Vapor lock? Need help!
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Vapor lock? Need help!
Hello,
I have a 85 F150 4.9 auto with carb. and I can't figure out this problem. The truck starts fine when cold but, after running awhile if you shut it off it won't start until it sits awhile. Has anyone else had this problem? For the life of me I can't figure this one out. Please help. any suggestions? Of course I thank you for your help.
I have a 85 F150 4.9 auto with carb. and I can't figure out this problem. The truck starts fine when cold but, after running awhile if you shut it off it won't start until it sits awhile. Has anyone else had this problem? For the life of me I can't figure this one out. Please help. any suggestions? Of course I thank you for your help.
Posting Guru
Joined: May 2004
Posts: 2,223
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From: Billings Montana
I myself have never had this problem, but I've seen quite a few threads about this problem and more than one person has suggested that it was the ignition module. When the module gets hot, it starts to soak up the voltage going to the coil and it causes resistance. Now, it could very well be vapor lock, but I would get a voltage tester, start the truck and check the voltage output of the ignition module when it gets warm.
If the module checks out ok, look to see if any fuel lines pass by exhaust manifolds, headers, etc. If it is vapor lock, you can get sleeving material that will reduce the temperature of the fuel line.
If the module checks out ok, look to see if any fuel lines pass by exhaust manifolds, headers, etc. If it is vapor lock, you can get sleeving material that will reduce the temperature of the fuel line.
Last edited by big hoss 29; May 9, 2005 at 11:59 PM.
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Originally Posted by big hoss 29
I myself have never had this problem, but I've seen quite a few threads about this problem and more than one person has suggested that it was the ignition module. When the module gets hot, it starts to soak up the voltage going to the coil and it causes resistance. Now, it could very well be vapor lock, but I would get a voltage tester, start the truck and check the voltage output of the ignition module when it gets warm.
If the module checks out ok, look to see if any fuel lines pass by exhaust manifolds, headers, etc. If it is vapor lock, you can get sleeving material that will reduce the temperature of the fuel line.
If the module checks out ok, look to see if any fuel lines pass by exhaust manifolds, headers, etc. If it is vapor lock, you can get sleeving material that will reduce the temperature of the fuel line.
Thanks for the reply, I'll check this out and reply!!!
Senior User
Joined: Jan 2005
Posts: 121
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From: North Central Montana
Could it be flooding? If it is, lower the float level, fuel may be vaporising and then dripping into the intake manifold. Also, a spacer under the carb may help to lift the carb farther away from the heat of the manifold.
Elder User
Joined: Oct 2004
Posts: 984
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From: Winnipeg, Manitoba
The easiest way to check vapour lock. When it's hot and won't start, try cracking your fuel cap opened. If it hisses, and the truck will start right after, you're suffering from vapour lock. This can mean a few things. Your fuel tank vent is plugged, fuel filter is plugged, or your fuel pump(either in tank or on the frame rail) is on it's way out. If you replace the fuel filter, have the fuel system pressure tested(or do it yourself if you have the tools and the know-how) and replace as neccessary.
I will agree that ignition can also be the problem, as well as poorly routed fuel lines with no insulation between them and intake/exhaust manifolds.
Recommendations...try the cheap stuff first. Insulate all fuel lines that come near a source of heat, change your fuel filter, test your ignition. If all pass, then look at the pumps, they're not cheap.
I will agree that ignition can also be the problem, as well as poorly routed fuel lines with no insulation between them and intake/exhaust manifolds.
Recommendations...try the cheap stuff first. Insulate all fuel lines that come near a source of heat, change your fuel filter, test your ignition. If all pass, then look at the pumps, they're not cheap.
New User
Joined: Mar 2005
Posts: 12
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From: Thornton, CO
Originally Posted by the breeze
Hello,
I have a 85 F150 4.9 auto with carb. and I can't figure out this problem. The truck starts fine when cold but, after running awhile if you shut it off it won't start until it sits awhile. Has anyone else had this problem? For the life of me I can't figure this one out. Please help. any suggestions? Of course I thank you for your help.
I have a 85 F150 4.9 auto with carb. and I can't figure out this problem. The truck starts fine when cold but, after running awhile if you shut it off it won't start until it sits awhile. Has anyone else had this problem? For the life of me I can't figure this one out. Please help. any suggestions? Of course I thank you for your help.
Senior User
Joined: Nov 2004
Posts: 163
Likes: 2
i have a 86 f150 witha 4.9. i did a bed swap and wile doing so i had to swap filler necks. my filler neck is from a fuel injected model and thats what caused my vapor lock
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Senior User
Joined: Apr 2005
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From: Oregon
when my module went out it killed the truck compleatly and wouldn't start again at all till i got another one. its justa pull out and put in procedure but at 40 bucks i would make sure it wasn't that. if it is though you might have a hard time getting the module out. you might need to grind down socket to get it to fit.
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To check vapor lock - on my 84 300-6. I had a manual fuel pump (mounted on low on teh driver's side). I carried a gallon jug of water (didn't have to be cold). When the truck died and wouldn't start, i popped the hood, and slowly poured my gallon all on teh manual pump. This instantly cooled the pump and recopndensed the fuel - allowing me to restart the truck, with out waiting.
Now I say had a manual pump, beause I replaced it with a new electric pump. Mount it out of the way and route new lines to stay clear of the engine heat.
Now I say had a manual pump, beause I replaced it with a new electric pump. Mount it out of the way and route new lines to stay clear of the engine heat.
Moderator
Joined: Jan 2001
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From: Virginia
You don't have to guess at this type of problem.
When it won't start, take the aircleaner off, and while looking down the carb throat, push the throttle back with your hand. You should see two strong streams of gas pour into the engine. No fuel, or puny dribbles mean you do have a fuel problem.
If you have fuel, then take a sparkplug wire off, and stick a screwdriver up into the end of the wire. Hold the screwdriver near the engine block, and crank the engine. You should see a spark jump from the screwdriver to the engine.
If you have spark and fuel, I would be suspicous that it is flooded. A clue to this would be black smoke pouring out when it finally starts.
When it won't start, take the aircleaner off, and while looking down the carb throat, push the throttle back with your hand. You should see two strong streams of gas pour into the engine. No fuel, or puny dribbles mean you do have a fuel problem.
If you have fuel, then take a sparkplug wire off, and stick a screwdriver up into the end of the wire. Hold the screwdriver near the engine block, and crank the engine. You should see a spark jump from the screwdriver to the engine.
If you have spark and fuel, I would be suspicous that it is flooded. A clue to this would be black smoke pouring out when it finally starts.
Fleet Owner
Joined: Sep 2005
Posts: 28,609
Likes: 18
From: Lake Tahoe, Nevada
Franklin2, right on and straight to the problem.
Mine did have the same problem. After a couple thousand miles of Mexico's washboard secondary roads my carb was falling apart. I had it rebuilt and then developed the vapor lock problem. Turns out I only had a paper gasket between the carb and manifold. Bought an insulator block for it and that ended the trouble.
Also there are several ways for the ignition module to fail. In my case, the timing just didn't advance. New module and all was well again.
Mine did have the same problem. After a couple thousand miles of Mexico's washboard secondary roads my carb was falling apart. I had it rebuilt and then developed the vapor lock problem. Turns out I only had a paper gasket between the carb and manifold. Bought an insulator block for it and that ended the trouble.
Also there are several ways for the ignition module to fail. In my case, the timing just didn't advance. New module and all was well again.
Post Fiend
Joined: Mar 2004
Posts: 24,418
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From: San Francisco, Ca.
I must have a weird truck (81 F150 4.9)
Never had any problems with fuel or any kind of that stuff.
Still have the original fuel pump, starter, Ignition module, computer module, distributor module, coil, even the locking gas cap that I bought when I bought my truck new.
But I'm glad to read these posts just incase I do have a problem, or some people I know that this info. would help.
Never had any problems with fuel or any kind of that stuff.
Still have the original fuel pump, starter, Ignition module, computer module, distributor module, coil, even the locking gas cap that I bought when I bought my truck new.
But I'm glad to read these posts just incase I do have a problem, or some people I know that this info. would help.
