ianbsears

Curious if anyone can help me understand the time delay on engine shut off. My understanding is that the engine should run on for 4-5 seconds after the key is turned off as the spark continues while the fuel cuts off in order to clear the system of any residual fuel.

When I turn off the key my car sounds like it sputters off rather than running normally for 4-5 seconds and then stopping. Is this what should happen (the sputtering off) or is it a bit more graceful? What controls this?

Also would this have any impact to hot starts? The car fires up perfectly when cold and not perfectly when hot.

Thanks everyone!

Ian
92 964 turbo - polar silver

Black_Hat

When hot, are there any fans or other electric motors running ( rear fan, front oil cooler, AC ect) when you shut off that are not running when you do a cold shut down ?

ianbsears

I think so but not on every shut down but on occasion after a decent drive and if it is hot outside. I can't tell you which one is running as I am not yet familiar with every noise.

heliolps2

Your fuel system pressure might just a tad high. Try changing your fuel filter first. No, it's not normal.

Black_Hat

an electric motor will keep spinning when the ignition is switched off- at this point it can act like a alternator back feeding into the system. An old motor or aftermarket or something related that is grounding when its
not supposed to.
I've run into this condition many times especially with aftermarket fans (oil cooler, AC ect) in this case a diode can stop the flow of electrons from flowing in the wrong direction.

Metal Guru

Absolutely not.
You turn the key to off and everything goes off. Having the engine run on would be a safety issue.
As for residual fuel, the fuel pressure is 40 psi when the engine is off. This prevents fuel from vaporizing in the lines during a fuel soak event (called vapor lock) and enables a quick engine start at key on. Every car with fuel injection has residual fuel pressure for this very reason. A long crank time is indicative of a fuel accumulator failure.
It would be possible for dieseling to occur if you have one or more leaky injectors bit it would be running on roughly for a second or two, really feeling like it's running on two cylinders.
My car shudders a bit when I key off but but it's nearly instantaneous.

ianbsears

@Metal Guru @heliolps2 @Black_Hat
Ok, by way of background I just replaced the fuel pump and accumulator which seems to have made this less noticeable. I am attaching 2 videos. In the second video you can hear some of the residual run on and the first video the car shuts off as metal guru suggests it should. I really can't imagine that the car is supposed to turn off as in the second video. The first video seems good however I have read conflicting things regarding how the engine is supposed to shut down. I read here https://www.adrianstreather.com/docs...20overview.pdf on page 7 under Ignition Delay that the engine should run for 5 seconds after the key is turned off. Additionally my tech at Porsche Marin who is also the shop foreman (and exceptional mechanic) said the same thing regarding the engine running for 4-5 seconds after you turn off the key.

Multiple opinions but with that said have you guys heard or seen what is going on in the second video?

heliolps2

I'll try to explain my experience with your issue. On my 1992 964 turbo, I wanted to raise the system pressure by adding a shim to the fuel pressure valve on the fuel head. By doing that I had the same issue with run-on that I didn't have before. At times it was worse than others. When I finally removed the shim and put the system pressure back to stock and the run-on delay went away. Not saying that its what's going with your car, But it could be related to fuel system pressure, That's why I suggested replacing the fuel filter in the prior post. I am curious about the fuel pump you used as a replacement, was it a Porsche OEM part? as was the Fuel accumulator? Can you give us the part numbers to cross-reference for you? I'm guessing if the fuel pressure is just a little off in may cause your issue. I also read Adrians notes that said it was normal. I don't think it is. My car doesn't do that and I'm most people on this forum would agree I'm sure. I hope this helps

Helio

Metal Guru

Helio, I think you found out that if you raise system pressure over nominal, it shows the weakness of worn-out injectors. The pintle springs get weak over time as you would expect. They allow leakage into the cylinders plus the gradual contamination they accumulate prevent them from properly atomizing fuel properly.

There's a small test that one can do to verify this: take a can of carb cleaner and heat it in hot water. Then, using the supplied straw, force the carb cleaner through a used injector and a new one. It's easy to get the liquid through the used one and impossible to force it through the new one. These injectors are wear items, no doubt about it.

Another knock-on effect is a loss of sensitivity in fuel metering. System pressure is applied to the top of the metering piston in the fuel head to damp it's movement. Increasing system pressure makes it hard for the fuel metering flap mechanism to overcome the counterbalance force. The movement becomes much less linear to the point that you just run full rich under load. The engine will bog down and blow black smoke. I have experimented with system pressure in the past and these are my findings.

Ian, DO NOT TAKE A WORD THAT STREATHER WRITES AS BEING EVEN REMOTELY ACCURATE. I can't emphasize this enough. The guy clearly guessed at 50% of what's in his book. It is rife with errors, especially about Turbos. He got chased off of Rennlist because of this.

The two technical references I recommend are Bentley's "Without Guesswork" manual for 964's and the Charles Posbt book on CIS.

Did you measure the fuel volume before you replaced the pump? Did you measure residual system pressure before you decided to replace the accumulator? If either one of these components were going bad, the result would be no power (pump) and/or hard starting (accumulator).

These cars represent a great way to hone your troubleshooting skills because if you don't, they will bankrupt you

Buy new injectors and crush washers. I'm certain that will solve the issue.

ianbsears

thanks @heliolps2 . Both the fuel pump and accumulator are Porsche OEM, 911-110-197-02, 911-608-102-02 and a Bosch restraining valve 893-906-093. Also replaced the fuel filter 928-110-147-11.

bweSteve

Unfortunately that seems wrong. Mine doesn't do that, ... several of my friends (here on RL too) whose cars I have heard fire up & shut down both cold & hot, have never done that. The consult advice you'll get here with Paul, Helio & of course Cobalt, is far better

If that dealership mechanic is a Porsche Classic trained technician, and he is advising on a 4-5 second run-on,.... well then....

=Steve

Black_Hat

Hope this clears things up. Porsche official doc's should be relatively free of errors.

Stefan

bweSteve

never, ever heard of that

Metal Guru

News to me but I still stand by what I said about worn, leaky injectors. I imagine that feature is a safety feature to protect the cylinders from being washed by fuel from a leaky injectors.

ianbsears

Hum........well, something is still a bit off on my car. It either shuts down cleanly or sputters off after 1-2 seconds. It was worse before the new fuel pump and accumulator; additionally before these new parts it also suffered from a laborious hot start (5 seconds of cranking and then sputtered to life) which now seems to be largely gone. While it doesnt start up as quickly as my 997 (probably never will) it certainly seems to be getting much better. So for now I am going to take the Porsche shop manual and my technician at Porsche Marin who is an air cooled specialist as the gospel. Either way something isn't working as it should but whats more interesting is that there isn't a consistent view of what is right vs wrong across our community.

Thanks @Black_Hat for providing the smoking gun (shop manual). Does your car runs for 5 seconds after key off? And if so how would you describe the noise? Sputter or a clean run and then stop?

@Metal Guru - yes we (ok, my technician) measured fuel volume and pressure. I think the volume was fine and I can't remember what he said about pressure. I think you're right on learning to diagnose - probably need to up my game or this puppy is going to be $$$$. Been focusing most of my wrenching on the small and easy stuff that requires less diagnostic skills (I'm only a good surgeon @ this point). Also, I think you are probably right about the injectors.

Thanks everyone!