What could it be? I'm about to poop on the floor.

[pearlmoto]

Hello everyone,

My name is Thomas from Dallas and I have a lovely all original 1986 K30 Silverado with 23K miles, 5.7L LT9.

It came from an estate and I have been using this truck as my daily driver.

I have ironed out a few wrinkles that can be expected from a truck that wasn't used very often, but I have recently been presented with a problem I simply cannot figure out.

I recently restored function of both gas tanks by replacing the fuel selector valve. I had run about 500 miles between the tanks, alternating between left and right. I filled up and about a quarter mile down the road, the truck dies.

It feels like a fuel starvation issue, but the accelerator pump is working and further tests had the fuel pump sending out an acceptable volume of gas - it is an AC Delco pump I replaced about 1000 miles ago. I set the fuel pump up on a remote gas can to eliminate the selector valve or lines before it as suspect - the truck ran the same off a remote can. I concluded that I could focus on the fuel system from the pump to the carb.

I figured putting an unused gas tank back in service may have put some silt through the system, so I pulled the carb back off the truck and rebuilt it (I had already rebuilt it about 1000 miles ago). I did find some silt in the bowl, and also a brass breather tube had come loose from the airhorn in the carb. I fixed these issues and reinstalled the carb. There was no change when I tried starting the truck.

I did the next logical thing - replace the fuel pump. Again. I go to start the truck - same behavior.

There's got to be something I'm missing. There's ignition verified by 12V to the coil - the ignition control module, cap, rotor, plug wires, and coil are all new and all AC Delco. I replaced the plugs about 600 miles ago and checked a few - they look good.

The vac canisters are all new; I replaced them when I did the selector valve because the failed valve bled gasoline through the vent line and fouled the canisters.

I have also verified 12v to the choke, and the choke mechanism is also new.

Here's a video of my trouble. Can anybody voice their thoughts on what to attack next? I was so frustrated yesterday I chucked my ratchet across the yard.

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[Georgeb]

It may go without saying but have you checked the fuel filter. The one in the fuel inlet fitting of the carb is rather small and easily plugged.

 

[pearlmoto]

Yes - I replaced the fuel filter when I rebuilt the carb, it's brand new.

The carb rebuild included a new needle/seat, seals, gaskets, etc.

I installed a new brass float when I rebuilt it the first time.

I used spray cleaner and compressed air to blow out all orifices, replaced the accelerator pump, and ensured the idle pickup tubes were free of obstructions.

I removed all the jets and cleaned up the rods.

I didn't forget the checkball.

 

[crpntr78]

Do you have spark? Ignition module perhaps? Another thing, check the selecter switch to make sure it didn't get bumped into a neutral position(ie.. sitting between the right and left side) happened to my Pop's 87 once.

 

[pearlmoto]

Yes, I have confirmed 12v at coil, and spark. Everything in the distributor is new, including the ICM. The pickup coil is the only thing I have not replaced.

The tank selector switch is new and will only toggle between left to right. Regardless, I think that given the fact the truck runs no differently off a remote gas tank, bypassing all the factory tanks and lines, I believe the problem lies some from the fuel pump forward to the carb, if it is indeed fuel related at all.

 

[1987 GMC Jimmy]

No cracked rubber lines and no kinked steel lines? You mentioned that a good volume of fuel is getting past the pump. I'm assuming you removed the line from the inlet and saw the fuel pumping. What about vacuum leaks? I don't want to suggest intake, but EGR valve can be a big one. I'd check carb base with starter fluid to see if the idle changes to make sure the carb isn't warped since I'm assuming new base gasket and good bolt torque. Quadrajets have been known to warp when idiots don't handle them properly (not you, you clearly know what you're doing). Besides that, just making sure all the lines and ports are either connected or plugged. I had a 454 that ran like that once, and when I started to fix the many vacuum leaks, it started to run a whole lot better. I think a vacuum gauge would be invaluable in this scenario. You can test the EGR by moving the plunger to make sure it's not stuck and depressing the plunger and capping your finger over the nipple to see if it holds vacuum. Or you can use a mityvac to test the vacuum holding if you have one.

 

[pearlmoto]

Nope - rubber and steel are all in good shape. Nothing is deteriorated or leaking - the truck was stored and everything is still very pliable and fresh looking.

I verified fuel flow at the line at the carb - got 16 oz. with 15 seconds of cranking. Thought the pump may be fouled but was still sort of working, but not under pressure. I replaced it and no change.

I wondered about the EGR - but my understanding is that a blocked EGR would cause a rough idle but not a no run scenario like I'm experiencing. I have a new one on the shelf if necessary, and I do have a mighty vac to test. I tested it just recently before this event and it was working normally as it should.

Carb has a new base gasket. I bolted the carb down according to Rochester's sequence. I've been instructed to ensure flatness with a straightedge across the airhorn at the idle tubes, as too much arc will apparently cause "late tip in of the main circuit". It's also been suggested to run the motor as in the video until it stalls, then pull the airhorn to ensure fuel is in the bowl and to double check float height.

As far as vacuum systems go, I have replaced all of the thermal vacuum switches in the manifold as well as both vac canisters. I removed and resealed the intake manifold about 300 miles ago, there is no leaking at the intake.

 

[Jrgunn5150]

I was going to suggest the same, when it stalls, is the float bowl full?

 

[1987 GMC Jimmy]

Nope - rubber and steel are all in good shape. Nothing is deteriorated or leaking - the truck was stored and everything is still very pliable and fresh looking.

I verified fuel flow at the line at the carb - got 16 oz. with 15 seconds of cranking. Thought the pump may be fouled but was still sort of working, but not under pressure. I replaced it and no change.

I wondered about the EGR - but my understanding is that a blocked EGR would cause a rough idle but not a no run scenario like I'm experiencing. I have a new one on the shelf if necessary, and I do have a mighty vac to test. I tested it just recently before this event and it was working normally as it should.

Carb has a new base gasket. I bolted the carb down according to Rochester's sequence. I've been instructed to ensure flatness with a straightedge across the airhorn at the idle tubes, as too much arc will apparently cause "late tip in of the main circuit". It's also been suggested to run the motor as in the video until it stalls, then pull the airhorn to ensure fuel is in the bowl and to double check float height.

As far as vacuum systems go, I have replaced all of the thermal vacuum switches in the manifold as well as both vac canisters. I removed and resealed the intake manifold about 300 miles ago, there is no leaking at the intake.

A dirty/clogged EGR valve acts as you said, but a stuck open or non-sealing valve is a nasty vacuum leak. I'd do the straightedge test on the carb because a previous overtightening or improper torquing may have damaged it. The starter fluid being sprayed at the base will suck it through and idle it up if you want to try that. And the intake gasket is okay? That one would cause super low vacuum reading. You've been really thorough. Unless something's been left out, I feel like the issue's gonna be found in the engine bay, especially if you're getting good fuel at the carb.

 

[Georgeb]

I wonder what would happen if you started disconnecting and plugging vacuum ports as a process of
elimination to try to find the issue. What if for some reason it will start with no vacuum it will run but as it runs the increase in vacuum pulls the egr or somthing open and it stalls.

 

[1987 GMC Jimmy]

What's your base timing set at also? I'd say something about the ESC module, but I don't think federal emissions 350s got them.

 

[MrMarty51]

I don`t rememer, do these have a sock over the fuel pipe inside of the tank ?

 

[pearlmoto]

Hey guys,

Thanks for the replies and apologies for the late response.

I ended up disassembling the carb again to verify fuel level and check the airhorn flatness. Everything checked out.

A neighbor had even suggested the truck may have jumped time, and as unlikely as that was, I did investigate that with no discoveries.

Something I had neglected to do was purge the lines before I ran my initial gas can directly off the pump test, so I decided to redo this job since the gas sitting in the bowl looked like absolute trash.

This time the truck fired up and ran. Here's a comparison of what's in the tank vs fresh:

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So - after one tow, 3 weeks of downtime, one fuel pump, 2 carb disassembles, one ignition control module, 3 fuel filters, and 30 gallons of bad gas to dispose of, I have learned that bad gasoline is a very real thing.

Next step is to drain both tanks, drop them, clean them and replace the sending units while they're out.

 

[74 Shortbed]

Yes it is a real thing, anymore this crap they call gas after 30 days it starts to go downhill..

 

[Jrgunn5150]

I was really in suspense over whether or not you pooped on the floor.. And I still don't know!