Fuel pressure issues

[Redchevy87]

I’m having issues with fuel pressure. I have a gauge right before the carb and sometimes it doesn’t even come off of zero when the truck is running and sometimes it jumps around but rarely gets above 2 or 3 psi. Sometimes the truck seems to be running fine and sometimes under load it falls flat on it’s face and feels like it just ran out of fuel. I replaced the pump and the filter with no change. I ran a line from the pump to a gas can to try to rule out it being a bad pump even though it’s new, and I got a rock solid 6.5 psi on the gauge. The only thing I can think of is the sock in the tank could be clogged. Does it sound like I’m on the right track? I just though I would get a couple more opinions before I go through the hassle of dropping the tank, which is of course full.

[cwcarpenter98]

When was the last time you replaced any rubber lines in the system? Have you checked the hard lines for any pinholes allowing air into the system? If those are good, I'd check the pickup in the tank next

[AussieinNC]

I would be suspecting the strainer sock on the bottom of the tank pickup....

There are also two rubber hoses under the floor where the tank lines exit the cab and start their journey forward....

I have seen these hoses collapse and allow the feed line to the pump to cause a kink in the rubber hose, effectively acting as a restriction.

[Redchevy87]

Just realized I didn’t mention I’m talking about my 87 c10. I think the only rubber lines I haven’t had my hands on are the ones between the sending unit and the hard lines and that also means dropping the tank.

[AussieinNC]

If you have a compressor and air nozzle drop the air pressure down to around 10 psi and remove the rubber hose from the pump that is heading back to the tank...

remove the gas cap

blow air slowly thru the gas line and any trash that has s collected on the pickup sock in the tank should dislodge.

If the truck runs ok after this, you should drop the tank and clean out the trash or it will just do the same thing in the future.

[Redchevy87]

Ok, I tried that and it didn’t have much of an effect. I could hear air going into the tank, but when I hooked everything back up I still had barely enough fuel pressure to keep it running.

[AussieinNC]

Sounds like the pickup tube in the tank has split/broken....

How much fuel is in the tank?

Do the symptons go away if you run a line from the pump direct to say a gas can full of fuel?

[Redchevy87]

The tank is about 1/2 full. I ran it off of a gas can and it was a solid 6.5 psi on the fuel pressure gauge. The only other thing I could think of was maybe the pump is faulty and is weak and couldn’t pull all the way from the tank. But I hooked it up to a vacuum/pressure gauge and cranked the engine- I got about about 15 psi on the pressure side and it held good vacuum so I think it’s ok.

[AussieinNC]

Looks like its time to drop the tank and pull the sender / pickup...they sometimes crack where the pickup tube connects to the mounting plate.

[verdell]

Sounds like a good time to replace all fuel lines, filters, clamps and connectors from inside the tank all the way to the carb. Maybe even pick up a new pressure gauge.

[Redchevy87]

Thanks. I assumed I was going to be dropping the tank, just wanted to make sure I wasn’t overlooking something before I got into it.

[AussieinNC]

It may pay you in the long run to replace the old tank with a new unit....

Aftermarket tanks run around $90 and new sender units and pickups run around $60 depending on how many pipes go into the tank...1 or 2 or 3...

If you do go with new tank, make sure you order the anti squeak strips...item 14 on the page.

See LMC page as an example

https://www.lmctruck.com/1973-87-che...and-components

[Redchevy87]

Mystery solved. The previous owner that switched my 87 from tbi to carb used a piece of fuel line to rig the tbi sending unit to work with a mechanical pump in it basically turned to black goo and split. Fingers crossed, it seems like replacing that with a proper mechanical pump sending unit seems to have solved my problem.

[AussieinNC]

Yep...ethanol additives in todays fuels will destroy old rubber lines as you have just found out.

Might be an idea to change any other older rubber lines anywhere in the system, especially the return line if you are using one.