Engine pulse

[325e '87]

I've figured out that my engine goes through a steady pulse where the rpms go up and down very slightly.

after up to operating temperature

idle - rpm fluctuates by approx 25rpms up and down at a steady tempo
cruising with a steady throttle position, same thing - rpms up and down very slightly
acceleration, same thing, as the rpms rise, they rise in a steady cycle at the same tempo, up-hold-up

i estimate that the tempo of the pulse is approx 72 beats per minute, so, each pulse is less than one second but not by much.

ive done a lot of refreshing to the car and it's running much better than when we got it two weeks ago. The pulse cycle I have now is the only issue I have left.

any suggestions are appreciated very much. I've learned and relearned a lot here the last couple weeks and hope to become more of a contributor.

[Kershaw]

Have you replaced your:
- coolant temp sensor (blue)
- crankshaft position sensor
- harmonic balancer
- injectors
- air flow meter

[325e '87]

First, thank you for responding.

Coolant temp sensor, yes, new oem
no, cleaned and re installed
no, cleaned and re installed
no, haven't touched them
no, cleaned and tested, I need to test it again though, I'm not confident I got good information but it did give me passing results.

I will keep at it though. It runs very nicely and is close to being "right".

im curious, what goes wrong with the harmonic balancer?

[Kershaw]

I have no idea. But I know I had a bad one at one point. The car couldn't get down the street it was bucking so hard. I had replaced everything else in the engine bay so I thought eff it, why not. And that turned out to be the issue. Visually there was nothing different between the bad one and the good one. Cleaning your CPS does nothing unless it's literally covered in magnets. I always replace it as soon as I buy an e30, along with the timing belt.

The spray pattern on on one injector might not be 100% ideal, leading to a slightly different firing periodically.

Now that I think about it, I doubt it's your AFM since it's seen at idle with your AFM not moving at all.

Could also be a slightly weak ignition coil.

[ELVA164]

You may want to look at the idle control valve also.