fuel pressure problems?

[Da Burb]

1996 Suburban. 140K miles. I am trying to narrow down a problem. The truck starts sometimes starts after it sits for a while. Today it did because I didn't have to go to work. The last two mornings it didn't. I got it running yesterday morning by pouring a little gas down the intake and it fired right up. It was hard to start yesterday afternoon when I leaving work. It has been spitting and sputtering with anything more than 1/4 throttle making merges on to the highway interesting. It also hesitates up hills trying to maintain its speed. I changed the fuel filter, plugs, cap, rotor and wires. They were due anyways. No engine codes that stick. I got an O2 sensor a while back but I cleared it and it hasn't came back.

Today I checked the fuel pressure and at idle it is 54 and about 2000 RPMs it is still 54. I pinch the return line and it goes to 62. It holds pressure when I turn it off. When I let it sit for a few minutes it doesn't drop below 50.

Pump or regulator? Any other idea?

[inafogg]

Originally Posted by Da Burb

1996 Suburban. 140K miles. I am trying to narrow down a problem. The truck starts sometimes starts after it sits for a while. Today it did because I didn't have to go to work. The last two mornings it didn't. I got it running yesterday morning by pouring a little gas down the intake and it fired right up. It was hard to start yesterday afternoon when I leaving work. It has been spitting and sputtering with anything more than 1/4 throttle making merges on to the highway interesting. It also hesitates up hills trying to maintain its speed. I changed the fuel filter, plugs, cap, rotor and wires. They were due anyways. No engine codes that stick. I got an O2 sensor a while back but I cleared it and it hasn't came back.

Today I checked the fuel pressure and at idle it is 54 and about 2000 RPMs it is still 54. I pinch the return line and it goes to 62. It holds pressure when I turn it off. When I let it sit for a few minutes it doesn't drop below 50.

Pump or regulator? Any other idea?

what is the pressure KOEO should be 60-66psi
whats the pressure w/regulator vacuum unpluged engine running

[Da Burb]

52 psi with key on. There isn't a vacuum line to the regulator it is under the upper intake manifold.

[inafogg]

pump pressure is too low,sounds like pump is getting weak.
try to prime it a few times before you start it see if that helps
but that would only point to a bad pump.make sure its a delphi or delco

Good luick

[j cAT]

Originally Posted by Da Burb

52 psi with key on. There isn't a vacuum line to the regulator it is under the upper intake manifold.

the regulator will no cause your pressure to be too low.

with high vacuum at idle the pressure is reduced slightly, with no vacuum on cranking engine when starting you get max pressure...with your 52-55psi this normally will not start your 1996 engine..

as mentioned you need the 60-66 psi of fuel pressure.

change fuel filter then replace the fuel pump with the delphi pump ///NO AIR TEX PUMPS ....

[Da Burb]

Today I took the upper intake off because I started it before I read JcAT's post. It was fine because I have had the intake off three time and never changed the fuel pressure regulator, silly, I know. While I had the intake off I looked at the injectors and they were not happy. One was totally covered over with smoots and the others were "dirty" to say the least. I cleaned them with a toothbrush and put them back. I cleaned the intake, changed all the gaskets, and put it back together. I checked the pressure again, KOEO and still got 53 but it wouldn't hold. Dropped immediately. I got it running with some gas down the intake and it the pressure NEVER moved off 52ish no matter what the throttle position was. I apparently have a few problem that I am working on. Fuel pressure regulator is new, injectors are clean and I made sure all my connections and gasket wont allow any air in. Next is the pump. I know JcAT, you said to change it, I will. :-) I went out last night to buy a Delphi pump because that is what was recommended, after buying a airtek. I will take the airtek back. Long story short, am I missing anything? Changing the pump after work tomorrow. I will update then.

[MT-2500]

Originally Posted by Da Burb

1996 Suburban. 140K miles. I am trying to narrow down a problem. The truck starts sometimes starts after it sits for a while. Today it did because I didn't have to go to work. The last two mornings it didn't. I got it running yesterday morning by pouring a little gas down the intake and it fired right up. It was hard to start yesterday afternoon when I leaving work. It has been spitting and sputtering with anything more than 1/4 throttle making merges on to the highway interesting. It also hesitates up hills trying to maintain its speed. I changed the fuel filter, plugs, cap, rotor and wires. They were due anyways. No engine codes that stick. I got an O2 sensor a while back but I cleared it and it hasn't came back.

Today I checked the fuel pressure and at idle it is 54 and about 2000 RPMs it is still 54. I pinch the return line and it goes to 62. It holds pressure when I turn it off. When I let it sit for a few minutes it doesn't drop below 50.

Pump or regulator? Any other idea?

Low fuel pressure can be caused by a bad/weak fuel pressure regulator or weak fuel pump pressure.

Also with intake cover off prime up fuel pressure nand look for leaks in there.

The fuel presure regulator should hold pressure at 60/66 lbs if the fuel pump is capable of producing 95-105 lbs of full direct pressure.
Block of return line or connect direct ti in line at fuel filter and check fuel pump dirrect - fuel pressure.
Only run fuel pump a second or two at full pressure.
If not up to 95 to 105 lbs. then replace fuel pump.

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 around 100 lbs pressure.
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 100 lbs or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

[Da Burb]

I took out the tank and removed the pump. The sending unit was looking old and warn so I replaced that too. I was there and it wasn't too expensive, good insurance. It was original. Put it all back together, put the gas back and it fired right up. I let it do its thing for a minute and work the air out and checked the pressure. It was 62ish depending on which way you tilted your head. I pinched the return line and it jumped to over 90 and the engine hesitated. The old pump didn’t know when I pinched the line. The test drive was good. It hasn't run this good in a long while.

Thanks for the help.

[MT-2500]

Originally Posted by Da Burb

I took out the tank and removed the pump. The sending unit was looking old and warn so I replaced that too. I was there and it wasn't too expensive, good insurance. It was original. Put it all back together, put the gas back and it fired right up. I let it do its thing for a minute and work the air out and checked the pressure. It was 62ish depending on which way you tilted your head. I pinched the return line and it jumped to over 90 and the engine hesitated. The old pump didn’t know when I pinched the line. The test drive was good. It hasn't run this good in a long while.

Thanks for the help.

You are welcome and thanks for posting back how it went.
If you replace it with the delphi Fuel pump module the fuel gauge unit comes with it.
The block off return line pressure test should be done with engine off.
Good luck

[j cAT]

Originally Posted by Da Burb

I took out the tank and removed the pump. The sending unit was looking old and warn so I replaced that too. I was there and it wasn't too expensive, good insurance. It was original. Put it all back together, put the gas back and it fired right up. I let it do its thing for a minute and work the air out and checked the pressure. It was 62ish depending on which way you tilted your head. I pinched the return line and it jumped to over 90 and the engine hesitated. The old pump didn’t know when I pinched the line. The test drive was good. It hasn't run this good in a long while.

Thanks for the help.

your repair of this issue with this vehicle is quite common with the testing results you gave in post #1.

good luck and keep the fuel above 1/4 ...