Well I took an extended lunch today and put the new O2 sensor in. Went very easy. Took longer to get all the shrouding and rear tire off than the actual sensor.

Anyway, I started it up ran fine and turned on the air (as you recall the reason I went down this road was because the car ran like sh** when I turned the air on AND the car was warmed up, but, the car ran fine with A/C while cold). Well it ran great up until about 7-8 minutes into the test drive when it proceeded to run like crud again. Turned off the A/C and it's fine. Turn it on at fan speed 4 and all goes to hell. I disconnected the brand new O2 sensor and it runs good again on any A/C setting.

WHAT GIVES? Everything about this ponts to a bad O2 sensor, but, the one in there is brand new now. Could the CAT be clogged (with mice)? I've always heard that "stuff" comes out of your exhaust pipe when the CAT is plugged/bad. I have no such thing happening. How long does a CAT usually last (live)?

 

Slant: Don't know what to say that might be helpful. Sounds like you have eliminated the O2 sensor as the problem. You now have on your hands a nearly new, hopefully perfect O2 sensor, and one old one that may still be functional. May have to bite the bullet and ask for a dealearship to diagnose it. I know if there is not enough "flow" by the sensor it would cause a faulty sensor type problem. Can you beg, borrow, steal a "test" pipe from anyone. Your "plugged cat" theory may be one to follow up on. Don't know where you are, but if you are anywhere near me, you could borrow mine for a test. Only takes 10 minutes to switch out

 

oops. I see you are from Los vegas. Sorry. a bit too far.

 

John;
If you can not find a cat, I'll send you mine off my RSA, low miles and it worked. Just pay for shipping and when your done ship it back. Last resort.

 

Horst - That's "Lost Wages". I mean Las Vegas. Thanks anyhow.

Kevin - More than I could ask. Not sure what I want to do next. Don't know that a new CAT is the answer. Is this a part you actually need? Or have you gotten rid of it from your car? Will an RSA CAT work on my 90 C4? Just thinking that if it does work I might want to keep it (for a price of course).

 

If you haven't done so already you may want to check the wiring associated with the O2 sensor.

I'm NO EXPERT on the O2 sensors, but the 3-wire types are usually heated lambda sensors. My guess would be that your car is operating in open loop mode when the AC runs OK, and then when it enters closed loop mode (when the car is warmed up and the DME decides it's going to start paying attention to the output from the O2 sensor) it screws up. The DME monitors a potenial difference (voltage) from the O2 sensor that will vary as the F/A ratio changes. If one of the wires running between the DME and O2 sensor is broken, internally shorted, or has a bad contact somewhere in the path, the DME will get a faulty reading. Whether it's really a 3-wire heated type or not is probably irrelevant should this turn out to be the problem.

I think a likely cause for this could be a break in one of the wires (UNDERNEATH the insulation where you can't see it) going between the O2 and one of it's wires going to the DME, or a bad connection SOMEWHERE in the path. A short could also do it but I think it would be less likely. Unfortunately, this could be any wire that runs between the DME and O2.

Just a thought

Bill Wagner

 

I've been asking around a bit and don't you need to have a cat "fire up"in order to have an O2 sensor operate correctly,to where a test tube may solve one problem but create another?

 

Slant, I think Bill Wagner may be on to something. Here is something else, quite simple minded: If you look into the plug of the O2 sensor, you will see that it goes together correctly only one way. I am an expert at screwing things up. Believe it or not I ahve snapped this thing together the wrong way, and outwardly it looked right. Have you checked that you have made this connection properly. It does sound like with the a/c involvement that there is something electrical going on.

 

Hello there,

I could be that you have a problem with the ground/mass [31] or [-] of the sensor.
Try to make wire from the cataliytic converter to engine mass/-.
Between exhaust pipes there comes rust/dubris/carbon, what means that your o2 sensor has no good contact on engine and Motronic ground/mass/31 [sorry I don't know the right name for that] and that means that the o2 sensor gives wrong information to the Dme/Motronic.
Connect one side of the wire/cable on the heat sheild [clean the screw and contact first] between the cat. and sheild.
Put the other side of the cable on the screw who holds the bumper bracket. [make good contact there to and do some elektrical grease on]

Now the o2 sesor has good contact with body and so with the Dme/Motronic.[even better, find engine mass somewhere]

I hope this helps

Arjan Brinkcate
964 C2 Black 1992

 

John;

In regards to the cat, I have 2 spares, I tend to be a pack rat. The RSA cat will fit and work. Oh, yes, its all for sale. By the way, what does your exhaust smell like? Does it smell like rotten eggs, Sulfur? Thats a good sign that things are pluging up.

 

Interestng recommendations. I will follow up on some of them tomorrow when I have a chance. As far as the smell of exhaust goes. It does smell bad a little bit. But, not real bad. However, when the O2 sensor is disconnected it smells real bad, but, the car runs good.

Horst - I appreciate the info. I too can screw simple things up. I've checked that it is in there the proper way..

I'll try the ground tomorrow and report back. I may want to get a CAT from you to at least try Kevin.

Thanks all again.

John

 

Dear Slant,
If you go back to the original thread you started on this subject (and it got very long) I did recommend a check of the O2 sensor wiring. Especially the connector and where it enters the engine bay. You should smell fuel with the O2 sensor disconnected because the 964 will run rich. The fact that you are smelling fuel with it connected, suggests you are still running rich. "Too Rich". This still points to the O2 sensor circuit is not functioning, being ignored or being overridden. When you turn on the Aircon there is an earth provided to the DME which commands a RPM Map change.
Maybe you could try an emissions gas test. This may help in diagnostics. Get the analyzer hooked up to the exhaust pipe and get some readings. Maybe it is also time to hook the Hammer up and get a fault code readout,
Ciao,
Adrian
911C4

 

Dear Slant,
A thought came to me today whilst I was working on my daughters 944. When did you install the bulb for the engine check light. Was this before or after this problem started. I cannot explain unless I have about five hours but may I suggest that you remove the bulb from the circuit again. Disconnect it and see what happens. Obviously it is up to yourself but I would advise returning the system to original,
Ciao,
Adrian
911C4

 

Adrian,

I noticed this problem before messing with the check engine light. I have since put the system back to normal (no light). I have not however done a "reset" of the DME since taking the light back out of the system. I will do that today to make sure the DME is getting the proper info.

 

Dear Slant,
No problem. It is always a good idea to eliminate the changes first. I should have mentioned it earlier. Remember the C4 in Malaysia with the flashing PDAS light. It turns out the lamp that is flashing is actually the green PDAS lamp which should not be connected. Some owners install lamps and then when the traction control activates the light flashes, then they think they have a fault. This lamp is not described in the owners manual. It was disconnected in March 1989 by Porsche because it was deemed annoying and confusing.
However I think this C4 has a problem because the ! warn keeps coming on as well. I suspect the lamp has been transferred from the red warn to the green indicator.
Anyway, back to your problem, did you reset the DME when you replaced the O2 sensor. This will obviously be achieved today anyway.
Good luck,
Ciao,
Adrian
911C4

PS: I believe you have the old intermittant wiring problem syndrome. Mess about with the wiring and it is fixed only to come back later.