O2 sensor help for 87 911 3.2 Carrera Euro Version
 

Hi:

I got an 87 911/3.2 (UK version) recently, which I fell like the car has some strange engine running behave. The oil pressure fluctuate randomly (sometimes has it for 1 minute, then back to smooth running). So I think it may be a good idea to check the O2 sensor.

But when I looked into the engine bay and Cat, there was no O2 sensor on the Cat, and I only found this connector in the car. I seems like an O2 sensor connector but shorted wired.

I’m now in UK so I don't think there will be any issue with exhaust regulars, but is this car comes without an O2 sensor? Or do I need to re-install an O2 sensor, also will the O2 sensor helps the car engine run smoother?

Many thanks.

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In '87 there were 3 versions of the 3.2, the 930/20 & 930/26 didn't use a cat the 930/25 did. So what version is in your car?

The engine code is 930/20, dose that helps anything?

That means there is no Cat in this car?

No cat, no O2 sensor and that has nothing to do with your oil pressure issue anyway.

JR

Thanks for the reply.

It really comes down to the version of the DME installed in your car and more specifically the software (chip inside). Unless you are the first owner of the car with complete history I would highly recommend checking both since more often than not someone has changed things around in the past on the hunt for some more ponies. I see so many mix and match cases where the car is not even close to its original configuration.

From a hardware perspective there are two items in the DME that can differ between all the 3.2 boxes out there:

- EPROM socket size either 24pin (earlier) or 28pin (late)
- O2 sensor electronic present or not present

Then, based on above there at least 10 different software versions (stock chips) over the years with some using the O2 signals and others ignoring it. In addition there is a jumper on the DME harness and an altitude box on some. And then the rotary switch inside the DME that slightly alters timing and mixture. That about sums up the playing field.

In your car you appear to have the O2 harness and you have a bridge which pugs into the harness instead of the O2 sensor. That might indicate your DME can process the O2 signal and requires the plug not to get confused.

The only way to be sure is to check the DME and look whether it has been opened before. If you see any sort of pry marks then you need to open it to check if there is a stock EPROM or any kind of aftermarket chip installed. That will tell you what you are dealing with.

Opinions differ wildly on whether the car runs better with or without O2 sensor. I believe it is fair to say that a well-maintained O2 sensor is an advantage since it helps for good mixture in part-throttle. However, once the sensor breaks or reaches end-of-life it leads to poor running. And that is why folks postulate "remove the sensor and it runs better". Go figure.

Ingo

The oil pressure gauge on these cars is not electronically dampened. The needle shaft of the gauge runs in oil for mechanic dampening. Once the oil dries out with age the mechanic damping is gone and the gauge needle is quite nervous and reacts instantly to any spikes in the signal. Two things make it jump:

- with the engine stone cold the oil pressure can reach 5 bar quickly at low RPM and the relieve valve opens releasing pressure, then closes, then opens. This makes the needle jump.

- the wire attaching to the pressure sensor on the engine makes poor contact. This results in spikes on the signal and the needle reacts to it.

Ingo

As per javadog, no cat, no 02 sensor if it is a UK car.

No 3.2 (or earlier car) that was factory designated as a UK delivery car (i.e. C16 on the option sticker on the underside of the front lid & sticker in the handbook) had a cat or 02 sensor.

But, this does not apply to all RHD cars and so you need to check it is originally a UK car and that it has not been modified at some point.

Thanks for the detailed response, Ischmitz.

The problem is: the oil pressure gauge fluctuate, and the engine output fluctuate with it. So it seems like it isn’t just the gauge’s problem. It could be something related to oil pressure, or engine management.

Yes like you say so, this car do seems to have the O2 sensor connector and wiring originally, so I assume its engine management system should / could be able to process the O2 level. I do wish to put the O2 sensor back even it does nothing to help the oil pressure issue, I believe that will help the engine running one way or another.

I’ll do some detail checking this weekend, hopefully I can figure out something.

Where are you going to install the busines end of the O2 sensor if there is no CAT?

I know nothing about this era of Porsche, but the connector that you are holding in your hand in the photo looks like a loop back connector. Did harnesses from this era all have provisions for O2 sensors, and these were just looped back to the DME for configurations tht did not require them?

I plan to buy a Cat.. Anyway the cars entire exhaust seems quite rusty and it needs to replace soon.

Also, it do looks like it has the Cat, that section in my first photo looks like the Cat and it still has the O2 sensor hole/socket on it, just covered by a white rubber cap.

An O2 sensor will get installed upstream (before) any CAT. A muffler shop can drill a hole into the exhaust pipe at the correct location and weld in a thread. The original O2 sensor has the opposite end connecting to the 3-pin plug. In your case the "thing that looks like a CAT" could be a gutted CAT where a broomstick was used to smash out the ceramic core. And the plug should be metal. No plastic would survive the temperatures. Cars with no CAT have a straight tube instead of the CAT body there.

Back to your engine fluctuations. It sounds like you are describing a hunting or unstable idle maybe? In that case you need to start with basic maintenance: make sure ignition (plugs, rotor, cap, wires) are in good shape. Next, check for vacuum leaks anywhere on the intake that cause lean conditions. Then on to idle switch, CHT sensor, air flow meter and ICV to ensure proper idling. Once all these components are verified in good working condition it should idle stable.

While I agree that adding the O2 sensor and CAT is a good idea in the long run its absence isn't what causes the unstable idle. Adding it now only adds another variable to the matrix.

When you do a search here you will find lots of threads that deal with Motronic troubleshooting. Everything from no start to rough running is covered in great detail.

Cheers,
Ingo

On an umolested 930/20 they used a premuffler that has a bung for an O sensor, this will look like a cat externally.

As Ingo mentioned if the chip in the DME doesn't take an O sensor input it makes no sense to install one, it will just be an expensive null input.

As an unmolested 930/20 never had a cat and won't use one if it is added later, I'd follow Ingo's advice and check the DME and chip for originality.

As for the wiring harness Porsche only has one wire harness that is used for all versions of the car it includes all the possible options and possible variants, some get used some don't.

Also there are multiple maps on the oe chip, these are accessed via what are called variant coding pigtails,, these pigtails are either grounded or connected w/ specific resistors to access different maps.

I don't own a 3.2 so am not intimately familiar w/ the 3.2 variant coding details but if you do a search there have been some discussions in this forum about them

It is the engine hunting, I'm going to check the Idle Air Control Valve next.

Thanks for the explanation. I'll put aside the Cat and O2 sensor idea.

How do you properly install the 02 sensor? I can't seem to get the rubber grommet to seal the hole and stay in place.

KY Jelly (warming sensations), a screw driver and two fingers.