E28 M30 Help Needed - Engine Not accelerating correctly after running for 15 minutes

[DoobieZuben]

So, here is my situation.. Any suggestions are certainly appreciated !

Recently acquired a 1985 535i.

Problem: Lack of power during acceleration after 15 minutes of running.

Engine accelerates beautifully during first 15 minutes of running after starting cold. Engine temperature is within normal limits within 5 minutes of starting, and stays the same even after decrease in power is noticed. I'm guessing that an external part i.e. air flow sensor itself is heating up for example after 15 minutes and perhaps causing an incorrect fuel air mix?

Symptoms: Diminished power in rhythmic cycles of about one second or so. . Engine has minor surges of power and lack of power as i accelerate. Especially noticable after about 3500 rpm.

I have replaced injector seals, adjusted valves, replaced O2 sensor, replaced spark plugs, many vacuum hoses, Distributor cap and rotor.


My questions:

Could the fuel regulator create the rhythmic power decrease surges (does the fuel pump move fuel in that cycle)? The air flow sensor not sending a constant signal to the DME?

What should the fuel pump psi be? I could put a gauge inline and check for dimished performance after running for a while.


THanks ALl for your comments and questions to me to help further track down the problem.,

Zuben

[Gus540i]

check to see if your spark plug threads are stripped, who knows, when cold everything seals and when she warms up the expansion causes the threads to give up seal, is there a "knock" present when the lack of power occurs? If yes then maybe his is your dilemma