I have a 1987 short bed GMC half ton 4x4. The passanger side tank works fine. As soon as I switch over to the drivers side tank the truck shuts off. I would appreciate and ideas on this, before I dive into it face first. There is gas in the tank. The truck is fuel injected and the sending units are mounted on the tanks. Thanks for any ideas
Yes I agree with "stardog" un less your running duel pumps & the pump not working . or the Sending unit is on reading corect & there not enough gas in the tank to suck it up.
I had the same problem on my truck. The problem was the wire running from the CPU to the fuel tank was melted
fuel problems between tanks? What does the CPU ? sorry. I am trying to get my driver side tank to work now. I am just getting into the problem this weekend to fix it. It runs on the passenger tank and just wont switch. Was the wire near the exhaust pipe? Thanks
Possible problems that I can think of, assuming the fuel system is stock: Bad selector switch in the dash. Ground is missing on the tank. The large O ring that seals the sender unit in the top of the tank also electrically isolates it from the tank and the rubber straps that hold the tank in place isolates it from the chassis. Originally there was a small tab on the sending unit that a female electrical connector will slide onto. This is where the ground for the fuel pump and sending unit is connected. These are notorious for getting corroded and falling off. Bad fuel pump. A stock 87 fuel system will have a pump in each tank. Bad cutover device The hose that connects the output of the fuel pump to the metal supply line going out of the tank has gone bad. (Inside the gas tank). This is another frequent problem. If someone has replaced this hose and did not use a good quality piece of hose it will get soft and kinda melt away because it is submerged in gas all the time. Fuel pickup screen in the tank is clogged. The sending unit for the fuel gauge is bad and the tank is empty but the fuel gauge falsely shows it’s not. I would start by determining if it is an electrical or mechanical problem. Locate the fuel pump relay, it is in the engine compartment, on the top of the firewall, next to the AC evaporator or where it would be if you don’t have AC. There should be a red wire about 6 inches long coming out of the plug on the bottom of the relay that is not connected to anything. If you connect a wire from the positive battery post (using an inline fuse about 10 amps or so) to this lead, you can run the fuel pump without turning the key on or trying to start the truck. Much easer and safer way to troubleshoot. Connect this lead up and switch to the good tank and you will be able to hear the fuel pump running. Now switch to the bad tank, if you hear that pump running you have a probably have a mechanical problem. Still could be low voltage caused by a bad connection or flakey switch but I would start looking at the mechanical stuff at this point. If you don’t hear the pump running it is for sure an electrical problem or a bad pump. If it is an electrical problem it is somewhere between the selector switch in the dash and the pump in the bad tank. If you can’t find anything post the results of what you have done and I will try to help you troubleshoot. Don
Fuel problems in tanks Thanks Don for your information. I will be working on the truck tomorrow and will let you know what I find. I was looking at a wireing diargram and see the oil sending unit says as its tittle is ( oil/fuel sending unit.) I just change my electrical oil gauge to mechanical . I took out the sending unit. Could that be tied in to the system some how to cause this problem? I just received in them mail today a new switch for the dash and the new swithover unit that goes under the passengers seat. A new filter and hoses to make up the 3/8's tubing. Hope one of these parts fixes it. Thanks again for any and all info you can give me. Roy
On an 87 EFI system you will have an oil pressure sending unit and an oil pressure switch. The sending unit is located down by the oil filter and sends an electrical signal to the oil pressure gauge. The switch is on the top, back of the engine, almost under the distributor. The switch provides power to the fuel pump once the engine has started and has oil pressure. Here’s how it works. When you turn the key to start the engine, the ECM (computer) comes to life and sends a signal to operate the fuel pump relay, contacts in the relay close and send 12 volts to the fuel pump. The ECM removes the signal to the fuel pump relay after about 3 seconds allowing the relay to deactivate. By this time the engine should have started or spun enough to get oil pressure. Engine oil pressure activates the switch on the top back of the engine. Contacts in this switch close and send 12 volts to the fuel pump. The system will stay in this configuration until you turn the key off or the engine looses oil pressure. The 3 second operation of the fuel pump by the relay is to prime the fuel system, this makes the engine easier to start. The pressure switch is in the circuit as a safety feature. If you have fuel line burst or get cut in an accident, the engine will shut down due to the loss of fuel pressure. With the engine shut down the oil pressure will drop to zero and deactivate the switch, shutting down the fuel pump. Now you are not pumping out massive amounts of fuel onto hot engine parts. If you are installing a mechanical gauge you need to replace the sending unit down by the oil filter, not the switch on the top of the engine. You can make it work by shorting the wires that plug onto the switch but I highly recommend that you don’t do this because you are defeating an important safety ferture.
fuel tanks Hello Don: Say got around to working on that 87 chevy fuel tank problem. I first put back in the sending unit on top of the engine. I used a Tee and now have the sending unit back in the system and the mechanical oil gauge next to it. I then tried to switch tanks and the right one runs fine but as soon as I switch the tanks from the dash switch ( new also ) the engine begins to slightly stumble, slight miss. I switch it back and the right one smooths out. I think the next thing I will do is change the switching unit under the passenger seat by the right tank. I had added some fuel injector cleaner a week or so back and wondering if that may have stuck the valve. The say they are putting in ethanal in the gas here now up to 10% and just a thought. I know I had a small engine 0 rings swell up on me on my lawn mower. I see if you buy 91 octane it is free of ehanal. Hope this fixes it. I did not make any difference before I put the sending unit back in place so I am on to something with you help. I thank you again for your guidence in this matter. I will owe YOU one . Thanks and I will keep you up dated this week as soon as I do this. I will check back here to see if you have some tips or things I need to know and follow. Thank Again Roy
switch over device I think my switch over valve is not operating correctly. It runs good on right tank and cuts out as i try to change the tanks over. Hope this will fix it. I was wondering can one clamp the fuel lines off and make this change with out dropping the tank? And the ground wire is that on top as well? Thanks for any info.
I agree with changing out the cutover device, these things fail all the time, if you got a new one, change it, good PM, but I don’t think it will fix your problem. Based on what you have done so far I suspect that you have low fuel pressure coming from the left tank. The pressure should be somewhere between 9 and 13 PSI. I strongly suspect the small hose that connects the output of the left tank fuel pump to the metal supply line coming out of the tank has failed. Unfortunately this is not an easy thing to check out. You must drop the fuel tank or remove the bed in order to gain access to the top of the tank. I would opt for dropping the tank. Here are your options from this point: 1. Install a pressure gauge in the system in order to confirm that low fuel pressure is the problem. There are test kits available that install in place of the fuel filter that allow you to verify the pressure on the supply line. I have made one up from parts purchased from the local Home Depot. 2. Drop the tank or remove the bed and check the hose. If you drop the tank, replace the hose even if it has not failed yet. If you pull the bed, replace both hoses. This is a very common problem and will bite you in the a__ at some point. Please post what you find; I’m very interested. As for the one you owe me, Michelob Ultra please.
I'm curious, is pump power switched through the tank valve? I have a 1983 K30 that runs factory dual tanks. I don't have fuel injection, obviously. I believe the tank valve is a screw motor. As the tank switch is pressed to seat for the the other tank, the valve switches over to close the fuel port for the previous tank and open for the selected one. Simultaneously, the contact(s) for the sender opens for the previous tank and closes for the selected tank. While the motor is running, have you listened for fuel pump operation for the bad side, switched to the right tank or to the left? My tank valve works so-so. While running on one tank and switch to the other, one of three things can happen: 1) The tank switches and my gauge switches as well. 2) The tank switches but the fuel gauge reads way past full as an open circuit and I am unsure of my fuel level. 3) The tank is switched, the fuel gauge switches but apparently the valve did not fully operate as the vehicle will stumble and run out of gas. Obviously, at that point, I have to switch back to the other tank. The electric fuel pump, which I don't have, adds another parameter and I'm not clear on the logistics. I assume the pump power is switched based on tank selection as I would be suprised if both pumps would run as only one tank is feeding. Perhaps my experiences may shed some light on something. Maybe your valve is not fully switching, like mine.
On an 87 with fuel injection the fuel tank selector valve switches four things, supply line, return line, pump electrical feed, and the gauge lead. Guam52 stated that when he switches to the left tank the engine stumbles and misses. Based on this statement I think the power for the left pump is getting switched. If the pump power didn’t switch, the engine would die immediately. Stumbling and missing are signs of low fuel pressure. It seems to be common problem with these selector valves that when they fail they will continue to switch the supply, pump power, and the gauge lead, but the return port hangs up blocking the return path to either tank. The electric pump is capable of putting out over 30 pounds of pressure and with the return path blocked off that stresses all of the elements in the supply line. Typically the weakest link in the supply line is that small hose that connects the output of the pump to the metal line that exits the tank because it is submerged in gas all the time. That hose will split causing a low pressure condition. You can elimate the selector valve by bypassing it. Replacing the selector valve will correct the return blockage problem, but you will also have to replace the hose in order the get the fuel pressure up where it should be. I may be all wet here, could be a weak pump or flakey valve, but I have seen this problem on a couple of trucks I have messed with. GonicGM, I have never messed with a duel tank system on a carbureted truck but base on what you have stated, it sounds to me like the selector valve or the switch is bad. I would try the switch first; it’s less expensive and pretty easy to change. Hope I don’t sound like a smarta__ or a know-it-all, I’m just trying to help you guys fight the battles that I have already fought. Don
On an 87 with fuel injection the fuel tank selector valve switches four things, supply line, return line, pump electrical feed, and the gauge lead. Guam52 stated that when he switches to the left tank the engine stumbles and misses. Based on this statement I think the power for the left pump is getting switched. If the pump power didn’t switch, the engine would die immediately. Stumbling and missing are signs of low fuel pressure. It seems to be common problem with these selector valves that when they fail they will continue to switch the supply, pump power, and the gauge lead, but the return port hangs up blocking the return path to either tank. The electric pump is capable of putting out over 30 pounds of pressure and with the return path blocked off that stresses all of the elements in the supply line. Typically the weakest link in the supply line is that small hose that connects the output of the pump to the metal line that exits the tank because it is submerged in gas all the time. That hose will split causing a low pressure condition. You can elimate the selector valve by bypassing it. Replacing the selector valve will correct the return blockage problem, but you will also have to replace the hose in order the get the fuel pressure up where it should be. I may be all wet here, could be a weak pump or flakey valve, but I have seen this problem on a couple of trucks I have messed with. GonicGM, I have never messed with a duel tank system on a carbureted truck but base on what you have stated, it sounds to me like the selector valve or the switch is bad. I would try the switch first; it’s less expensive and pretty easy to change. Hope I don’t sound like a smarta__ or a know-it-all, I’m just trying to help you guys fight the battles that I have already fought. Don
Fuel switching unit!@! Ok I had time to get under the 87 chevy 3/4 ton 4x4 and change out the fuel selector valve with all those hoses. Some job to ! I also changed the switch on the dash as well. Before the switch under the passengers seat so that told me something there. As soon as I turned the key on and tried to change the tank the fuel gauge jumped over to the full level. So I switched it back and it jumped over to the 3/4 full for the right tank. I drove it tonight for about 5 miles and it was smooth and gauge was looking good. I will take it for a drive tomorrow and burn up some fuel in the left tank and see if it is really using it from that tank. If so the gauge will start to drop down. That will be the next test. But the stumbling has stopped , so maybe the switch was not opening up all the way. I know that((( just before all this happened ))))I could see the fuel gauge was ((slow to move)) each time I changed tanks. Now its instant jumps to the next fuel level of the tank I select. Very quick readings. So I will get back to you tomorrow after my drive and fill up on that tank. Looks good so far. I thank you very much for all these answers and tips and just your thoughts and opinions. This is a great place to help us chevy guys out. Wish I could repay you some how! Thanks Roy
Don, As everything happens at the valve except the 12 v feed and ground that you swap when you press the selector switch which is momentary, we both know that the switch works, for 81-up that is. Apparently, the valve doesn't operate to 100% limit leaving the fuel sending unit contact(s) open (reads over full on gauge) and a restricted/closed fuel port to feed the pump, so it hesitates, runs out of gas. I know mine does this, for years, and know to expect it. I just switch back to other other tank, then switch back again and most of the time it is okay. I have too many other things that need greater attention to mess with this now, probably ever. Thanks for your contributions to the thread! Gregg
Dual Tank Switching Methods by Years GM used at least three different tank switching techniques for factory dual tank trucks. 1973: had a mechanical valve, dash controlled with cable(s). I'm unsure or don't remember how the sending unit swapped. This setup may have been used in 1974, I'm not sure. 1974 or 1975 to 1980: these trucks had an electrical valve. It was a metal case valve, self (case) ground with one wire to it. The dash switch switched everything. The switch did not have the spring momentary depress like the 81-up. It just clicked to one tank or the other. The valve was a solenoid. It was either energized or it was not. I do not remember which tank was default. Anytime the ignition was off, it would switch to that tank, selected or not. If it was selected to the non-default tank, it would switch back over once the ignition was turned back on. The sender was actually selected at the switch. If you had a faulty tank solenoid valve, you would switch to the other tank, the tank would not be physically swapped but the sender would as the switch was thrown. You would run down the gas till you run out while the gauge never moved. There were obvious problems with these. 1981-1991: these have the plastic valves. Power and ground are swapped at the switch, valve not self (case) grounded. The sender swaps at the valve. If the valve doesn't operate, the sender is not swapped. The fuel gauge will read the tank it is feeding from. The switches for these are two position with momentary spring depression for each, to momentarily energize the valve. It is not constantly energized like the 80-back. Without looking at harnesses, I can't be sure but I believe the 81-86 had a seven wire connector, two power, two ground, sending unit return, left sender, right sender, IIRC. I have an 87 harness but don't remember how many pins the connector was.
Its working great now. I got time to use the truck and the left tank gauge seems to be working. I had not been able to us any gas out of that tank for a while , but now it is working fine. I changed the switch and the fuel switching valve under the passenger seat. Its great when you can get help like the guys on here, chip in and share answers and tips. Great place to have and to use and count on. Hope what I have shared has help someone else to. Thanks Again Roy
I have a problem with my 87 too. My left tank works and as soon as you flip it to the right tank the truck dies. I replaced the switch in the dash with a new one from GM didn't change anything. When I switch it from tank to tank the fuel gauge does change from tank to tank so I don't think the valve is the problem. I personally think the fuel pump in the right tank is bad I can never heard it when it's selected to right tank. So, does it sound like a bad fuel pump to you guys too?
I you have duel pumps and it does not make any noise check the conection at the pump good clean contacts and grounds. if all is good then the pump could be no good.
