Fuel pump help

[arveetek]

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

[DmaxMaverick]

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.

[bmoeller]

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

[arveetek]

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

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

[DmaxMaverick]

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

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.

[ToddMeister]

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.

[JohnC]

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

[phantom309]

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

Nick

[arveetek]

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

[Robyn]

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

[arveetek]

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

[JohnC]

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.

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...

[arveetek]

Drove close to 75 miles today, no hiccups.

Still not sure what I did....

Casey

[Warren96]

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.

[arveetek]

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.....

[rameye]

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.

[JohnC]

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.

[arveetek]

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

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

[arveetek]

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

[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 ??

Keep us posted

Missy