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1991 M5, 52k miles, my garage queen. All maintenance up to date. Car has been driven frequently all summer, no problems at all.

Symptom #1 started while I was driving under wide open throttle, a loss of power at 5500rpm. Above and below 5500rpm the car was fine. This occured for 2-3 days.

Symptom #2 several days later, sudden complete loss of power at idle, very difficult to keep car running, I had to keep RPM above 2k to prevent stalling. Below 2000rpm, no acceleration response.

Action#1, I replaced fuel filter, no improvement.

Action#2, bottle of Techron fuel injection cleaner into 1/2 tank gas, I kept car running for a while, no improvement.

Action#3, I checked distributor cap, rotor, plugs, etc. all seem they are in really good condition.

Action#4, Drained gas tank completely, opened gas pump access in trunk, checked for water, etc. inside tank, everything looks very clean. Added 5 gallons fresh 93 octane gas, car ran rough at first, then ran good, then started running rough again, so overall no improvement.

Observations:
Car runs semi-fine when cold, as air pump operates, once air pump stops, car runs very rough, stumbles, etc. Car seems to run well for short periods but then runs rough again.

Any suggestions greatly appreciated; I want this resolved before the New York winter kicks-in.
 

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First, I would check my throttle cables and linkage, then check your idle control valve and your throttle bodies themselves. The throttle bodies may not be synced anymore, and may be producing these weird symptoms. After that, I would check the fuel pressure regulator if it's still being all wonky.
 

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the water-temperatur sensor (blue) on the right-hand outside (driving direction) of the engine is known to create similar symptons when failing, it also is a limited investment to replace BEFORE you go into other much other cash demanding details - :byebye:
 
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