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78porsche924 · Posted: Fri Jun 27, 2003 8:03 am Post subject: Engine cutting off

Ok my car runs fine...for about 5-10 minutes. Then it acts as though it is only getting fuel intermittently. It missfires hard and the car starts to jerk and buck. If i take to long to get it into a neighborhood, it acts as though soemone shut the fuel pump off(never thought to listen for it untill now). It seems to be heat related, but i am not positive. I was thinking possibly vapo-lock so i insulated the oil line going to the turbo. I also put insulation around the fuel lines in the places that they pass over head sources. I also tied them as far away from the head as possible. Had the same problems. It has been really hot here lately and this is around the time that this problem started. Any ideas for me to try?

Thanks
_________________
90 944 S2
78 924 NA <---now sold and killed by new owner
snailshell trans
Bae turbo kit
to check out my 944 S2 http://www.cardomain.com/member_pages/view_page.pl?page_id=388139

Lizard · Posted: Fri Jun 27, 2003 8:30 am Post subject:

put your hand on the pump and see if it jerks, I doubt that it is a vapour lock but to test, get some ice (maybe dry) and cover a 1 foot section of the fuel line with it, this will cool the gas and eliminate vapor lock, on a 931 just before the fuel dizzy would be perfect
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3 928s,

78porsche924 · Posted: Fri Jun 27, 2003 8:35 am Post subject:

Ok i will try that. i thinking it might be those spark plug wires again. They seem to be getting hotter then normal so i dunno.
_________________
90 944 S2
78 924 NA <---now sold and killed by new owner
snailshell trans
Bae turbo kit
to check out my 944 S2 http://www.cardomain.com/member_pages/view_page.pl?page_id=388139

Lizard · Posted: Fri Jun 27, 2003 8:44 am Post subject:

it could be the plug wires but I doubt it, they normally aren't affected by heat short of getting fried on the header
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3 928s,

78porsche924 · Posted: Fri Jun 27, 2003 8:46 am Post subject:

I figured that. Would the WUR failing be a possibility? The PO got that replaced from this incompetent porsche junkyard. Looks almost brand new and it only cost him $178. Does this sound like a possible culprit?

Thanks
_________________
90 944 S2
78 924 NA <---now sold and killed by new owner
snailshell trans
Bae turbo kit
to check out my 944 S2 http://www.cardomain.com/member_pages/view_page.pl?page_id=388139

Lizard · Posted: Fri Jun 27, 2003 8:49 am Post subject:

possible but to eliminate it, unplug the plug from it
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3 928s,

78porsche924 · Posted: Fri Jun 27, 2003 9:21 am Post subject:

Oh ok, didn't know u could just pull the plug. Thanks.
_________________
90 944 S2
78 924 NA <---now sold and killed by new owner
snailshell trans
Bae turbo kit
to check out my 944 S2 http://www.cardomain.com/member_pages/view_page.pl?page_id=388139

Doodle · Posted: Fri Jun 27, 2003 9:26 am Post subject:

Sometimes ignition coils will do this when they are failing. Really high ambient temperatures = even higher under hood temps. Does the car start and run OK after an hour or so to cool down. If so you might want to look at replacing the coil.
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Doodle
Pre 77.5 Mars Red

78porsche924 · Posted: Fri Jun 27, 2003 10:19 am Post subject:

brand spanking new msd blaster 2 coil in there. I was having trouble before so i guessed might be the coil so i replaced it already. It will run ok after about 5 min of cooldown with the hood open, then the cycle repeats itself. If i turn it off then right back on it won't start. But it is so bad i can't even move in 1st gear from a dead stop. But i have noticed my temps are a lot higher then usual, but it was also i think 97 degrees here today.
_________________
90 944 S2
78 924 NA <---now sold and killed by new owner
snailshell trans
Bae turbo kit
to check out my 944 S2 http://www.cardomain.com/member_pages/view_page.pl?page_id=388139

78porsche924 · Posted: Fri Jun 27, 2003 10:59 am Post subject:

Took it out tonight when it was nice and cool. No problems whatsoever. Unless it is coincidence then it is definantly a heat related issue. Couldn't test the vapo lock or wur because of this though.
_________________
90 944 S2
78 924 NA <---now sold and killed by new owner
snailshell trans
Bae turbo kit
to check out my 944 S2 http://www.cardomain.com/member_pages/view_page.pl?page_id=388139

Sleykin · Posted: Fri Jun 27, 2003 11:16 am Post subject:

I don't know if the 78 has points or a pickup coil in the dizzy but I have had the pickup coil in other vehicles cause symptoms like what you describe. Go to an electronics supply house (or even Radio Shack) and get some "Freeze Mist" or circuit cool or other spray cooler. We use it frequently to chase down heat related electrical failures. You could also use it to cool your fuel lines for a test of vapor lock. Usually when a car suddenly develops a vapor lock it is because someone recently re-routed the fuel lines to a hotter environment. (intentionally or otherwise)

HTH

78porsche924 · Posted: Fri Jun 27, 2003 11:45 am Post subject:

The fuel lines are in the right spot but i have an oil line running with them to the turbo that isn't there stock. i insulated the oil line to keep temps there down. I also used some wire to tie the lines away from the head. I will try that cooling spray stuff.
_________________
90 944 S2
78 924 NA <---now sold and killed by new owner
snailshell trans
Bae turbo kit
to check out my 944 S2 http://www.cardomain.com/member_pages/view_page.pl?page_id=388139

5150 · Posted: Fri Jun 27, 2003 4:30 pm Post subject:

Check the leads to the coil, the earth's (especially the one between engine and alternator) and the wiring around the alternator and starter.

When my 78 was going mental it would start and idle *sometimes* then not others, sounded like the engine was about to explode when you tried to drive (juddering and mis-firing badly) and the rev counter was leaping around erratically...

I started with the usual stuff - points, condensor, coil, ballast resistor etc and finally sorted it out by replacing all the wiring to the alternator and starter, it was all badly corroded and the earth lead between alternator and block almost fell apart in my hand.

Good luck

924RACR · Posted: Fri Jun 27, 2003 10:30 pm Post subject:

Excellent points Sleykin, especially like the idea of using cooling spray!

I had a heat-related poor-running condition in my daily-driver '82 931. It ran beauttifully when cold, but as it warmed up, it ran rough, hesitated hard off boost.

I tested a lot of things. Also had the mixture a little off, didn't help. However. I worried about the coil; swapped in the Blaster 2 from the racecar, no help. Found that the fuel pump fuse was getting very hot. Had to replace the connection, the actual spade terminal, on the back of the relay mount for the fuse pump relay. The relay was also getting hot. The problem was that there was heavy corrosion of the wires where crimped into the terminal. The heavy corrosion created very high resistance, which would pull through more amps and really heat up the connection.

I think what happened was a previous fuel pump failed, before I got the car, and drew heavy current on its way out, heating the wire and burning off the insulation (it was clearly burnt/melted). This then allowed water in to corrode the wires, setting me up for the later failure.

This would reduce the power to the fuel pump, decreasing fuel pressure (my hypothesis), and putting off the engine management.

It's all better now. Except I still have to set my idle properly.

Pull out the fusebox and look at the connections on the back of the relay block, see how they are. If you have corrosion, cut it out and replace the terminals.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

NINEEAGLES · Joined: 25 Apr 2003 Posts: 219 Location: SW PA

I had the same problem during very hot weather last summer....I replaced the fuel pump relay and the problem was solved....I tried replacing the old one it ran fine again so it must be a heat related problem

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79 924 "Missy Grey"