Joel Lester

Just bought an '86 911 coupe about six weeks ago and I'm just loving it! My first Porsche. It is white with a black interior. The car is in unbelievable shape and is super clean. The guy I bought it from was the second owner and he only put about 10K miles on it and owed it 9 years (total miles is 130K). So, it sat in the garage a lot over the last decade which is probably not a good thing. But it has been very well maintained (had it checked out by a Porsche shop prior to purchase and everything was good).

Anyway... my question is this...

So far, there have been three to four times that I'm driving along (low speed - not driving hard) and the car just quits running. All the lights work and when trying to start it again the starter has plenty of "juice". A couple of times it starts right back up again but other times it doesn't. Try it ten minutes later and it starts again no problem. Hmmmmm. It's been cold outside but not that bad.

Most of the time... when I jump in the car in the morning... it starts right up. Other times it doesn't and I sit there for a few minutes trying to start it ... then suddenly it starts right up.

The battery is new and the starter motor turns the engine over at will.

When the car is running - it runs great.

Any idea what the heck is going on?

It's either electrical or fuel. I doubt the fuel is a problem because it runs so good when I drive it (doesn't spit and sputter etc.) Where do I start as far as the electric stuff?

Are these 911's prone to any common problems that I'm not aware of?

Steve Weiner-Rennsport Systems

Joel:

I'd bet you "dollars to doughnuts" that your DME relay is flaky. This little device found under the driver's seat provides power to the fuel pump and DME (brain). These have been a common problem
on the Carrera's and stranded quite a few people who didn't know to carry a spare.

Buy 2 (one for your glovebox spare), and plug in a new one.

Joel Lester

Thanks! That is exactly the kind of response I was looking for.
What does DME stand for?
FYI... I see your from the Portland area... that is were I bought this car!

Thanks again,

Joel

Steve Weiner-Rennsport Systems

Joel:

Digital Motor Electronics; this is the Porsche trade name for the Motronic Engine Management system.

Keep me posted on your progress.

rbcsaver

joel,
You can't do better than living that close to Steve Weiner. I agree with his diagnosis. Worst case, if this is not the problem, then you will have an spare part (DME Relay), which you should always carry iunder your bonnet anyway. I have been there to witness that failure on the highway, out of state. It is located under the driver's seat, about 1" x 1" x 2.5".
regards,
Steve

Joel Lester

I have called the local Porsche dealer and they have just one in stock. I've asked them to hold it for me and I will pick it up today. Hope that fixs it. Thanks to you guys for all your help. I could have been chasing my tail on this for months!

Joel

Joel Lester

Huge thanks to Steve Weiner at Rennsport for the tip. I replaced the DME relay for $50 from the dealer and installed it. She runs great and haven't had a problem since!

Great job. I could have been pulling my hair out for months trying to figure that one out.

This forum thing is awesome.

Joel

madmmac

Quick, now go get another one for an in car spare!!

Cowhorn

Does anybody have a picture of where exactly the DME is located and how you can get to it?

carrera

Picture this

a black box directly under the drivers seat, open it up take a look, get to know your dme.

ChemMan

Joel,
I asked the same question as to what the DME actually is a couple of months ago. I aslo have a 86 carrera. Do yourself a favor and get the Bentley manual. It has been a huge help to me. This is the guy I got mine from:

https://rennlist.com/forums/911-forum/243553-if-you-are-looking-a-bentley-manual.html

Well...its in this post.
Mike

Joel Lester

Mike,
Funny you mention that manual... Steve (the guys who told me about the relay problem) strongly recommended I get that too. I have got it on my shopping list.
Thanks,
Joel

PAS

Does anybody know if the DME relay is the same relay for the 993?

sourcandy

I have an 89 911 C2 with the exact same problem been driving us crazy. Finally called a mechanic friend who works at Auto Assests in Ohio he told us the same thing.. DME relay however, no avail, still no spark.

So far the coil, cap, rotor, speed and reference sensors have been replaced. As well as the DME relay the other night. Car still doesn't have spark.

We are starting to really miss her..

Also... the only way we check the senosr.. was whit ohm reading from a voltmeter, however, even with the new ones we still don't get the proper ohm readings from them. Is there another way to test?

Thank you,
-Ed


EDIT:I should mention i just recently acquired an oscilliscope (this morning actually) but I don't know how to properly use it yet. I figured it would come in hand for electrical problems though.

theiceman

Damn computers ... trade it in on an SC ...... just kidding

As an SC owner I really can't help with your particular problem , as an electrical engineer I can definitely help with some troubleshooting.
the osc is a fancy voltmeter is all , it can react to very fast signals like those used in cars , it is great for monitoring varying outouts , cyclic outputs and so on. Usually sensors have about 3 lines ( basic sensors anyway depending on the type. , a ground , a constant 12v , and a signal out ( usually 0 - 12v ) that is the line you scope. the schemtic will be a big help here.
What I do not know is what signals and sensors your car uses to " allow" ignition. If your car uses a crank reference sensor and / or a hall efect sensor on the cam to tell the computer it is at TDC ( permision to "fire " ) I would start here. If the car does not receive these signals it will never fire. Perhaps someone who knows that engine really well can chime in. But I would definitely investigate these sensors first.

Good luck ..