Fuel delivery issues (Air in fuel line)

[86 K10]

Hello folks. This is the first time posting so if it seems like I am leaving something out feel free to ask me. I have and 86 Chevy k10 with duel tanks and a 350 SBC. Started out everything was running smoothly and then went to start the truck and wouldn't start. Checked the fuel pressure and had none so I assumed the manual pump had went out. Replaced the manual pump with still no fuel coming through and the pump had suction as the engine turned over. Had to use the truck that day so I bypassed the manual pump with an electric one and it started. Ran for a day or two and quit while going down the road. Since then and up to today, it makes no difference which tank is being used, if the tank is full or empty, the truck might run perfect or die out until it finally decides to start pumping fuel again. I can hear the electric pump picking up air and then pick up fuel again. Long story short I have dropped both tanks and replaced the pickup units in both, replaced the fuel tank switch valve, tried a different 5lb electric pump, replaced carb with a new edelbrock thunder series carb, replaced steel fuel line that runs along the frame, replaced the lines going from the fuel tank switch valve, and made sure the vent tubes on the tanks are clear. The fuel tank switch valve switches perfectly and you can audibly hear it change positions. Currently I am completely out of ideas. To the best of my knowledge I have replaced everything in the fuel system. So if you have any suggestions let me know. Thanks!

 

[kenny78]

I hate to say it cause I have been in similar troubleshooting dead ends, but: you missed something, something has the same failure mode as the previous part, or it's not actually fuel related.

What kind of hose clamps are you using? Have you bypassed the fuel selector valve with a double hose barb? Where was/is the electric fuel pump located?

 

[Cuba]

You said you replaced both pickup assemblies in the tanks... were they a reputable brand? No kinks anywhere? Need to start back at the beginning unfortunately. If it's still picking up air whether or not tanks are full, this is not sounding right.

You've done a lot of work so far and lots of $$ thrown at it. You did all the work yourself? Just wondering. You can prove work has been done, unlike a shop or a friend, you never know.

Since you have an electric pump already, you can bench test the tank pickups by pulling them again, to make sure there are no pinholes anywhere. Use clean buckets for fuel, and just pump it back into the tank, listening for air pockets or vice versa.

How long before it starts acting up? Before air is getting sucked up?

 

[1987 GMC Jimmy]

I would pull fuel from a can using your mechanical or electric pump to rule out everything north of the pump. I'm thinking it will be between the pickup and the selector valve if it's a fuel issue, which I don't see why it wouldn't be. One thing you didn't mention was tank venting. Do you either have a fully operational evap system, a deleted canister with the evap line in the engine bay inline filtrated and a newer style fuel cap, or a completely deleted system with a vented cap? I'm not sure that would cause your issues, but that's the only thing I could think of that you didn't mention. I would look ahead to a spark issue just in case. I'm not saying I don't believe you, but you've clearly been very thorough, and this sounds just too clean to be a run of the mill fuel issue, and the fact that it's intermittent also vexes me because you mentioned it's either perfect or dead. Fuel issues in my experiences have yielded loss of power, bogging, backfiring, chugging, and stuff like that. I know they can result in your symptoms, but just driving down the highway and poof - it shuts off sounds a lot like a ignition control module or pickup cool on the fritz.

 

[86 K10]

First, THANK YOU ALL for making suggestions and asking questions about it. If nothing else, brainstorming with more people might enlighten me on something I might have over looked! I did forget to mention earlier that I also put a clear inline filter directly before the carb so I could watch the gas and see if it was a continuous flow of air or if it was intermittent. It will have a solid flow of gas into the carb and when it picks up air, its like the fuel tank isn't even connected to the truck and zero gas is flowing. Then it will pick up gas again either constant or have a solid flow then cut out sporadically even while sitting still. Also, that the entire emission control system was gone as I listed for 1987 GMC Jimmy user.

Kenny78: I have not bypassed the tank selector valve with a barb fitting yet just simply due to that if it does work I would more than likely just leave it due to being at wits end with it. However, I am at that point now so I will try that more than likely today. All the hose clamps have been converted to the screw type clamps. And the fuel pump is mounted to the inner fender on the passenger side. The fuel line runs directly to the electric pump which bypassed the manual one completely. The only question I had about that is if the fuel return line might be causing an issue since it is completely bypassed. Didnt really see why it would but that's the only remaining line I haven't tampered with.

Cuba: I have done all the work myself. I will get an urge to work on it on weekends and tear into it. Its more along the lines of a personal vendetta now instead of a project. I bought the truck with the motor disassembled in the bed and went through and completely rebuilt everything. At which point I drove it a year before any issues what so ever. When I had the tanks both off previously (for the 3rd time) I cycled the gas through the tank fitting perfectly for about an hour. And as far as how frequently it will do it, well that's the question. If I could get it to repeat it regularly I could eventually track it down. As it stands now, it might work perfect for a week then the next time its driven it will start smothering out due to lack of gas. I have used the switch while its doing this to see if it is an issue with just one tank but it carries over to both so I am thinking its not a pinched line anywhere. When I started replacing all the fuel lines it was because I figured the ethanol had deteriorated the inner lining of the hoses and had a "flap" hanging in there blocking the fuel. I even tore the new carb apart to see if the float was sticking or anything.

1987 GMC JImmy: The engine already had ALL of the emission control and charcoal filter removed before I got it. The vents in the tanks are clear and I am 100% positive that its not a spark issue. I completely went through the CDI system when I rebuilt the motor. When it does die, its a gradual smother out condition just like the fuel tank is empty. That's when I can hear the electric pump picking up air and then it will pick up fuel. It might cycle back to picking up air or it might start pumping fuel again.

 

[86 K10]

Does anyone by chance have access to a wiring schematic for the terminals on the fuel tank switch valve? If so I can check the voltage and see if it might be trying to return to the center position and cutting the fuel off that way.

 

[Cuba]

Does anyone by chance have access to a wiring schematic for the terminals on the fuel tank switch valve? If so I can check the voltage and see if it might be trying to return to the center position and cutting the fuel off that way.

I came across a thread on here that had all those pdfs, I believe. They're even searchable. Let me try to dig it up real quick.

Here's the link to that page. Download the whole wiring .pdf anyway for future reference. Can't beat it!
http://www.gmsquarebody.com/threads/1973-1991-service-manuals.14499/#post-293629

 

[kenny78]

I'm not an expert but I see a couple points. The electric fuel pump wants to push fuel-not pull it. Try putting the electric pump on the fame rail near the selector valve. I have learned a hard lesson about hose clamps. It's counter intuitive but do not over tighten them. A proper hose barb and spring clamps can provide a better seal than worm gears on a cut pipe.

What pressure electric pump are you using?

Is the mechanical pump completely removed from the system?

Do you have the 3 port valve or 6 port? Is your switch a simple toggle switch or a rocker that says to depress fully?

 

[86 K10]

It's a 5psi electric pump and the only portion that the manual pump is still connected to is the return line. The actual fuel supply line goes straight from the fuel tank switch valve to the electric pump to the carb. I will see about relocating the electric fuel pump closer to the switch valve more than likely tomorrow evening. The valve is a 6 port with the rocker switch that says to depress fully. It has the 3/8" line and a 1/4" line going to it from each tank and then the corresponding ports that go to the carb. Thanks again people!

 

[kenny78]

Total wag

The suction from the electric pump is pulling air from the return line through the selector valve.

 

[crazy4offroad]

If your xfer valve is plastic it could be a crack in an inlet/outlet. First thing I thought was fuel pickup line was hooked to vent or return, like it would run on a full tank but once some gas was ran down it would quit. Ken's post makes sense too.