How do I test to see if my truck is vapor locking? And how would I fix the issue.

Tiny
CARFREAK105576
  • MEMBER
  • 1990 FORD F-150
  • 65,000 MILES
When its cool or cold weather my truck will run fine. When its hot out (90-110) it will start to spit and sputter while losing power until it wont idle without feathering the gas pedal. Then after it sits and fully cools off it will start and run fine again for a while. The first time it did this I shut the truck off then it wouldnt start. It was not getting fire. So I replaced the ignition control module on the side of the distributor and the coil and it ran fine for about 2 weeks. Then started to do it again but this time the 2nd in line catalytic converter was smoking and glowing red. So I replaced both cats and muffler. It ran fine for a couple days then started to do it again. And its always the same symptoms. I tested my fuel pressure while truck was cool and it showed 40psi. My truck is supposed to have 50-60psi but runs fine on 40psi except when pulling a trailer or such up hill. Could it be a vapor lock issue? And how would I test to find out? If it is how would I fix a vapor lock issue?
Thursday, July 12th, 2012 AT 1:29 AM

12 Replies

Tiny
RASMATAZ
  • MECHANIC
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Don't sound like a vapor lock to me-burning up Cat's to me its running too rich-injectors are sticking open and causing your low fuel pressure-try flushing the fuel system and see what happens
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Thursday, July 12th, 2012 AT 2:30 AM
Tiny
CARFREAK105576
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How would I go about flushing the fuel system? What would I use?
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Thursday, July 12th, 2012 AT 4:20 AM
Tiny
RASMATAZ
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Take it to a shop or use a fuel injection cleaner and run it thru and see what happens
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Thursday, July 12th, 2012 AT 4:57 AM
Tiny
CARFREAK105576
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I've used cleaners fuel stabilizers and octane boosters all made by Lucas.
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Thursday, July 12th, 2012 AT 5:00 AM
Tiny
RASMATAZ
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What liter engine is this?
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Thursday, July 12th, 2012 AT 5:15 AM
Tiny
CARFREAK105576
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4.9L 300 straight 6.
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Thursday, July 12th, 2012 AT 5:11 PM
Tiny
RASMATAZ
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Test the ignition coil's primary and secondary windings resistances could be it heating up and breaking open-let me know the readings
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-1
Thursday, July 12th, 2012 AT 7:49 PM
Tiny
WRENCHTECH
  • MECHANIC
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Let me point out a couple things. First, vapor lock is not even possible with this system.

Second, if you really only have 40 PSI of fuel pressure, that has to be resolved before going any further.
That should never go below 50 with the engine off and 45 with the engine running. That will cause a world of problems. The cats were glowing because of how the engine was running, not because there was anything wrong with the Cats, although getting them that hot can melt the insides.
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Thursday, July 12th, 2012 AT 8:21 PM
Tiny
CARFREAK105576
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How do I test the coil windings? Ive actually never done that. And its a new coil. Only about 3-4 weeks old. The old one smelt like burnt wiring.
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Friday, July 13th, 2012 AT 5:14 PM
Tiny
RASMATAZ
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Go to this link: http://www.youtube.com/watch?v=rJYJ3KvPhhY
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Saturday, July 14th, 2012 AT 3:15 PM
Tiny
MILTON PLYMALE
  • MEMBER
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Hmmm-. Not to recommend it, but for old catalytic converter issues, perhaps run a 1/2 gallon of paint thinner per 1/2 tank of gas. Start with a 1/2 tank, run it nearly dry, I hear. Seems it clears the system-
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Saturday, July 22nd, 2017 AT 9:25 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
Great addition to this thread! Please feel free to help out whenever you are on the site :)

Cheers, Ken
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Monday, July 24th, 2017 AT 11:45 AM

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