94 Stumbling No Acceleration (1 Viewer)

[davework]

My 80 won't accelerate under power at all all of a sudden. I hit the gas pedal and it just sputters. It mostly idles fine and you can rev the engine sometimes without sputtering, but sometimes it sputters even with no load. No codes have been thrown by the computer.

I have about 30k mi on it since a full engine rebuild, when I replaced most everything with new OEM.

I just did all of the following with no luck:

Removed and cleaned upper intake and tbody
Tested VAF Sensor per FSM
Tested IAC and TPS per FSM
Tested EGR VSV per FSM
New OEM Fuel Filter - replacing this made the idle much better, I thought I solved it until trying to drive around
New Plugs - old ones looked good
Replaced Rotor - Cap and Wires were still like new
New PCV
Checked Valves while in there - all within spec
Checked intake hose for cracks - none that I can see
Moved engine wire harness behind EGR around while running, no change - I wrapped this with heat loom and it is well protected during rebuild
Swapped ECM with a spare I have

I am stumped. I ordered a EGR and Vacuum Modulator, but with only 30k mi on the ones in there, I hate to replace those already. I plan to test when I get home thought to see if they are working per the FSM.

Also thinking maybe the fuel pump? I did not replace that, so it now has 255k on it. Will also bypass/test the Fuel Pressure Relay and Resistor after reading up on Mud a bit.

Could the fuel pump be causing this?

I have a spare ECT I'll swap in just to see if that helps.

O2s only have about 5k mi on them, replaced those (again) and the cats hoping for better MPG last summer.

I'll also check for vacuum leaks again. All the hoses were replaced during the rebuild.

Any other ideas?

 

[SUMMIT CRUISERS]

Fuel pump strainer/sock is my guess

 

[davework]

I did replace the sock with new OEM during rebuild.

 

[SUMMIT CRUISERS]

Next would be fuel pump if original. Fuel related issues don't usually throw codes.

 

[davework]

Thanks, I am hoping that is it, easy fix if so. But expensive. Is there any way to test the pump? I see the on/off test in the FSM, but if it's slowly dying, I imagine this might not pop up

 

[LS1FJ40]

Have you checked all vacuum lines? I had sputtering in my 92 a while back. It felt like it wasn't getting fuel. There was a crack in a vacuum line that when the motor torqued it gapped open. I happened to be about two blocks from AutoZone. swapped the hose and all was good.

 

[ZackR]

Did you check the wiring harness next to the EGR? Although not as crippling as what you are experiencing my cruiser had major hesitation due to shorted wiring that had leaned up against the hot EGR pipe. It took me a long time to figure it out since I was busy trying everything else (plugs, wire, filters, etc..).

 

[davework]

I checked the vacuum lines as best I can. I plan to try the propane test tonight to see if I can find anything.

I didn't unwrap the wire harness behind the EGR, I checked it when I rebuilt and it was solid, and then I wrapped with heat loom, so it is well protected. I also wiggled it while trying to duplicate the problem with no response. It could still be that I guess. May have to unwrap and take a look.

 

[SUMMIT CRUISERS]

Not exactly sure how to test fuel pressure on a 94, but since you stated that replacing the fuel filter made it idle much better I still am leaning towards a fuel delivery issue.

 

[davework]

My thoughts exactly. Ordered a Denso pump, and going to borrow a used OEM one this evening to test out the theory.

 

[davework]

Fuel pump didn't help.

But I also borrowed a VAF meter and that was the problem. Back to running as it should, at least for a short commute. Fingers crossed that was the problem. VAF tested fine on the bench, so not sure what it was throwing that was causing the problem. Off to the classifieds to find a used VAF.

 

[erose54]

I have a question about the stumbling issue. I have a 91 that has been desmogged cause I just could not get an EGR or any of the components, although I did not check for used ones here. I figured it was easier to just remove the pump and EGR items. But just in the past couple of weeks since I replaced my brake booster that my rig has been stumbling/hesitating when it is cold. In park or neutral it will rev and be a little rough, but as soon I put in gear and try to accelerate it will sputter, sort of back fire and have no power. Once it gets hot it seems fine. I replaced the temp. sensor and temp. gauge, which seemed to help but couple days later same issue. I have not tested the VAF but noticed today on the way home from work I would cruise about 50 mph and gradually try to accelerate and it wouldn't really do anything unless I accelerated hard.

Ideas think it might be the VAF?

 

[erose54]

I have a question about the stumbling issue. I have a 91 that has been desmogged cause I just could not get an EGR or any of the components, although I did not check for used ones here. I figured it was easier to just remove the pump and EGR items. But just in the past couple of weeks since I replaced my brake booster that my rig has been stumbling/hesitating when it is cold. In park or neutral it will rev and be a little rough, but as soon I put in gear and try to accelerate it will sputter, sort of back fire and have no power. Once it gets hot it seems fine. I replaced the temp. sensor and temp. gauge, which seemed to help but couple days later same issue. I have not tested the VAF but noticed today on the way home from work I would cruise about 50 mph and gradually try to accelerate and it wouldn't really do anything unless I accelerated hard.

Ideas think it might be the VAF?

I checked the ohms on my VAF and everything looked good. So I am guessing that my VAF isn't the issue. I probably have a vacuum leak some where or one of the vacuum lines is probably not in the right spot.

 

[SmokingRocks]

You should put a pressure gauge on your fuel rail to see what you're getting when the incident occurrs.. That way you know if it's a fuel issue or spark issue anything else and you're just guessing and throwing money away. That can easily become a very expensive method of problem-solving

yeah that was the way I started trying to fix problems early on in my wrenching life. it will ultimately save you money to correctly diagnose the problem

 

[MountNGoat]

You should put a pressure gauge on your fuel rail to see what you're getting when the incident occurrs.. That way you know if it's a fuel issue or spark issue anything else and you're just guessing and throwing money away. That can easily become a very expensive method of problem-solving

yeah that was the way I started trying to fix problems early on in my wrenching life. it will ultimately save you money to correctly diagnose the problem

Smoking, can you elaborate on how to put a pressure gage on the fuel rail and monitor while driving in order to try to correlate the stumbling with changes in fuel pressure? I've got a '97 that is experiencing these same issues.

 

[Rifleman]

Not exactly sure how to test fuel pressure on a 94, but since you stated that replacing the fuel filter made it idle much better I still am leaning towards a fuel delivery issue.

It's a bitch to test the fuel pressure, you have to remove the rear banjo bolt from the fuel rail. Then you install a banjo bolt/Schrader valve in place of the factory banjo bolt. That valve is what you attach your pressure gauge to, but there's not much room to screw the pressure gauge onto the fitting back there. Here's a photo of what the valve looks like. You can buy this valve from Actron for 8 bucks plus shipping.
Banjo Bolt M1215 | Automotive Parts & Accessories