M50 Blows back out intake manifold!

[RickyJ]

The ’92 525I M50 is running rough. The idle is not too bad, but when driving, it chugs and pops. Yes POPS. It blows back loudly through the intake manifold. Obviously it is a problem with the valve, spark or fuel timing, but right now I’m at a loss. The things I did today were:
Ignition: The plugs and coils look good, no oil. Coil resistance were good. When idling, my inductive troubleshooting tool flashes close to each coil. The voltage variation at the coil and the DME look normal.
Fuel: The injectors look good, not covered with crud. There resistance look good, and they all click. The voltage at the DME looked good. I did not do a spray volume test or fuel pressure test yet.
Air leaks: one small one corrected, no change. Grounds at DME show 20 mV when idling, >.2 ohms when connector pulled.
I cleaned a couple of grounds, and DME connectors. The MAF glowed on shutdown , the TPS had a normal resistance curve, and all the voltage readings at the DME matched readings from good operation a few years ago. Crank position readings at the DME are normal. I have not done a compression test. New air filter.
Could it be a sticking valve? If so what should I do. Any help would be appreciated.
RickyJ
’92 525iA 144k

[Derek A.]

sounds like you are running lean, or your timing is way retarded. I would check your fuel pressure next.

[winfred]

a running condition like that can be caused by a dieing air mass sensor, i've had a cam sensor on the way out cause funky running but that's rare they usually just die, it's hard to say exactly what it is without getting my hands dirty, have you done the stomp test? http://www.bmwe34.net/E34main/Trouble/Engine_codes.htm

[RickyJ]

:(

[bjl4776]

I had the same exact problem with the same engine. I took it to a shop with a computer and found out that two of my coils weren't putting out enough voltage and some of my plugs were misfiring. I know you said you checked the voltage but it sounds as if if thats your problem

[RickyJ]

I used a induction pickup tool that is used to check 110V outlets. This tool has an all plastic exterior. Putting it about 1/4 inch from the top of the coil packs caused it to light up showing high voltage, but there is no direct evidence of the plug firing. The plug coloring looked normal, but that does not rule out an intermittent problem. I'll check tomorrow some more.
Thanks

[bjl4776]

My problem was in the 2-3k rpm range and was more evident under load. Try new plugs if you haven't already.

[bjl4776]

My problem was in the 2-3k rpm range and was more evident under load. Try new plugs if you haven't already, mine looked fine too. What kind of plugs are you runnings, i was running Bosch Platinum when i had the problem, so im not anymore.

[RickyJ]

They are fairly new.
This morning I pulled the plugs again after testing it yesterday. #3 plug ceramic below the gap now has some black discoloration with a few very small black specs and the rest have the usual slight light brown color. I pulled off the rubber boots with the 1.8k ohm printed on the side between the plug and the coil. They all measure 1.85 to 2.03k. The plug resistances are all the same.
Right now I suspect #3 coil.
I'll keep you posted.
Thanks

[winfred]

i've had one coil screw with the rest, we chased a under a load miss on a m50 for a while till we figured out that one coil was doing some to the others, don't know if it was drawing too much power shorting out or what, the resistance test in the bentley is useless, i've had bad coils pass with the same readings as good coils

[RickyJ]

to one common wire to ground. If there is a bad ground in this common wire, the plugs can be in series! Yes, one coil firing multiple plugs. I've soldered the common connections and cleaned the ground today. This one little brown wire runs over to a screw on the thermostat housing.
Rick

[winfred]

i think i remember theres something in that wire, a fuseable link or resister or something, it's been a while and i can't remember what car i was looking at but there was something on the diagram


This one little brown wire runs over to a screw on the thermostat housing.

[RickyJ]

Bentley shows nothing. page EWD-45
I did a fuel pressure test and a compression test today. They both look good. I'll order a coil tomorrow.
Thanks,
Rick

[Fivfingerz]

Try switching coils first, swap #3 with #6 or #1 doesn't matter. Make sure you clean the spark plugs up good and drive it for awhile or make it do what it was doing a few times if its undriveable and then re-check your plugs.

[RickyJ]

I put a new coil and boot in #3 and the problem was corrected. The Bentley coil test showed that the problem was not with the primary winding. The dealers have coil test equipment, but if you want to stay away from the dealer, allow the car to idle rough for 10-15 minutes. Pull the plugs and look for the dirty one. I suppose now the rest of the coils have all reached their lifetime and the rest will go.
Thanks for the help
RickyJ '92 525iA 144k

[632 Regal]

but since its running good it gives you a chance to shop around for the rest of the coils, I dont think you can beat BMA but the coils are still pricey.