M50 running like crap. Did I fry something with a super cool backfire??

[Dave M]

Background:

1991 525im. The car sits in a nice dry garage over the winter and I start it/drive it occasionally. This winter, after sitting for long periods its developed starting issues. It soaks the plugs (possibly leaky injector(s)). After a few attempts, it usually starts rough and smooths out. So, on the weeekend, after flooding and not starting, I pulled the plugs, dried stuff up and tried again. When cranking it over, it answered back with a mean ass backfire that sooted the area behind the car and had a neighbour pop in to investigate. I pulled all the plugs again and let everything dry again.

The New Problem:

Once I finally got it started, it had a hunting idle (not out of the ordinary given the circumstances) and sounded/felt like it was missing. The exhaust is 'put put puting' rather than smooth. When driving, under any load it was definitely hurting. Theres a lack of power and vibration associated with any real level of throttle.

What I've Checked So Far:

- Code pulled = 1221 (O2 sensor) - haven't checked the sensor output.
- All spark plugs are toasty brown
- All the coil leads have good voltage
- All the coils have proper resistance
- All the fuel injectors have proper voltage (and vary with throttle).
- No noticable inatake/vacuum leaks

Are the symptoms consistant with O2 failure (backfire cooked it?) Is the test in the Bentley good at determining if its bad? (I seem to recall that its not a great test).

Any help would be much appreciated.

Dave

[Russell]

I have been told by my indy that the m50 does not like to be started and run briefly than parked. it runs very rich on start up and stopping it before fully warmed up leaves a lot of unburnt fuel.

I have noticed that t starts and runs very rough on a restart after say backing out of the grarage and quickly turnng the car off to wash it. Then restarting the car 1 hour later. Almost always a rough running engine.

Also mine for 6+ years has backfired on a cold start a few times a year. have no idea why or if it harms anything. BTW, this car has had plugs, coil connectors, ox sensor and knock sensors in the past 18 months.

Hope my experience helps.

Oops is the 91 a M50 eninge?

[Dave M]

I have been told by my indy that the m50 does not like to be started and run briefly than parked. it runs very rich on start up and stopping it before fully warmed up leaves a lot of unburnt fuel.

I have noticed that t starts and runs very rough on a restart after say backing out of the grarage and quickly turnng the car off to wash it. Then restarting the car 1 hour later. Almost always a rough running engine.

Also mine for 6+ years has backfired on a cold start a few times a year. have no idea why or if it harms anything. BTW, this car has had plugs, coil connectors, ox sensor and knock sensors in the past 18 months.

Hope my experience helps.

Oops is the 91 a M50 eninge?

Yep, its a M50. I tend to believe your indy following this bout of trouble. Unfortunately, I've since driven it for a half hour and had it to operating temp and its still running like crapola. I wonder what I've done?.............

Thanks,

Dave

[Ferret]

Nest of mice in the intake, that you've just converted to black powder?

I like the way your neighbour popped in!

I dont think this is an O2 issue - they're being ignored at startup, though the question is, did the O2 sensor code appear before or after the backfire? It's possible it's become brittle with being parked up over the months and it's been blown away by the backfire. Question is though - does it run like a dog from stone cold, or just running rough after the O2 sensors have come online?

Try unplugging the MAF as usual and see if it sorts it out - maf wire could have corroded with being laid up.

Aged fuel, or split intake boot?

Just some thoughts...

EDIT: Hunting idle almost -always- signifies air leak as it's the ICV overcompensating... maybe a hose/boot has dried up during the layup, that's where I'd start... unplugging the MAF will tell you though.

[Dave M]

Nest of mice in the intake, that you've just converted to black powder?

I like the way your neighbour popped in!

I dont think this is an O2 issue - they're being ignored at startup, though the question is, did the O2 sensor code appear before or after the backfire? It's possible it's become brittle with being parked up over the months and it's been blown away by the backfire. Question is though - does it run like a dog from stone cold, or just running rough after the O2 sensors have come online?

Try unplugging the MAF as usual and see if it sorts it out - maf wire could have corroded with being laid up.

Aged fuel, or split intake boot?

Just some thoughts...

EDIT: Hunting idle almost -always- signifies air leak as it's the ICV overcompensating... maybe a hose/boot has dried up during the layup, that's where I'd start... unplugging the MAF will tell you though.

Good onfo Ferret, thanks. I did look at the intake, but I'll try again and disconnect the MAF. And yes, it runs like **** right from start up.

Dave

[repenttokyo]

burnt valve?

[tim eh?]

could you have damaged the injectors? I'm picturing a leaky injector with some gas soaked deposits going up all at once ... good luck...

[bubba966]

Check the plastic connector that connects the big ribbed vac line from the ICV to the intake manifold. The crap design of the plastic connector leads to the retaining clip breaking off and the connector working it's way out creating a large vac leak. This large vac leak will cause it to run like crap.

Had friends '95 525iT do the exact same thing. Had a nasty backfire one morning that ended up blowing the vac line connector out of the intake manifold. Unfortunately I didn't figure this out until we'd got & tried a used MAF, new MAF, new injectors, new plugs, etc. Spent a good $700+ before I found out it was this damn little $6 part that is so prone to breaking the local dealers all carry it as a stock item.

Part #9
http://www.realoem.com/bmw/showparts.do?model=HJ63&mospid=47414&btnr=11_1124&hg=11&fg=40

Easy way to check it is to pull the throttle body off and look into the manifold. Then you can see if it's fully seated when looking into the intake through the throttle body port. You could also stick your hand underneath the intake when the throttle body is off and try to see if you can insert the connector farther into the intake if you're not familiar with how it should appear when fully seated into the intake.

Hopefully it's just this stupid ass connector that's causing your problems as it's cheap and easy to find.

[Dave M]

Check the plastic connector that connects the big ribbed vac line from the ICV to the intake manifold. The crap design of the plastic connector leads to the retaining clip breaking off and the connector working it's way out creating a large vac leak. This large vac leak will cause it to run like crap.

Had friends '95 525iT do the exact same thing. Had a nasty backfire one morning that ended up blowing the vac line connector out of the intake manifold. Unfortunately I didn't figure this out until we'd got & tried a used MAF, new MAF, new injectors, new plugs, etc. Spent a good $700+ before I found out it was this damn little $6 part that is so prone to breaking the local dealers all carry it as a stock item.

Part #9
http://www.realoem.com/bmw/showparts.do?model=HJ63&mospid=47414&btnr=11_1124&hg=11&fg=40

Easy way to check it is to pull the throttle body off and look into the manifold. Then you can see if it's fully seated when looking into the intake through the throttle body port. You could also stick your hand underneath the intake when the throttle body is off and try to see if you can insert the connector farther into the intake if you're not familiar with how it should appear when fully seated into the intake.

Hopefully it's just this stupid ass connector that's causing your problems as it's cheap and easy to find.

When I did my major engine work a few years ago, I saw the 'crappiness' of this connection and took matters into my own hands before re-assembly. Given your experience, I will have a look at it in the next day or two to make sure it hasn't found a way of releasing. I'll also check the other end of the macaroni to make sure its still attached.

Thanks,

http://img.photobucket.com/albums/v11/dave_macisaac/Bimmer/Engine/Block_Swap/ad6f03cd.jpg

[bubba966]

I should probably do something similar to that on her M50. I've had to replace that damn connector again. I think I first did it a little over two years ago. Next time I pull that intake manifold off I'll have a look see at modding the little bastard. Hope to not have to anytime soon. But if this problem it's currently got isn't fixed by a different fuel pump I will have to pull the intake off.

[93 525 Paul]

When I did my major engine work a few years ago, I saw the 'crappiness' of this connection and took matters into my own hands before re-assembly.


Glad I read this thread. Will do the crappy connection update (CCU) when I reassemble the m50. Sort of incredible you had quick access to this photo Dave.
Kudos.

[DanQ]

Hi All,

I know this thread is a few months old but I had an observation/question.

It seems that this happens more frequently as the car ages.
I'll be on my second connector in less than 20k miles.

I suspect that the connector is getting blown off when the engine backfires which then breaks the tab that's supposed to hold on the connector.

I wanted to get others thoughts on this as well.

What are the possibilities for causing a backfire through the intake?
Do the injectors start to leak after age/miles?

My car is a 1993 525iT with 205,000 miles.

Thanks

[Tiger]

Timing slip... leaky injector and carbon buildup... you need all three to backfire through intake.

[Ross]

I think your backfire is a symptom rather than the cause of anything. Unburned fuel that had been dumped into the exhaust lit off that's all. Be glad you still have a muffler.
Leaking injectors as you suspect sound likely. Observe fuel pressure after shutdown, if it drops off you probably have some dribblers.
Long cranking time?

[Dave M]

Since its been brought back from the dead, I'll provide an update.

The miss was a bad plug, thats it. I didn't look hard enough at the plugs the first time I pulled them out (nor the second time apparently). After replacing injectors (which were leaky and causing laboured starts and backfires), checking all sources of intake leaks and testing everything under the sun, including MAF, coils and leads, TPS, fuel pressure etc., I finally found a plug with cracked ceramic. Replaced it with an spare and off we went.

I learned a lesson. Thanks for the all the help and discussion,

Dave

[genphreak]

Hah! I've done everything on my M50B25 too, but I have intermittent, sometimes temp-related, bad running as well as backfires. Replaced coil packs, injectors, pipes, connectors and I still don't have the root cause.

It very seldom backfires now, but the crappy running is still common.

Those connectors are a weak point so if you keep breaking them, it might be backfiring in the intake (on mine it happens occasionally during warm cranking) and you can relaly hear it. I can never fit the new ones, the little clip is never long enough. I usually have to cut the return a mm or so off the little barb on the clip. The last culprits I have to go are: Crank position sensor, Cam Position sensor, Fuel Pump Regulator, FP and... F@#! knows what else, probably the root cause whaetever it ends up being...

[DanQ]

Anyone come up with a solution to the backfire?
I put on a new connector and it lasted 3 days until the next backfire blew it off and broke the clip off!
I pulled the cold injectors out and left them on the fuel rail, put cardboard under each one after turning on the key and testing them to make sure they each inject. This was to see if there were any leaking on to the cardboard. I didn't get anything from that test.
I then ran the car to get it heated up since it seems this only happens after a drive in to work and the car sitting all day. No apparent leaks from the bottom of the injector but there was one at the fuel rail which I suspect was from pulling the injectors from the intake manifold.
The backfires happen only on start and very quickly too. Usually well within a second of starting to crank. It doesn't always happen so I suspect it's not happening on all cylinders or it would happen on every start.
After this, I'm not sure what else to check except all timing related items.