Speed varies slightly

[David 84 911]

If I keep gas peddle to a constant speed for example 40MPH, I feel the speed vary like the car is rocking forward 1-2 MPH's. Would this be a vacuum leak? Oxygen sensor? bad fuel filter?

Any suggestions welcome.

Gear Head in training...

[MikeDL]

I was told that this is due to how lean the 3.2 DME engine runs. On my '84 911 I disconnected the oxygen sensor and the sensation stopped.

[PatrickB]

If disconnecting the O2 sensor works, you are only putting a band-aid on the problem... Even if that does help, your gas mileage will go down the toilet! BTDT!! I've been chasing the same problem in my '85 Carrera for a yeaqr and a half. There are about 20 different ways to try to skin this cat.... My problem is intermittant though. Also it will bog under light to medium acceleration between 2800 & 3200 RPM's. Under hard acceleration, sometimes I will get a similar bog around 4200 RPM. Try the simple stuff first. Vacume leaks, Plugs, Fuel filter, spraying intake cleaner in the air box with engine on, battery connections, battery, main ground wire near tranny, DME Relay, CHT sensor, Speed sensor. Those are the easy things. Then you can remove the intake and replace the gasket on that... I've done all the above, and still have the same problem... Good luck!

[MikeDL]

"Even if that does help, your gas mileage will go down the toilet!"

That's not been my experience. If my mileage went down at all, it's been by less than 1 mpg.

"Also it will bog under light to medium acceleration between 2800 & 3200 RPM's. Under hard acceleration, sometimes I will get a similar bog around 4200 RPM."

After disconnecting the sensor, my car did not get these symptoms either. The slight power variations at steady speeds are very typical of the 3.2 DME due to how lean it tries to run for emissions reasons.

Search for a thread (disconnect AND oxygen AND sensor) "Help Please...'85 911 seems to pulse or surge..." started by 97*Sunshine and you will find a great discussion of this phenomenon.

I'm not saying that's the cause in this case, but I would try the sensor disconnect before anything else.

[efhughes3]

Sure sign that the engine is shot, and needs a full rebuild.

[PatrickB]

Originally posted by efhughes3
Sure sign that the engine is shot, and needs a full rebuild.

What is a "Sure Sign" that the engine is shot, and needs to be rebuilt?

[efhughes3]

Originally posted by PatrickB
What is a "Sure Sign" that the engine is shot, and needs to be rebuilt?

The symptoms outlined at the start of this thread....




(It was a joke )

[PatrickB]

Originally posted by efhughes3
The symptoms outlined at the start of this thread....




(It was a joke )

Good! I didn't need that big lunch anyway...

[Tyson Schmidt]

The surge you're describing is a lazy oxygen sensor.

As they get old, rather than cycle from lean to rich very rapidly, they cycle very slowly. So slowly that you can feel it as you're driving.

The cycling is done to keep the catalytic working at optimum temperature. The cycling is supposed to happen so quickly that you can't feel it, and it averages out at stoichiometric which is ideal fuel mixture for fuel mileage and low emissions.

When they start slowing down, driveability and mileage suffer. So disconnecting the O2 sensor is a viable temporary fix. But you really should install a new O2 sensor, or switch to a Euro DME set-up.

Steve Wong can switch your DME brain to use the 28 pin chip, and give you a custom Euro chip that eliminates the need for an O2 sensor. He can also do the same thing with a U.S set-up and utilize the O2. Either way will really help driveability.

[David 84 911]

This is excelent feedback, thanks.

I am a "tie wearing gear head in training" so if anyone has a picure of their O2 sensor and where it is located, I would love to see it. I have a Porche mechanics book, but the location is not shown.

CHEERS !


David

[ken_xman]

its on the top side of the cat converter