High idle but only in P or N

[Majestic7]

I just got done with a big project replacing the rear OSV (to resolve a vacuum leak) heater hoses and the driver's side knock sensor (the one under the coolant tube). During reassembly I noticed my TB wouldn't close all the way so I messed with the throttle cable and cruise control cable adjustments quite a while until the throttle plate would close completely - as far as I could tell.

Upon starting the car, it idled at 1,200 rpm but once put into D it went to 600 rpm and drove fine. I drove it over an hour total.

After reassembly, my cruise control cable came up weirdly short and was tugging on the throttle. So I disconnected it and loosened the throttle cable until the return spring is at rest. It helped a little but the idle still is at 1,000 rpm in P or N.

The knock sensor and vacuum leak codes are gone but I have these codes now:

P0B28 (BMW code 121) "Mass or Volume Air flow circuit range/performance problem."

P0505 "Idle air control system"

P1550 & P1509 "Manufacturer control vehicle speed idle speed control auxiliary inputs"

• I've sprayed carb cleaner all over including at the injectors and can find no vacuum leaks
• The battery was disconnected during the project
• Gas cap, oil filler cap and dipstick are all tightly secured
• Idle air control looks clean, doesn't have any dark build up on it
• Connectors on the TPS look fine
• I double checked all the connectors
• Tried the "TPS reset procedure" key in pos2 and floor accelerator for 30 seconds etc and it didn't change.
• I didn't have any idle trouble before I started the project and the car has had vacuum leaks for years

Any ideas what could be amiss??

[740iSport303]

Did you take off the IACV and roll it back and forth to make sure it was nice and loose inside? You should hear the valve clunk as you roll it back and forth.

[cazal740i]

I say dirty or bad MAF sensor

[Majestic7]

Thanks for the input sport303 and cazal. The MAF and IAC were in my dusty dirty detached garage for weeks so those could be good contenders. I have MAF cleaner so I'll try that and do I just stick a screw driver flat blade into the IAC hole and push to get it to roll back and forth?

Once again thanks for your input. I'll be sure to report back once it's resolved.

[xboss]

Make sure you remove the maf from it's boot before you clean it. That stuff will dry up rubber in a hurry!

[Majestic7]

I think mine is held in its plastic tube by those one way screws. Mine is the early kind, how do I get it out?

[cazal740i]

use vise grips if you don't have the Security torx head bit to remove the MAF from its housing

[740iSport303]

The IACV is removed from the car and you can simply use carb cleaner into it. Roll it around, pour out and repeat until the valve moves freely. DON'T force movement with a screwdriver.

[Majestic7]

740iSport303 you may be in to something. I removed my IACV from the car and cleaned it with carb cleaner but when I shake it it's still silent, no clunking at all. I guess I'll keep cleaning it some more.

However when it was installed and I put the ignition in the "on" position i can hear it make a buzz sound.

My guess is the IACV is stuck in the open position, that would explain why my car feels like it only has 100 hp.

Any tips on getting the valve to unstuck? I understand I'm not supposed to force it with a screwdriver.

[740iSport303]

Carb cleaner, lots of carb cleaner. Spray into the valve, block the openings and shake, roll around, empty and repeat until you can roll it and feel/hear the valve clunk.

I had one that I bought Berrymans ChemTool in a jug and poured in as much as I could and let it sit for an hour in my vice. Then when I poured it out it finally started moving inside again. The solvent I poured out was solid black. Looked like super thin motor oil.

[Myro]

so this was happening to my 98 though it was also causing a shifing issue where it would not shift automatically it turned out that i had switch 2 connectors on the throttle body ... just food for thought as now my car runs nice and smooth and quieter

[Majestic7]

Thanks very much for your insights 740iSport303. I've started to notice the car idles fine on cold start up but goes back to 1,000 as it warms up. I used probably half a can of carb cleaner, a bunch of brake cleaner and Q-tips everywhere they would reach to clean the IACV. Tomorrow I'll try filling it up and leaving it for an hour.

I reinstalled the IACV and started up the car cold, unplugged the IACV and it was open. I put it back, let the engine warm up and unplugged the IACV and it was still open. I really think that's the whole problem.

And Cazal740i thanks for your tips about the MAF. I used vice grips and got the actual sensor out and cleaned it thoroughly with MAF cleaner. Reinstalled it and drove around and it didn't fix it unfortunately.

My brother-in-law also has an E38 up in Cleveland and a parts car. He's sending me both a IACV and a MAF to try and narrow down my variables. My hypothesis is it'll run great with his IACV installed. I shall let you know.

[Majestic7]

Thanks Myro, I had that thought as well because I couldn't remember which connector was for the TPS and which for the elbow TSC throttle plate. Based on a picture i found online the connector with the boot on it goes on the TSC plate and the connector without the rubber boot goes to the TPS. I switched them just to try and my TSC light on the dash came on.

I double checked it on my brother-in-law's E38 when I was in Cleveland this and I believe I have the connectors right. Though they do physically connect to either so somebody reading this in the future it is something to check.

[Majestic7]

Update:

The other code i have is P0430 CAT efficiency below threshold bank 2. It's been there for 2.5 years so I didn't mention it. The other thing is my car feels like it has 100 hp. I thought that was because the vac leak and bad knock sensor but it's still down on power even though those are fixed.

My brother-in-law couldn't find his AICV but sent me his MAF. I tried it and no change. I got a chance to talk to him tonight and he said it's not possible for the IACV to cause me to be down in power, because it is by definition metered air. So he thinks I have 2 problems. A clogged CAT would explain why I'm down on power (I've been down on power for like 2.5 years now that I think about it, same time the CAT code has been tnere).

And he found his IACV and it doesn't clunk either. He's going to put it on his 740 to check that it works and send it down to me. Next weekend I might make the trip up to his shop and he's going to cut off my CATs and see if they're clogged.

[racer2086]

Your cats are probably clogged. Time for a new one and your power will come back

[740iSport303]

Realize an IACV will not "clunk" if you shake it. Roll it back and forth between your hands and you can hear the valve move.

[Majestic7]

I went up there to Cleveland at my brother-in-law's and tried his IACV and at first it still idled high and we were about to try other things but then all of a sudden it leveled out! Restarted it and still 600rpm. Took it on a test drive and it continues to idle perfect!

740iSport303 you were spot on. The car runs better now than any time I've owned it in the last four years! And for the first time in four years I have no check engine light! We have a baby due in three weeks so the 740iL is about to become my daily driver 3/4ths of the year and my 350Z less so since it has no back seats. Thank you all for your help and for folks reading this in the future I hope this case being solved aides you as well.

[740iSport303]

Glad you got it figured out! That's great to see.