geolab

Hello Mike,

The issue here is unconstant, the problem is arousing when engine is hot, or not always, as you described. So resistance test or timing light will show normal, until the circuit to cyl. #6 heats up and the short happens.

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'97 Carrera S 100% original

TomF

Mike, I think that since this appears to be driven by heat, that a resistance check would not work. By the time you got the cable out and tested, you would be back to room (garage) temp. Before tearing into the #6 wires, I would do a real thorough examination of the caps at #6... It could have been a manufacturing defect that only hit a couple of sequential caps on the line. You could also swap out the easier of the two #6 wires and then do the same timing light tests that you mentioned above.

Davies

Mike, swap out those #6 plug wires. About three years ago, I had a misfire that was traced to a plug wire. It was arcing off another wire and caused all kinds of pinging and a CEL...so it looked like the two of them were bad, but it was actually only one of them that had a weak spot in it. The only thing my small brain can surmise, is when the car got hot, the spark took advantage of the natural softening of the (rubber) outer casing - due to heat(?) - and jumped from the weak spot to the other wire. It was a new wire but I'm guessing it either had a small defect or it was inadvertently damaged or crimped in some way during installation. It looks like you're already headed down that road, so does sound similar to your issue. Hope that's all it is...
Good luck!

Mike J

Thanks everyone...here is my plan.

1 - Check the caps/rotors for wierd marks/wear on six. if looks normal then...

2 - Run a spare plug wire for the easy #6 (bottom) first (which I have from my old harness) and heat test....if good then its the wirs. If that does not work then..

3 - Wait for the engine to cool down and try replacing the top plug wire. If that does not work then...

4 - Replace the caps/rotors with some spares that I have.

I would like to see how it runs with the other spark plug wires tonight. I can run them temporarily and see how that goes. Of course I have to get to #6 upper which is a real bear.

I will report in if I learn anything...

Cheers,

Mike

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!

chris walrod

Quick question, doesnt the 993 utilize a 'batch fire' setup?-- meaning the injectors on each bank are fired at the same time?? I thought I had read that somewhere..

If this is the case, you may try to swap (given ample loom length) 5 and 6 injector connections.. Just a thought..

Mike J

From my understanding they are sequential since each of the pistons on one side are 120 dgrees out from each other. I am not sure where the injectors fire as compared to the piston position though....

Cheers,

Mike

jw97C2S

Mike
Not sure if this will help but I had a serious misfire on a previous '86 911 caused by a faulty coil. The coil tested fine but wasn't. Good luck!
Jim

JasonAndreas

The ECU will run batch mode if you disconnect or have a problem with the hall sensor on the dual-distributor. I think there might be a few other "fail-safe" modes that switch the ECU from sequential to batch but that is the only one I've found.

chris walrod

Thanks Jason, I was hoping you would chime in. The fail safe or limp home modes must have been what I had read about. My memory is fading fast these days.. Am I am few years away from 40

Mike J

Keep in mind when the car is running well (i.e. cold start to hot) it runs really well. Another rennlister came over and we drove our two cars back to back on the same course...there is nothing wrong the way my car is running until it sits for quite a while after getting hot. This is why I suspect the fuel check valve. The problem I am having is finding the specific attachment that mates to the test port on the fuel rail. Anyone have a good solution on how to hook up a pressure guage other than spend the outragous money that Porsche wants for the adaptor?

Cheers,

MIke

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.

I figure a good week of driving with no symptoms and I can put the engine back together (i.e. put the new plug wires back in, heater fan, etc.).

Here is a picture of the check valve for those who are interested...



Cheers,

Mike

TomF

What does your gut tell you: that it was the check valve or that maybe you relieved the vapor lock by opening up the fuel system... if that is possible. Any fuel system experts out there?

Mike J

Well I doubt if opening the system did anything since the check valve is at the fuel pump just underneath the tank. I went for a longer burn today now that the engine is getting some milage on it (heh 300 miles and climbing) and it ran great. I have not seen the problem yet. I have noticed at some times I have to give it an extra crank to fire up...not sure what that is about.

My guy is telling me that I was/am chasing a few different problems at once...and they are waiting to launch themselves just before I do the 6 hour drive from Vancouver to Portland to attend the 2006 Parade..

Cheers,

Mike

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.