Engine stumble/cutout getting on my nerves

[vortec94]

UPDATE: i finally got around to checking my fuel pressure. with the engine off and the ignition cylinder in the "run" position, i get 10-11psi and 11 psi with engine running. however, i'm getting no residual fuel pressure. it drops to zero within a few seconds of the pump shutting off. i was expecting at least a few minutes of maybe 5-10 psi. no gas smell, so injectors and lines aren't leaking. most logical thing seems to be fuel dumping into the return line too quickly. bad pressure regulator, or maybe a check valve?


1994 c1400 4.3L TBI with 4l60e

it was running terribly several weeks ago, nearly dying from stops and on the highway whenever i didn't baby the throttle. no codes. i swapped the MAP sensor and things are much better. however, today, i dropped pushed the throttle near WOT to pass on the highway and it cut out BAD but fired back up in a second or two. at that speed, i could feel the affect on the driveline and i cringed thinking about it.

i've replaced: fuel filter, MAP sensor, VSS
i've checked: TPS, IAC, EGR valve and solenoid, O2 sensor, ignition, vac leaks,

I have not messed with the knock sensor or crank position sensor yet.

ideas?

thanks in advance

[93V8S10]

Don't forget to check the fuel pressure. Keep us posted. Best of luck.

[vortec94]

yeah, that's another thing i was going to do. i just need to pick up some fittings and a gauge. I don't know of another way to test it on TBI systms. most kits i've seen require a schrader valve on the line or just "T" into a rubber line for carbs.

[lo-n-slo]

They sell a kit at autozone for about $25-$30, that that goes in between the fuel line and and the filter or injector pod.

[93V8S10]

I've got the one from Auto Zone too. One my 91 S10 I just left the adapter fitting in-line so I can check pressure easily.

[lo-n-slo]

It was made to go between the fuel filter and line, But I installed it on the injector pod inlet and left it there. Now, I dont have to jack the truck up to check the Fp, keeps it alot cleaner too.

[94silveradoecsb]

Is your motor stock? I have the usual bolt on's on my truck and its doing some similar things as yours. I have a wideband o2, it shows the motor is going dead lean when it happens.
If you stomp it does it run ok once the drama stops?

[vortec94]

Is your motor stock? I have the usual bolt on's

yes. i haven't changed it much at all because it's my dd and i need good reliablilty and mpg. mostly exhaust and a few bolt-ons.

I have a wideband o2, it shows the motor is going dead lean when it happens.

my air/fuel ratio gauge was reading way lean b4 i replaced the MAP sensor.

If you stomp it does it run ok once the drama stops?

i rarely go WOT, but when i do, that's when it starts to cut out. that's why i initially changed the fuel filter and ran a bunch of seafoam through it. i thought the injectors or filter were clogged. that doesn't seem to be the case. i wanted to install a fuel pressure gauge in the cab years ago, to see numbers under load, but didn't want to drop the money on an electrical. mechs are cheaper, but the isolator drives the price back up. this annoying problem is making me feel like spending the money, though.

[neevnav]

I would say either fuel pressure regulator may be shot, fuel pump, or distributor module gone bad causing week spark.

[tom ward]

hey my 5.7 was runnin like crap cuttin out and stumbling too, i replaced the ignition cap and rotor and it runs like normal again lol, its a cheap fix too mine was 25 bucks and runnin

[vortec94]

hey my 5.7 was runnin like crap cuttin out and stumbling too, i replaced the ignition cap and rotor and it runs like normal again lol, its a cheap fix too mine was 25 bucks and runnin

yeah, i swapped my cap/rotor about 6 months ago. i even tried swapping the old stuff back on and there was no change.

[vortec94]

Ttt

[lo-n-slo]

Take the air cleaner off and the plastic/metal ring in between the air cleaner and the throttle body. Start it and watch the injectors, you should have a nice conical, umbrella spray pattern, then you can rule the injector filter screens out. If they drip fuel or have abnormal spray patterns put a rebuild kit on it.

Did you try reading the fuel pressure when you are driving and got WOT? I've had pumps show decent pressure at idle but drop way off under a load. Just a few things to try.

[93V8S10]

UPDATE: i finally got around to checking my fuel pressure. with the engine off and the ignition cylinder in the "run" position, i get 10-11psi and 11 psi with engine running. however, i'm getting no residual fuel pressure. it drops to zero within a few seconds of the pump shutting off. i was expecting at least a few minutes of maybe 5-10 psi. no gas smell, so injectors and lines aren't leaking. most logical thing seems to be fuel dumping into the return line too quickly. bad pressure regulator, or maybe a check valve?
Theres a foot valve at the base of the pump, if it fails, the fuel will run back into the tank immediately, when the power is shut off. My advice, change the fuel pump before it takes a crap someplace bad.:eek: Also you'll need a new in-tank filter as well.

[vortec94]

Start it and watch the injectors, you should have a nice conical, umbrella spray pattern, then you can rule the injector filter screens out.


did that. it's how i ruled out clogged injector a few months ago.


Did you try reading the fuel pressure when you are driving and got WOT? I've had pumps show decent pressure at idle but drop way off under a load. Just a few things to try.

yes i did. little to no pressure change, regardless of rpm or load. our temps got near 80 today, so i couldn't replicate the problem, at least not as bad. we had temps in the 30s the last few weeks and the engine is not compensating fully for the cold dense air. i left the t-fitting in place so that when the temp drops (next few days) i'll be able to stick the gauge right back on and try it again. it seems solid and leakproof enough to keep on there, at least for a while (actron from autozone).


Theres a foot valve at the base of the pump, if it fails, the fuel will run back into the tank immediately, when the power is shut off. My advice, change the fuel pump before it takes a crap someplace bad. Also you'll need a new in-tank filter as well.

that sounds exactly like what it's doing. sticking the valve in the pump instead of at the injector bod base was a pretty bone-headed thing to do. last time i swapped a pump on one of these, there were 4 of us and we pulled the bed off. made things easy, but one of the guys dropped his corner :eek:. i won't be going that route.

[93V8S10]

that sounds exactly like what it's doing. sticking the valve in the pump instead of at the injector bod base was a pretty bone-headed thing to do.
Sorry, actually it wasn't. The valve keeps some fuel in the pump to help prevent dry pumping and to prime the pump.:cool:

[vortec94]

that does make sense. i guess i owe the general an apology.