A/F mixture gauge readings

Bill in OKC · 08-23-2005, 10:32 AM

Hi - I am a first time Porsche owner - its an '84 3.2 Carrera. The car runs OK - I think... I found an air/fuel mixture gauge tucked under the driver's seat and now it has me worried. It has a row of LEDs and when the car is first started/cold/fast-idle the mixture indicates really good. Then as the engine warms up and the idle starts to drop the mixture indication starts to wander all over the place from full rich back to full lean. I can hear the engine run a bit rougher when the mixture goes to full lean. At full throttle the mixture pegs rich. I was thinking maybe the O2 sensor, but then did a search on this forum and found things like it is not too accurate and only gives lean/stoic/rich readings... Should I be worried? Is this normal?

Thanks, Bill

Early_S_Man · 08-23-2005, 11:13 AM

Bill,

Sounds perfectly normal for a DME-engine running on the O2 sensor -- the back-and-forth wandering is exactly what closed-loop operation on Motronic is supposed to do to optimize fuel mixture and economy. Full-throttle operation always goes full rich and open loop -- until you back off, then back to closed-loop goes the DME!

Bill in OKC · 08-23-2005, 11:52 AM

Thank You!! That is good news. I was a bit worried. I am expecting some challenges with this car - but I am hooked

sammyg2 · 08-23-2005, 12:04 PM

It should swing back and forth, but it should do it rather quickly. If it spends more than a second on either side it may be indicating a problem.
It might be an old and weak O2 sensor, it might be a dirty or corroded or damaged wire connection going to the sensor, or it could be that the O2 sensor just isn't hot enough at idle to give a good reading.
Is this a three wire heated sensor?

Walt Fricke · 08-23-2005, 12:50 PM

Sammy

The 3.2s came with the 3 wire heated O2 sensor.

Walt Fricke
(wife used to own an '84)

Bill in OKC · 08-23-2005, 01:34 PM

I have had the car one week now. It has had some modifications done which I am clueless about - a MAF tuner (It is going to be removed because I know nothing about it) along with the A/F gauge. I can tell the MAF has been changed from stock(from pictures - this car has a round piece where the pictures show a squarish silver piece) along with what looks like a K&N cone filter. I am going to try to get this car back to stock someday. I can only assume that it has the stock O2 sensor in it. I have been REAL busy fixing all kinds of things.

Yes, the leds sequence back and forth relatively quickly. I guess it was the very steady/perfect cold readings that made me worry about the readings after it had warmed up a bit. I expected maybe a smaller variation but I've never seen one of these things before. Thanks for your help!

sammyg2 · 08-23-2005, 02:04 PM

I'm no expert on Motronics but I would venture to guess that when cold the O2 sensor doesn't come into play, the system going into a standard cold enrichment condition similar to an automatic choke on a carbureted engine (non-911 type). As it gets warm enough for the O2 sensor to take over, you start to see the swinging. Sounds normal to me.

ausgezeichnet · ausgezeichnet's Garage

My 84 3.2 came with a 2 wire not a 3 wire 02 sensor.