Over-rev at idle M5 S38B35 ***SOLVED

Specific conversations and info for the BMW E28 M5 and M535i.
Post Reply
arthurK
Posts: 3
Joined: Sep 10, 2020 2:29 PM
Location: Edmonton, AB

Over-rev at idle M5 S38B35 ***SOLVED

Post by arthurK »

My first post..
Recently I installed do Holy Grail Labe fuse box replacement and all was working just fine. Great product by the way!
Then last weekend while driving all of a sudden the engine RPM shot up like the throttle was stuck at fully open.
Expect it wasn't - cable is file and throttle linkage operates OK.
I tried to start it, but it revs up like crazy and I need to shut it down immediately. Throttle confirmed closed - I can see it with open hood.
The engine is stock S38 with Dinan chip.
I did check the AFM and the resistances seem to be OK: Term 2 & 3 556 ohm, varies with the flapper position, thermistor term 1 & 4 3300 ohm at 18 C.
TPS switch was not closing at the throttle idle position (term # 18 & 2) but at full-throttle (term 18 & 3) was OK.
TPS model # is 280 120 301. In order to have the idle switch closed, I have to rotate the switch body and the mounting screw will no longer fit the bracket.
Couple questions:
1. Any idea of the cause of this over-revving?
2. Can the lack of close contact at the idle be the cause?
3. How to open the TPS? I see the locking tabs, but is it glued together?
4. Bad ICV might be a cause?

Arthur
Last edited by arthurK on Oct 20, 2021 11:47 PM, edited 1 time in total.
Galahad
Posts: 496
Joined: Sep 06, 2016 9:08 PM
Location: Beverly/Worcester, MA
Contact:

Re: Over-rev at idle M5 S38B35

Post by Galahad »

Always happy to see people who like the fuse board!
I'm pretty sure you have a stuck ICV, possibly a major vacuum leak.
The throttle switch is used to tell the ECU to run the ICV, I don't see a way that itself could cause the issue.
In a 528e the idle computer only controls the air valve; the ECU fuels to match and doesn't have any idea what the "target" RPM is. If the ICV sticks open, the idle computer can't limit airflow and the ECU is too dumb to care. The m30 and s38 ECUs have the idle computer built-in instead of in a different box, but I'm pretty sure it acts the same way.
Based on your description it doesn't sound like a vacuum leak because your engine would run like crap if that were the problem.
Here's a link to a clickable version of the M5 ETM: https://www.armchair.mb.ca/~dave/BMW/e28/e28_88.pdf
austin8753
Posts: 1444
Joined: May 16, 2010 1:37 AM
Location: Portland, Oregon, USA

Re: Over-rev at idle M5 S38B35

Post by austin8753 »

make sure the throttle cable hasn't come loose from the isolator/grommet, bolted to the throttles. i've seen that a few different times. look really closely at all those.
austin8753
Posts: 1444
Joined: May 16, 2010 1:37 AM
Location: Portland, Oregon, USA

Re: Over-rev at idle M5 S38B35

Post by austin8753 »

double-post
Last edited by austin8753 on Oct 20, 2021 2:22 PM, edited 1 time in total.
arthurK
Posts: 3
Joined: Sep 10, 2020 2:29 PM
Location: Edmonton, AB

Re: Over-rev at idle M5 S38B35

Post by arthurK »

Solved - one of the butterflies was partially open. Cleaned all the linkages and it's working again as it should.
Still need to solve that TPS issue - I have close contact at idle, but WOT contact doesn't engage, I might damage it while trying to adjust.
Luckily I have one spare from my late E30 (Bosh 280 120 321) and will do surgery on this one.

Thanks to all who responded!
Post Reply