bmw misfire

Pete Guthrie · **Posted:** Thu Jul 16, 2009 6:19 pm

hi all. our white ETA bmw 2.7l. has a misfire.. we think it is cyl 2. when we pulled the plug boot.. the rpm did not respond. our flywheel sensor is also not so good.. maybe that is it yes.. i dunno.. we put new plugs in it, from autozone.. still no luck. the flywheel sensor went bad on my mothers 525i.
the timing consequently went waay out of whack. Once replaced, it ran great.
but if the timing was off, all the cylinders would be bad.. so either bad injector or sticking valve i think.. maybe i am wrong eh.

Steven Carter · **Posted:** Thu Jul 16, 2009 10:51 pm

just a SWAG, but...

If changing the spark plug doesn't help, you need to make sure the plug wire resistance is still within spec. Then check the ignition rotor and cap for oil/grime or broken contact points. My 325is' rotor was pretty bad off, and the engine appreciated a new cap and rotor, along with plugs and wires....

If all that checks out, maybe it is the fuel side of the equation as you suggest.

Any CELs from the DME?

Why do you think the sensor is bad? According to the Bentley manual, on the 325e the resistance between terminal 1 (yellow wire) and terminal 2 (black wire) of each sensor connector (not the harness side) should be 960 +/- 96 ohms. If not, the sensor is bad. (Bentley section 5, p 22--for those reading along)

(Further reading in that section suggests that the two sensor connectors can be mistakenly swapped on the harness, and the reference pin on the flywheel can malfunction, so check that too?)

Good luck

Charlie Guthrie · **Joined:** Thu Aug 28, 2003 8:14 pm **Posts:** 832

Thanks for the input Steve. The compression is 150 +/-2 psi on all 6. The cap rotor and wires are all new. The cap is a few weeks old but the wires just arrived. New plugs properly gapped and still there is that misfire. I listened to the injectors with a stethoscope and all are clicking merrily. The most convincing bit of information leading me to suspect that the injector is fouled is that the misfire is much less when first started, I think that the cold start injector may be adding just enough extra fuel to mask the lean injector. As soon as it warms up it begins to skip a little, but really becomes noticeable under hard acceleration.
I was tempted to swap injectors around (and may yet do so), but the hassle factor may be low enough to justify the $80 for an injector.
Depends on how my weekend goes.
Charlie G

Steven Carter · **Posted:** Fri Jul 17, 2009 12:09 pm

Hi Charlie, Pete:

Sounds like the ignition side has been taken care of, so fuel issues may be the cause as you suspect...If it is a dirty injector, would you try to clean it before replacing it? replace the filter also? Does one clogged injector make you think about whether the others should be serviced?

Sorry for the questions, but my 325is needs some fuel/intake work also, and I've limited experience with E30s. Obviously, I'm interested in how your car turns out.

Hope it's an easy fix for you!

Charlie Guthrie · **Joined:** Thu Aug 28, 2003 8:14 pm **Posts:** 832

Just in case anyone is out there wondering, after completely disassembling the injectors and test spraying them, the problem was traced to the new distributor cap. The cap reads continuity from the #2 electrode to the plug wire connection, but it just will not pass spark. One dead cylinder. In a truly desperate shade tree mechanic move, I swapped distributor caps with my track car. The track car cap was really in need of replacement (heavy electrode erosion) but it still delivered spark to all cylinders evenly and now the miss is no more. The only thing that I can figure is that the new cap was molded slightly off center with the internal gap to the #2 electrode just a little off. I used calipers to measure the internal diameter (distance between opposite electrodes) and they were the same for both caps. This falls into the realm of "stuff I've never seen before". The new cap worked at 100% for long enough to develop the typical spark-eroded finish, but then it apparently just started fading away.
By the way, pulling the injectors is a PITA if they have been in for 20+ years. A liberal dose of carburetor cleaner applied a couple of times helps free the stuck O-rings. The work space getting to the injectors is pretty small, too.
Charlie G

George Bright · **Posted:** Sat Jul 25, 2009 6:08 pm

I had that trip me once on my Datsun 210. I did a tune up and developed a miss after a week. I swapped the old plugs and rotor back in, put on a new coil, and swapped the distributor module. Replacing the week old cap fixed it. Of course this was done over 3 rainy days without benifit of a garage.

Pete Guthrie · **Posted:** Sat Jul 25, 2009 7:31 pm

the bmw is running well again. turns out the distributor cap.. for what ever reason.. was bad. took the one off the 325is track car.. ran fine. now we need some more parts for the 325is, and we are good t ogo. odd odd.
anyway. maybe the season ender auto x will feature two guthrie cars.

Steven Carter · **Posted:** Sat Jul 25, 2009 8:49 pm

glad to see it's working better again. I had a similar problem with my Z. The replacement brand new rotor was slightly malformed such that it wouldn't carry a strong enough spark to the cap and it had misfire despite "all new" ignition components. Ordered a different brand and voila! It worked.

Anyway, glad you didn't end up buying new injectors only to figure out the cap was the culprit.

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