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Roger, BMW Master Tech
Category: BMW
Satisfied Customers: 3044
Experience:  10 yrs. experience as BMW Master Technician, shop foreman and dealer.
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I have a 1989 BMW 750il V12. Both throttle bodies were

Customer Question

I have a 1989 BMW 750il V12. Both throttle bodies were cleaned and are in good oder, It starts up every time no problem but the exhaust is really loud and it backfires under hard rev. It revs smooth and seems to have power in park but when you get out on the road it bogs down and won't accelerate. I have a peak tester and it throws no codes. I am wondering if I remove the mufflers, cats and O2 sensors will it help or hurt? Could the cats being plugged up cause this? Could the O2 sensor wires be mixed up? How can I troubleshoot this?
Submitted: 1 year ago.
Category: BMW
Expert:  CHEVYEATER351 replied 1 year ago.

Hello my name is ***** ***** I will help you with this issue. Here are a few suggestions to try. Cats could cause this issue for sure if they are clogged, but with the tester you should be able to read the O2 Sensor sweep. If they both sweep then you most likely do not have cat issues. Will the vehicle rev fine in park? I ask this because these engines are known for the high pressure fuel pumps going bad. This will give you the lack of acceleration you are describing. If memory serves me the V12 has one on each side and they are quite expensive to replaced. If possible I would check the fuel pressure when you are driving the vehicle to see if the fuel pressure drops.

Customer: replied 1 year ago.
It revs fine in park, but the exhaust will pop and sounds loud until it warms up. I replaced the fuel filters but did not check pressure since it seemed to rev fine and seems to have good power in park. It will take off quick but then just loses power and won't accelerate.
Expert:  CHEVYEATER351 replied 1 year ago.

That is a symptom when the vehicle doesn't have enough fuel pressure under load. Can you check the sweep of the O2 Sensors on your scan tool? I would not look pass the fuel pressure, it is hard to say if the fuel could be air raiding causing the popping as well.

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