second opinion on stumble

[vortec]

i made a post about this over at fullsizechevy.com and got what i think is a good answer (supported my worst idea) but i'd like to see what answers this board provide.

the situation: 94 4.3L c1500

engine idles pretty well, but as rpms increase, it feels weaker and starts to stumble. if i open the throttle quickly at all it stumbles and almost dies, regardless of speed or rpms. under normal operation, my air/fuel ratio gauge should swing from lean to rich constantly as the computer adjusts, but it's not making i to the rich side anymore. i checked fuel pressure yesterday. 11psi, which is right in the middle of factory spec. but, i have no residual pressure. turn the key out of "run" and it drops to zero in seconds. i think the fuel is draining into the return line too soon, so enough fuel is goint to the injectors for light use, but it can't handle any extremes, like compensating for cold air (30s) or WOT.

i'd love it to be the pressure regulator, but one of the guys over there suggests the foot valve in the fuel pump, necessitating a pump swap. i'd like some more input before i go drop my tank.

 

[dyeager535]

Does fuel pressure stay the same under heavy load?

 

[vortec]

yes. it remained pretty constant regardless of speed, rpms, or throttle.

 

[dyeager535]

It should certainly keep fuel pressure when you shut it off, but that doesn't sound like the issue. Residual pressure is so it's quicker starting.

My thought process is that if it were a pump issue, pressure would drop as fuel demand increased. Not the case. If it was a regulator issue, pressure would be low if it was "dumping" fuel back into the return line too fast, which again would be shown with a drop in pressure.

Timing is correct? Ignition is all in good shape, all plug wires routed correctly? Vacuum lines in good shape?

 

[vortec]

it were a pump issue, pressure would drop as fuel demand increased. Not the case. If it was a regulator issue, pressure would be low if it was "dumping" fuel back into the return line too fast

yeah, that's exactly what i was thinking while i was checking. i expected it to always be low or to be insconsistent.

ignition system is fine. i've never actually seen a faulty ignition system cause a lean condition, anyway. there's probably some way it could, but that seems backwards since it would be dumping fuel into the exhaust in most cases.

i've checked for vac leaks around the throttle body, intake manifold and vac lines several times. if there is one, it almost has to be on the back of the intake manifold because it's the hardest spot to do a thorough check.

 

[Thunder]

Sounds like a sensor problem to me. Do you have a scanner?
Have you checked trouble codes?
From your description problem sounds like the MAP sensor. Un plug the MAP sensor wires. If your engine runs better you found your problem.
Beings you are holding fuel pressure under load i do not think you have a fuel pump or regulator problem.

 

[vortec]

i swapped the MAP sensor about a month ago. made a big difference, but i still have WOT stumble and the air/fuel ratio gauge reads leaner than it used to. i checked the o2 sensor and it seems to be in spec.

no codes at all.

 

[k20]

EGR gone screwy?

 

[Blue85]

i've never actually seen a faulty ignition system cause a lean condition, anyway. there's probably some way it could, but that seems backwards since it would be dumping fuel into the exhaust in most cases.

What you are saying is correct, except that this condition is still measured as lean. An oxygen sensor, doesn't measure, mixture, it measures oxygen. When you have a misfire, you not only get raw fuel in the exhaust, you also get a whole charge full of fresh air. So ironically, if the mixture is too rich to fire, the sensor reads it as lean (same as with any ignition-related misfire).

 

[Blue85]

Shouldn't the fuel pressure regulator hold the pressure in the system?

You are describing running problems from transient conditions. This would make me suspect TPS, a vacuum leak, or especially MAP.

What kind of A/F system are you using? I assume it's just a gauge connected to your stock (narrowband) O2 sensor? Remember that these things work in a very small range. They can be helpful, but listening to how the engine runs is always more reliable information.

 

[noJeepshere]

maybe a leak in the line? just a small one that would let the pump do its job and keep the pressure up, but starve the engine of fuel on demand. that would also explain the instant drop in pressure when its shut off.