325i Misfires from 2500-3000 rpm

Elroy Jetson · 04-12-2011, 04:16 PM

Well....I'm stumped. The beastie is a 1994 325iC, 5 speed. Regardless of speed, she misfires and stumbles badly from 2500-3000 rpm -- all gears, including neutral. By about 3100 rpm she's hitting on all 6 and pulling strong. The stomp test comes back with no faults. The Peake tool indicates bad coils on 2 and 5. I reset the error, swapped the 2 and 5 coils to 3 and 4, and still got the bad coil error for 2 and 5 -- clearly not really the coils, and the plugs on 2 and 5 looked good. Pulled the DME - it was clean and dry in and out with no evidence of any past moisture and the connector was also clean. The specific speed of the fault makes me think that it's an electrical issue, but to be sure, I changed the oil (maybe the VANOS is sticky?) and put an aggressive cleaner in the fuel, thinking maybe an injector or two was flipping out at this particular frequency -- no improvement yet. Anyone have any ideas? Thanks.

coolbear1 · 04-12-2011, 08:43 PM

My 93 did the same thing, I replaced everything but the paint. It was the O2 sensor. Never got a bad o2 code though. I got missfire on this and that cylinder and a few knock sensor codes, but no O2. The reason I did not suspect the sensor was It was fairly new. I changed it reluctantly after there was nothing left to blame.

Good luck with it.

Elroy Jetson · 04-16-2011, 08:15 AM

Thanks for the O2 sensor suggestion. For grins, last night after I changed the fuel filter - with no improvement -- I pulled the connector off the O2 sensor and ran her up. Still that misfire from 2500-3000. Disconnecting the O2 sensor made no difference...and did not illuminate the check engine light, either. Pulling the O2 sensor heat relay did illuminate the check engine light, but still the misfire. It just feels like an intermittent ignition issue, but always in that particular RPM range. Any ideas, anyone? Thanks.

bostogrun · bostogrun's Garage

Usually O2 sensors cause a too rich or lean issue. They cause misfires because they have nothing to do with ignition at all rather fuel metering. Also o2 sensors don't come into play until the engine goes into closed loop (fully warmed up).

Because the misfire didnt move with coils it could the stationary wiring to those coils. (small chance but possible)

It could also be a vacuum leak. Did you connect a vacuum gauge and see what it says.

Did you check the big elbow in the intake system they sometime crack and cause a big vacuum leak.

I had a similar problem with my 328i. It ran like it had a miss and kept on getting PO 305 (#5 miss) I replaced the plug and the coil pack and it still happened. I ran a multimeter test on injector and all the resistance readings were right so I assumed it was good. Finally out of desperation, I replaced the injector and the problem went away forever. Sometimes those multimeter tests are not effective because microscopic cracks in the windings often open up as the component heats up causing an open circuit like condition when it cools it returns to a normal yielding an acceptable reading.

If the misfire happens more than 10% frequency the ECU will shut off the injector off so it won't deliver any fuel to the injector so as not to ruin the cat.

I never had any luck with those in tank injector cleaners. In order for contaminants to get the injector they would have to pass through the fuel filter sock, the wire mesh filter on the bottom of the fuel pump and the foot long paper cartridge filter on the gas line. Any contamination would come from the air intake side. The idea of some how putting six ounces of solvent in your fuel and it miraculously cleans a microscopic passages ten feet away is quite a stretch.
Don't forget gasoline, especially with 10% alcohol, is already a solvent.

To clean an injector you have use a special fixture to force pressurized carb cleaner into it while 12 volts are applied to activate the internal solinoid so the cleaner can pass through

Elroy Jetson · 04-23-2011, 10:43 AM

Thank you for the thoughtful reply. I'll check the vacuum this weekend, and look into the injector issue -- I'll check the wiring first. The problem occurs with the engine hot or cold and in all gears, including neutral. Always in the 2500-3000 rpm range. Above and below that band, everything is strong and normal. At least it's not intermittent anymore -- it was for a while. Anyone want to buy a misbehaving convertible?

Elroy Jetson · 05-06-2011, 09:34 AM

Looks like it's the ECU. Everything else checks fine, and when I removed the ECU circuit board for inspection, it had clear signs of water damage. Reflowing the solder did not fix the problem. Picked up another ECU from ebay, Red Label 413, just like the one I removed which came off an early 94 525 with an auto xmission. Now the car won't start at all. Starter motor turns, and I smell gas, but no ignition. Before I return the ECU as faulty, is there a way to re-initialize this box to my 94 5 speed 325iC that I'm missing? I don't think it's related to EWS as my car doesn't have it, and the new ECU is red label. Thanks.