Vapor lock?

chewy012 · Mar 20, 2017

Dude,

I've been battling this issue with my truck dying on my way home for a year now. I've replaced every sensor, injectors, coil module... Can't figure it out.

On my hour drive home, coming off the highway, the truck died. Sputters and stalls out. I believe it to be vapor locking based on pressing the bleed valve and vapor coming out. Also, I've watch the truck died on my scanner with live data stream, when it starts stalling, injector pulse width jumps from 1.5ms to 10. The truck does not run hot.

I've tried running different octanes, additives, nothing seems to help and I'm at my wit's end.

Anyone had this issue?

4.0 2wd '92

Thanks

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chewy012 · Mar 20, 2017

Could it be the Inertia Switch?

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RonD · Mar 20, 2017

Vapor lock could happen but very very unlikely because of the fuel pressure at 30psi

Have you tested fuel pressure to see it with engine running, 30-40psi
And then see if it holds that pressure above 25psi, for a few months, so shouldn't start slowly dropping at all.

Maybe pump is just dropping pressure after an hour or so.

Also try unscrewing the gas cap a bit, long shot but EVAP system could be sucking too hard on the gas tank so pump can't force enough fuel out after a hour of driving, like a vapor lock, lol.
Or just unscrew the cap after the stall, listen for a BIG rush of air into the tank

Vapor lock was not an uncommon problem with mechanical fuel pumps on the engine, fuel line from tank to pump had 0psi fuel pressure, so on a hot day and metal fuel line running near exhaust system the gasoline could "boil" so mechanical pump could not "pump it", it could pump fluid not air/vapor.

But as you raise the pressure of any liquid you raise the boiling point, that's why a pressure cooker cooks food faster, water boils at 212degF but if you add pressure boiling point goes up so hotter cooking temp

chewy012 · Mar 20, 2017

Yeah, I'm scratching my head on how it would vapor lock.

Is your inertia switch suppose to have a slight rattle on the inside, assuming I have the plunger secured...

I have check the rail and get 35 PSIG, and it holds it.

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RonD · Mar 21, 2017

Inertia switch is an ON/OFF setup so can't reset itself, not sure if it should rattle like a PCV valve, lol, but never shook one?

I would try the gas cap

How long does truck need to sit for a restart and OK running?

chewy012 · Mar 21, 2017

Usually 1-2 minutes is all, and then I can be on my way. But I've had it where it stalls, wait, start up, drive a mile'r two, stall...

I read one case of a guy who had seemingly similar intermittent failure that was due to warm contacts in the switch, not the trip mechanism...

As you've not shaken one before, I assume you haven't otherwise fondled one much.... I gave it a light wrist flick which tripped it. I reset it and was able to repeat the trip fairly easily. To my understanding, these things are build for "life" but really only tolerate one trip.

Not sure where I'm going with this, but I think I'm suspect of it....

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chewy012 · Mar 21, 2017

There's a good write up on here but I can't remember how I found it....

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Angie · Mar 22, 2017

inertia switch can't be the problem if it dies and the switch isn't disengaged. it is there to kill power.... no pump will run if it is tripped. so if it needs to be reset then it starts... this will answer that question.

can you hear the pump running? maybe your problem is faulty ground to the pump.... or wire problem, however figure out if the pump is working.

chewy012 · Mar 22, 2017

Oh yeah the pump works, you can hear it nice and strong.

The truck tried to die today when I was coasting to slow down in 4th. I did stall for a only few seconds, but I punch the throttle and it kicked back on after holding the pedal for a few seconds.

Typically when this stall situation hits, pressing the throttle barely raises the revs to 900 and just sputters like it's trying but never snaps out of it.

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chewy012 · Mar 22, 2017

That's usually what causes the stall, coasting in gear, and sitting at a stop light too long after coming off the highway

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chewy012 · Mar 23, 2017

So while I don't think the fuel pump is the issue... It's only like $40 so I'm thinking about just doing it.

I was thinking unbolt the bed and Jack it up? Torx 55 yeah?

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RonD · Mar 24, 2017

Yes, depending on the rear bumper setup you can loosen but leave the back 2 bolts in place and then tilt up the front of the bed.

But if you have a helper sliding the bed back is better
Remember to unplug the tail lights

Did you try that 1 hour drive with loosened gas cap?

chewy012 · Mar 24, 2017

I did, morning and afternoon. Afternoon commute is typically warm and dry, which is prime weather for the condition to occur.

Yesterday's PM commute was been unusually cool and humid, so it's tough to say if the loose cap helped.

Also, instead of the normal buzz from the fuel pump, when I had a loose cap, I'd hear bubbling/gurgling.

Does the fuel cap try to maintain a certain pressure, or or is suppose to seal/vent open/close?

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RonD · Mar 24, 2017

Gas cap seals the tank, no vent

Bubbling and gurgling with engine on or off?

EVAP solenoid may be stuck open.
EVAP sucks gas fumes from the gas tank, computer opens and closes solenoid to keep some negative pressure in the gas tank while engine is running.
If too much negative pressure builds up in the gas tank then fuel pump has a harder time pumping out fuel.
When you decelerate and come to a stop, throttle plate closed, the engine vacuum is the highest, so that could cause quick fuel pressure drop and a stall, this could also pull open return fuel valve(fuel pressure regulator) enough to drop pressure at the injectors

This is very unlikely, but not impossible

If you can't get problem to happen again with loosened gas cap then not so unlikely a problem.
And you would also be correct that it is a "vapor lock" just a negative pressure vapor lock

chewy012 · Mar 24, 2017

Nice, I cleaned out that valve awhile back thinking it was that, but the solenoid may be on it's way. I'll snag one from a yard. Thanks Ron!

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Vapor lock?

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