Exhaust and Oxygen/Lambda sensor

[chrism964]

My 1990 964 c4 has an exhaust system (fitted when I bought the car) which does not have a threaded hole to attach the oxygen/lambda sensor to the exhaust system. The system apart from this looks standard with no bypass pipes.

Someone has therefore made up a connector that is just a wire from pin 1 to 2 effectively bypassing the oxygen sensor input to the engine management by making a permanent connection.

I discovered this as I was investigating poor fuel economy and the heavy smell of fuel that occurs at idle.

Is this a recognised modification or some sort of half arsed attempt at increasing power by so called 'tuning'?

I can't see how this would help anything other than forcing the car to run permanently rich.

Any help appreciated. (Or is this just the excuse I need to pay Haywood and Scott a visit?)

Thanks

Chris

 

[chrism964]

Having spent a few hours trawling the internet. I have come to the conclusion that someone has fitted a cat bypass pipe without oxygen sensor port and plugged off the connector. If this is the case then there is no problem with running the car as is.

 

[Dave Wilkinson]

Chris,

I'm not so sure that your complacency is so well founded. If the connector to the O2 sensor has been shorted (i.e. pins 1 and 2 connected) it will give the DME a completely erroneous signal. When fitted, the O2 sensor varies the signal voltage to the DME from a little over 0 volts when the Lamba is > 1 to almost 1 volts when the Lamda is < 1. With pins 1 & 2 connected I'm assuming that will put a 12 volts supply on the signal wire to the DME - creating a situation where the DME will constantly try to lean off the mixture. In this situation the engine will run very hot.

If pins 1 and 3 are shorted it will ground the signal wire - forcing the DME to constantly enrich the mixture - which sounds like it backs up your experience with the fuel smell and rich running. In this situation the fuel economy will be awful and the performance mediocre.

There is a 3-pin jumper connector next to the DME itself (under the seat) where one pin can be grounded to tell the DME that there is no cat fitted (for markets where no cat was fitted). I would assume that will prevent the DME from assessing the O2 sensor input - but I'm not sure that would make a great deal of difference.

Given that the DME makes constant changes to the fuelling mixture partly based on the input from the O2 sensor it's difficult to imagine how the engine can run at anything close to correct air/fuel ratio without it fitted. The only times that the O2 input is ignored - in what is referred to as "open loop mode" is during the first 90 seconds of running and when the wide open throttle (WOT) switch is active.

I'm sure there are loads of 964s running around with no O2 sensor, but I can't imagine it's a benefit. Some of the cat bypass pipes on the market DO have the O2 boss (my Dansk unit has it).

Hope this helps.

Regards

Dave

 

[chrism964]

There is reference to this modification on the Ninemeister website (they mention a plug to the oxygen sensor connector when fitting a cat bypass which I assume is what has happened here.)

I thought pin 3 in the connector was a 12v supply for the heater in the oxygen sensor so that it could be at working temperature almost as soon as the car was running. Connection to this would give a constant voltage and cause the DME to think the engine was consistently running weak and lead to over fueling.

I will have to see if the DME has been grounded to earth at the branch out from the control loom (as per option M150). As I can't see how jumping the oxygen sensor can just 'fool' the DME and allow the car to run normally.
I have already bought an oxygen sensor to fit but as I mentioned the cat bypass pipe fitted has no port to screw it into.

Performance doesn't seem to be impaired at all . Although admittedly I have not driven two 964s back to back for comparison. I did, for a brief period, run the 964 alternate days with a Clio v6 and the Porsche blew that out of the water.

Haywood and Scott it is then I think.

Thanks for the help.

 

[Dave Wilkinson]

Chris,

You're right about the pins. Acording to Adrian's book, pin 1 is the ground, Pin 2 is the signal output to the DME and pin 3 is the 12v supply to the heater. I'm sorry if I misled you. Apparently, the O2 sensor GENERATES it's own voltage - it's not derived from the heater supply.

As far as I can tell, you're not quite right about the effects - grounding pin 2 (from pin 1) would cause a low (0v) voltage to the DME (indicating a lean mixture) and it would compensate by attempting to enrich the mixture. Linking pin 2 to pin 3 (12v) would give the opposite effect causing it to lean off an over-rich mixture.

I agree with you in that jumpering the O2 sensor (in either direction) can't give you "normal" running.

A cheaper solution than a full Hayward & Scott system might be just to replace the cat bypass with one which DOES have the O2 boss. The Dansk unit I bought from Design 911 has it and I know there are others which do.

Regards

Dave

 

[chrism964]

Dave,

Sorry for the pedantism(?) with regards to which pin does what.
I have just checked the option codes for my car and guess what? It has option M150 from the factory (for cars running in countries without unleaded fuel!)

I guess that means that if I want to get the Oxygen sensor incorporated into the system I need to remove the jumper from the harness into the DME. I think I probably will at some stage. I think that if I'm going to do it will get a full stainless system.

However it looks like I don't need to immediately.

Chris.