Idle problem! Please help!

E28 technical advice asked and given! Troubleshooting, modifications and more.
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Rocketflip
Posts: 17
Joined: Dec 30, 2020 12:20 AM
Location: Illinois

Idle problem! Please help!

Post by Rocketflip »

https://youtube.com/shorts/uRdDUgRHdr8?feature=share

My car is a 1985 535i it has been swapped to a manual by a previous owner.

Here is a video of my idle problem. When I start driving it it runs fine. Idles below 1000, but after it gets up to temperature, it constantly revs between 1000 to 1200 rpm over and over and over.

When it drives it seems fine, except for some excessive engine breaking rolling around parking lots in first or second gear.

I should mention, it didn't do this before I changed the head gasket. So I had the whole thing apart and put it back together. The head gasket blew so I got the head decked and new valve guides put in.

Now this happens.

Here's what I have done to it.

New external fuel pump
New coil
New ICV
New temperature switch.
I have checked for numerous vacuum leaks, I found out that I didn't have the cabin temperature sensor plugged in, so I put a screw in the end of it to plug it up. This had no change.
I've adjusted the throttle position switch.
I've tried a different OBC

I've installed a rebuilt set of fuel injectors.

I found out the bottom of my air box had a bunch of holes in the bottom , so I switched it out with a used one .

New spark plugs, gapped correctly.

New Fuel pressure regulator.

New alternator.

New battery.

If I unplug the O2 sensor nothing changes.

There is another issue, which is that my auxiliary fan doesn't seem to come on when my air conditioner is running. Probably won't if the car gets too hot either. However it does turn on if I jump the wires, and I even installed a new resistor.

I need the Dr.House of 80s BMW's please accept this challenge and help me resolve my issue.
demetk
Posts: 8431
Joined: Aug 09, 2007 8:58 AM
Location: CT, USA

Re: Idle problem! Please help!

Post by demetk »

Could be the TPS going open when it gets hot. Try unplugging it and jump pins 1 and 2 on the connector while idling.
Mike W.
Posts: 26872
Joined: Feb 12, 2006 12:00 PM
Location: California Whine Country

Re: Idle problem! Please help!

Post by Mike W. »

It could be the ICV, but I doubt it. But it's getting air somehow. Everyone doesn't seem to have had the same experience, but if my TPS was out of adjustment and not making contact at idle, idle speed was unaffected, but it would tend to die coming to a stop, especially with the A/C on. Pull the intake boot off and check the butterfly clearance. It might be open just a bit. Make sure the linkage isn't making contact at the stop, it shouldn't be touching. Read the O2 sensor and see what's happening there, a vacuum leak will tend to show up lean. Check with the O2 both plugged in and disconnected.

Aux fan. Which wires do you jump to get it to work?
Rocketflip
Posts: 17
Joined: Dec 30, 2020 12:20 AM
Location: Illinois

Re: Idle problem! Please help!

Post by Rocketflip »

Mike W. wrote: Nov 28, 2021 4:02 PM It could be the ICV, but I doubt it. But it's getting air somehow. Everyone doesn't seem to have had the same experience, but if my TPS was out of adjustment and not making contact at idle, idle speed was unaffected, but it would tend to die coming to a stop, especially with the A/C on. Pull the intake boot off and check the butterfly clearance. It might be open just a bit. Make sure the linkage isn't making contact at the stop, it shouldn't be touching. Read the O2 sensor and see what's happening there, a vacuum leak will tend to show up lean. Check with the O2 both plugged in and disconnected.

Aux fan. Which wires do you jump to get it to work?

Are you saying the butterfly valve should be slightly open ? Or all the way closed?

Or are you saying the piece that the the throttle cables shouldn't be touching

Because it appears the butterfly is all the way closed AND the stop plate is touching.

https://imgur.com/a/Lauaa56

Here are some pics
Rocketflip
Posts: 17
Joined: Dec 30, 2020 12:20 AM
Location: Illinois

Re: Idle problem! Please help!

Post by Rocketflip »

demetk wrote: Nov 28, 2021 9:12 AM Could be the TPS going open when it gets hot. Try unplugging it and jump pins 1 and 2 on the connector while idling.
If I take unplug the tps the idle rises like crazy, jumping those pins (assuming I got pins 1 and 2 right only caused it to return to the problematic pulsing idle.
Mike W.
Posts: 26872
Joined: Feb 12, 2006 12:00 PM
Location: California Whine Country

Re: Idle problem! Please help!

Post by Mike W. »

The butterfly is supposed to have just a few thousands of an inch clearance. .012 sticks in my mind, but I'm not sure. But looking at the pic, the right hand cable, closest to the center of the engine, looks like the adjustment knob is screwed way in. I can't remember, but I think it's the CC cable, but regardless, it looks like it's preloading it some, and that alone could be your problem.
TommyBahama
Posts: 35
Joined: Jul 27, 2020 12:50 AM
Location: South Florida

Re: Idle problem! Please help!

Post by TommyBahama »

Hey Rocket, were you ever able to resolve this pulsing idle issue? I have been experiencing the same issue for the past 6 months. I also replaced many of the same parts, ICV, idle computer, thermo switch, temp switch, 02 sensor, MAF intake sensor, plugs, fuel pump/filter, and it's STILL happening. I have smoke tested and multi-metered, but everything checks out. Money is no issue so I don't mind throwing parts at it, but I don't even know where to go next. Never thought I'd miss OBD so much lol
TommyBahama
Posts: 35
Joined: Jul 27, 2020 12:50 AM
Location: South Florida

Re: Idle problem! Please help!

Post by TommyBahama »

I wanted to post an update for anyone in the future that may be having this problem. I went to start the car last week and the starter clicked once, then nothing. "oh great", I thought. "First the car doesn't idle right, and now the starter is dead". Before ordering another starter though, I decided to verify what my issue was. I tested the battery, multimeter said 12 volts. I removed the starter from the car, and put 12v directly to it, and oddly enough, it was working fine. The starter was spinning and extending every time without issue. "Maybe it was just a loose ground?" I reinstalled the starter making sure I tightened everything up. Turned the key and *click*, nothing. :brickwall:

Luckily I have a close friend who is an engineer and skilled with electrical systems. He came by and verified the battery was good, but he noticed the starter is only getting about 7 volts. He traced the ignition wire off the starter solenoid to the fuse box, connector C101.

Image


First he checked that output on the fuse box side was good, and it checked out. He then tested the resistance of the other pins in the connector side, and noticed they were all showing about 4 ohms of resistance. He informed me this is most likely due to a short somewhere in the harness off this connector (C101) that is causing my no-start issue. What other systems run off this connector?

Image

The starter, cold start valve, and thermo-time switch! It looks like there's also wiring to the diagnostic connector and motronic DME, so I have a lot of wiring to check next weekend. It may take me a while but eventually I'll update here once I find exactly what the issue is. I'm praying it won't require major surgery to find the issue, as a lot of the wiring is routed under the intake manifold, through the firewall, etc. Hopefully this helps any future E28 owners: test the C101 harness (or equivalent in your year and model) to make sure you don't have a short! I found my wiring diagram here:
https://www.armchair.mb.ca/~dave/BMW/e28/
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