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91 F-150 stalls when hot


Old 04-06-2006, 07:59 PM
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91 F-150 stalls when hot

My '91 F-150 5.0l has an erradic problem of stalling when it gets hot. It seems to occur most often when I have driven at highway speeds for an hour or so. It will start by "chugging" intermittently and then seems to get worse. It will power out but if I switch off the OD I can maintain a speed of 35 mph or so but can't get back up to speed. If I stop it will run really rough and stall out and then won't start back up. ONce it has cooled down for several hours it will restart and seems fine. I have checked numerous things but don't want to start replacing without a more definitive direction.

New plugs, wires cap, rotor, alternator, fuel filter and fuel pump relay.

Any ideas?
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Old 04-06-2006, 08:22 PM
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Location: San Antonio, Texas
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Sounds like you have you have narrowed it down significantly.

Sounds like there is a problem in the trans. lockup converter maybe?

I would find a reputable trans shop (if THAT's possible) and explain what you are experiencing to them.
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Old 04-06-2006, 08:48 PM
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Location: Topeka Kansas
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Vapour lock,
Vapour lock with weak fuel pressure or
weak fuel pressure.

Not being able to start the vehicle until the fuel lines have cooled down is a classic indicator for Vapour lock.

Strong fuel pressure compresses the Vapour so that it is kept under control in most circumstances.


Last edited by temp1; 04-06-2006 at 08:52 PM.
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Old 04-07-2006, 12:02 PM
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Thanks for the replies. What would classify as weak fuel pressure? From another source, someone mentioned that the fuel pump may be going, but I have tried switching tanks and that doesn't seem to make a difference, and I can't see that both pumps would be geting weak at the same time. Although I suppose that could happen. this might explain the weak fuel pressure that you mentioned.

If it is vapour lock, how do you cure it?

Could you explain how the trans converter may be causing this? Just don't understand how it would cause the rough idle.
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Old 04-07-2006, 12:55 PM
Join Date: Sep 2005
Location: Topeka, Kansas
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If it is vapour lock, how do you cure it?


Two tanks might eliminate the fuel pressure part of the equation and leave the possiblility of vapor lock but the fuels lines would have to be very close to a exhaust manifold or hot engine part to cause vapour under good fuel pressure.

Another possibility is that some electrical part is getting hot and giving the PCM bad readings. The pcm perhaps can't start the vehicle while the sensor is out of range.
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Old 04-08-2006, 12:11 AM
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i've seen ignition modules cause symptoms like yours when they get hot. also the fuel filter is common to both tanks, so if it is getting plugged up it could be causing the problem.
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Old 04-08-2006, 03:04 AM
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Location: Arizona
Posts: 198
I doubt it is vapor lock-- EFI setups hardly ever get that, that's a carbeurated problem. If you're having issues on both tanks I'd say it's probalby not a fuel pressure issue either unless they both use the same fuel pressure regulator (which I am not sure about, I have never owned/worked on a dual tank truck before). If they use the same regulator, that might be going and causing the issue.

If that checks out OK, take a look at this thread: http://fordsix.com/forum/viewtopic.p...ghlight=shield. It looks like there is an air-cooling device which is supposed to prevent heat soak. Do you have one of these in your truck, and is it operational?

If not, you might want to get one or a heat shield off a newer truck and put that in there...see what that does. Make sure too that your computer itself has a heat shield.

Last edited by Clifford f150; 04-08-2006 at 03:13 AM.
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