Dual tank switch kills the engine

I believe you said your truck DOES NOT have EFI, so the fuel pumps probably aren't in the tanks. I'd check the solenoid valve that switches the tanks to be sure it's shifting the flow to the passenger side tank. However, if that's the problem the truck probably wouldn't die "INSTANTLY" and should run for a bit off the fuel in the carburetor's float bowl.

Dan Brown said:

I believe you said your truck DOES NOT have EFI, so the fuel pumps probably aren't in the tanks. I'd check the solenoid valve that switches the tanks to be sure it's shifting the flow to the passenger side tank. However, if that's the problem the truck probably wouldn't die "INSTANTLY" and should run for a bit off the fuel in the carburetor's float bowl.

Click to expand...

Actually, he did in post #7. He said the truck is stock and in 87 TBI was stock. Why would you have electric in-tank pumps on a carb'd setup? Moving on.

I would have swapped leads on the switch and see if the problem switched sides. That would rule out the switch and the only thing left is what's downstream from there.

I pretty much have your same truck. 87 tbi 350 with saddle tanks. Replace the passenger fuel pump, relay under the hood and the switch over that’s under the truck. While you’re there might as well drop both tanks and replace the driver fuel pump too. Also check your fuses under the driver dash.

Based on OPs further description, I'm going with bad fuel pump as most likely. Then relay and/or wiring if pump is good.

You guys are great. Once fixed I will give an update on what I found.

Update. Let me start by saying I barely know how to use a multimeter, but I thought I would poke around the wiring on the dual tanks and see what I could find. I started with the bad passenger tank. I found a wiring harness that goes from one tank to another following the frame crossmember. On the passenger side of the harness there is a connector with 2 wires (appears to be pink and gray) . These wires go to the top of the tank. I assume this is wiring to the sending unit/pump. I unplugged the connector and checked continuity to the sending unit and the multimeter read 045. I then checked the wiring harness side of the connector and read nothing. So I plugged it back together

I scooted over to the good driver side and found the same connector set up. I think the wiring harness splits here and goes to the driver side with a split to the passenger side. I unplugged it and found the gray wire is broken/missing from one end of the connector. Its the end going to the crossmember wiring harness to the passenger side. I think this is my problem Do you think if I cut open the wiring harness, find the gray wire, reconnect it to the connector plug that the passenger tank will get power and the sending unit and pump will work? I assume I should get a new connector. See pic.








THE GRAY WIRE ON THE "A" SWITCH TO AUX TANK IS THE WIRE MISSING

It’s very likely that you have found your problem, and repairing the gray wire is the fix.

I think they sell those wire connectors in the parts store, and it would be easier to splice on a new one than it is to work with that short broken wire in the old one. I don’t know the name for that connector, but GM used them a lot.

If you want a quick fix for troubleshooting, use two scotch-lock connectors and a short piece of wire to connect to the gray wires on either side of the connector. If the truck then runs, you definitely found the problem is the broken wire.

Bruce

Hello. I am new to my 85 GMC pickup which just came a week ago. I love it as it reminds me of driving our new 79 Jimmy back in the day. This truck has saddle fuels tanks. I am not a mechanic. I love these older vehicles for the history as I grew up with my grandfather (a small town GMC dealer) in the 1960's before his death in 1972. Since this truck is a 1985, it is my understanding that there is one fuel filter and one mechanical fuel pump to service both tanks. Not individual pumps in each tank. From time the truck arrived, the left tank was pegged way above full (dealer said he would send fuel). Right tank was close to empty. I took it to the station and added 10 galls to right tank which brought the gauge close to F on that tank. I drove about 50 miles on each tank first couple days I had it. No problems. Left tank gauge finally started to drop from above F telling me the sending unit was working and not a bad ground or something. On Saturday, we went to a show. A friend drove the truck as I took more than one vehicle. On way back to garage, he said it stalled repeatedly on left tank. I told him to switch to right tank which got him back to storage. I drove it some Saturday and was able to duplicate the issue but you might drive some distance on left tank before it would stall out gradually. It would fire back up with right tank. Left tank seems to falter the most when heavy acceleration (this is only a 305 ci engine truck which is fine with me) either when already driving or when starting from stop. Right tank still seems to be working strong. I even drove it enough to need to put 10 more gal of fuel in right tank and this time filled it up. The gauge at full on right tank is not as far above F as it is for the left tank when it arrived. In what order would you mechanical experts start your troubleshooting process. I have read where it might be the on dash switch or perhaps the valve that controls the flow from one tank or the other. I know you are not here to test it or crawl underneath but appreciate your feedback. Remember I am a vehicle history lover and not a mechanic. Have a great day

@85 red GMC
Welcome!
Before trying anything else, loosen the fuel cap on the left side and make sure it’s not pulling a vacuum from the tank and stopping fuel from being pulled. If it repeatedly dies slowly after running for a while, this is a possible cause.
Or if truck sat, the fuel pickup could be plugging.
Imo the fuel tank valve isn’t bad or it would happen all the time.

Thank you Grit Dog. I had the same thought as one possibility. I not only checked the left tank cap, I switched them Monday when I was doing more drive testing. Since reversing the gas caps (not same brand but look to have similar structure) I have not noticed any differences. Right tank still dependable.

Then I’d lean towards the fuel pickup sock getting plugged up.
Or maybe return line to right tank is not working? Seems possible but not sure that would cause a vacuum situation in the tank.

I purchased the valve, dash switch and wiring harness if the fuel process / switch system is determined to be bad per our mechanic. I can return parts if not needed although maybe some day I will. Last week I put some fuel conditioner and water eliminator in the left tank to see if the problem might be bad / wet fuel since the non fuel injected truck has only one mechanical pump and one fuel filter (to the best of my knowledge). I let it sit outside and idle for about a half hour to get better fuel through that line. I then drove some around parking lots with no stalling. I took it out on the road and did not have any problems Friday and Saturday. It did stall once Sunday when driving so I went back to right tank for awhile. Unless mechanic comes up with a better idea, I will try to carefully burn out the fuel in the left tank with careful driving over next few weeks and keep replacing it with new / clean fuel (non ethanol) to see where we go with it. Thank you

Hello, I am having the same problem as a previous poster, my truck would die instantly when switching from the right tank, [ which worked great, ] to the left tank then it died like you cut the ignition off. After being mad for a while I got underneath the truck and jiggled the wire harness and all of a sudden both tanks worked. 2 days later I went to drive it and nothing. Would start with starting fluid for a second then nothing. I have been checking wires and have found nothing yet. I did take out the dash switch and checked for voltage but nothing. checked fuses and all are good. I used my test light to see if I had any power to the wires going to the tank selector on the frame and I was getting gas seeping from the plug. No power but gas coming out of the plug.Any ideas ? The truck is a 87 c20 all stock with only 90,000 on it. Thanks in advance.

Deutsch connector .. and you should be able to find one with a pigtail on it most auto parts stores . I have a kit at work for them . Although the tool to crimp is expensive. The pieces are reasonably cheap. And yep electric fuel pump will kill engine instantly with no power