85 Carrera Warm Idle Surge and 02 Sensor

[CraigDseattle]

Another question about vacuum leaks - can they be ruled out if my idle drops when I remove the oil fill cap?

I've done that test, and there is a definite change when I open the cap, and marked improvement with the cap reinstalled.

I've seen other threads where that was recommend as a vacuum leak check, but I don't know if that is an effective approach.

thanks again!

[rusnak]

That may or may not be the case. I would still check for vac leaks. No guarantees here.

[Bill in OKC]

Make sure to check on the back side of the TB. There is a thermo vac switch back there. There are 3 vacuum lines to it that get cracked/disconnected and it is all hidden unless you can get a mirror back in there.

[techman1]

ISV / Idle Stabilization Valve
ICV / Idle Control Valve
Same thing.

Looking for a photo for the other thread.

[Jcslocum]

Originally Posted by CraigDseattle

Another question about vacuum leaks - can they be ruled out if my idle drops when I remove the oil fill cap?

I've done that test, and there is a definite change when I open the cap, and marked improvement with the cap reinstalled.

I've seen other threads where that was recommend as a vacuum leak check, but I don't know if that is an effective approach.

thanks again!

Not ruled out 100% but pushed down the list a bit of possibles. You test is a good one for vacuum leaks. These cars don't tolerate false air into the system. These are also many sensors to check out. Cylinder Head Temp is another to consider.

[CraigDseattle]

I really appreciate the comments. Seems like I need to put some energy into checking for vacuum leaks. I will do it this weekend, after an oil change for the motor and transmission.

I am attaching a photo of some cracking in a rubber hose that links a tube to some intake plumbing. I did smear some RVT sealant around the cracks, but maybe not well enough.

Could this contribute to my situation?

Somewhere in the forums I've seen photos where people have cut thru the copper tubing, and replaced the integral hose with a piece of bulk hose with clamps on either end.

Is this a common fix that avoids replacing the whole tube/rubber hose assembly?

Does the clamped version work well?

Thanks guys!

[rusnak]

Yes, if it's cracked all the way through. There is a BMW part that will fit and replace just the rubber tube. I don't have the part # though. If it's not cracked all the way through, then you can fix it. I've used JB Weld and some Gorilla Tape to fix that part until the new one arrived. Vaccum leaks can be numerous however. You might notice a bit more throttle response when they are all sealed.

As others have noted, you might have other problems contributing to the unstable idle. You should ohm the idle switch to make sure that it is working too.

[CraigDseattle]

Tomorrow I will be doing an oil change - engine and trans - and I will also investigate vacuum leaks....

I will keep my fingers crossed.

Thanks for all the help and ideas.
Craig

[Jcslocum]

That hose I'd stepped so it's not easy to replace with just a piece of hose. RTV and electrical tape will do until you can replace it.

[dshepp806]

After you get your vacuum leak testing completed (or repaired), let me know. Should you (then) be interested, I could get my old-used but functional when removed- ICV (from my 89) to you for test purposes. I had cleaned mine MANY times over the years,..she never really was problematic (honestly) and I decided to load her with a new one and tuck the original away. So:

Lemme' know if I can help there. You'd only need to pay for return shipping-tracking required. I'd pay for it to go to you.

Good luck.

BEST!

Doyle

[dshepp806]

As Rusnak mentioned: you should verify your idle switch is functioning properly,..and do it with a meter (not the auditory senses of "click").

Easy to do.

BEST!

Doyle

[CraigDseattle]

....but don't cure idle issues ......but it's good when everything is slippery...

While fiddling around today, I thoroughly cleaned the ICV again, with no noticeable improvement in surging when warm. If anything, I think I may be feeling a much reduced surging even with the O2 sensor disconnected, that I didn't notice before.

I also snooped around for vacuum leaks. All the hoses are attached, and seem to be in good shape. I really don't see or feel anything thing that looks suspect, but then again I don't think visual inspection is the best in this case. I was struck again by the definite drop/change in idle with the oil fill cap removed, so it seems like things are pretty well sealed up.

Doyle - thanks so much for the offer to try your ICV. Can we correspond off the board about that? I certainly would pay all the shipping costs.

I think I need to do the tests for the switch that are mentioned here. Does the Bentley manual have a good description about doing this, or are there some threads here?

Once again - thanks everybody for all the great help.

Hope everybody is having a good weekend.

Craig

[CraigDseattle]

I was just reading another thread about idle adjustment and I have a couple of questions. So, my 85 should be set at 800rpm apparently, but my mechanic set it at 880 rpm. It holds that really well up through the initial warming up period, up until the surging starts.

One of the posts mentioned that having the idle set too high will mess with the DME since it is programmed for 800 rpm, and that surging idle is a byproduct.

Is this a possibility? Should I attempt to adjust the idle to see how it goes?

Thanks again

[kidrock]

Craig,

Yes, it's true that an improperly set base idle can cause surging idle issues. However IIRC, if you have a SW chip, your idle should be set at 880.

I don't think that it would hurt to set your base idle to 800, just to see if it solves your issue. Don't forget to have the car fully warmed, then jumper your B & C plugs at your diagnostic port to "center" the ICV before adjusting your base at the throttle body. Very simple procedure.

[Hunt3R]

880 is too high for an '85 unless you've updated the chip. And yes that alone may cause the surging. Check the idle switch first, it's easy:


Then set your base idle to 800 with B-C jumpered, ignore the CO adjustment procedure here. Hopefully your base air/fuel mixture is already good, although if not that can cause surging too just as it can when vacuum leaks cause a lean mixture:


And if you want to test your ICV:

[CraigDseattle]

I continue to be amazed by the insights and helpfulness of everybody here. Thanks for taking the time to provide input and manual pages, etc.

I know what I will be doing this afternoon!

So now another silly question. Is there a good spot in the engine compartment to attach the leads of a basic tach/dwell meter? Obviously with the battery up front, that's not the spot.

Have a great Sunday folks~
Craig

[john walker's workshop]

shrink tubing works well on that cracked hose.

[CraigDseattle]

With all the suggestions, I decided to investigate the base idle setting situation.

I started by checking the idle switch by sound and by multimeter, then I checked resistances at the ICV terminals - - all looked good.

I drove long/hard enough to get all warmed up, and watched the idle rise and felt the surging/hunting slowing increasing. I popped in the jumper (BTW a 3.5mm mono jack fits great), and messed with the "screw" shown in the picture below. I was looking for an idle adjustment screw like on an old carb, but there is none. This seems to be the adjustment for the base idle speed.



With the jumper in place, and the O2 sensor connected, the surging was apparent, and I ultimately had to really screw the hex head "screw" in quite a was to get the idle to soomth out in the 800 - 850 rpm range.

Question - what is a good "bench setting" for this screw? Like close to the right, then backoff to the left by X turns.

It seems that mine was very far out, and I rotated it in quite a ways to get the idle where it should be. When I removed the jumper, the RPM dropped 50ishRPM, but seems to be running and idleing really well.

So now, with O2 sensor connected, fully warm idle a bit less than 800 rpm, there seems to be no surging and the performance feels really good all around.

I don't know how/why my adjustment ended up so far out, and I hope I havent totally missed the boat on this adjustment.

With the screw - is right/in lean or rich? is left/out lean or rich?

I'll get her out on the road tomorrow to see how things go.

Thanks for all the help
Craig

[DRACO A5OG]

do you know what kind of chip you have?

as I recall, right is rich, left is lien, but I go by the rpms at the tach, jumpered at b &c.

my 85 has the sw chip base idle at 880.

[Hunt3R]

Sounds like you're doing it right. Clockwise lowers idle, Counter-clockwise raises idle. The jumper is disabling the ICV so that it doesn't attempt to adjust the idle while you're setting it and I'm not sure if it's necessary but before putting the jumper in the B-C test port I turn the car off, put the jumper in, and then start back up. While adjusting I blip the throttle a lot to be sure it falls back down right where it should, and then when done adjusting turn her off, pull the jumper out, and start back up and maybe drive around a little more to be sure it's holding good.