Wierd driveability issue... Help!

REM88Carrera · 09-12-2007, 07:41 PM

Pelican people:
This is my first time on so please, cut me just a bit of slack. That said, I have an '88 Carrera coupe, 3.2, with ca. 92K miles, recently had the 90K service performed by my local Porsche dealer. I purchased the car in February out of Charlotte, NC, excellent shape, recent top end rebuild (84K), etc.
While driving from Denver to SoCal last week, the car was running fine at 75 to 80 mph only to feel as if the engine were shut off then back on. The tach showed 3200 rpm's, lights, radio, etc., all working fine, just no motor. The motor then came alive anew only to repeat the process again and again. It never stalled out or failed, just would slow to about 60 mph and start running again. A buddy checked the Pelican site which made mention of the DME relay as a probable cause. I obtained one through Pelican and installed it. No help.
While in Salinas, I had Mobile German Auto Repair come and look at it. They installed a new coil and two new carnshaft position sensors because the insulation was pretty much gone on my original pieces. The car ran fabulously for about five hours only to have the same problem return. I made it back to Denver but you could have bought this car very cheaply last weekend!
Ok, the mechanic claimed to have checked the fuel pump pressure but I don't know if he was ever able to duplicate the condition because it is very intermittent. Rather than start swapping parts willy-nilly, has anyone had a similar situation and, if so, what was the culprit? I would really rather not start throwing parts at it just because. Fuel pump, brain, some other sensors? Any suggestions will be welcomed!
Thanks!

Quicksilver · 09-12-2007, 08:20 PM

When the fuel pump goes it will make great pressure except when it cuts out. When mine cut out it was like flipping a switch. I coasted to the side of the road and then a minute later it was running whole hog again. 5 miles later it cut out again. Unless your mechanic is checking it when it doesn't want to run then the test means nothing.

It sounds like the fuel pump or the DME relay. You have replaced the DME relay so I would suspect the pump. Instead of just putting in a new pump, how about the mechanic swapping in a good used unit to TEST it before throwing your money at it.

This points out something I hate about the vast majority of people working in the auto repair field. They aren't troubleshooters and they aren't repairmen. They are parts replacers. Any fool can replace parts until the problem goes away. Charging you for his incorrect guesses is just wrong.
Example: Replacing the crank sensor. The tach gets its signal from the DME. If the crank sensor was bad and the DME lost the signal the tach would have dropped when the engine cut out.

One good thing that you now have is a good spare DME relay. Keep it in the car!

REM88Carrera · 09-13-2007, 06:54 AM

Wayne,
Thanks for the response! I agree with your suggestion and have always felt it may well be the fuel pum given vaguely similar happenings with a '96 Land Rover Defender. Your comment re: crankshaft positioning sensor replacement. All the times the engine would cut out, I could watch the tach slowly lose rpm but it never went to 0 rpm; the motor would catch and then build rpm's in a normal fashion. I would prefer not to mention what the evaluation and crank positioning sensor replacement cost!
While in Salinas, I spoke to my mechanic in Denver regarding the problem. He mentioned connecting a remote fuel pressure gauge to the fuel injection which would allow one to see a pressure drop when the problem occurs. Only issue there is the intermittent presentation... I can drive the car for hours before it presents itself or it can show straight away. I like your idea of trying a known, good used fuel pump.
Have you ever heard of this problem being caused by the computer itself?
Wayne, thanks again for your input!
Richard

Quicksilver · 09-13-2007, 12:16 PM

The DME signals the fuel pump to run by triggering the DME relay. You could hook up an indicator light to the fuel pump power lead on the relay to see if it is turning it off. If it cuts out and the light stays on you know that it isn't the DME.

A fuel pressure gauge would tell you what you need to know but you would have to find one and then hook it up in a way that you can see it when it is running. Kind of a pain. It would be best if your could just swap the pump off of a different car for a bit.

If it is the pump, a thing to look for is the rubber hoses that connect to it. They tend to start bulging with age so they should be inspected before you have "Carrera Flambe".

PatrickB · PatrickB's Garage

CHT ~ Cylinder Head Temp Sensor. I had the exact same symptoms, replaced this part never to have it happen again...

Quicksilver · 09-13-2007, 02:37 PM

You can disconnect the head temp sensor and the car will still run. Not well, but it will run.

REM88Carrera · 09-13-2007, 07:19 PM

Patrick and Wayne,
Again, thanks for the input. BTW, loved the Ben Franklin quote! Amen!
One thing of interest, I presume the cylinder head temp sensor sends info to the computer reflecting engine load, for example, going uphill, more heat. Curiously, the problem is more prevalent ascending hills but still will present on level highway and even descending grades with the accelerator not depressed. Does that info help with your thinking?
Thanks again guys!
Richard

4flyboy · 4flyboy's Garage

Hey Rem88,

I had this precise problem going to Vegas one summer in my '74 Carrera. It was if someone was flipping a switch on and off. I coasted into Baker at 5 pm on Friday afternoon certain that my weekend was shot. Every garage was closing and completely disinterested except for one. A kid that specialized in VW Bugs. He listened to the symptoms popped the hood and pulled cable that ran between the coil and distributor off. He walked around behind the shop and pulled the same cable off a junker VW. 5 minutes later I was back at eighty. The wire looked fine from the outside but had obviously corroded or broken inside the sheathing. Hope your problem is as simple and cheap!

Scott

Bill Douglas · 09-13-2007, 08:17 PM

Ha, don't let this bit of bad luck put you off the car. It sounds a really nice one abeit with a few teething pains. My money is on it being the cylinder head temp sensor. Maybe do some research because I think they slightly changed them and it's best to have the new variety. The contacts to the fuel pump can get fairly ugly so you could try cleaning and reconnecting these, Bill.