Fuel pump help

I developed a hard start/stumbling issue last week.

Symptoms:

Once in a while it's hard to start, either warm or cold;
Idles just fine, will run fine for a while;
Develops stumbling and hesitation after hard acceleration;
Will smooth out when light on the throttle;

My first thought was a clogged fuel filter, but I only had 8,000 miles on the filter currently installed. However, to rule that out, I changed the filter, but it didn't help. In fact, it seemed to get worse.

Initial tests of the fuel pump showed good: I opened the fuel drain valve with the engine running, and it didn't stall. However, after changing the filter and driving it for a bit, I did the test again, and this time it did stall.

I have Kennedy's OPS harness, so I can tell when the pump is running. I can also easily push a button to manually run the pump, which aids in filter changes. I was able to bleed the system after changing the filter, but it didn't seem to have as much flow as normal out the drain valve.

I decided that it must be time for another lift pump. The first one I installed when I bought the Tahoe lasted 35K miles before it quit. This one had 45K miles on it.

I replaced the lift pump, it started right up, and ran great. I drove it hard several miles, and never stumbled.

Today I drove it again since changing the pump, and after a few miles, the same symptoms appeared. Now when I open the drain valve, the engine stalls again. It's hard to restart. When I do get it started it idles great, but after holding RPMS above 2500 for a while, it starts cutting out and acts like it is starving for fuel.

I don't seem to have normal flow out of the drain valve when running the pump manually. I added 5 gallons of fuel to the tank to make sure gauge is reading correctly (now shows 1/2 tank).

Fuel tank and pickup tube/sending unit were replaced 15K miles ago due to a leak in the tank.

I did verify that the pump is running when the engine is running. In fact, when I unplug the pump harness, the engine stalls out pretty quickly.

So, did I purchase a faulty pump? Or do I need to look elsewhere for a problem?

My plan tomorrow night is to disconnect the outlet side of the pump and try to pump fuel into a bucket to see what kind of flow I have. I'm thinking I have either a restriction on the suction side of the pump, causing it not to be able to pull fuel from the tank, or a restriction on the filter side of the pump, causing it not to be able to push fuel to the injection pump.

Any other insight would be helpful.

Thanks,

Casey

Go through with your LP test. It is possible you got one that's not up to par.

After that, if it checks out, I'd suspect the IP.

I have had the "sock" in the tank plug up and really restrict the flow.

Quote:

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Originally Posted by bmoeller

I have had the "sock" in the tank plug up and really restrict the flow.

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I thought of that, but I just replaced the whole pickup/sending unit assembly 15K miles ago. It's hard to believe that the sock could be clogged after 15K miles when the first one went over 200K miles (I assume it hadn't been replaced before).

You never know, stranger things have happened!

Casey

Quote:

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Originally Posted by bmoeller

I have had the "sock" in the tank plug up and really restrict the flow.

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The sock is nothing more than a clod sifter (coarse screen). His is new, or it would have been suggested. It takes many years for the sock to be an issue. If the system were so contaminated to plug a "new" sock, other very obvious symptoms would be present, and long before the sock plugs. A first indicator, for example, would be immediate fuel filter plugging, and obvious evidence of the contamination in the filter bowl, and on the filter element. I've seen this happen on a few occasions, and there will be no doubt of the source.

If clean fuel at the minimum pressure (anything above zero) is being supplied to the IP, stalling or poor idling/running conditions can be narrowed to the system between the IP fuel inlet and the injectors, or the return system. Excessive fuel return restriction/resistance can cause this. Something as simple as a failed fuel filler cap vent can cause this. Injectors themselves won't cause stalling on an engine up to operating temp. Bad injectors will generally cause very consistent poor running, across the board. If it starts cold, the injectors aren't the problem. This isn't to say the injectors are not due for replacement, just that they are not the cause of the current condition.

Maybe run a bit with the fuel tank cap loose. Sounds like the cap is not venting the tank correctly. That would explain the reduced fuel flow and stumbling after a hard run.

This is an easy thing to try first.

Could also have an air leak between the tank and the lift pump inlet.

IP is going bad,. my exact symptoms ,. then it quit while idling and wouldn't start till Ip change,.
best of luck,.

Nick

Finally had another chance to look at it today.

It's definitely a lift pump issue. IP is fine. I've also ruled out the fuel cap.

I've narrowed down the symptoms to this: I can bleed the fuel filter, start the engine, and drive normally for a few miles, and then it starts cutting out. I can stop the engine, check the bleeder valve on top of the filter, and air comes out. I can then prime the filter again until fuel comes out of the bleeder valve on top of the filter, and then drive normally for another few miles.

So, should I start checking for an air leak between the pump and tank, or look for a restriction past the fuel filter?

Casey

You likely have an air leak between the tank and the lift pump.

Could be a fuel line (metal with corrosion) or a rubber connector hose or possibly the tank pickup safety valve is leaking air.


Missy

I'm not sure what I did, but I seemed to have fixed the problem.

I went ahead and dropped the tank to check the rubber hoses and the fuel pickup tube. I took the sock off and cleaned it, seemed fine.... I made sure that the sock sat parallel against the bottom of the tank. Hoses looked good. Put it all back together, took the outlet side of the pump loose and turned the pump on. Seemed to have adequate flow. Hooked the pump back up, and everything seems great!

I noticed right away that now I have pressurized fuel from the bleeder valve on top of the filter. Before I had air coming out first, taking several seconds of the pump running to get fuel to appear. I could also tell the pump sounded different when running.

Drove it around for quite a while and didn't have a single hiccup.

The only thing I noticed was that there was an o-ring sitting on top of the tank when I dropped it down. I'm assuming it fell out of the rubber hose to tank connection when I took the hose off. Surely it couldn't have been sitting there for the last 15K miles, with no o-ring in that connection??

Either that or I didn't have a tight connection at the lift pump....

Cross your fingers and hope I eliminated whatever problem it was...

Casey

Quote:

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Originally Posted by arveetek

I can bleed the fuel filter, start the engine, and drive normally for a few miles, and then it starts cutting out. I can stop the engine, check the bleeder valve on top of the filter, and air comes out. I can then prime the filter again until fuel comes out of the bleeder valve on top of the filter, and then drive normally for another few miles.

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How are you bleeding it? are you sure the lift pump is running when the engine runs? Pressure from the lift pump should keep the air out, unless the leak is between the tank and the pump, but then I wouldn't expect bleeding to fix it...

Drove close to 75 miles today, no hiccups.

Still not sure what I did....

Casey

My "94 had the exact same symptom when the supply line from the tank to the pump got a pinhole leak and let air in. So I have to agree with Robin on this one;The supply line was either leaking or it had a loose connection. Check and see if you have any air in the filter anymore, maybe that is gone also. If it isnt, keep looking, that leak will bite you on the butt again.

Well, I have driven the Tahoe for 3 weeks straight without a single hiccup, until last night, when it started cutting out under hard acceleration. Then, this morning on the way to work, it was running pretty crappy again. When I got to work, I checked the fuel filter bleed valve, and the filter was full of air again. Took several seconds of priming the pump manually before fuel appeared. Air is still getting in the system.

So, whatever the problem is, I have not fixed it yet. The same symptoms are present after installing a new pump; but after dropping the tank and putting it back up, it has run great for 3 weeks.

Is it possible that the replacement pump is not pulling/pushing fuel properly? Or since the replacement pump did not fix the problem, that I should look elsewhere?

The fuel line between the tank and hard lines is a special hose w/steel swivel fittings. I assume this will probably be a special order hose...

Hmmmm.....

I know of one way to rule out the pickup and hose assembly including the feed line. Drop the tank, remove the pickup, plug the tank pick-up input and put the whole assembly under vacuum and see if it holds....or charge the assembly and hoses to 5 psi and hit it with bubble spray.

You have a leak before the pump 99% most likely ...unless the pump is failing and admitting air ( very rare).

It probably got better because you moved or retightened something and it worked loose again with time and vibration.

Stay at it.

Does it work when the tank is full and fail when it is (say) 1/4 full?

If the pump is working then it pretty much has to be on the suction side.

Even if the pump was not working, air can't get in unless there is a leak somewhere.

Quote:

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Originally Posted by JohnC

Does it work when the tank is full and fail when it is (say) 1/4 full?

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I think you have led me to the problem, John.

I hadn't noticed at first, but the problem does seem to only appear when below 1/2 tank. This last time it started failing between 1/4 and 1/2 on the fuel gauge.

Trying to find an air leak on the suction side, I took an old fuel cap and fashioned an air valve into it. I then pressurized the fuel tank to about 10 psi. No leaks found anywhere.

I then decided to drop the fuel tank again. I manually ran the lift pump to pump the tank dry. Hmmmm...upon pulling all the fuel I could out of the tank, the gauge still showed 1/8th full.

I dropped the tank, and found quite a bit of fuel still sloshing around. I measured 4.5" of fuel still sitting in the tank. I measured the pickup tube on the sending unit, and it showed to be around 3" from the bottom of the tank. I bent the pickup tube so that it was closer to the bottom of the tank. I also closely inspected the whole pickup tube assembly for any leaks, and could find none.

I was then able to pump out more fuel, leaving only 2" of fuel in the bottom of the tank. However, at this point, I checked the ohm reading on the sending unit, and it was still not showing empty. I tweaked the float arm slightly so that it now shows empty when the tank is actually near empty.

I put it all back together and filled the tank up. Seems to be running great again. Time will tell.

At this point it appears to be a combination of a mis-adjusted float and a pickup tube too far from the bottom of the tank. Basically, I was running out of fuel. Why it didn't give me problems until now, I'm not sure...unless the sending unit has changed resistance slightly over time causing the gauge to fool me for a while...

Casey

I'm still having issues. I've finally narrowed it down to being a problem only when I reach a 1/4 of a tank or less of fuel. It runs great until the fuel gauge shows 1/4. Then it starts sucking air. Last week I hit 1/4 tank returning from a Jeep trip towing my Jeep on a flatbed. Had ran great all day without a hiccup until I got down to 1/4 tank. I stopped for fuel, bled the system, and all was great again.

Last night on the way home from work, I got down to 1/4 tank again, and it started sucking air once again. I had to stop and bleed the system on the way home, and then again on the way to work this morning (have not fueled up yet). Then it died in the parking lot at work. Had to bleed it again to make it the rest of the way in. I've also left the fuel cap loose during this time period to rule that out once again, but it didn't help.

I removed the sending unit and pickup tube several weeks ago, and I could not find a problem, other than the pickup tube seemed to be off the bottom of the tank, which I adjusted. But there has to be a pinhole or something in the pickup tube that causes it start sucking air when the fuel level drops.

I guess I'll see about ordering another sending unit assembly. This one is only 15K miles old. I replaced it because I replaced a leaking fuel tank, and the old sending unit was making the gauge dance around sometimes.

Casey

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 ??

Keep us posted

Missy

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.