possible vapor lock?

I’ve searched and read quite a few similar posts but nothing matched my problem.

First, I have a ’91 and have replaced both the fuel pump and filter…..Feb. ‘05
It runs fine at idle and at speed……no problems.
Later in the day (long wheel’n day) the fuel pump starts whining…….. the old pump would whine after a long hot day as well……I’ve gotten use to hearing it and it never seemed to get worse.
Just a few weeks ago it started whining like it always does but then while idling, it stumbled and died like it ran out of gas. It would cough and sputter while trying to restart, but it would eventually start. It would only remain running if I kept my foot in it otherwise it would just cough and die. If I sat there and for the lack of better description, “let it cool down” it would restart fine and run down the trail fine, but would eventually start to cough and sputter. Next day, fine…runs like a top.

I’m thinking it could be “vapor lock”……it only seams to occur during a long hot day of slow rock crawling, but later after letting the rig cool down it runs just fine.
I’m confident that the fuel pump is fine.
The fuel filter is only a few months old but if it were bad the symptoms would be more active.
From what I’ve read, a bad fuel pump relay would simply keep the fuel pump running after shutting off the engine…..that’s not the issue.
A bad fuel pressure regulator would / should be like the filter…. the symptoms would be more active.
With the fuel lines running near and along the exhaust header and very little air movement under the hood, to me it seems like the perfect condition.

Does this sound like a viable theory?
Could there be another problem I’ve overlooked?

Thanks for the help!

If it's fuel related I say your pressure regulator is going bad, vapor lock don't happen with FI with the pump in the tank. Fuel is pumped from the tank, into the injector rail and back to the tank, keeping it cool. Try opening your gas cap when it starts to act up. Could be a plugged gas tank vent.

ok, i can buy that........the fuel is under pressure and may not be susceptible to heat. however, wouldn't the pressure regulator, if bad......present more problems than what i'm experiencing?
i also like the idea of the gas cap. i've relocated the type and location of the filler neck...(TJ type)....possibly pinching the vent tube........i'll look into it.

thanks for the input

Warthog said:

ok, i can buy that........the fuel is under pressure and may not be susceptible to heat. however, wouldn't the pressure regulator, if bad......present more problems than what i'm experiencing?
i also like the idea of the gas cap. i've relocated the type and location of the filler neck...(TJ type)....possibly pinching the vent tube........i'll look into it.

thanks for the input

Click to expand...

Ah ha, the gas tank filler neck should have a vent that runs back up front and into the charcoal canister so it can get air.

get a pressure gage and check it when its acting up.
Also have you ever noticed if the amount of gas in the tank has an effect?
Try keeping it over 1/2 tank some time (shouldn't need to but if the pump is over heating it may help)

there are two vent lines off of the tank itself and yes they are connected to the charcoal canister via a single line.........i'll inspect that.

the tank was full........its always full when the pump starts to whine.

but what is causing the pump to whine? it never whines any other time...only on a very long hot day of wheeling
thats kinda why i was thinking vapor lock.......bubbles in the line....maybe causing the pump to cavitate.....possibly dropping or losing pressure.
if the tank isn't venting or equalizing the pressure...this could cause the pump to work harder........basically trying to suck the tank walls in on itself.

A plugged vent would be worse with a full tank, because there's less air in the tank.

when you changed the pump, was the tank clean? because a new filter in a dirty tank is useless and will produce these symptoms. when you shut it off the filter lets go of the larger sediment, and it will run again till it clogs back up. also is the top gasket on the pump assembly sealing? if you were to top off the tank can you see fuel pouring down the front of the tank? that is a possible source of sediment.

and yes more fuel in tank makes a bad vent a worse problem.

the tank was not drained and cleaned when the pump was replaced......the tank was dropped to aid in the pump replacement and could have caused any sediments to move and sucked up. the filter is cheap enough and worth changing just to be sure.
when time permits, i may drop the tank and do a thorough cleaning and inspection of the pump screen.
i'm liking the vent issue and will pursue that.........as a side note and for my own edification.....i will also rap and insulate the fuel lines near the header.

thanks again for your input

ok........thought i would follow up.....in case anyone would care.
wheeled the wee out of the rig this weekend......and as always......pump begins to whine....soon after the engine begins to stumble.

first, i opened the gas cap to help vent......very small change in pump whine but still not effective.

second, i popped the hood and pulled the fuel pump regulator.......can you say hot!!......almost too hot to hold. so i dropped it into the beer cooler, had beer while the regulator cooled then popped it back in.

worked like a charm......so i'll pick up a couple new ones......keep one as a cool spare.

thanks again for everyones help and input