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Discussion Starter #1
The problem is very abrupt. The engine runs as flawless as ever until around 3800 to 4200 rpm, then chokes hard, almost like a rev limiter but worse. It will happily regain its composure and idle and almost immediately run at 3600 without issue. This is a persistent issue, all the time, every time.

My gut says its a fueling issue, the first thing I replaced was the fuel filter, no luck. Wondering now if I should move on to the pump and pressure regulator. I've also needed to do some tune-up maintenance, so I replaced all the ignition works, plugs, wires, coil, rotor and cap. Threw in a new air filter while I was at it. The issue prevails, though idle is buttery now.

I should mention, this first happened on a road trip I was taking, 900 miles from home.. just after a run to 167mph. I was quite proud of the car until she gave me a cough.. then I started to worry....

-Jason
 

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That cough you describe is not usually a good sign. It seems that horrible things usually happen after a long hard strenuous top speed run in these cars.
 

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Discussion Starter #7
I doubt a cylinder is dead. The engine runs very smoothly... plus when I changed the plugs they all looked nicely browned and even across the board.

I have a fuel pump on its way.. its easy enough to swap out along with 20 year old vent hoses etc.. If that isn't the problem I have another project to use the pump on anyway.

The damper/trigger wheel theory is certainly an interesting one! If I find time I'll throw a timing light on it and observe what the timing does when it coughs.
 

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Interesting thread...looking forward to the outcome. Can you pimp this thread a little with some nice pics? I can assure you it will trigger a few other members as well:):)
 

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Discussion Starter #9
Boo.. changed fuel pump this afternoon and no luck, though I found a pile of aluminum shavings under the pump, so it might have been worthwhile after all.

The situation is slowly degraded, over the past few weeks the engine as started to stumble earlier and earlier in the rpm range.. now at about 3200.

I'll have to do more investigation with fuel pressure..

On my list of suspicions is the cat, fuel pressure regulator, fuel pump relay, or possible crank pulley issue...

Bad news is my 850i has a serious coolant leak now! I might have to drive the dreaded '96 Ford Ranger (4 cyl). Please someone save me.
 

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When the car hits this limit, do you have to completely release the throttle before you can apply it again, or can you just reduce the revs slightly to cure it?
 

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I had the same issue on mine, felt like a rev limiter when accelerating above 4000rpm but was it`s normal self below that. Engine would rev no problem when not in gear but the minute it was loaded up, splutter splutter.
Main dealer (under warranty!) threw the parts book at it and eventually cured it after 2 or 3 weeks of effort by replacing the ecu.
I don`t know if this was 100% the original cause or if it was just an accumulation of new parts that cured it but the ecu was the final part to be replaced.
 

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Discussion Starter #14
I finally have a bit of time to play with this car again.

Updates:

I've since bought a parts car which has an engine that runs great, and has half as many miles.

I've got all new ignition, coil, wires, plugs, cap, and rotor. No change.

I've swapped the ECU, no luck.

The condition is slowly getting worse, now the engine can't rev past about 3400rpm.

As soon as I can get the two cars side by side I'll start swapping other parts.. if I can't fix it in a day, the engine is coming out for a swap. I already have some oil leaks that need to be addressed, so might as well.

Happy motoring,
-Jason Miller
 

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Then i am pretty sure your damper pulley is loose since it is getting worse. That 30mm bolt is supposed to be torqued up to 450NM with an old bolt, but you should rather replace with a new strech bolt.
The key might have sheared already and now your ignition timing is out. Dont drive it any further, or you might cause serious damage to the timing chain sprocket and damper hub
 

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Discussion Starter #16
The more I thought about it, I suspected it may be the harmonic balancer, the trigger wheel is vibrating independently from the crank and the ECU is losing its position. This would explain the erratic tach behavior, then its recovery at lower rpm.

Well, I just took a look.. most of the rubber is completely gone and there's a crack in the rubber around at least 70% of the perimeter. I can wiggle the trigger wheel with only my finger!

I'm so very excited to have discovered the problem, now to order another balancer and get her back on the road (above 3000rpm!)

-Jason Miller
 

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That is surely the cause.
Nice going. luckily you can remove the harmonic balancer without taking off the centre bolt.
While you there, just check if it is still tight. (you would need a locking tool to keep the engine from rotating)
 

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Discussion Starter #18
I shall do so, I have the tool on hand.

The hub might be removed anyway if it looks like the crank seal is leaking. I will disassemble soon, she's still too hot to play with tonight, plus its 2:40 in the morning.

-Jason
 

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What a bonus that you found the problem , and well done Mr Mac , pinpointing the issue early doors !

D
 
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