Fuel pump help

Quote:

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Originally Posted by Robyn

The Pickup tube has a little valve in it part way up that is spring loaded.

This little device will allow the lift pump to take in fuel if the sock gets clogged.

If this valve leaks air (and many do) as soon as the fuel level gets below the valve it starts sucking air.

Get that pickup assembly out and have a good look at it.

My bet is that valve is faulty.

Was the new pickup assembly a GM unit or aftermarket ??

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Ah! I was not aware of that valve. When I took the sending unit out last time, I did not see anything like that. I still have the old sending unit, I will look at it tonight and see if I can see that valve. That definitely sounds like the culprit for sure.

Is that valve inside the pickup tube itself? I thought mine looked like a simple tube only.

The new assembly was an aftermarket Spectra brand. It was only $80 vs. the $500 + the dealer quoted me! However, I see I can get one from GMpartsdirect.com for less than $300.

Casey

Well, it's definitely a fuel pickup issue. I filled up the tank this evening, and she's back to running great again with no air in the system.

When it gets to 1/4 tank, I can barely keep her running. She sucks a lot of air into the fuel system. I can only drive about 6 or 7 miles before she starts sputtering. I can pull over, shut her off, and bleed the air out of the fuel filter, and she'll run good for another 6 or 7 miles. Topped her off with fuel, and all is well again; no more air in the fuel filter.

And the gauge is reading correctly as well. I figured that if I truly had a 1/4 tank of fuel left, it should take around 22.5 gallons to fill her up. I was able to squeeze in 22 gallons. So the gauge is correct.

Somehow, somewhere, the pickup tube is allowing air into the system at 1/4 tank.

Since it takes me nearly two weeks to burn through a tank of fuel, it's been hard to narrow the symptoms down!!

Casey

FYI: I have a fuel sending unit that we build from scratch. It totaly encloses all the electronics in a stainless steel mast. It won't help with your problem of sucking air, but it is much better than the stock fsu. I call it a Metrum Rod Fuel Sending Unit or MRFSU. Its designed so you reuse your stock pick up and just replase the FSU with ours.
I can email you/anyone the instructions with very good pictures also.

http://www.pmdcable.com/index.php?ma...products_id=19

Quote:

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Originally Posted by Robyn

The Pickup tube has a little valve in it part way up that is spring loaded.

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I inspected the old pickup assembly that I took out of my tank a while back. Just as I remembered, there is no valve in the pickup tube. The tube is just a solid tube from the top to the bottom.

However, after noticing how the sending unit is spot welded to the pickup tube, it makes me think that a possibility exists of the new pickup assembly having a pinhole in the pickup tube where the sending unit is attached. I didn't see anything abnormal when I had it out last time, but that's about the only thing left I can think of.

Right now, as long as I keep the fuel level above 1/4 tank, everything is peachy and rosy! :D

I'll have to see about ordering a new pickup assembly. In a previous experiment, I rendered my old OEM assembly useless.... :( Oh well.

Casey

Okay, I'm really starting to scratch my head now....

Yesterday, with 3/4 of a tank of fuel, air got into the system, and I've had to bleed the fuel filter twice now.

I'm really about to pull my hair out! Where is the air coming from?!?!?!

I have a new 'diesel fuel only' green fuel cap, but there is still a slight vacuum when I remove the cap. There's no other vent on the fuel tank, except the fuel cap, correct?

Casey

PS...just to update everyone on the symptoms: Engine runs great until it starts to stumble. Shut engine off. Open air bleed valve on top of fuel filter. Manually run fuel lift pump. Air comes out of top of filter. Takes 10 to 15 seconds of pump running to get steady stream of fuel coming out top of filter (and when fuel does appear, it shoots a healthy, steady stream). Close air bleed valve. Start engine and drive away, and everything is fine, until symptoms reappear sometime down the road.

I thought the air in the system only appeared when fuel level reached 1/4 tank or less. Yesterday it did it with 3/4 of a tank.

Sometimes it will go a couple of weeks without symptoms. Occasionally I will open the air bleed valve on top of the fuel filter with the engine off just to see what happens, and sometimes it will spew fuel out as if under pressure, which it ought to be.

The air leak problem seems to be more and more random. Read previous posts above, and I feel like I have ruled nearly everything out.

Is there anything in the fuel return system that could these symptoms?

Tank sock getting cloged? then when it does the next week link shows up as an air leak?
Maybe a rubber hose from tank to hard lines or hard lines to FFM. Also orings on FFM. I have nitrile oring replacements if you need a set LMK.
You can blow air from LP suction side of hard line back to the tank with fuel cap removed to knock any crud of the tank sock (atleast temporarily).

Are you sure you're not mistaking a small amount of pressure for a vacuum?

Eliminate the cap as a potential cause by leaving it loose.

As long as the lift pump is working, the only place air can get in as on the suction side of the lift pump.

I agree - there should be NO sign of a vacuum at the filler.

"Sock" is new, only 15K miles on the pickup assembly.

I have checked, rechecked, and triple checked hard lines, soft lines, o-rings, etc. from fuel tank to lift pump. I have dropped the tank several times now. I have pressurized the fuel tank to check for any leaks between tank and lift pump. Everything checks fine.

Yes, there is vacuum in the tank. When removing the fuel cap, if I slowly twist the cap off, no sound is heard until I pull up on the cap, pulling it away from the filler. Then I hear and feel the hissing. Letting go of the cap causes it to be sucked back down to the filler. It's not pressurizing the tank, because I would have the opposite signs.

It's not as bad as it was before I replaced the fuel cap, but it is still there.

I'll try leaving the cap loose for a while to see what happens.

Casey

For diagnostics maybe try a temp fuel system. Use a marine 5 gallon fuel tank & pump plumbed straight to the FFM and bypass everything else?

After pondering it a bit more....

Maybe I should re-phrase what I've been stating.

The reason I say that air is getting into the system is because the engine bucks and will almost stall, and then I have to manually run the pump to fill the fuel filter housing back up.

Looking at it from another view point: when the symptoms appear, the fuel filter housing is nearly empty. I open the bleed valve on top, run the pump until fuel appears, and then close bleed valve.

What's happening is that the fuel filter housing is not filling up with fuel. The engine is basically running out of fuel. I have checked every single component for air leaks. What if the system is not sucking air....what if the lift pump cannot overcome some other issue and is not able to pull fuel from the tank? Whenever I stop, open the bleed valve, run the pump, etc... it is overcoming whatever obstruction there is, pulling fuel from the tank, and filling the filter.

I have been assuming that the lift pump has been sucking air, since fuel is not present in the filter housing. If I DON'T open the bleed valve before manually running the pump, then of course it does try to compress the air already in the filter. If I open the bleed valve after running the pump, then air does come out. But if I open the bleed valve first, and then run the pump, it simply runs for a while before fuel appears.

So maybe I'm going about this all wrong. Maybe the problem is not an air leak. What could be preventing the lift pump from pulling fuel and pressurizing the filter housing?

It's as if the engine is using more fuel than the lift pump can supply. Eventually the housing empties of fuel, and I have to stop and manually refill the housing.

When the problem first surfaced, I assumed a failing lift pump. Replacing the lift pump did not make the problem go away.

I left the fuel cap really loose on the way to work this morning. By the time I got to work (22 miles), it was starving for fuel again.

I sure do appreciate all the help given so far!!!

Casey

PS....at the very beginning I verified I had a running lift pump when the engine was running...I better double check that again.

Dirty fuel sock!
What pressure do you have on LP? IMO a fuel pressure gauge is the most important gauge for diagnosing problems. You can red neck it and put a gauge on the drain from FFM & run it to the windshield so you can monitor PSI while driving. A LP might give normal #s in the driveway, but on the road is where you need to know. Coincidently I sell a LP & pre filter and LP bypass fittings if it turns out you need a LP LMK. Check your sock first or remove it and install my kit.

Leroy

Like I have mentioned a couple of times, the sock is nearly brand new. I have pulled it out twice, and she's clean as a whistle.

Only takes one fill up of bad fuel. Its got to be LP, dirty sock or air.
One more thing and you would know this with a gauge is the OPS could be faulty, but then again I believe your runs off the PCM after start up?

What year is your truck?

I installed the Walbro FRC-10 Fuel Lift Pump and bypass fittings from PMD. I also installed a Ricor pre-filter.

I posted some pictures under "coxfmly" showing the set-up with a bracket assembly I fabricated.

I did some more diagnostics tonight. I really wish I had a fuel pressure gauge.

Here's some more info:

With the engine off, running the lift pump manually produces a steady, healthy stream of fuel out of the top of the filter with the bleeder valve removed. It nearly shoots out the top of the filter about an inch.

With the engine idling, the fuel dribbles out of the top of the filter with the bleeder valve removed.

With the engine at high idle (1200 rpm), there is barely any fuel visible coming out of the top of the filter with the bleeder valve removed. Sometimes there is no fuel at all coming out, sometimes it dribbles out. I would call this 0 psi at high idle.

I verified that the lift pump is running when the engine is running. I installed a small light on the fuel pump circuit. So it's not an OPS or harness issue.

I'm really starting to lean towards a weak lift pump. It seems to be able to provide pressure when engine is off, but little to no pressure when engine is running. It doesn't seem to provide enough fuel at higher rpm.

Is there anything else in the fuel pathway or Injection Pump that would cause the fuel lift pump not to be able to keep up with demand?

I've been chasing this problem since before Thanksgiving. Perhaps I just replaced a failing lift pump with a new faulty pump. :(

Casey

On edit: The new pump is an Airtex brand. The previous pump was the same Airtex and it lasted 45K miles. The pump before that was an AC Delco and it only lasted 35K miles.

Sure is sounding like a weak pump. Get a cheap 0-15 psi gauge and hook it to the FFM drain with a hose long enough to reach windshield so you can see it while driving. OR, I do sell a Fuel Pressure Tap that screws onto the filter. It provides a very easy way to see LP pressure.
http://www.pmdcable.com/index.php?ma...&products_id=2

http://www.pmdcable.com/index.php?ma...products_id=17

http://www.pmdcable.com/index.php?ma...&products_id=9

How are you powering the lift pump with the engine off? The engine does not use that much more fuel at idle than when off, so something else is causing the difference in flow.

You could try powering the pump however you do when the engine is off, when the engine is running to see if it makes a difference.

Quote:

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Originally Posted by JohnC

How are you powering the lift pump with the engine off? The engine does not use that much more fuel at idle than when off, so something else is causing the difference in flow.

You could try powering the pump however you do when the engine is off, when the engine is running to see if it makes a difference.

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It's a 95. The pump will run anytime the key is in the "start" position. If the gear range is anything but P or N and you turn the key, the pump will (should) run with no cranking.