Subject: long running problem – 1986 3.2L revs up and down while idling
 

I have searched and seen similar problems, but I haven’t seen a solution for my specific problem…

The car is a 1972 911 with a 1986 engine and transmission. The engine revs between idle and a little over 1K. Just sits there continuously revving up and down while at idle. Sitting in the garage. At stop lights. It does it at first start up and after thoroughly warmed up after a 30 minute drive. Once underway the car runs great.

I’ve had this problem over a year now. Have brought it to a reputable (expensive and seemingly thorough) shop several times.

The following is a list of fixes over three shop visits (some don’t apply but just in case I’ll include them):

Spark plugs
Air filter
Fuel filter
Engine bay fuel lines (x 3)
Fuel pump, fittings and seals
Fuel tank (it was pretty rusted inside)
Fuel filter
Intake manifold gasket
Vacuum elbow
Intermediate flange (I don’t know what this is but I included it to be thorough)
Alternator

Each time they insist the problem is fixed. But it never is. They are obviously thinking fuel supply. I notice what’s not mentioned in the list is the ECU. Could it be the shop was looking in the wrong place? I bought the DME/ECU from Vertex Auto in 2003 when I had the new engine and transmission installed. The engine is stock although I did delete the catalytic converter. I've owned the car since 1992.

I’ll mention a second problem because it may be related. This problem is annoying but not as annoying as the first problem. As I slow down in gear the car (engine) starts shuddering as the RPMs go below 3K. Hit the gas and it smooths back out. I should be able to go below 3K as I coast to a stop. This is new problem that seem like popped up around the same time as the first problem.

Any suggestions?

Thanks,
Matt

Picture of the 1972 car in question. The black one.

http://forums.pelicanparts.com/uploa...1717284279.jpg

It's a fuel mixture problem or the air screw in throttle body is mis-adjusted, or a non-stock chip.

Not good!

Fuel shut-off occurring (on/off), typically less than 2K RPM. It could be a mis-adjusted throttle switch, or the DME ECU.

Remove the idle air control valve and clean with electronic parts cleaner.

Remove the air flow meter and clean with electronic parts cleaner, minding the thermistor in the passage way.

Turn clockwise the idle air bypass screw at the front of the air flow meter and count the number of turns it takes to bottom out. Then remove the entire screw to inspect the o-ring integrity. If bad, replace the o-ring. Clean the orifice. Replace the screw to the prior setting. Should land some place between 2 and 3 turns out from bottom.

Make sure the auxiliary air valve is operational. If not working properly it may result in high idle at working temperature (stuck open), which the iacv and computer then may try to fight to correct.

If you are running an O2 sensor, and it's anywhere near 60k mi old, it's time to replace.


These are all the items I addressed in ridding a hunting cold start idle (reduced from 4 to 2.75 turns on a new idle bypass screw) and the just off idle parking lot hurky jerky drive (new O2 sensor finished off the last of the jerky throttle).


That's all predicated on the idea that you're running a complete motronic system.

Also think about the cylinder head temp sensor. I just had a URO branded one go bad right out of the box.

Post the work receipts.

What have the reputable and thorough shop checked and diagnosed each time to "fix the problem" ?

I had to replace my idle control valve to solve this problem. Cleaning it didn’t help, but I didn’t do the above procedure. You might have better luck.

Be sure to use only Bosch or genuine Porsche parts when repairing the fuel injection. There is a difference.

Hey guys,

Thanks for the replies and info. I am sifting through it all. I apologize that I have not been responsive. I very much appreciate any and all advice but I've been extremely busy. I obviously need to figure this out so I will be back.

I forgot to mention the fuel injectors were also replaced. Again, the shop was thinking fuel delivery as the problem.

mysocal911 - regarding your comment about the purchase of the DME from Vertex in 2003... "not good". Are you saying the DME is old and perhaps the problem or that Vertex provided it? It's worked well for many years...

Thanks again,
Matt