Hey everyone.
I seem to have a very specific problem with my engine, and I could really use some experienced insight. I hope I don't come across as wordy; I'm just trying to be as specific as possible for you all so you can perhaps help me diagnose my problem.

Okay, I am the proud owner of a 1987 Porsche 944 S. My car will start just fine and idle at 900 RPM. The car does not have a wandering idle. The engine idles and runs smooth with no backfiring or missing. She can sit in my driveway and idle all day without a qualm. When on the road, my engine only stumbles during very specific conditions. When the engine is at load, always in fourth gear at speed, and I depress the clutch and let off the throttle, allowing the engine to drop to idle speed, the engine likes to drop way below 1000 RPM and try to die before returning to normal. This will often kill the engine. Again, the engine is at load, and when I remove the load and the gas at speeds from 40-50 MPH in fourth gear, RPM drops to nothing and the engine chokes. This is the only time the engine does this.

I can also provide specific information on what happens after the engine dies. After this happens and she is stopped, when I try to start her, she will fire right up, briefly rev to 2000 RPM, and then die again. However, if I give it gas right as it is doing this, I can easily keep the engine alive, and it rapidly returns to normal, idling just like normal without me applying any throttle pressure.
This happens the same way every time.

I have also noticed (this may or may not be pertinent information) that when I hit a bump in the road, the engine RPM sometimes likes to jump about by several hundred RPM.

I am currently running her without the o2 sensor. I have had a look over my vacuum system, and everything seems in order. This has really stumped me.
Any knowledgeable insight would be GREATLY APPRECIATED.

Thanks a ton,
Jacob

 

First thing I'd check is the ISV. Its the 'T' shaped object underneath the intake manifold with ~1" vacuum lines running to it. It may be stuck open, or closed, which is then messing up the afr's for the DME. Lots of these symptoms can be related to a problem with it.

The other things are as mentioned - vacuum lines/leaks. The fact that when you hit a bump your RPMs change leads me to believe ISV too, but it could also be the TPS (underneath the throttle body, black box).

Take a look at both of those, as well as clarks-garage for more troubleshooting steps to figure out exactly what's causing this.

 

Thanks for the speedy response, Equilibria.
The reason I don't believe that it is necessarily the ICV sticking is that the engine drops below proper idle speed only specifically when mentioned. For instance, revving it up to 2000 RPM just sittting on the driveway for a while, and then letting it go back to idle will not cause any upset. It seems like a sticking ICV would cause this to occur sporadically, in other words the problem would not be so reproduce-able.

 

I had the same problem on my 87 S roughly 4 years ago, never did manage to figure it out as the car was involved in an accident shortly thereafter, but one thing I did to make it driveable was disconnect one of the small vacuum hoses to the intake. By leaving one of the small vacuum hoses open it seemed to run fine, which should not have been the case but for some reason worked.

Give that a shot, just as a diagnosis, and see what the results are. It's a quick and easy test.

 

Sorry to hear about the accident, that must have properly stunk. I find what you say about the vacuum hoses interesting. Is there any chance you remember the hose you disconnected? I'll be sure to give this a quick try... It couldn't hurt.