Fuel line has air mixed with gas

[cris2004]

I replaced a failed fuel pump in a 4.3, 1998 truck. The engine ran fine for three hours of repeated starting and driving.
The problem is a loss of power, the engine acts fuel starved. It wont run much above an idle.
I have a pressure gauge reading 55 pounds when all is ok, but a rapidly fluctuating 55 pounds when performance is poor.
There is air in the line I can bleed out. There is no air when running properly, of course.
I have replaced the filter, no codes lights are on, the vacuum is a steady 20 pounds at all times.

Anyone know how air can get in the fuel rail? This never happend before the pump died.
thanks so much

[rhandwor]

I think your pressure regulator is acting up. Search the forums Blue Bowtie has pictures and tips for replacing the regulator.

[cris2004]

Thanks.
I havent found that thread yet, but I have seen most regulator failures associated with leaking and low pressure. I understand there is some type of bleed off system on some cars, but I'm mostly surprised there's air in there.
More reading to be done!

[rhandwor]

My son has changed the pump twice on his 1997 5.8 and we never bled it. The 1998 pump I ordered the wrong year pump had a small bleed on the pump and a small hose. I just put it in the fill tube and it worked fine has been about six months.
The pump is submersed in the gas and should not be pumping air. All excess gas goes through the return line to the tank. Check your lines for leakage.

[cris2004]

I haven't found evidence of any leaks, the pump holds pressure nicely after shutting off motor.
It is submersed in gas, with three lines attached. I suppose one is the return you mentioned, maybe the third is a vent misbehaving somehow.

There is also a reservoir tank portion of the pump I was suspicious of. I thought it may need to fill. When the truck first acted starved, I filled the half tank up and it then ran great, till a sudden stop caused it to be starved again.

Thanks again! I may have gotten a bad pump as well.

[MT-2500]

Originally Posted by cris2004

I replaced a failed fuel pump in a 4.3, 1998 truck. The engine ran fine for three hours of repeated starting and driving.
The problem is a loss of power, the engine acts fuel starved. It wont run much above an idle.
I have a pressure gauge reading 55 pounds when all is ok, but a rapidly fluctuating 55 pounds when performance is poor.
There is air in the line I can bleed out. There is no air when running properly, of course.
I have replaced the filter, no codes lights are on, the vacuum is a steady 20 pounds at all times.

Anyone know how air can get in the fuel rail? This never happend before the pump died.
thanks so much

For it tio get air it has to be sucking air someplace.
Your tank may be low on fuel.
Add 5 Gallon or more of gas.

If it still does it and pressure is not up to par.
Pitch the aftermarket pump in trash can and get a good ac-delco/delphi fuel pump.
Good luck
MT

[cris2004]

When it started running poorly yesterday, I pulled over and bled the bubbles out of the rail. The truck will stall when I do this with the air in the lines, but not when all is well and the rail has pure gas.
I re-started the truck and bled the air out a second time with the residual pressure, and was able to drive normally another fifteen minutes.
I took the truck 13 miles to work again today without event.

The air may just be somehow trapped in the lines or in the pump body from the install.
The tank is full and the pressure is always the required 55 running pounds. There aren't any leaks, but a friend did suggest a schrader valve might leak are in like a venturi. I don't believe that the case here since I'm running with the pressure gauge attached.

If it keeps up and I don't find another solution I intend to finger the pump as you say. It's not a Delco as you guessed!
Thanks! I appreciate the help.

[MT-2500]

Air should bleed out first start up.
if it is getting air you have a fuel pump problem.
Your pressure is way to low.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

[cris2004]

It's a 1998 4.3 CSEFI system, the book says 55-61 pounds operating is correct. (I realize that isnt 'all that' lol)

I have 60-65 key on only, not starting
65 pounds at start up
58 pounds running, a steady needle when bled, fluttering a pound or two when running incorrectly.
The pressure doesn't drop after shutting off for ten minutes, drops 3-4 pounds after at least that long.
The lines are all steel, so I havent found how to test all out pressure. I do have an access panel in the bed so I bet I could run directly with some rig up but I have some confidence in the pump after seeing the other values.

I haven't check driving pressure yet, best done heading home tonight.

The trucks starts and idles fine at all times. It acts fuel staved running until I bleed off the bubble filled fuel, then works for some time again.

I'm hoping it's cleared up but I may just have to replace the pump again.
I'm wondering if a connection at the pump or tank seal could cause this.

Thanks

[MT-2500]

58 RUNNING IS IN DANGER ZONE.
The only way a fuel pump can get air in system is from where it sucks in the pump.
Bottom of tank or in the fuel pump resovior canister.
Run the full pressure check on the pump and after a 20-30 drive,
And make sure the pump puts of full direct pressure.
Good Luck
MT

[rhandwor]

Follow MT guidelines check full pressure if this meets specs some regulators can be adjusted. The new pump and old pump may have different full pressure output.
If you have a full tank and no leaks in the fuel line you don't have an air problem. Did any air blow out when you tried to bleed.
Your just running in circles getting no where. MT does a lot of chevy repair.

[cris2004]

Your right about the circles!
I havent had a chance to do the full pressure test yet, I was driving it to work and had to go back as it wasn't working well at all.

I did notice I could shut the truck off and it would run a bit after restarting.

I bled the system again thru the presure relief port, and 106 degress today it was a lot more air than gas in the line. It does run better after doing that but not for long.

There arent any leaks in the lines, and it hold pressure well, but I did see a bit of gas seaps out at the o ring seal on top of the full tank. I doubt thats normal, but could it be the cause? As you say the fuel is drawn from the bottom of the tank.

I'll have to do the pressure test at home.
There is definately a lot of air in the fuel line, after what you two have said it seems only the pump can do this outside of a leaking line. There may be a malfuntion internally, drawing air in from another line? I underatand the value of the full pressure test as it would expose this type of possibilty.

The access to the pump is very simple, I'm going to put the test hose in place of the presure line immediately out of the pump, is this acceptable or do I need to do it another way?

Thanks very much!

[rhandwor]

It maybe easier to do it at the fuel filter but I have an adapter.

[MT-2500]

Take all pressure reading at the test scrader valve.
The direct hook up at the pump is only for the pump full pressure test.
Do not run it at full pressure over a few seconds.

[cris2004]

I have been taking all the others at the valve, I didnt see a way to pinch/cut off the steel return lines to do the test.

Testing peak at the filter sounds like a good idea, so I wont risk the plastic fittings on the pump.
I'll get a peak amount, if it's not the 75-85, I'll politely return the new pump after getting a Delco.

Thanks gentlemen.