Taking "Bets" on Rough Idle Diagnosis

[Johno]

So, I have a very slightly rough idle on my M3. It has newish plugs, new coils and new o2 sensors. Plugs look fine.

A local indy shop checked and their computer located a misfire in the number 5 cylinder. Compression checks out perfect. (yeah!) Current Diagnosis: Fuel injector. Since there was not time today for the shop to simply swap injectors from one cylinder to the other and confirm the diagnosis, we will confirm with a new injector tomorrow.

Thanks to great customer service at Europsport, I have a new injector arriving at the shop in the morning. I'm displeased that the injector allegedly failed after only 5 months of service, but life ain't perfect, eh?

Soooo...do you think the new injector will solve the problem, all things considered?

Could it be ANYTHING else?

Thanks,

[vinnymac]

Make sure you idle control valve vacuum hoses are connected correctly and getting a good seal.

Heck, it wouldn't hurt to clean the ICV and then check all the vacuum connections while you're at it.


If you suspect a bad fuel injector...move it to a different cyclinder and see if the error code follows it.

[Balthazarr]

Easiest thing to do is squeeze the return line, momentarly.
Pressure change should force the bad injector to allow cylinder to fire normally.
Predicated on that the inj is clogged rather than failed.

[Johno]

vinnymac,
The shop says they checked for vacuum leaks and their computer said it's a misfire on 5.

INteresting idea about squeezing the line.

I am sorry they did not swap the injector and see if the trouble followed. Apparently there was not time for that final step today.

[BeSeen]

Weird, normaly a Misfire is caused by a bad coil or spark plug. Are the Coils OEM?

[Johno]

Weird for sure.

The coils are from bimmerparts.com, and they appear to be made by the same company that BMW uses.

However, the problem was there before I removed my BMW coils, and still there after I installed the new ones. Plus, the new coils check out fine per shop diagnozis. Since the problem was unchanged between BMW and bimmerparts (femi?) coils, I surmize it was not the coils.

I have heard that these "little" problems can be the toughest to diagnose. My brother said he went through coils quickly on his cadallic, and they caused significant problems when they failed. So I am hoping German engineering failures are more graceful than GM engineering failures.

Meanwhile, I hear from M3/M5 that based on the other parts checking out (plugs, coils, compression, O2 sensors(, there's nothing else it can be other than the injector.... M3/M5 knows his way around a BMW engine. But I won't rest easy until it's solved.

[Balthazarr]

You didn't mention the plug boots.
Are those ok?

[BeSeen]

If your getting a misfire there are several things that can be happening either not enough fuel is going in or you have a vaccum leak somewhere in the car. I would definately try the Injectors though, but if not that your gonna have to try other stuff like Air Mass, Fuel Filter, Throttle, ect.

[Johno]

I forgot to mention new plug boots.

Would a vacuum leak not make all cylinders rough, or can a vac leak affect a single cylinder?

[Balthazarr]

I forgot to mention new plug boots.

Would a vacuum leak not make all cylinders rough, or can a vac leak affect a single cylinder?

I think it would affect more than one randomly as the pistons were moving on the compression stroke.

[BimmerM3inGA]

Could be an injector o-ring. A friend of mine chased a bad idle for months and spent thousands on parts and labor he didn't need. It turned out that a couple of the injector o-rings were dry rotted.

Of course, this was on a 1994 325i with 180K miles. But still, it's worth checking.

[Johno]

And the current diagnosis is:

Intake gasket leak.

Per smoke test.

Parts arrive Monday. Now I have a spare injector that is good in case another one ever "fails."

Thanks to Josh for being there to handle this quickly.

I will be interested to see if this really does the trick, and if so, whether there will be a noticeable change in the butt dyno. It would restore my faith in FI if this repair encounter vindicates my feeliong that there was a performance loss.

[THATDONFC]

You should've taken some brakleen and sprayed it around the intake manifold/anywhere there could be a vacuum leak. If the rpm's get higher as you are spraying you have a leak.

[Balthazarr]

Intake mani leak...never thought about it.

[Johno]

I listened to somebody here who said to use a propane torch (unlit) to check for vacuum leaks. That didn't work.

If this happens again I'll check more diligently.

[Johno]

Yep, Intake gasket. Total cost for this expereince was about 571, plus a week in the rental car.

[Balthazarr]

Yep, Intake gasket. Total cost for this expereince was about 571, plus a week in the rental car.

$571...how long did it take him? :eek:

[Johno]

It took 8 hours, and he cut it back to 1 diag and 4 install.

JO