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Sucking Air

A

alfadoc

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Warning, this is going to be long, but I think the chronology of events is important.
I have owned my ‘98 K2500 for fifteen years. Aside from one PMD failure (long ago relocated), and a couple of lift pumps, it’s been the paragon of reliability. My background, ASE Máster Tech, 40 years.
This summer I had the truck painted. While the bed was off, I thought I’d replace the fuel filter sock in the tank, with a genuine GM part. Got the truck put back together, and it ran like crap under heavy load. I assumed the injection pump had finally done its last, as it had presented symptoms about a year prior of getting ready to go. I had purchased one last year in anticipation of needing it, so I went on and swapped it out, a reman done by Cardone. Also did a new set of Bosch injectors, and Tygon return lines. No improvement. I felt certain I had a fuel delivery issue, so I pulled the bed back off, pulled the sender, and discovered the GM sock had collapsed. I threw it away, with a plan to install an aftermarket spin on filter before the lift pump.
Now, the fuel system seems to be sucking air. The truck stalls out after a short drive. If I jump the lift pump relay and crack the bleed on the FFM, I get air, then fuel. The return lines have bubbles.
I pressurized the fuel system via the tire valve in a fuel cap, and heard air escaping from atop the tank. Bed back off, I found a pinhole in the vent pipe that runs from the top of the sender to the fuel filler neck. I cleaned it up and soldered it closed. I also added a sealant around the sender lock ring, as it had from the factory. No help, same problem.
The steel lines running forward from the tank are not corroded, and seem in great shape. Pressurizing the tank produced no fuel leaks. I’ve also replaced the rubber line from FFM to injection pump, with a GM part. No visible fuel leaks anywhere when pressurized.
I have to get this truck running. What did I miss?
 
Welcome to TheTruckStop @alfadoc Glad to have you join!

it looks like you have done a lot of pleminary checks here, I assume you already have some clear hose on the IP (injection pump) return to see air bubbles in the line. when you had it all apart did you replace and verify the o-rings in the fittings for the fuel sender line and both sides of the LP (lift pump). lots of times these are overlooked.

do you still have the factory t valve drains installed for the FFM (fuel filter manager) ? if so, attach a hose to it and lead into a soda bottle. open it and trigger the LP to run. see how your flow of fuel is. I can't remember but there is a spec on the amount of fuel per time it had to pump out for it not to starve the IP. also get you a pressure gauge tied in line at the IP inlet hose and check your pressure. you can use a boost gauge for this as a temporary remote gauge so you can drive and watch pressures under load.

with key on you should see at least 8 psi for a factory LP. and then around 5 psi in normal driving. the PSI will decrease with the increased load but should NEVER go into a vacuum. having a vacuum will introduce air from any connections between the IP and the tank but those areas that can suck in air will never leak when pressurized as lots of us have found.

I'm sure others will chime in soon with more knowledge and help. plus explain things better than I can LOL.
 
dbrannon79 said:
I assume you already have some clear hose on the IP (injection pump) return to see air bubbles in the line.
Click to expand...
I do
dbrannon79 said:
when you had it all apart did you replace and verify the o-rings in the fittings for the fuel sender line and both sides of the LP (lift pump). lots of times these are overlooked.
Click to expand...
I did not detach the hoses from the sender when I removed to from the tank. O-rings on the lift pump fittings? I admit that I missed that, though I’ve just now seen a pic of it on another forum. I’ll be checking for that today.
dbrannon79 said:
do you still have the factory t valve drains installed for the FFM (fuel filter manager) ? if so, attach a hose to it and lead into a soda bottle. open it and trigger the LP to run. see how your flow of fuel is. I can't remember but there is a spec on the amount of fuel per time it had to pump out for it not to starve the IP. also get you a pressure gauge tied in line at the IP inlet hose and check your pressure. you can use a boost gauge for this as a temporary remote gauge so you can drive and watch pressures under load.

with key on you should see at least 8 psi for a factory LP. and then around 5 psi in normal driving. the PSI will decrease with the increased load but should NEVER go into a vacuum. having a vacuum will introduce air from any connections between the IP and the tank but those areas that can suck in air will never leak when pressurized as lots of us have found.
Click to expand...
I have a lift pump that functioned just fine for lots of miles. I might swap it back over today. The one I have on the truck right now is a new one from O’Reilly, and while I have not tested for pressure or rate of flow, I have observed that the volume looks pretty punky when I open the T handle.
if it turns out to be the lift pump, I’ll jump for joy. I’ve been battling this for months, and I’m completely out of ideas. Thanks for the suggestions. I will report my findings!
 
alfadoc said:
I do

I did not detach the hoses from the sender when I removed to from the tank. O-rings on the lift pump fittings? I admit that I missed that, though I’ve just now seen a pic of it on another forum. I’ll be checking for that today.

I have a lift pump that functioned just fine for lots of miles. I might swap it back over today. The one I have on the truck right now is a new one from O’Reilly, and while I have not tested for pressure or rate of flow, I have observed that the volume looks pretty punky when I open the T handle.
if it turns out to be the lift pump, I’ll jump for joy. I’ve been battling this for months, and I’m completely out of ideas. Thanks for the suggestions. I will report my findings!
Click to expand...
That's where a fuel pressure gauge shines. It can alert you to an issue, sometimes before it's noticable
 
Welcome Alfadoc. Sounds like the lads have you on the right course. For a temp fuel pressure gauge, connect at the T-Valve and run the hose so you can tape the gauge to the windshield. Obviously a purely Temp measure but it will give your pressure status as a before and after LP swap.
 
Swapped out the lift pump for what I believe to be a known, working unit. Bled the FFM and the injection pump, got the truck started. Tons of bubbles in the return line. Truck faltered and stalled. With the lift pump relay jumped, I got 4 psi at the tee.
I can’t a good steady stream of fuel out of the FFM at this point. It just sputters out of the bleed. Obviously there is a massive air leak somewhere, but again, with tank pressurized to 5psi with the test cap, there is no leakage of diesel fuel from under the truck.
I’m beginning to wonder if the pickup tube inside of the tank is cracked. Any thoughts?
 
I thought the sender unit was tested outside the tank when the initial pin hole was discovered and repaired. Well done on that by the way.

Could be one more inside the tank but, to make that sudden of a change it would have to be some hole. Seems odd that this issue would present itself suddenly after the sock swap and subsequent collapse (gas sock? Parts suppliers have made that mistake before).

I've never heard of it but, is it possible the lines are compressed at the tank outlet from the bed swap or, at least maybe collapsed (weakened from LP draw against the sock)?

Line check? If you're pressuring the tank at 5psi, are you seeing 5psi at the T (LP is flow on fail so, engine off)?

If not, you may want to walk the issue backwards to the tank. If there's weak flow at the T, then try to gauge flow/pressure at the IP inlet. If no change...rinse and repeat at the LP outlet.

With the sender out or, at least the draw line disconnected at the sender, blow the lines out. Cap the line and blow again with soapy water on the lines for the bubble reveal.

Obviously, I'm not ASE certified anything (even as an observer). As a novice though, these are the steps I'd be trying.

Please keep us posted.
 
That I didn't think of, but the pickup tube in the tank having a pin hole is very possible. something you can try before going though all the trouble of pulling the bed or tank again is disconnect the line off the back side of the LP and plumb in a jerry can of diesel but be sure to have the jerry can up at least the height of the frame or maybe even set if in the bed of the truck. re-prime using the jerry can as the fuel tank and see if that eliminates your air intrusion issue.

This will point you to the tank or the lines up to the LP as the problem area.
 
the LP can't pull fuel only push. you need a slight gravity feed to the LP. once the LP gets fuel and starts pushing, it will continue the flow like a siphoning effect on the tank.
 
I've used lift pumps to suck out the FFM and other stuff.

I never elevated the containers when I checked the lift pumps for flow by eliminating everything before the lift pump.
I didn't drive around or anything. I just used one of the adapters from the help section, put a hose on it and stuck it in a can of fuel
 
ak diesel driver said:
Technically an LP can suck fuel but most pumps do a much better job pushing vs pulling.
Click to expand...
That's fact. If I had a lift pump that was having trouble sucking out of a container on the floor, with 4 - 5' of hose, I'd be replacing it.

I also disconnect the discharge end of the lift pump & pump container to container.
It doesn't take long.
 
Just went through this in September on a '99 K2500. It was the seal on the feeder line at the sender on the tank. You can get the seal for a few $ at Napa, or they come with new sending units. I replaced both seals, fuel tank, feeder and return lines, in addition to the lift pump.

New Lift, Still Biting, Splash of Air

If you're interested in bypassing the stock FSU, I installed this draw straw kit on my truck. Have yet to use it as the IP died shortly thereafter and truck has been parallel parked ever since, but I was personally excited to get away from the stock setup...
www.thetruckstop.us

This sucking air was exposed when I installed an Air Dog Raptor lift pump with pre-filter. The old OEM style lift pump had failed and the increased capacity of the Air Dog Raptor exposed the faulty o-ring seal and was sucking a lot of air. I had done this same job on a '94 K2500. At this age of the trucks, it's best to replace everything.
 
Hooked a Jerry can of diesel up to the lift pump, bled the FFM and IP, started the truck up. After about 60 seconds of running, the bubbles cleared out of the return line. Finally some real, diagnostic progress.
Most of my automotive career I’ve worked on Italian and German cars. A diesel pickup has been a bit of a learning curve. I’m really grateful to everyone here who has helped point me in the right direction. I’ll report back when it’s finally solved. My plan now is to replace the fuel pickup/gauge sender, and both the rubber lines that attach to it.
 
Glad to hear you've narrowed the problem down some.

Not sure if you're in the salt belt or not, but I personally can't stand the saginaw flare & o- ring fittings on the tank pickup/ sender unit. They seize up in salt country and usually leads to replacing sender and lines. If you're feeling motivated and inclined to upgrade a simple fix is to cut off the fittings, slide on a new tube nut FIRST, then flare the lines using a simple flare tool for a less problematic connection.
 
just an FYI on replacing the fuel pickup tube / sender in the tank. all of the aftermarket ones out there will work (application specific) but your fuel gauge will bounce around when on the road due to the fuel sloshing in the tank! I experienced this when I replaced mine. used the Spectre brand from RockAuto. there is an alternative that iirc Leroy Diesel sells called the metrum rod sender. it's a bit more pricy than the factory replacement.

I've just dealt with the gauge movement on mine, even though it moves with the stop and go traffic, I have gotten used to it and know when I need to fill up!

take a good look at the rubber lines that connect to the sender assy too and replace all the O-rings.

not to mention look at the inside walls of the tank real good. if you see any rust or what looks like shiny hairs in the tank, that is the lining coming off. the tank will need to be replaced. RA sells the complete tanks for around $130 or so.
 
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