New to me help me put these symptoms together surging won't rev
 

Back to 944 ownership. Funny logging in to find "last visited 10-16-2005." Guess it's been a while :eek:
New to me 89 944 base, 155k, timing belt done at 145k.

Drove it about 200 miles with no problems.

Started it yesterday morning and it cranked for 4-5 seconds before catching. Before this it was always turn the key and immediate start. No driveability issues all day, 50 miles. Got gas midday. Tank showed half full but since this was the first tank I don't trust the gauge yet.

Last night after driving about 10 miles I noticed the tach jumping around. Steady cruise 3000 rpm the tach would jump 200-300 rpm but did not translate to any driveability change. Push clutch in and tach again wouldn't respond normally. Would rise or be slow to drop to idle range.

Then the surging started. Random and intermittent at first, then to the point of audible backfire pops, wouldn't rev past 3000 rpm and bad bucking/surging. I found a happy steady state where I could keep it moving about 30mph. Made it home making all of the lights, rounded corners on the gas and the first time I pushed the clutch in to turn into the drive it died.

Help Porsche experts?

Clean your grounds, run some Seafoam through the intake, test your AFM.

I'd look for vacuum leaks, replace O2 sensor, and replace the dme temp sensor.

Check the reference sensor.

I have not started the troubleshooting yet but I went out and it cranks but won't fire. The cranking does sound normal, just not firing up.

If that helps any, otherwise I'm ordering a DME since I need a spare anyway and then will begin to troubleshoot.

The jumpy tach was the part that makes me curious.

is the tach jumping when you crank it?

Regarding the jumping tach. Last night the first sign I had of impending trouble (after the long crank time first thing in the morning) was the tach needle jumping at steady driving. Cruising at 50 MPH, around 3000 rpm in 4th, the needle started to fluctuate 200-300 rpm high and low with no change in the driveability i.e. no surging, just the tach going wacky.

I just went out and cranked the car - the fan immediately turned on so I couldn't hear the fuel pump but the tach needle did not move a bit while the car was cranking.

Ordered a DME because I should have a spare anyway, and the two crank sensors. Hopefully this will be an easy fix.

That sounds like a fault with the speed sensor (the sensor closest to the firewall, on the bellhousing), a bad DME interpreting the signal, or a gauge problem.