Is this a fuel or an electrical problem?

[Dawg667]

I have a 91 K1500 with a 350. When I'm driving and its under 2k RPMs and I step on the gas pedal it starts to act like its running out gas and it has a full tank. If im easy on the throttle it doesn't happen. What could case this?

 

[magimerlin]

I have a 91 K1500 with a 350. When I'm driving and its under 2k RPMs and I step on the gas pedal it starts to act like its running out gas and it has a full tank. If im easy on the throttle it doesn't happen. What could case this?

What have you done to it? What have you already checked? What's the fuel pressure at idle?, and when you step on the gas does the fuel pressure stay constant(ish) or jump way down and not recover fast? Last fuel filter done when? Sensors tested?

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[Dawg667]

I just changed the fuel filter on Monday and it didn't make a difference. I really don't know what to check.

 

[magimerlin]

Get yourself a Chilton book if you don't have one yet as it will have all the specs in it.

--Last tune up done(plugs, wires, cap, rotor, air filter, pcv valve, etc)
--test, tps sensor, iac, egr, etc
--check fuel pressure(at idle and under load)
--check base timing(with wire under dash by the glove box disconnected)
--clean the throttle body real well
--Get some seafoam(1 into the tank, 1 into the intake via the brake booster hose, look up how to do that here on the site, do not just pour it into the intake)
--check intake for leaks
--check throttle body for mounting gasket leaks


--that's kinda the short but easy list...


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[redfishsc]

Fuel filter is usually the first thing I do when this happens, but since you've already done that, the next thing I would be looking for is a serious vacuum leak, and also probably clean the MAF.

I've had bad plug wires and cracked spark plugs cause the same problem as well, but they seem to cause the problem pretty much across the whole RPM range, stealing worse at higher RPM since it's misfiring a lot more times per second.

 

[Nathaniel2g]

To me it sounds like a fuel issue - I'd suspect your culprit is low fuel pressure, however I would assume it would cause problems all the way through the power band.

 

[90halfton]

91 won't have a maf sensor. Check fuel psi while driving, at different loads and speeds. Actron makes a good cheap psi tester with a hose and Schrader valve. Snake the hose down through the crack between the hood and windshield and you can see it while driving. Should be between 10 and 13 psi I believe. Also check timing and vacuum leaks.

 

[90halfton]

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[Dawg667]

Thanks for everything I will start checking tonight.

 

[Baighn92]

Man, I had that exact same issue. I checked everything.

Long story short......it was the plug wires (even though they looked new).
The heat shields were rotted off my exhaust manifolds and made the plug wires crispy and cracked near the manifold (out of view). When under load the engine has more pressure in the cylinder which requires a higher spark voltage to arc across the gap in the plug. The higher spark voltage would shoot out of the cracks in the wire/insulator instead and ground out on the exhaust manifold. During idle and low load, engine cylinder pressures are lower which requires less spark voltage and the spark would not have enough power to arc to the exhaust manifold.

You can check for this if you pop the hood and make your shop pitch dark, load the engine and you might see the arc jumping (misting the area with water can help show it too).

Mine would sputter:
- On idle, in gear, Air conditioner compressor on.
- When cruising and I touched the gas to slightly speed up.

Put new NGK wires on, problem fixed.