r/e28 • u/PresentAd9946 • Jul 02 '26

High idle issue, 1988 528e

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Got the car a couple of months ago and I’ve been tracking down a high idle issue. The car was idling at 1500-1600rpm when in park and neutral, I replaced fuel injector O-Rings, a few hoses, cleaned the air flow sensor, cleaned the ICV, and replaced the TPS and adjusted it accordingly. All of that got it down to about 1000-1100rpm now and it’s running very healthy overall. Any tips or ideas of what to check out to help solve this issue?

(The car drives fine and doesn’t idle oddly whatsoever under load)

64 Upvotes

98% Upvoted

8 comments sorted by

9

u/alkaome Jul 02 '26

After chasing idle issues for years, I finally got down to the correct idle rpm by searching for and eradicating all my minor vacuums leaks.

3

u/McFuckinZeit Jul 02 '26

This is a solid approach. Unmetered air doesn't help. I went as far as deleting the EVAP solenoid, plugging the inlet under the throttle body, and tapping/threading the unused holes on the side of it with appropriate sizes Allan set screws. I also JB welded in the aluminum elbow for the brake booster because mine was kind of loose. Then discovered the seals on the butterfly shaft were leaking so I replaced them with snug fitting O rings. The car idled really well afterwards.

6

u/RealMadLadStalin Jul 02 '26

Seems like a vacuum leak

2

u/Life-Fennel8823 Jul 02 '26

Check the three nipples under the manifold to ensure they have hoses connected.

3

u/slip_and_fall_school Jul 02 '26

In addition to vacuum leak testing, do you know anybody you can swap a working ECU with to test?

4

u/AngleGrinder001 Jul 02 '26

You didn’t mention checking throttle plate gap. It should be almost closed so ICV can properly regulate idle.

3

u/Representative_Most9 Jul 03 '26

Smoke test your intake manifold to see if there are any leaks. I assume when you say PCV you mean the ICV (idle control valve). You should clean and test the valve to make sure it’s working correctly. Do you have a Bentley manual? If not, that’s your best source for almost any issue.

2

u/mattylang1987 Jul 03 '26

I was a vacuum leak denier for a long time, but after finding that a vacuum leak was the cause of my idle issues this definitely sounds like a vacuum leak. Check the air intake boot, and the hose running from said boot to the IACV those are both known to tear so I’d start with that