be a bad ECM, plugged catalytic converter, or bad fuel pump, but these are all beyond my diagnostic/repair confidence at this point. Any suggestions?



Thanks!



Christopher

Is this transmission shifting ok. If it is not I think I have your fix. I will wait to hear from you

The shifts are sometimes clunky, with an audible and sensible thud.

But it will shift out of 1st hmmm. Well a commom mistake is to reverse the wires for the throttle switch and another sensor right at he snorkel. It will cause the cylinder walls to wash down so starting may involve pulling the plugs and oiling things up everytime, and the no-shift out of first condition. This mix up is more commonly found on early V-8’s and it is easy to do as the wires are in two seperate two wire looms but have no type of indexing connector. I must admit it does not really sound like your cndition.

Yah, doesn’t sound right. I have the straight-6, and she starts fine; the stallout occurs under load. I should also mention she idles just fine.

Thanks for the brainstorming though! Any other ideas?

This might be a problem with the MAF sensor. Do the mechanical test to make sure the vane is free and not touching the case. Then do the electrical test on the vane potentiometer and the air temp sensor. If those check out okey, I would check the Coolant Temperature Sensor resistance and the Throttle Position Switch, just to rule out problems there. There appears to be a potentiometer connected with the TPS but I cannot find the electrical test for that.

Good luck on this. Post back when you find the solution.

Dagnabbit. I ran the mechanical and electrical tests this weekend, and replaced the MAF sensor yesterday. Also checked the coolant sensor and adjusted the TPS (it’s a simple on/off switch, and you just have to rotate the TPS so that it switches at the right times for idle vs. wide open throttle). So we’re thinking the same way, but sadly I was one step ahead. Again, good brainstorming.

The fact that it stalled out under load, wouldn’t restart for a while, and has carboned up plugs really makes me think it’s the cat. But I can’t rule out the ECM, and don’t want to make unneeded repairs/replacements, as both are about $500 each.

Sorry, can’t help with your car’s problem but the bike is a Beemer; the car is a Bimmer!

Aha, live and learn. Thanks! Looks like I can’t change the discussion’s title, but I’ll know for next time.