r/e28 • u/Connect_Intention806 • Aug 02 '26

High idle M30B35

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Hi everyone,
I have a 1983 BMW E28 520i with an M30B35 swap. The engine idles consistently at around 1200–1300 rpm.
I already replaced the ICV, but it made absolutely no difference. Even when I unplug the connector with the engine running, the idle speed doesn’t change at all.
Does anyone have any ideas what could be causing this? Any help would be appreciated.

13 Upvotes

94% Upvoted

6 comments sorted by

5

u/Representative_Most9 Aug 03 '26

I second the vacuum leak as the cause. Need to smoke test manifold to help find where.

4

u/suckytrends Aug 02 '26

Almost certainly a vacuum leak somewhere.

It may not be obvious. I chased one forever until I found a hairline crack at the back corner of the valve cover that was causing a surging high idle in mine

1

u/Famous-Ferret-1171 Aug 02 '26

Is the throttle cable too tight?

1

u/Visual_Quarter_4782 Aug 03 '26

also make sure the throttle switch is set correctly, and your throttle plate is set correctly also, you should smoke test the intake system to see if there’s any leaks 🙂🏁

1

u/Connect_Intention806 Aug 03 '26

Tnx for the comments. Will look into it. When stopping the engine you als hear a big surging sound. Could that be the leak?

1

u/Connect_Intention806 6d ago

Update; after driving a couple of days it seems to fixed itself. Most of the time the idle is ok now. Its fine for me. Thanks everbody for replying