Originally Posted by JasonAndreas

Failing solder joints in the DME relay can be one cause, that would also explain your weird running problem if it was affecting all six cylinders.

Originally Posted by Mike J

Well, this is pretty subtle....

After replacing the plugs, wires, caps, rotors, spark plugs with no avail Dennis (aka C4S_fan) came up with the idea of a vapor lock.....so I thought about that. Cyl 6 is at the beginning of a very long section of rubber hose that runs across the engine from the left to right side. So I got to thinking...what would cause fuel to vaporize in the fuel distribution network? The system is normally running around 50psi and the pressure is supposed to be maintained after shutoff for at least 20 minutes which would raise the vapor point quite a bit...

...but if the pressure drops off faster then the fuel could get to atmosphere pressure and be way more likely to vaporize with the engine heat...

...and my car did not exhibit the symptoms until it sat for at least 20 minutes head-soaking..

so I decided to measure the fuel pressure.....then I find out SOB Porsche have a wierd fitting on the fuel rails to put my trusty gas pressure guage on. It has a safety ball bearing valve which most testers will not push in...so I spent a week trying to build a test jig....

and then gave up. I talked to Weissach, the fuel pressure check valve is only $20CDN...cheaper than getting the adaptor to measure the fuel pressure so I ordered it and installed it (which was fun with fuel dripping out of the fuel pump, you gotta be quick!!)

That was four days ago. After several cycles of running it/cooling/starting the car has NOT exhibited the symptoms at all and it has been running fine.

So what's the conclusion?:

1 - The problem went away by itself and its a total fluke that I put in the check valve at that time

2 - The ambient temps a bit lower so the condition does not trigger

OR

3 - Its fixed.
<snip>
Cheers,

Mike

Originally Posted by Mike J

Ok, my latest update:

I have determined the following pattern (at least 6 times now):

1 - the car consistently runs great from a cold start right to full operating temperatures including full thottle and pulls up long hills.

2 - the car consistently runs great if its been shut down from a full temp run within a few minutes of being stopped

3 - the car consistenly starts a major miss on piston 6 if the car has "cooled down" for at least 20 minutes

4 - the car will reset back to (1) if it sits for 4+ hours

I have also done without changing the characteristics of the problem:

1 - Replaced spark plug wires to piston 6 (including the $#()$)* upper)
2 - Replaced the caps and rotors
3 - Replaced the spark plugs for piston 6

Once the car is in a "missing" condition it a regular misfire...very regular what what I can tell from the exhaust. Once I shut down the car and let it cool down quite a bit (almost cold) the problem goes away.

So my current theory(s) are:

1 - That once the car is sitting my fuel check valve is not holding pressure, some gas vaporized in the fuel rails and #6 is on a hot spot that seem to get a fuel bubble. When the engine cools the bubble gets reabsorbed and it all works again.

2 - Something on cylinder 6 is causing the ignition system to misfire in some way. The Hall effect sensor in the distributors can be heat sensitive.

3 - The wiring to the injector for Cylinder 6 is somehow heat -sensitive only at a certain temperature and coming from a hotter state !?

Since the condition has not appeared when the engine is running at full operating temperature from a cold start, and it takes time with the engine off to get it to happen I am now leaning towards (1). At least its an easy check...I just need to get the adaptor to put my fuel pressure test kit on the fuel rail test point.

Does anyone know an easy way to hold down the pressure ball in the gas rail to test the fuel pressure while the gauge is attached? I don't seem to have the specific fitting that is required. The adaptor might be a VW part...

Anyways that's about it for now.... **sigh**

Cheers,

Mike

PS: This engine runs really nice from a cold start to full temp under different loading conditions...between a tight new motor and the RS flywheel it certainly feels quicker, likley 95% because of the new flywheel!

Originally Posted by george996

Hi Mike,

Interesting thread. I am chasing a problem with similar characteristics.

When your car was in the "missing mode" did it ever go away after driving for a while?

Thanks

George

Originally Posted by SwayBar

I would not think so since he reported good compression.