Vapor lock?

[curtis73]

About three times in the last year, my 94 B4000 has a hiccup. It cranks and starts and runs rough and barely idles at about 500 rpm. I can usually get through it with modest throttle or just waiting a bit. It will run rough for 30 seconds or so, then kicks in with a misfire on one cylinder for a few seconds, then runs great. Fuel pressure tests great, right in the middle of the spec.

The other day I pulled into a restaurant for dinner. Came out, started, sputtered, died. Crank but no run. Occasional fire on a couple cylinders but not enough to keep it running. I started poking around under the hood. Pulled a plug wire and got good spark, but when I pushed the schraeder valve on the fuel rail I got vapor/air for the first couple seconds. I cycled the key a few times, got more vapor/fuel. Once I got mostly fuel, I tried to start it but still no-go. Had it towed to my buddy's shop. Three days and she couldn't duplicate the problem. She used it for parts runs to get it up to temp, tried it cold, tested pressure. No issues.

I took it back and used it yesterday and it acted up one more time (sputtering idle for a bit) OBD1, so no CEL.

Ideas?

 

[RonD]

If you had pressure at the schrader valve, even air pressure, then it can't be Vapor Lock

Does the problem only happen when fuel gauge is 1/4 tank or less, you could have a broken/cracked lift line inside the tank, so when fuel level gets low and pump is off, fuel can drain out, into the tank, and then air replaces it, so on next start up you have air in the line

But just out of curiosity, is it extremely hot outside when this happens?
Because.........if you do lose all fuel pressure when key is off, you shouldn't, then at 0psi in the fuel line near exhaust system, fuel could vaporize in the line, would be a very very rare occurrence with fuel injection setup with nylon fuel lines

Vapor Lock on older carb systems was because there was no pump in the tank, so 0psi pressure in the fuel line from tank to mechanical pump on the engine, so after shut down on a hot day the extra heat from exhaust system could heat soak the METAL fuel line causing the problem
And the problem was the mechanical pump can't pump air/vapor, so when the air hit the pump, the pump stopped working and carb ran out of gas

With fuel injection there should always be 15-25psi pressure in the line, but even if not, when pump, in the tank, comes on, it would push cooler gasoline thru the hot lines so no more vapor lock, so engine would take a few cranks to clear the air but should start up with no issues

 

[curtis73]

I didn't notice the other times, but the one that caused the tow a few days ago the tank was 3/4 or more.

The one the other day, it was pretty hot outside... mid 80s, but it has also happened at 6am and 50 degrees when I first start cold. The number of times I cycled the key (20 or more) should have purged any vapor/air, but it didn't seem to. It was as if somehow vapor/air was introduced into the fuel rail and after cycling the key I kept getting less air/vapor before I got liquid gas. After all those key cycles I finally got almost all liquid, but it still wouldn't start. My initial thought was a dead spot in the fuel pump and it just intermittently works, but I was getting pressure in the rail (even though it was mostly air)

Next time it happens I want to get an analog meter on the CKPS and see if it wobbles the needle. I also thought about a stuck/gunky IAC, but that would be affected by throttle input. The throttle doesn't seem to make any difference other than once I get past about 1/4 throttle it just dies... which leads me to believe its a fuel issue

 

[curtis73]

Here were her observations on fuel pressure (had to dig out the printout she gave me).

KOEO x1 = 35psi
KOEO x2 = 38psi
KOEO x3 (and subsequent times) 38psi.

Maintained ~38 psi for 30 min.

Running idle = steady 32psi
Running fast idle/throttle sweeps = 38 - 42psi

I don't think she checked pressure the next morning before key-on, but it started right away.

So... assuming the pump is working the same all the time (doesn't have a dead spot or intermittent short/low voltage) there shouldn't be a fuel pump issue. It is definitely not a source voltage issue, but I haven't crawled under to check voltage near the tank or tank ground. It's my daily driver, so yanking the bed isn't an option right now.

 

[rusty ol ranger]

Could be the pickup line in tank/sock filter getting plugged, or loose, or broken.

 

[RonD]

Maintained ~38 psi for 30 min.

If this is with engine off then no leak in the tank

Check the vacuum hose on the fuel pressure regulator(FPR) for gasoline, if found replace FPR