Vapor lock symptoms
Thread Starter
Instructor
Joined: Aug 2008
Posts: 110
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From: Le Roy NY
Vapor lock symptoms
I ran my 1980 on the hottest day of the year up here in upstate NY. While at cruise speed, the car began running rough and stalled. No fire upon cranking. After several minutes it would re-fire, but with with carb back fire and flooding. Eventually it would clear up and run OK... for awhile. I've seen this happen now several times. I checked spark from the HEI and it was OK. The car never did this before and, as you can tell from my numerous posts, I'm chasing down several possible leads. Is vapor lock a reasonable cause? It has done it again in slightly cooler weather (mid-80's).
Pro
Joined: May 2010
Posts: 737
Likes: 3
From: Pennington NJ
If it starts and runs cold fine but when it gets hot it stalls and dies then refuses to start until cool, it is most likely your hei module. The exact thing i desribed happened to my friend and we replaced it with a napa part and it starts and runs fine now. Although you said you have spark that is what his and others ive worked with did it has spark but doesnt actually function correctly.
Will
Will
Le Mans Master
Joined: Jul 1999
Posts: 9,206
Likes: 17
From: Sewell NJ
I doubt that your experiencing vapor lock since later C3s had a recirculating fuel system. I would look more towards fuel boiling in the carb. You didn't mention what your engine temp was but I'll bet it was up there if it was as hot as you say. An easy low cost fix for this would be to put an insulating spacer between the carb and manifold to see if it cures the problem. My 74 specifies the thick insulator as factory equipment so GM must have been aware of an overheated carb problem. Good luck in tracking this down.
Thread Starter
Instructor
Joined: Aug 2008
Posts: 110
Likes: 0
From: Le Roy NY
Great feedback. My vette does have a fuel return line, but an overheated carb is plausible. The problem first occurred on the hottest day of the year and has done it twice since under milder temperatures. The hot day might have been a coincidence. The carb/manifold gasket is a thick one too.
The HEI module failure is, perhaps a stronger possibility. I was running a Mallory 6AL when the problems occurred. I put the GM module back in and haven't seen the issue again, albeit only being run for a short time under more normal temps. Engine temperature has never been out of the normal range.
The Mallory folks say intermittent failures are not common. They either die or they don't. I'm not real sure about that. I've also changed the spark coil to an old one when I re-installed the GM module. Earlier this year I pulled the dizzy to clean it up and might have messed up the pick-up coil. You always have to go back to the last thing you did when troubleshooting, so it's on the list.
So I think I've narrowed it down to 1) the 6AL ignition box, 2) pick-up coil, and 3) spark coil. Now it's just a matter of swapping parts to see what happens. I might pull the dizzy again to have a good look at the pick-up col wires.
The HEI module failure is, perhaps a stronger possibility. I was running a Mallory 6AL when the problems occurred. I put the GM module back in and haven't seen the issue again, albeit only being run for a short time under more normal temps. Engine temperature has never been out of the normal range.
The Mallory folks say intermittent failures are not common. They either die or they don't. I'm not real sure about that. I've also changed the spark coil to an old one when I re-installed the GM module. Earlier this year I pulled the dizzy to clean it up and might have messed up the pick-up coil. You always have to go back to the last thing you did when troubleshooting, so it's on the list.
So I think I've narrowed it down to 1) the 6AL ignition box, 2) pick-up coil, and 3) spark coil. Now it's just a matter of swapping parts to see what happens. I might pull the dizzy again to have a good look at the pick-up col wires.
Pro
Joined: May 2010
Posts: 737
Likes: 3
From: Pennington NJ
Oh yeah i completely forgot, when we rebuilt the distributor, we also replaced of course the cap, rotor, module (which was the problem) and the coil (could have also aided the problem) on his HEI. I would try replacing the coil, cap, rotor and module, specifically the coil and module, that should fix your problem. We got all napa parts to fix it and they work great.
Good luck!
Will
Good luck!
Will
