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Old 03-27-2015, 05:24 AM   #5
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Join Date: Jan 2015
Location: az
Posts: 15
Originally Posted by b55er View Post
I had my 1998 986 sit in the garage for quite a while. Car has 25k on the clock.

Upon starting it up, and after temp reached 180F, idle would sit around 1100.

Okay, I thought.. something is wrong. I take the car to the local Porsche dealer. Dealer says fuel trim levels are off and needs a new MAF. No codes no CEL. Dealer puts in a new MAF, resets all adaptations, etc, and sends me on my way; yet the car still idles at 1100. Dealer tells me that adaptations need to be set by driving about 60 miles. I take a nice 80 mile drive, car still idles around 1100. Back to the dealer.

I drop the car off at the dealer. Dealer hooks up car to their laptop, rechecks everything (fuel trim looks okay), takes off throttle body and says it's squeaky clean in there. No vacuum leaks. Dealer tells me that their computer states idle is at 850 and that engine management is working fine. Tech's ears also confirm 850.

Dealer now tells me there is nothing wrong with the car, and that the engine RPM sensor is working correctly as reported by the DME. Fault must lie in the tachometer.

Dealer tells me that nothing can be done to calibrate the tachometer. They suggest calling a VDO repair shop.

I call Palo Alto Speedometer (VDO) shop. They also tell me there is nothing that can be done to calibrate the instrument cluster. All other functions on the cluster work fine. Palo Alto Speedometer tells me to buy a used cluster (eBay?), and they'll flash my milage onto it (~$175).

Odd that the tach would be so much off. The instrument cluster has never been disassembled. With the car off, the tach needle rests in the same location as 3 other 986's that I observed at the dealership.

Any thoughts or ideas? Anyone seen this before?


Wish I had found this earlier. My symptoms: car ran perfectly, would idle at 1000 to 1100 rpm. When the ignition was shut off, the tach needle would bounce for a second around 0 rpm, before stopping at the 200 rpm mark.

After cleaning/replacing the maf, replacing the aos (it was ok not needed), checking for vacuum leaks, cleaning the icv everything seemed ok with no check engine light.

Finally looks like the tach is reading 200 rpm high, explains why it did not read 0 rpm with the key off.

I need to verify this with a diagnostic tool. A lot of work, and misdiagnosis, just for a tach gauge problem.

I'm now looking for a used, good, tach.
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