1987 V10 leaking fuel after changing gas caps

[Gander]

So here’s what happened. Bought the truck a few weeks ago, noticed just the faintest hint of a gas smell a couple days ago. Took a look at both gas tank caps and they looked to be the OG ones with cracked gaskets and looked like garbage so I replaced them. This morning when I started the truck up I noticed a drip of fuel from what looks to me like one of the lines off of my LH fuel pump/sending unit. Switched to right side tank and no drip so I’m good for the moment but just doing some thinking. It would have to be the return line with the problem, right? I mean, the 1987 trucks with dual tanks have fuel pumps that run all the time so if it was a fuel supply line then it would still be leaking regardless of which tank was selected? I don’t think removing the bed is going to help much here since my leak is up under the cab. I wonder if the return is plugged and the treasure in the tank is pushing fuel out where rubber hose is clamped to metal? Maybe I could blow the return line out but then wouldn’t that just be blowing garbage back into the tank.

Anyway, thought? Suggestions?

 

[Ricko1966]

9So here’s what happened. Bought the truck a few weeks ago, noticed just the faintest hint of a gas smell a couple days ago. Took a look at both gas tank caps and they looked to be the OG ones with cracked gaskets and looked like garbage so I replaced them. This morning when I started the truck up I noticed a drip of fuel from what looks to me like one of the lines off of my LH fuel pump/sending unit. Switched to right side tank and no drip so I’m good for the moment but just doing some thinking. It would have to be the return line with the problem, right? I mean, the 1987 trucks with dual tanks have fuel pumps that run all the time so if it was a fuel supply line then it would still be leaking regardless of which tank was selected? I don’t think removing the bed is going to help much here since my leak is up under the cab. I wonder if the return is plugged and the treasure in the tank is pushing fuel out where rubber hose is clamped to metal? Maybe I could blow the return line out but then wouldn’t that just be blowing garbage back into the tank.

Anyway, thought? Suggestions?

2 separate pumps 1 in each tank so no fuel is going through left side supply or return line when you are running on right tank. I'd be changing all the fuel lines on the left tank,and making plans to do the right tank not to long after.

 

[Snoots]

I'd be changing ALL rubber fuel lines. The old ones aren't rated for that dam ethenal incested fuel.

 

[Grit dog]

Only 1 pump runs at a time. Just put new hoses on.
Don’t spend time thinking about weird scenarios like the end of your first post. And this isn’t related to switching gas caps unless maybe the vent lines are actually plugged which could pressurize the tank from heating up in the sun but would cause a vacuum theoretically while using fuel. But you’re gonna remove and replace hoses so blowing thru the vent lines is easy at that point.
Realistically you just bought an almost 40 year old truck that may or may not have been run a lot recently. It surely hasn’t had the fuel system gone through or you wouldn’t be replacing nickel and dime parts. Hoses are old. Hoses leak when old sometimes. Replace all hoses. Move on to next project.
Look at what you got. May be original or may be replaced/cobbled together? But iirc, LH tank has short hoses from sender to hard lines across to tank valve the. Short hoses again to valve. RH side similar.
Regardless you’re going to have to get to the senders which requires dropping tanks or moving the bed.
This is for you to decide which is easier. And what else you could accomplish if you dig into it either way.
Some are proponents of dropping tanks vs wrestling bed bolts. Again, think about what you want to do to the truck and plan accordingly.
If you’re gonna do any real work to it now or in the future, I’d test the bed bolts first and see if they are frozen. If you got a real clean rust free truck good chance they will come loose. Both my 86 and 77 were not froze up. If they are, new boot kit is like $20. Cut the old ones off and save yourself the headache for next time, imo.
Tank bracket bolts could be rusted too. But better chance of not spinning them. Tanks And senders could be on their last legs, or not. If everything works like it should which you allude to, I’d probably consider not touching the tanks/senders until you have an issue with one of them or just want to refresh the system.
That lends itself to moving the bed back. It’s like this. If it’s old and you touch it, much greater chance of having to replace it. If you just give it a little side eye and gently work your way around it, less likely to have to dive in and redo it….now anyway.

So it’s back to what are your plans for the truck and what can you get done much easier now while you replace some hoses and prevent extra effort in the future.

 

[Grit dog]

PS, if you go for the bed bolts, at a minimum have someone over 150lbs standing on a block on the carriage heads to keep them from popping up and rounding out instantly if they break loose. Or if you have the technology and the bed floor needs painted anyways, tack weld them to the floor to prevent spinning.

 

[Gander]

Yeah, it looks like I’ll be changing all the rubber lines on both sides. The truck is up off the ground a bit so I think I’ll drop them instead of taking off the bed, especially since it’s just me around here. Tanks actually look pretty clean to me, almost like they’ve been out before and not too long ago. Looking up at the lines running from the tank on the left side I see screw-on hose clamps so someone’s been there before. Anyway, yall are right, only way to do it right is to drop both and replace all the rubber lines. I’ll run the right tank dry, siphon out the left and fill the right back up, run it out again, so then I’m dropping 2 empty tanks.

Any recommendations on brands of pumps/sending units? Parts numbers? Might as well replace those while I’m there, I definitely don’t want to do this job and 6 months later be doing it again because a fuel pump went bad.

 

[Grit dog]

Well those are good signs! Also if both pumps sound and work well and gauges too it’s not likely they’re original.
I’ll add Summit is very good about returns. I’d consider buying all the parts from them. That way if you get it down and find new lookin pumps and feel good about them, Summit will take them back no problem. I’ve returned stuff half a year later because I forgot and no issue. They cool.

 

[Ricko1966]

Yeah, it looks like I’ll be changing all the rubber lines on both sides. The truck is up off the ground a bit so I think I’ll drop them instead of taking off the bed, especially since it’s just me around here. Tanks actually look pretty clean to me, almost like they’ve been out before and not too long ago. Looking up at the lines running from the tank on the left side I see screw-on hose clamps so someone’s been there before. Anyway, yall are right, only way to do it right is to drop both and replace all the rubber lines. I’ll run the right tank dry, siphon out the left and fill the right back up, run it out again, so then I’m dropping 2 empty tanks.

Any recommendations on brands of pumps/sending units? Parts numbers? Might as well replace those while I’m there, I definitely don’t want to do this job and 6 months later be doing it again because a fuel pump went bad.

You have just as much chance of a new part failing in 6 month as you do a working good used part these days. The days of I'm going to change it,just because it's old are over. When you go to drop your tanks. take pruning shears under with you. Unplug the sender/fp harness unbolt the filler from the bed,the 2 tank brackets from the frame and as you lower the tank with everything still attached and intact reach up with the pruning shears and snip the fuel lines. It will all come down and go up as 1 piece filler,brackets and all.

 

[Broken85]

I stall with an extra foot of hose. Just let it coil up on top of the tank. Don’t try to make 3” pieces of hose work. GM did that with the cab off. It’s even almost impossible with the bed off.

 

[Gander]

Well those are good signs! Also if both pumps sound and work well and gauges too it’s not likely they’re original.
I’ll add Summit is very good about returns. I’d consider buying all the parts from them. That way if you get it down and find new lookin pumps and feel good about them, Summit will take them back no problem. I’ve returned stuff half a year later because I forgot and no issue. They cool.

Pumps both work fine, PO only had it a year but one of the things he had done was replace the valve/solenoid/switch/whatever it is that selects between tanks so that works fine too. The sending units both work but not great. They’ll both read full when I fill them up but they drop down to 1/8 or so when there’s still 1/2 tank full of gas. Doesn’t bother me since I’ve just been filling up every 1/2 tank and then switching over to run the other tank down.

I’ll get all these rubber lines changed and then onto the next thing! Thanks!

 

[gmbellew]

if you are dropping the tanks and the sending units are not working right (assuming you diagnose it to bad sending units) id definitely replace them while the tanks are out.

 

[Gander]

So I solved this problem not too long ago and just remembered that I’d made a thread about it so here’s what the problem ended up being and how I solved it.

My truck, when new, was sold with RPO NA6 “High Altitude Emissions Equipment”. From what I can see, that means a slightly more complicated EVAP system. Instead of just having a purge valve on the charcoal canister it has a second valve on the vent line just ahead of the tank selector solenoid. With everything hooked up and correct, anytime vacuum is applied, that “tank pressure control valve” opens, allowing vapors to pass through the charcoal canister and on into the TBI.

Anyway, so the gas caps I replaced weren’t just worn out. They had a tiny hole drilled into each so the tanks would vent from the cap. Also, I didn’t know that a previous owner had disconnected the hose to the “tank pressure control valve” and left it loose. What this meant is that whenever vacuum was applied, the TBI was just sucking in dirty air from under the truck through the charcoal canister and into the TBI and since vacuum never reached the “tank pressure control valve”, as soon as I put those new gas caps on the tanks they began to build pressure until a line on my left tank popped and relieved the pressure.

The solution was to drop the left side tank to replace the line, AND to go from the TBI to the charcoal canister and on down to the tanks checking every line, checking every connection, making sure no lines were plugged and every connection was clamped and that both the purge valve and the “tank pressure control valve” opened up with vacuum. With all that done, it’s all good.

One more thing. My issue with incorrect fuel gauge readings was fixed when I replaced the fuel gauge and printed circuit while putting together a tach cluster for my truck. Replacing that printed circuit also solved a twitchy voltmeter and temp gauge as well.