BMW 4.6 is poor acceleration and hesitation

[pezho405]

Originally Posted by Aman

I took car to a new garage
Car still has codes for misfire on cylinder 6 and 8 (originally was 5,6 and 8)
I know coils were definitely replaced by previous garage as I asked garage to leave covers off engine and I can see new coils on 5,6 and 8
Makes me wonder if original coils were fine after all (despite the first garage telling me they needed replacing)
To recap, plugs are all new
Today garage reset engine management light and at idle no faults came up (live test)
Need to bring car back for garage to investigate further

You should try to swap coils to different places, then see if that cylinder begins to have a problem

[Aman]

Originally Posted by pezho405

You should try to swap coils to different places, then see if that cylinder begins to have a problem

Thanks for your advice
The cylinders which had misfire (5,6,8) had new coils fitted.
24 hours later there was codes for misfire of cylinder 6 and 8, which suggests the coils are not the problem
I notice that the misfire is all on one side of the v8 engine - I'm not sure if this is just a coincidence?

[upallnight]

Originally Posted by Aman

Thanks for your advice
The cylinders which had misfire (5,6,8) had new coils fitted.
24 hours later there was codes for misfire of cylinder 6 and 8, which suggests the coils are not the problem
I notice that the misfire is all on one side of the v8 engine - I'm not sure if this is just a coincidence?

Perhaps an intake manifold leak on that side of the engine.

[nyum96]

Had the exact same problem, even with the new coils.

Finish changing all of them out. Started with 6, jumped to 7, then to 8, finally jumped to the other bank. Someone can correct me if I am wrong but there are 2 phases to the coil. Phase one can but tested but phase 2 (WOT) can not. Worth a search.

[Aman]

Originally Posted by nyum96

Had the exact same problem, even with the new coils.

Finish changing all of them out. Started with 6, jumped to 7, then to 8, finally jumped to the other bank. Someone can correct me if I am wrong but there are 2 phases to the coil. Phase one can but tested but phase 2 (WOT) can not. Worth a search.

Hi did you have the rattle like the one in the video I posted? (its on page 2 I think)

[bcredliner]

Bad cat will cause will throw weird engine codes. The previous bad coils were likely not burning the fuel, if they were even bad, which would contribute to a cat fail. Cats will rattle when they are shot and will be intermittent. Trace the rattle to the source. Bad cat restricts exhaust so engine will only rev so high as the engine can't suck in more air because it isn't getting out.

If they say a coil(s) are shot I would ask that they test the coils by moving a good one around and see if the faults move to other cylinders before buying new coils. I do like to change all the coils when one goes bad--but I am one that never wants to risk losing any amount of performance.

Now that it is missing at idle it should be easier to pinpoint the cause

As per Pezho and Aman

[Aman]

Originally Posted by bcredliner

Bad cat will cause will throw weird engine codes. The previous bad coils were likely not burning the fuel, if they were even bad, which would contribute to a cat fail. Cats will rattle when they are shot and will be intermittent. Trace the rattle to the source. Bad cat restricts exhaust so engine will only rev so high as the engine can't suck in more air because it isn't getting out.

If they say a coil(s) are shot I would ask that they test the coils by moving a good one around and see if the faults move to other cylinders before buying new coils. I do like to change all the coils when one goes bad--but I am one that never wants to risk losing any amount of performance.

Now that it is missing at idle it should be easier to pinpoint the cause

As per Pezho and Aman

Thanks for your help
After I took the car to the garage again today they reset the engine management light
Now when the car idles there is no misfire again
It seems me making the car go to 4000 rpm causes a misfire at approx rpm, if I do it enough times the EML stays on and the misfire stays, until the EML is reset... so it seems...

[bcredliner]

Originally Posted by Aman

Thanks for your help
After I took the car to the garage again today they reset the engine management light
Now when the car idles there is no misfire again
It seems me making the car go to 4000 rpm causes a misfire at approx rpm, if I do it enough times the EML stays on and the misfire stays, until the EML is reset... so it seems...

Your are correct, it will come back because the the computer will get the same signals that caused the EML to come on previously.

Are you still at the shop or is that all the shop did?

[Aman]

Originally Posted by bcredliner

Your are correct, it will come back because the the computer will get the same signals that caused the EML to come on previously.

Are you still at the shop or is that all the shop did?

That's all the shop did as they were only open half day
They didnt charge me though!
Should I take my car back to the BMW specialist and let them investigate or just go to an exhaust place and get the cats looked at?

[sethhatfield]

It's the catalytic converters are clogged and broken apart inside their shells. I had the same problem and had mine replaced with a pair of universals for $364 and I got all the power back. I wouldn't rev past 4000 rpm when flooring it. The muffler guy cut them off and chunks of crap fell out of them.