Hard starting after engine been running for a while

911mnypt · 06-21-2007, 08:58 PM

I have a 1980 911sc that is sometimes hard to start after running it for a while. The engine turns over strong but will not start. Eventually it will fire up, however it feels like it is flooded and is sluggish in idle. If I touch the throttle is dies out, if I let it go eventually the RPM will go up to where they should be. Any suggestion on what is the cause? I have good gas, new fuel filter, plugs & air filter...

Thanks...

yelcab1 · 06-21-2007, 09:04 PM

You need a fuel pressure gauge to measure hot control pressure. It may be off. Let's start with that first.

911mnypt · 06-21-2007, 09:19 PM

Thanks Yelcab1, I hate to sound naive, is this something that I need to take to my local Porsche mechanic?

cmonref · 06-22-2007, 03:07 AM

Sounds like a typical hot-start problem, which are caused by a faulty fuel pump check valve or a failed fuel pressure accumulator. The check valve and the accumulator conspire to trap CIS fuel pressure after engine shutdown. This residual pressure is necessary for a hot-start. Since the pressure bleeds down slowly anyway, eventually there is zero residual, but the engine is cold -- and starts nicely without residual pressure.

Shortly after engine shutdown (5-10 minutes), loosen the connection at the bottom of the accumulator. If it leaks, the accumulator has failed and you should replace it. (There is a current thread about repairing this accumulator, but it does not look promising.) If you get the connection all the way off and it is still dry, replace the fuel pump check valve.

More info thru SEARCH on "hot start", "accumulator", or "check valve."

911mnypt · 06-22-2007, 06:31 AM

Thank you very much for the detail break down Brian...I will give it a go and let you know what happens.

Best regards,

911mnypt · 06-28-2007, 05:31 PM

I loosened the bottom connection to the accumulator and fuel just spilled out! Found the part for $239.00 and replaced it. Car fires right up now after driving it for a while...problem fixed. After installing it and purging the air out of the line, the car blew white smoke for a while and then quit. I wonder if this is common?

cmonref · 06-29-2007, 02:37 AM

Hard to tell what your question is here. Did it "blow white smoke for a while and then quit" on your first start up after replacing the accumulator? How long was a "while?" Not "normal," but is depends on what happened next. Did it then "fire right up and now after driving it for a while ... problem fixed?" If so, I don't have an answer except to say it sounds like an anomaly to not worry about.

That said, if, while your accumulator was leaking fuel, you set your CIS control pressures and mixture, you may have to redo those adjustments again. With the accumulator leaking, it may have reduced the control pressure somewhat which would have richened the mixture. "Plugging the leak" will restore the control pressure to its higher value and you will have to reset the mixture. All this, of course, depends on the rate of leak and the amount of control pressure reduction -- which may have been negligible.

laughac · laughac's Garage

accumulator

911mnypt · 06-30-2007, 12:50 PM

Yes, it was the accumulator that caused the problem. Once replaced, the problem of not starting when hot has stopped. As I mentioned before, the car did smoke (white) for a few minutes when I started it after replacing the accumulator. That only lasted a short while and has not done so since.

Any time you break the fuel system open; do you always have to bleed the air out?

laughac · laughac's Garage

I've never had to bleed the air out of my system. In addition to replacing fuel filter and accumulator, I also ran out of gas once.

911mnypt · 07-02-2007, 04:20 PM

Well, my saga continues. Just today the car would not start after running it for a while…same thing as before it would just turn over. I did replace the accumulator which fixed the problem at first. Since I did not change the fuel check valve, is it possible that it could have failed due to the higher pressure from the “new accumulator”? Or could this have been a dual issue at hand?

willf · 07-02-2007, 05:19 PM

replace the oxygen sensor relay under the passenger seat... enjoy.

911mnypt · 07-03-2007, 12:34 PM

Willf, all i have under the passenger seat are crumbs...no oxygen sensor at all. Would it be somewhere else?

willf · 07-03-2007, 01:05 PM

the oxygen sensor itself would be connected to the exhaust. 80sc with stock CIS, I would expect there to be 2 items under passenger seat. the lambda ecu (spelling) and a small relay that is refered to as a oxygen sensor relay. did you look under the carpet? it should be clipped to the floor board. i will look for a pic and reply back to the thread. take a look again and let us know.

equality72521 · 07-03-2007, 01:49 PM

Ok, sounds like you're on the same path I'm on. Next time it doesn't start after you drive it try short cranking it about 20 times. Short cranking is where you crank it for about 1-2 seconds. Do that about 20 times and see if it starts. I replaced my accumulator and that fixed my hot start problem that ALWAYS occured. However, when the engine was hot, like after 2 hours of driving on a 90 degree day it wouldn't restart. I happened to be at the Porsche dealership when this happened and they worked with me, for free, on it and they tried the short cranking method and it worked. The problem is the intake valves are coked with carbon deposits and when the engine is hot enough the coke acts as a sponge for the fuel spraying in. I ran 2 cans of BG-40 and the problem is gone. Hopefully this isn't your problem but it's worth a shot to see.

911mnypt · 07-03-2007, 02:59 PM

When I purchased the car I tore out the whole interior, carpet and all and never found any hardware under the seat…just loose change. I believe the Oxy sensor hooks up to the catalytic converter? Nothing is plugged into it though.

Mark, I will give it a go and see what happens with the short cranking. The car has 93K on it so could imagine the valves could have major deposits on it. Did you replace your fuel check valve also?

willf · 07-03-2007, 03:05 PM

the oxygen sensor does connect before the converter; if its's not there, that woudl explain the missing items under the seat...

Scott R · Scott R's Garage

If your 80' was still original you would be having some serious issues with the Lambda control brain being missing. For one you have a frequency valve in the 80' that's driven by that unit. When that stops working you have major drive ability issues.

Now to get rid of these components takes backdating certain pieces of the CIS system. Mainly the fuel distributor and most of the associated plumbing. There is another 80' owner on this board that found the same thing, turns out he had a 78-79 CIS bolted into his 80' at some point.

911mnypt · 07-03-2007, 03:16 PM

Ok, I wonder what impact on the performace of the care this would have?

Thought it would be nice to visualize what we are talking about:

911mnypt · 07-03-2007, 03:19 PM

Scott, how do I determine if I have a different CIS on my 80?