Porsche 911SC 1978 warm run issue. Help.

I have a 78 911SC with 70000 original miles. It has never given me a days problem for years until yesterday. After driving 10 miles, I started her up to go home. She started, went into the cold-start cycle with a high idle, but when the cold-start cycle ended and she returned to normal idle she died. Would not start again. There is gas and abple battery power. Got towed home. :(

I tried to start her at home and she starts running, and immediateley dies like there is no fuel. In the morning she started perfectly but died again after cold-start idle cycle.

Any ideas? Must this not be the warm idle regulator? Warm control pressure? Cold start valve? Help?

Martin Folb

Reply to
martin
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Massive vacuum leak?

Reply to
Bernard Farquart

Fuel Pump?

Reply to
Vern

How do I diagnose a leak in vacuum? Most likely place?

Reply to
martin

I couldnt see any obvious vacuum leaks and played around with all pipes and tubes whilse she spluttered and eventually died. She starts up perfectly on cold start, runs to 2000 rpm for +-2 minutes and is responsive to revving as normal, then starts declining down to 900 rpm and then coughs, splutters and dies.

How do I test the fuel pump?

Reply to
martin

Vern,

I tested the pump for failure. It runs when I raise the air flow sensor plate. It also runs when I bypass the relay.

If you run out of gas, how much do you need to put in the tank to restart the 1978 911SC?

Reply to
martin

I think there is pressure cut off on the fuel pump. This usually means that your fuel filter is way past due for a changing. When did you last change your filter? That cut off was put in the circuit because they used to blow fuses when the pressure was too high and on my 1966 911 that fuse was under the gear shift.

Reply to
Bill

I put 5 gallons of gas in the car. Same result. Start first time, runs perfectly for 2 minutes, the second the temp rises enough, the car dies. It then starts, spins for a sec and dies.

Is there a fuel pump pressure cut off on a 1978 911SC?

Reply to
martin

Are you sure it isn't an electronics issue? It is not uncommon for chips and circuitry to have these kinds of heat related failures. When cool everything is OK. But as a circuit board, a chip, or solder joint heat up, the circuit expands and causes an open circuit. Let the offending circuit cool down, and connection is re-made and it miraculously works again until it reaches that critical temperature again. American cars with crank position sensors on the crankshaft a very prone to this. It is often caused by an engine running good and warm and an innocent run through a puddle of cold water which splashes up on the hot electronics and the quick change in temperature causes cracks and broken circuits that can fail like I described. Maybe you think it is fuel, and it is electronics instead. I would not think it is fuel if you say when cold it starts and runs fine, try leaving a plug wire loose to see if spark quits when it reaches the magic temperature. I was thinking for a bit, maybe warm air start mixing t-stat, but it would act the opposite to your problem.

Reply to
Vern

Haven't read the whole thread but there's plenty to go wrong outside the fuel distributor.

With CIS problems, or any FI for that matter, the first thing to do is change the fuel filter. These things need high pressure to work right.

Put a timing light on a plug wire and see if it is spark that is failing. If not, try disconnecting the lead to the cold-start valve. Maybe it is coming on and flooind the engine.

CIS is a pure mechanical system. (OK, there's Lmabda CIS with an O2 sensor but you don't have that.) Pretty simple at its core. It is electronics, as Vern points out, that are finicky with temp.

The CIS system has the warm-up regulator (WUR) and auxiliary air regulator (for higher idle when cold) that are affected by temp. But those have a gradual effect with temperature. It would run rough and then stall, not cut out suddenly.

Check these links for CIS info:

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OPnce you're certain it's the FI system, you'll need to check the flow rate from the pump and the operating pressures. That'll isolate the failure.

I'll be it's spark.

Vern wrote:

Reply to
someone

Thanks for the links, very helpful.

I have taken the car to a specialist who will diagnose the problem and let me know. I tried all the tests my tools would allow and couldnt find the culprit. I suspect a fuel pressure test would have been the right test but I didnt have the equipment. I suspect bad fuel pump. When I know I will post my experience and solution everywhere I can to help future 911sc drivers with this issue.

Thanks all.

Reply to
martin

It is possible that you will get a new fuel pump even though it might only be a filter.

Reply to
Bill

Turns out it was the CDU unit. Faulty after 26 years. Im replacing with a Permatune unit.

Thanks all for your help.

Martin

Reply to
martin

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