Help, SC fuel pump problems

tobeto87 · 06-10-2008, 01:20 PM

Hi all,
New registrant, long time reader, first post, from Denver. I have always enjoyed the posts and have found many entertaining and relevant.
I have an '82 SC that I had finally had time to take out of winter storage this past weekend, well attempted to take out of storage. After connecting the battery and cranking it over it would not start. I had replaced the fuel pump last summer because it was sounding tired. The car has never had a problem starting warm, hot or cold. I immediately thought the problem was fuel so I pulled the relay and jumped between 87a and 30 with a fused jumper. The fuel pump ran fine, replaced the relay, and again no start. I replaced the relay with my son's SC relay which I know works, still no start. I jumped the the 87a and 30 pins again fuel pump runs, the car starts and runs fine. Put the relay back in the car starts briefly, I assume from residual fuel and stops. I also put the relay back in my son's car and it works fine, so I can assume I am not shorting the relays. Any ideas, what am I missing. Would low voltage to the fuel pump cause these symptoms?
Any help would be greatly appreciated.
Below is a picture of my car.

Thanks for the input
Dave
Denver.

sabeo.m · sabeo.m's Garage

I had similar symptoms about a year ago. I had trouble starting after driving for a few hours and then letting it sit for 15 to 20 minutes with no fire up what so ever. After finally got my car started and in my garage, my car puttered and died again with no start up. I thought it was the WUR but figured out it was working just fine. I exchanged the relay for a new one and it as well seemed to fix the problem briefly. I siphoned out some old cheap gas, replaced it with high grade shell gasoline, replaced the fuel accumulator and replaced the fuel filter. I then performed a full tune up, replaced all plugs, distributer cap, plug and ignition cables with the better Magnacore ones. I haven't had a single start up problem. Freezing cold or a hot humid day, she always fire up. Your problem can be a simple fix as I described above. Also check your fuel injector cables they can be worn and cracked. Good luck and welcome.

Jamie79SC · 06-10-2008, 01:58 PM

Perhaps the problem lies in the socket where the relay plugs in. I can tell you first hand that a lose connection there will cause problems.

Pazuzu · 06-10-2008, 02:02 PM

You diagnosed that the high current half of the system works, and that the relay is fine, but you haven't worked on the LOW current side...

You probably have a short/break between the ignition switch and the relay. Jumpering the connector simply removes the low current half from the system, but doesn't tell you what's going on over there.

tobeto87 · 06-10-2008, 02:52 PM

Thanks for all ideas, it is appreciated. Mike how do I check the low voltage side of the relay.
Makes sense that may be where the problem lies.

Dave Detra

boyt911sc · boyt911sc's Garage

Dave,

The first thing I do for a non-starting CIS motor is to check the fuel pressures (control and system). If you don't have a CIS fuel gauge kit to measure the fuel pressures, determine the flow rate instead. A minimum flow rate of 2 liter per min. will deliver sufficient fuel pressure for your system. If your starter is able to turn the engine rapidly, you'll have compression. Next to check is your ignition. Assuming that your ignition timing is not too off and has no significant vacuum leak, your engine should start and run. Since your car has come out from hibernation, a supply of fresh clean fuel is critical.

Contact me at atd911@hotmail.com and will provide you pictures with detail procedures for your DIY troubleshooting. Good luck.

Tony

rusnak · 06-11-2008, 12:28 AM

I gotta subscribe to this one.

I started reading the Bentley manual to troubleshoot a buddy's 911SC. I decided to get a fuel pressure test kit for my car that will also work with 911SCs. I decided to start with fuel pressure testing, both system and control exactly as Tony laid out. From there, you can hone in on the WUR, fuel pump, check valve, etc.

I found the cold start valve after about an hour of searching for it with a mirror and a flashlight. If there are merciful Porsche gods, it ain't the cold start valve.

javadog · 06-11-2008, 03:29 AM

Guys,

The car runs with the jumper for the fuel pump wiring, so that pretty well rules out CIS issues...

Dave,

You need to get hold of a wiring diagram and work backwards from the relay. Mine are stored at my warehouse. If nobody else has one handy, I'll go fetch it later.

JR

Ron.G · 06-11-2008, 04:15 AM

I have had the same problem you are having.
I clean the socket for the relay and spread the pins on the realay with a small screw driver and that fixed the problem.

SC-targa · 06-11-2008, 06:01 AM

The car starts, but dies, right?

The fuel pump runs when you have the key in the start position (because it has a different power source), but not in the run position. If you hot wire the fuel pump the car runs.

I'm betting the problem is the air flow plate switch in the CIS metering unit. There is a switch on the plate that is isolates from ground by an insulating sleeve on one of the switch screws.

Unfortunately, you probably have to remove the metering unit to access the screw and its insulator.

You can clip a wire in the fuel pump relay to make it always hot when the key is on, but that is a temporary fix, though I believe the 73 1/2 through 74 or 75 models were wired this way.

Regards,

Jerry Kroeger

tobeto87 · 06-11-2008, 09:49 AM

This forum is a thing of beauty, I love it.
I agree with JR, because the car runs when jumped logic says it is probably not the CIS, the problem sounds like an electrical issue somehow associated with the relay. As I recall from reading the Bentley manual I think I can check for continuity of the air flow plate at the relay, does this sound correct.
When I get home tonight I will do a bit more trouble shooting.

Again, the input you all have provided is greatly appreciated.

Dave Detra

Aurel · Aurel's Garage

You can remove your air filter, turn the ignition on (no cranking necessary), and lift the air sensor plate. If the switch works, you should hear the injectors sparying gas and the pump running. If you hear nothing, you know there is a problem there.

Aurel

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by Aurel

You can remove your air filter, turn the ignition on (no cranking necessary), and lift the air sensor plate. If the switch works, you should hear the injectors sparying gas and the pump running. If you hear nothing, you know there is a problem there.

Aurel

This is the kind of test that should be done earlier to isolate the cause of your problem. After reading your post, I suggest you concentrate on the electrical side. Two (2) points of interests: (1). FP relay socket terminals, and (2). Air sensor switch grounding connections. Keep us posted.

Tony

tobeto87 · 06-11-2008, 04:40 PM

Success!!
I ran the test to check continuity at the air flow sensor plate and found that continuity was maintained when the plate was lifted. First thing I did was unplug and plug the sensor connection and it started right up and continued to run. Now I will clean that connector to ensure this doesn't happen again. I have to admit, that I first screwed around with the relay and checked connections from the ignition switch before running the last test.
Anyway, I want to thank everyone for your input, you all were very helpful in pointing me in the correct direction.

regards,
Dave Detra
Denver

SC-targa · 06-12-2008, 05:36 AM

I'm glad your car is running again.

Cleaning the connector does not seem like it would actually fix the real problem. If you had continuity (A short circuit), a dirty connector is not likely the cause. Most of the oil and dirt in the engine compartment is not conductive.

You may have wiggled the short so that it is no longer in contact, but your problem may return.

Regards,

Jerry