oscillating idle

[jbrand540]

If I have my o2 sensor connected the idle will oscillate by rougly 100 rpm constantly. If I disconnect the o2 sensor the idle appears to be steady, however, seems to idle somewhat rougher. Based on the tach I would place its idle in the neighborhood of 850-900. Not sure if this is relevant to this but when the car is idleing if I gun the engine to 2500 or so and let off the engine falls to 500 or so but quickly recovers, perhaps this is normal. Any suggestions or further troubleshooting suggestions I can do to narrow the problem down?

Also does anyone know what the plug in the below picture is for? I cannot see anything that it would plug into and can only assume that it may be some type of diagnostic plug.

[jbrand540]

I also was going to check my idle adjustment but in going to jumper the B&C ports I found that on my 89 they are not there. I have a hole in the metal where I think from the pictures that it should be but there is nothing there just the body of the car. Is this normal for an 89 or is something missing. Below is a picture of what I have.

[jbrand540]

Oops, forgot the pciture. Here it is

[Por_sha911]

Your O2 sensor could be bad, your ICV may be clogged. Do a search on both of these. You will find out how to test them. Do you have Bentley's repair manual?

[jbrand540]

Good suggestion, I will try cleaning the ICV and check the idle switch as well. I do have the Bentley's manual.

[jbrand540]

I foudn what happened to the test plug, it must have come loose from the flange and fell behind and got wedged up under the flange upside down. Whew, was worried thinking I had something extremely strange.

Any one have an idea on what that large black connector plug is?

[DonMo]

vacuum leak or mixture problem.

[rs6er]

I recently had the same problem. My P car mechanic put a CO2 meter on it and adjusted it. Now it's gone.

[jbrand540]

using a mirror I was eyeballing around and noticed that on the back of the throttle body there is two wire plug connector that I has nothing connected to it. Should there be something connected here? I cannot find anything about it in the Bentley manual. Cannot really get a picture of it either due to its location but it is dead center in the bottom back of the throttle body.

[jazzbass]

There are no wire connectors on the back of the throttle body, but there are a couple of vacuum connectors. One goes to the air valve that's over by the oil tank, the other loops back. Under that is the vacuum connection that comes out and splits off to the fuel pressure regulator and the fuel damper.

As for the plug, doesn't look like anything I've ever seen in a Carrera engine bay. The diagnostic port for the DME is right by the ICV. Does it go into the factory harness or does it look like it came from the factory?

[jbrand540]

looks identical to idle siwtch connector. Two blade connector sitting right in the middle on the back bottom of the throttle body. Wish I could get a camera back there to take a photo. Tried using the mirror to look around but I cannot see anything cables laying back there disconnected that would plug into it. Also cannot find a diagram that will show me that.

I will try a little harder to get a picture.

[jbrand540]

Here are the pics as best I can get (actually better then I thought), hopefully this helps.

[jbrand540]

and another from the camera by the firewall facing toward the back of throttle body

[jbrand540]

# on front of Throttle Body is 930.110.530.0R. Doesnt match pelican's throttle body part number for an 89. Cannot seem to find anything on that part number doing general internet searches.

[jbrand540]

Anyone have any thoughts on the plug a the top or on the throttle body?

[450knotOffice]

I just looked at mine (an '84) and found two small vacuum hoses attached. It's definitely the same port you showed. Maybe on the later cars (yours), the vacuum is converted to an electrical signal right there and sent to wherever it's supposed to go. In my car's case, I can't really tell where the vacuum hoses are going. Maybe they lead to a couple of transducers further down the line.

Sorry I can't be of any real help other than to offer a comparison to an older ('84) Motronic vs. yours.

[jbrand540]

Thanks for taking the time to check that Scott. It is puzzling as I have looked at all the part numbers and diagrams I can find for 3.2 carreras and across all years they show the same throttle body.

Noticed all this while trying to hunt down a minor erratic idle. Now I am just curious to know exaclty what this throttle body plug is for, and if it actually should be connected to something.

[450knotOffice]

You're welcome. By the way, I just searched my Porsche P.E.T and found no match for the number you provided. I am stumped.

Interestingly enough however, is that the parts diagram shows that piece that screws into the back of the throttle body (not with an electrical plug, however) and calls it a thermo valve, part number 930-207-231-00. It has a small tube protuding from it which connects to a small hose. That hose leads to an Air Valve (930-207-227-02). Find that Air Valve and see if it has a small diameter vacuum line attached to it (the Air Valve will also have a large diameter hose attached to it. Discount that one). If it does, trace it back and see where yours is attached. If it doesn't, maybe the thermo valve simply sends an electrical signal to the Air Valve instead of an air or vacuum signal.

Just something I would do.

[jazzbass]

Check these threads out for good info on the throttle body, AAV, etc:

911 3.2 carrera vacuum hose disconnected
engine bay buzzing noise

[rnln]

jbrand54,
Sorry I can't offer you any opinion but I jump in because I do have similar problem once in a while., most of the time when the car get hot, stucking in traffic. Rarely it happens right on start. I also have the black rounded plug (unluged) as shown on your first pic. It's laying under the rear aluminum relay panel. And my car is also a cab.
If you can find any info, fix on this problem, please update.
Thank you.