525i Expert needed: Surging while accel, no vacuum leaks, no MAF issues

[BMWFL]

Hello everyone! I am back again with the same problem. I tried all recommendations received but the problem persists.

I have a 525i M50 non-vanos manufactured in July 1991 with rhythmic surging while accelerating (a small spike in power every second or so), but doesn't seem to have it while idling or is not noticeable. It is present regardless of how hard the pedal is pressed, it is also visible in the fuel gauge, the tachometer and even in the speedometer. At a constant speed is not very noticeable (if at all), regardless if at low or high speeds.

I replaced all vacuum lines and made a smoke test to check for new leaks (there are none), I also replaced the MAF and swapped 3 different DME/ECU's without luck.

Is there anything else that can cause this surging?

Thanks!

[Younge23turbos]

Checked the fuel pump, filter, and fpr? How about any voltage issues too?

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[BMWFL]

Thank you Younge23turbos. I will test for fuel pressure drops. I have a new fuel filter, I do not think the issue is from the fuel supply line. The behavior is the same regardless of speed or how hard the accelerator is pressed, it is the same at soft acceleration or hard acceleration. I have not checked voltage variations, I will do that next.

Is there any other sensor that might produce this behavior? How about a bad oxygen sensor? I am getting no codes so everything seems to be running good, but then again these cars didn't have many codes to begin with.

Thanks again for the suggestions!

[Honolulu]

I'd like to think it's a classic sign of a vacuum leak, but you smoke tested it and found nothing, so a dead end there.

If you have access to a fuel pressure gage put the gage somewhere you can see it while driving. Pressure should hold pretty steady.

Similarly if you have a means to check the 02 sensor while parked, and while driving. Proper lambda values rise and fall between maybe 0.2 and 0.6 volts every second or two.

Unfortunately as I have lately discovered with my BMW bike, reliance on reading codes may not be as useful as we hoped it would be. No codes on the bike which is running quite rich and unrideable.

[BMWFL]

Thanks Honolulu, I will check the fuel pressure and the O2 sensor next.