Will start but won't run.

[gottaba5spd]

Posted something in urgent yesterday but I wound up being able to get the truck home, but after driving it around to confirm the problem is gone it came back, so here's the issue;

Yesterday after work I went to start the truck and it cranked but didn't stay running for more than a few seconds. Tested fuel pressure at the rail and got 20psi. I got a fuel filter for it, pressure went to 60 where it's supposed to be, but it still wouldn't run. With some advice from a friend I unplugged the maf sensor and it started up and ran like it's supposed to. Plugged the maf back in and it did the same thing, and after some maf cleaner and a new air filter it ran with a little bit of a rough idle. After about 50 miles of driving it and letting it get to temperature, pulled into my parking spot at work and it died and wouldn't start up again.

I'm pretty confident it's not the fuel pump, and maf sensors are 100$ so I don't want to just throw parts at it. Anybody experienced this kinda thing? Truck is a 98 b2500 2wd.

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[RonD]

Whats fuel pressure now, at the time it won't start and stay running?
It does read like a fuel pump issue.

Unplugging MAF sensor puts computer into Open Loop so it doesn't use most of the other sensors, it is running the engine from Factory tables in memory.
So unplugging MAF and having engine run better doesn't mean MAF is the problem, it could be just saying it isn't a valid test that MAF is the problem.

I would get a Volt meter and sewing needle/pin to test voltages at MAF and TPS(throttle position sensor).
Volt meter set for DC Volts
Sewing needle is to pierce wires while they are hooked up so you can test voltages.

MAF voltages here: http://www.therangerstation.com/tech_library/CleanMAF.shtml

TPS has 3 wires
Center wire has the Return voltage the computer uses to tell throttle position
With key on engine off
Center wire should have under 1 volt, .69-.99 is spec
As you manually open the throttle voltage should go up, should be steady no jumps up or down
At WOT(wide open throttle) it should read 4.5volts to 4.9volts

 

[gottaba5spd]

Well when I got to the truck after work it started like nothing had happened, so that was weird.

Haven't been able to test voltage on tps and maf but I re tested the fuel pressure after checking for replies on the thread, primed the pump 3 times, got 8 psi, 45 psi, and then 60 psi. A mechanic friend told me to run the test again and see if it held pressure but I actually watched the needle drop after priming the pump. I'll see if I can find my multimeter and check the sensors but I think fuel pump is more likely.

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[tomw]

The pump may have a bad check valve, allowing fuel to drain back to the tank rather than stay in the fuel rail, close to the injectors. Priming the system at cold start may allow quicker startups. If cycling the key to ON, pausing, then to OFF, back to ON, pausing several times makes it start better cold, you do have a leakdown problem.
When it won't start, if you've done the pump cycle mentioned, try stepping on the gas pedal a slight amount. If the IAC is jammed or in-op, the engine will be reluctant to start as it is trying to run with the throttle closed(IAC closed also). It will not(in general) idle if the IAC is not working.
tom