Gary C2

What are the symptoms of an O2 sensor failure?

Bill Wagner

A bad O2 sensor can create/contribute to the following symptoms:

1. Erratic idling
2. Poor pick up
3. High fuel consumption
4. Engine hesitation
5. Power loss
6. Poor hot starting

The above are out of the manual. Other problems can also create many of the symptoms identified above.

O2 sensors don't generally fail in a "binary" manner, with "binary" meaning that they are perfect one day and then completely shot the next (it can happen, but it's usually a wiring/cable problem). The problems usually start with one or more mild symptoms from the list above and get progressively worse.

If your car is equipped with the self diagnostic check engine light system that can spit out the failure codes, a failing O2 sensor will not trigger a light until the failure is really relatively severe. In other words, your O2 sensor may be in the process of failing but it won't trigger a check engine light and the Bosch Hammer likely won't pick it up either. If your car is in this state, and you really suspect a bad O2 sensor, www.AllData.com outlines the test procedures for testing an O2 sensor (you have to subscribe for about $20.00/year).

Hope this helps,

Bill Wagner

Adrian

A key point with a potential O2 sensor failure is timing. The O2 sensor is not used until it reaches operating temperature of 600F. When the 964 is started the DME is in what is called Open Loop. When the O2 sensor reaches operating temperature the DME will switch to Closed Loop. I will not explain how this is achieved a this time.
Now back to the timing. If everything seems to run okay for say 90 seconds and then plays up, this is an indication of an O2 sensor. Bills list of things which can happen is correct. A O2 sensor failure can also stall the 964 under certain circumstances. Completely trashes the idle. This is most noticeable on a hot engine start.
The O2 sensor is easily checked by either checking the voltage output at the sensor connector itself when at operating temp and then revving the engine to see that the voltage changes or use the Bosch Hammer. However the Hammer will only pick up a recorded hard failure,
Ciao,
Adrian
911C4

PS: O2 sensors do follow Murphys law. When you check them they seem to work. They are really only low voltage batteries.

Adrian

A couple of points I forgot. Messing around with the internal settings of a DME and messing around with the CO2 adjustments can create problems which appear to be caused by the O2 sensor but are in fact self induced,
Ciao,
Adrian
911C4

FlyYellow

Adrian,

I have cold start problems. My 964 is very difficult to start (must try 3-5 times) and initially the check engine light takes 20 seconds to go out. I read your open loop comment, but by that theory the car should be ok at cold start. Do you have any thoughts?

Thanks,

Boris

PS Once the car warms up the idle does wander upwards of 950 and down to 650 RPM. I've been told it is because I have a primary muffler bypass and it produces less backpressure. Is this true?

Adrian

Dear Boris,
Open and closed loop operations are not theories they are how the DME is set up and knowing this really helps in troubleshooting.
Cold start problems basically means thast you are either not getting enough fuel, too much fuel, not enough air or too much air. Inotherwards the air/fuel ratio is incorrect.
So what can cause this problem and I start with the simple,
1/. Starter motor not reaching 30rpm
2/. Bad spark plugs, or leads or coils
(broken distributor belt will not normally affect cold start, maybe a hot start)
3/. Oil contamination in airbox. Air fileter contaminated, air flow sensor stuck in the closed position.
4/. Air leak in throttle body or surrounds, such as the dipstick
5/. Fuel Pump not up to speed, pump or relay R41.
6/. The engine temperature sensor (not an uncommon failure) has failed
There are many more. If any of your temp sensors have failed you will be operating in what is called Emergency mode. Everything defaults ot 20 degrees C. This means all the DME temp inputs now use 20 as their operating figure and this never changes. If it is 0 degrees outside then you will have a hard time getting started.
The check engine light is telling you that you have a failure which corrects itself. Suggests a temp sensor or the airflow sensor freeing itself up,
Ciao,
Adrian
911C4

PS: If you fix this problem you will solve your RPM problem. The RPM problem also suggests and air leak or air related problem.
Also check the idle microswitch is activated with the foot of the pedal.

FlyYellow

Adrian,

Thanks for the info - I do have a question on the procedure for setting your air fuel mixture properly. My understanding is there is a potentiameter on the AFM which makes the adjustment. If this is true to what level of CO do you adjust on a car that has no cat converter?

Thanks,

Boris

DaveK

Hi,
I'd like to extend the question above slightly. I had a Cat bypass fitted last week and looking through some of the old posts I've seen a mail from Adrian saying that the Airflow sensor has to be adjusted for cars with no cat. I mentioned this to the mechanic but he said this was only the case if you no longer had the Oxygen sensor, because if you did the DME will still work it all out itself. Sounds like it makes sense - but is it correct?

Dave
90 C4

Bill Wagner

DaveK:

Is your mechanic correct? Well, yes and no. Yes, the DME should compensate for the cat bypass. The O2 sensors are mounted on the inlet side of the CATs (before them), so the only thing that will be different is any change in pressure or back pressure that exists because the CAT is now gone. The DME SHOULD be able to compensate for this, and should do so quite easilly (or so I'm inclined to think). In this case I think your mechanics quite right.

Your mechanic is probably wrong when he refers to a system where the O2 sensor is taken out completely. Unless someone is installing a DME replacement unit that completely ignores O2 sensor input (I suppose there are some available, but I would think they would be race-only, not street legal devices) the DME would register a fault and put you into "limp home" mode. On most contemporary cars, "limp home" mode can often be quite robust...so much so that if all you did was "about town cruising", you may never know anything was wrong with the car, but why bother owning a high performance car? On a 964 with a check engine light the elimination of the O2 sensor will DEFINITELY trigger the check engine light.

Hope this helps

Bill Wagner

Adrian

Dear DaveK,
If you have a bypass pipe you will still have the O2 sensor screwed into the bypass pipe. The lambda control or air/fuel mixture is primarily controlled by the O2 sensor output via the DME of course. However the design of the cat bypass pipe is VERY important to retain full control over the mixture. If you do not have the correct flow of exhaust gas through the O2 sensor you will suffer the consequences. This is why some people who install cat bypass pipes actually lose HP.
If you remove the O2 sensor as Bill says you will switch to Emergency mode unless you reconfigure to M150 status which is a 964 ohne Kat or emissions controls. They went to the Middle east mainly.
Now to the C02 adjustment. This is a mechanical adjustment in the airbox/throttle body. This can only be undertaken with an exhaust gas tester hooked up. You need a special socket for this as well if I remember correctly.
Do not go into the DME an start messing with the fuel quality settings. This is part of the DME set up at the factory in combination with the country and variant jumper plugs and the C02 adjustment settings,
Ciao,
Adrian
911C4

FlyYellow

Adrian,

I am interested in your comment about loosing HP by doing a cat bypass. Perhaps you can share your thoughts on my setup - I took a stock cat converter and had it cleaned out. I made sure that the entire honeycomb was out and that the inside was smooth. I am really interested in your theory because I believe that I lost performance when I made this modification. Any thoughts?

Thanks,

Boris

Adrian

Dear Boris,
Again what I have said is not theory it is a fact. However I do not have the fine details on this subject from personal experience. I would contact one of the recognised and respected manufacturers of bypass pipes and ask them. It is my understanding that a stripped out Cat is not the optimum internal flow design for a bypass pipe. I have hard dyno proven data from a RSA owner on this subject using hand built bypass pipes. To cut a long story short, correctly designed pipe was eventually installed.
I found the original data on a site on the web. It was a US site. Was not Porsche. I cannot remember now the site but it got me asking questions.
Wish I could help more,
Ciao,
Adrian
911C4

samk

FWIW I installed a Eurosport Cat bypass with
o2 sensor disconnected and several other mods and I went fro a 0-60 time of 7 sec to 5.6 sec (ave. of 5 runs each with G-Tech Pro). It runs rich and fast. 91 C2 cab/ Tip. Hope this helps. Sam

FlyYellow

Adrian,

Thanks for the info. I called powerhaus and they told me that replacing the cat with a bypass pipe made a 0hp increase on a dyno. Perhaps the cat bypass he used was poorly deisgned. I'm going to call fabspeed as well and see what they have to say. But I do think I lost hp when I changed to my cleaned out cat.

On the flip side, I have heard people who found the cat bypass pipes extremely performant and claim they increased hp significantly. Has anyone out there every dyno'ed the difference?

horst

I only have a "seat-of-the-pants"dyno result. I have a B&B bypass, with the O-2 sensor connected, an see very little, if any, difference. A friend with a 92 C-2 drove my car tonight- he has a cat still in the system, and he was kinda discouraged, as he felt my car was quicker than his despite the additional weight of the C-4. I believe I made him feel a bit better, when I told him that I thought C-2s were geared a bit taller in 1st &2ond (?) than the C-4.