Air in fuel system

[christie656746]

Out of ideas on this one guys. I've got an 85 suburban with a low mile crate 350, rv cam, nothing special. Truck has ran great previously. Suddenly driving around town it started bogging and wanting to die, fuel filter after pump was mostly dry. Thought it was vapor lock so I routed a return line just before the carb to keep things flowing. Cold the pump fills the filter right up but after a little driving it goes dry again. Replaced the pump, found rust in the tank so that and the sender got replaced. Replaced all rubber and metal lines for the supply and return. All new hose clamps on everything. Nothing has fixed this issue. Put another pump on today to rule out 2 pumps going bad but nothing. Is there something I'm missing? I've replaced everything before the carb and it still appears to be sucking air somewhere. Any ideas on this would be much appreciated. 25 gallon rear mounted tank with mechanical pump.

 

[squaredeal91]

Mechanical or electric pump? And if electric
is it in the tank?

 

[squaredeal91]

Sounds like mechanical tho

 

[christie656746]

Mechanical. Next step will be putting an electric pump closer to tank.

 

[squaredeal91]

I would be suspicious of the rubber hose inside the fuel tank. I've found them to be culprits allot.

 

[christie656746]

Old tank was rusted through and leaking when I pulled the bash guard. So tank and pickup/sending unit was replaced. Along with rubber connecting line to pickup.

 

[fast 99]

Read my other post on pumps earlier today. Vapor lock is almost always caused by a fuel flow issue. Yes, any restriction can cause it. One thing to remember, fuel today is much lower octane than when these trucks were made. It is not designed for carbed systems aggravating "vapor lock". Although not a solution higher octane may help. I don't run lift pumps in either of my trucks. One is AC so under hood heat is up there.

 

[squaredeal91]

So very strange. Probably a stupid question. Bur is there any way the new lines could have been crossed with return or vent?

 

[christie656746]

Nope. The supply is 3/8, return is 5/16. Paid very close attention to the routing. The line that would have been the vent to the charcoal can had a few inches of rubber then a diff breather. Redid it like this with new tube and breather. A few weeks before this sudden issue. I took the family on a 700 mile trip with many mountain passes without a hiccup.

 

[christie656746]

@fast 99
I replaced the gm pump with a speedway motors high volume 2 hole and ran a T for the return line close to the carb instead of at the pump. When the issue first occurred I thought it could have been a bad tank of fuel so I topped it off with some premium. Since then I have ran a few different batches of gas through the system to see if any of my changes have worked. Then today I put a new Delco pump on just to see if I got a bad speedway pump. Same issue. In the few years I've had the truck running it's never vapor locked even up and over 12,000ft passes. But now I only get a few miles of driving before it starts to sputter.
I'll still check out your threads. I'll take anything I can get at this point.

 

[Matt69olds]

Remove the gas cap and drive it. If it drive’s normally, there is something screwed up with the vent.

 

[Copymutt]

Mechanical pumps pull (suck) the fuel. I have had that issue caused by a hose on the pull side that would suck air at its connection, yet it did not display wet with fuel.

 

[christie656746]

Never had a problem before but after new tank the gas cap was sealing. I drilled a 1/16in hole and have driven with cap on and off and neither makes a difference.

 

[christie656746]

Thought the rust hole in old tank was my source of air. Would explain the gas cap suddenly sealing up, cap is new, truck didn't have one when I bought it. Did all new metal lines this weekend because I've read that the suction side won't visibly leak but will pull in air.

 

[fast 99]

Has the fuel pressure AND volume been checked before removing "defective" pumps. It is possible to have pressure without volume if the check valves get contaminated.

Although rare what about fuel pump pushrod length or cam wear? Just tossing out ideas.