924Board.org

Redandblack · Joined: 20 Dec 2007 Posts: 20 Location: Lewisville, NC

I have researched previous posts and I can find similar posts that describe my problem but I have not had much luck in finding resolutions. Here is my problem. I own a 1987 924S.

I can go out and attempt to start the car and it will simply turn over but will not crank. I can come back an hour or two later and it will crank immediately. Some mornings I go out and crank her up and everything functions fine for the entire day-no problems what so ever. I can use the car for a week at a time and experience no problems with cranking. My latest difficulty was Friday morning. I had an appointment to get to and thought I would take the 924S out for a spin. I got in, attempted to start her but it would not cooperate...it would just turn over. I switched over to my trusty 914 and went to my appointment. Upon return, I attempted to crank the 924S and it cranked right up-no problem. About a week ago, I was in the 924S in traffic at a stop light and it just shut down. I attempted to restart it but it would only turn over and nothing else. I waited for the tow truck for about one hour to pick me up and take me to the shop. In the meantime, I would attempt to start it up with no success. About one hour later, I tried again and it cranked right up. I cancelled the tow truck and drove back home. I have used the car for errands on and off since that incident and I have had no problem until the past Friday incident mentioned above.

I have already replaced the DME relay, the ignition switch, the fuel filter, etc., all recommendations from previous posts. What gets me is the completely random nature of the no-start problem. Is my problem electrical? If so, where should I start to track down the gremlin?

Paul · Posted: Mon Jul 13, 2009 6:53 am Post subject:

Just a little more info please:

You are saying that the starter engages and turns the car over at a normal speed, but it doesn't start, correct?

While turning the starter does the tach needle bounce?

Have you checked for spark while cranking?

Have you checked for fuel pressure while cranking?
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Redandblack · Joined: 20 Dec 2007 Posts: 20 Location: Lewisville, NC

Paul,

Thanks for the response. In answer to your questions:

The starter engages and turns the engine over at a normal speed but it will not crank.

I have recently replaced the sensor but yes, the tach needle bounces while turning the starter.

I have not checked for spark while cranking because I am usually in a hurry or in a location without tools which prevents me from checking. When I am back where my tools are located, it will usually crank and I cannot duplicate the conditions.

I have not check fuel pressure while cranking because I am usually able to get it cranked within an hour or so. I have not suspected the fuel pump because it will usually crank later for some unknown reason. That is why I keep going back to the idea that there is something weird with wiring that may be my problem. Could a fuel pump work on and off? I would assume that when a fuel pump goes, it is gone.

Paul · Posted: Mon Jul 13, 2009 8:11 am Post subject:

Always start with the basics:

Compression

Fuel

Spark

Timing.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Redandblack · Joined: 20 Dec 2007 Posts: 20 Location: Lewisville, NC

Paul,

Thanks again for your comments. Until the problem manifests itself again, is there some troubleshooting I can begin? I just went outside and cranked up the 924S and I had no problem. This car is my son's daily driver and I have had the car home in an attempt to diagnose the problem. I do not want to get it back to him and have him call in a week to tell me that the car is having the same issues.

There must be a half-way logical beginning out there!

Paul · Posted: Mon Jul 13, 2009 10:44 am Post subject:

Since your tach needle is bouncing you probably have spark.

The next time it won't start try pulling the plug on one of the injectors and see if it will start.

Also pull the plug on the DME and make sure there is no corrosion on any of the pins.

Also check the plugs and wiring for the ref mark sensors (near the brake booster).
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

masterdave · Joined: 10 Dec 2008 Posts: 183 Location: Harrison NY

could also be a grounding issue, there are a bunch of grounding points all over the car, i know of 2 one by each headlight there are a few more under the car and in the engine compartment i would advise cleaning those, just take the bolt out and sand the heads with a file or light grit sand paper till they are copper again. if your computer is not grounding correctly that would explain it.

Redandblack · Joined: 20 Dec 2007 Posts: 20 Location: Lewisville, NC

Thanks Masterdave for the input.

I have cleaned the grounds at the headlight positions and in the trunk. I have recently replaced the main ground from the battery to the top of the block. I have yet to find the illusive ground in the passenger compartment near the main fuse panel. I guess you have to pull the panel to get to it.

Paul, let me ask you another question concerning the injectors. You stated that I should pull the plug on one of the injectors when it is refusing to start. I understand the principle of pulling the plug on each injector while the engine is running to determine which one may be at fault but I cannot wrap my head around the idea of unplugging one of the injectors when it is refusing to start. Can you enlighten me? Thanks again for your continued input, Paul!!!

Paul · Posted: Wed Jul 15, 2009 9:03 am Post subject:

It's to reduce the resistance of the circuit that fires the injectors to see if the DME will fire the other 3.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Redandblack · Joined: 20 Dec 2007 Posts: 20 Location: Lewisville, NC

Paul,

Again, thanks for your thoughts. I have not forgotten about this thread...I am waiting to have starting difficulties again so I can try some of your suggestions. So far, I have been able to crank the car with no difficulties for several days in a row.

firemn131 · Joined: 16 Jul 2008 Posts: 57 Location: Lusby, maryland

Hey Paul. my 87 924s did the same thing foe about 7 months. It was my daily driver of 120 miles round trip each day. Until this started to happen. No pattern at all.
\
Changed relay fuel pump, fuel filter, fuel press. reg, AFM, and computer relay. These parts were changed out mostly due to good prices on someone else spares and it was about time to changed others.

It did not matter.

Then I changed the speed and reference sensors.

No problem since, and that has been about 3000 miles ago.

A guy named Paul first suggested this to me on this site a while ago.

The sensors were reported to be a pain to change out.

I was blessed. I had them out and changed in under 30 minutes.

the sensors are about 70. each. they are located on the back of the engine just at the firewall.

Any questions dont hesitate to ask. This problem drove me nuts for a long time. Like you it is my daily and could not get it to duplicate when i had an opportunity to test.

All the best.
John
_________________
1987 924S Daily driver 64 miles each way to work.
2001 F-150 my back up
1970 Mercury Cougar XR7 The Muscle.

Redandblack · Joined: 20 Dec 2007 Posts: 20 Location: Lewisville, NC

I was able to root around this morning to further investigate my problem and I concentrated on the reference sensor plug end that is coupled at the top of the intake manifold since I noticed that it had been wrapped in electrical tape. I removed the tape and found that the black wire had a crack in the insulation and the wire is frayed just a bit. The protective rubber boot is pretty much toast. This is the female plug end of the circuit that goes to the DME control unit I presume. It is not the sensor plug end which would be an easy fix.

There is not a whole lot of extra wire which would allow a simple re-wire of the plug. It looks like I may have to replace the wire all the way to the DME control.

Has anyone attempted this fix before or is there another way around fixing the frayed wire?

firemn131 · Joined: 16 Jul 2008 Posts: 57 Location: Lusby, maryland

how about splicing in a new plug from another car
_________________
1987 924S Daily driver 64 miles each way to work.
2001 F-150 my back up
1970 Mercury Cougar XR7 The Muscle.

Redandblack · Joined: 20 Dec 2007 Posts: 20 Location: Lewisville, NC

Thanks firemn131 for the comment...

That may be the only possibility. I looked at the cable and it appears to be a coaxial type with a center core of a white and brown (ground) wire surrounded by a wire sheath making up the black wire that has been gathered together to form the connection to the plug itself. I call it coaxial cable because it is similar to cable TV wire in that it has that metallic sheath that conducts electricity.

Anyone have any experience with this type of wire or do my eyes simply deceive me? I do not want to go too far.

firemn131 · Joined: 16 Jul 2008 Posts: 57 Location: Lusby, maryland

it is just a shielding and is splice able.

My thought would be to first find a spare and practice. if yours is bad you really have nothing to lose. Or a new harness.
_________________
1987 924S Daily driver 64 miles each way to work.
2001 F-150 my back up
1970 Mercury Cougar XR7 The Muscle.