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GMC Topkick 6.0 litre

Welder Dave

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I have a 1995 Topkick 6.0L gas with TBI. It has dual tanks and uses 2 different fuel pumps. The fuel pump doesn't come on when you turn the key. The gas was a little old so I drained it and got 2 new aftermarket fuel pumps. The passenger side pump doesn't have the electrical connector so needs to be modified slightly. Does polarity make a difference on a fuel pump? Also if one pump is bad will it affect the other one not working either in the fuel control or wiring? The drivers side tank appears to be the priority tank and wondering if the passenger tank even has to be used? Also wondering if the oil pressure switch controls the fuel pump in a Topkick? I wouldn't think so because normally the pump comes on for 2 or 3 seconds when you turn the key. Just trying to figure this stupid thing out. Put a distributor in last year and it was running great. Now no fuel. Very frustrating. Any help would be appreciated.
 

Welder Dave

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Yeah, everything I read says a DC fuel pump will run backwards if the polarity is switched.
 

Willie B

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Mount Tabor VT
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I don't know. A diaphragm pump only pulsates, the check valves control direction of flow. If it is a motor, it should be that field is in proper orientation to armature. It shouldn't be hard to check polarity & wire it correctly.

My C65 is older than yours, but two tanks with a manual valve to select. I believe it once had an electric selector valve.

Last week I was losing my mind, had equipment to move, and one hill was beating me. Tilt up hill, it'd die. Explored several possible issues, ended up days into the failure switching to electric fuel pump. It didn't fix the problem. I connected to key on power with an in line fuse. I had more than one problem. I now believe a cracked O ring seal around a fuel filter in the suction line hidden under the passenger floor was depriving the pump of prime. Tipping up hill was making the lift more difficult.
Choosing the low axle gear circuit in my ignorance made matters worse. When I needed fuel most, I was turning off the pump.

Mine like yours was built with a 366 tall block engine. I've been through a lot, mostly on that steep hill. Over 14 years, three engines, I've had many failures on one hill.

Timing was all over the place. One mechanic said there was so much cost fixing worn out valve timing, a rebuild would make sense.

A highly respected engine rebuilder said: "The 366 was tricky. Get them running just right, they had just enough power to drag a sick whore off a pi$s pot. All but the block bore was the same as 427. He had a block he would contribute.

A rebuilt 427 with a new distributer, (we discovered a tiny broken stop in the distributor) had all the power it needed, (except on that steep hill).

I now believe the cracked O ring on the filter may have been the problem 14 years ago when my air brake truck first died going up that hill. It died on a few other steep hills also.
 

Welder Dave

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I just use it as water truck so it has plenty of power... when it's running. Just frustrating that it was running great after the new distributor last year and now no fuel. A mechanic friend is coming out to take a look. If I could get it to run on just one tank I'd be happy.
 

Willie B

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I've used one 50 gallon tank 14 years, I figured twice the contamination with two.
 

Welder Dave

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Just not sure if it will work no pump hooked up in the other tank. It appears the right side tank transfers fuel to the left side tank and the left side tank is the priority tank to the TBI.
 

Willie B

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I'm not so sure how yours is set up. My 1976 had a mechanical fuel pump, and a T valve electrically controlled. Before I got it a mechanical valve had been installed. Lots of crawling, I found a hidden filter. It was a spin on canister that accepts a cartridge filter. Mine had a square cross section O ring in poor condition. I suspect it was the problem all along.
 

4x4ford

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aunts on the strip Currently drive a 1951 chevy pa
If I remember right on those it’s something to do with the fuel gauge that controls the pump to transfer but I don’t think there is any reason it wouldn’t run without the other pump hooked up
 

Welder Dave

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It has fuel injection so a little different than a carb version but having only 1 tank would be OK if it's possible.
 

Welder Dave

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Mechanic friend came out and changed the drivers side pump and it is working. Hoping it doesn't rain and he will do the other pump and fuel filter tomorrow. It was a huge relief hearing the pump run when the key was turned on. Apparently GM's are known for fuel pumps. Nice having someone familiar with disconnecting the fuel lines and stuff. He makes it look so easy.
 

Willie B

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I can't say much about yours. Electric fuel pump didn't help with mine. Like the stock mechanical pump, tipping up hill so it had to lift a considerable elevation distance, it'd lose prime. I had had trouble on steep hills numerous times. Several times I thought I had run out of gas when the tank was 4" from the bottom. I figured the lift tube stopped short of the bottom. Now it all makes sense. I didn't even know that filter existed. It's one of three filters. A square cross section O ring is supposed to seal, it had numerous tiny cracks.
 

Welder Dave

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Just an update on this. Put the pump in the main (drivers side) tank and truck ran great for about 10 minutes until I shut it off. Thought everything was OK and I could use only 1 tank. Next day truck started but didn't run as good and after about 5 minutes died. Never thought about it but went out today and used a rod to check fuel level in main tank. Almost empty, hmmm. Checked fuel level in passenger side tank that was bone dry and it now has about 3" of fuel. Figured the return fuel was going to the passenger tank. The fuel lines just have a tee. Mechanic friend came back out today and not only hooked up the new pump in the passenger tank but took the wire connector off the old pump and pried the mounting holes just slightly wider to fit the new pump. This saved having to cut the connector off and using 2 eyelets. He was leery to cut the wires because if it ever needed a fuel pump in the future it might require the connector. Sometimes it just pays to have someone with experience. Truck appears to be running great but will know better after I use it a bit.
 
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