FRporscheman

Hey Karl, I've checked out the vacuum lines using a non-lit torch and found no leaks. But today while working under the hood, my arm pushed the purple vacuum tube behind the firewall to the reservoir, and I heard a hissssssss, and after taking a look it is badly cracked. I decided to manually check every single vacuum tube with a hand pump and everything was ok except that one. I'll replace it ASAP - anyone know where I can get the right kind of tube? Can I use thicker tube as available from the local Kragen?

The cap and rotor are new. I think the spark is there, as it runs fine when hot... and I just cleaned the injectors 2 weeks ago with carb cleaner and my air compressor... they worked fine then. But I'll check for spark and injector pulse anyway.

I'd like to think the O2 sensor is ok since it costs so much, but I'll check it when hot. Also, I'll check the DME temp sensor again just to be safe. Thanks u guys, now I have something to go on again.

83na944

I agree with Mark, it is not vapor lock. That just doesn't happen on a pressurized system. Did you check the speed and reference sensors?

FRporscheman

I did a resistance check on the speed and reference sensors a couple weeks ago, and I got about 1000 ohms on each one, in both cold and hot condition. That matched the values givin in the haynes manual... is there also a voltage check or something?

Thanks everyone for the invaluable help!

FRporscheman

I replaced the bad vacuum line and drove the car today. The hesitation and noises from the exhaust at high rpm are gone! But, the starting problem isn't gone. I was told to look for the tach to bounce as I cranked the engine when the problem occured, and it doesn't bounce. I guess that means that's the problem right there - the car won't start because of the speed/reference sensors... should I replace them, or is the problem more likely with the DME? The box connectors near the firewall for the sensors are tightly connected.

PorscheDoc

I posted a reply at clarks also, but even if those sensor connectors seem tight, they usually still aren't making contact. Real pain in the ***.

FRporscheman

I tried wiggling the connections when it didn't start yesterday, and it made the tach needle start bouncing but the car still wouldn't start. I am completely at a loss now.

FRporscheman

School started and now I have only weekends to work on this car. I swapped in the DME out of my mom's 944 and drove around for quite a while. I got to my friend's house and hung out for 10 minutes, then when I went to turn it back on it wouldn't. This means I've ruled out the DME as the culprit, since my mom's car runs like a champ. I can't think of anything else that could be causing this...

FRporscheman

Just updating (for anyone who ever has a similar problem)

OK I think I'm on to something...
A bad O2 sensor would cause wrong amounts of fuel to be sent to the engine, right? That would cause my hesitation and the popping from the exhaust. Also, it could prevent the engine from starting by flooding or not sending enough to start. The problems only occur when the engine is warm... but what if it's really when the exhaust is hot? The O2 sensor is in the exhaust, and that would explain the 20 minute wait time (since it takes the exhaust about this long to cool-off). Also, I have noticed horrid fuel economy, which is another symptom of bad O2 sensors. On top of all that, the car just failed smog, having too much CO, and I've read that bad O2 sensors cause excessive CO emissions. Man, all this evidence has been right under my nose the whole time! DUH!

I ordered a bosch universal 13913 from paragon and I can't wait to get it in the mail. I'm praying...

Fishey

o2 sensor (since its a warm problem) I had lots of hesitation and stubleing when my O2 sensor went bad..

FRporscheman

I got a bosch O2 sensor (#13913) for like $40 from paragon and it fixed the problem! All this time... Well I'm glad it's finally fixed. It still stumbles once or twice on heavy acceleration after a hot start, but it goes away. Hey, IT STARTS, I can't complain!
Thanks to everyone who helped me troubleshoot, and I hope anyone with a similar problem will find this thread and save some time and effort.

EDIT:
Haha I was premature... the O2 sensor helped a LITTLE, but the solution to my problem was the FPR (fuel pressure regulator). Changed that and problem is gone. I drove it 150 miles without a hitch. It cost me like $40. NOW I can celebrate