99 fuel psi?

[1987ss]

My tahoe is acting up again. its a 1999 with the 350 vortec with 138k on it.... its hesitating after driving for 2 hours.i dont know if the a/c plays a part in it cause the last few times it happened the air was running and it was parked ideling for about 45 min i tested fuel psi while truck was HOT and it was at 50psi at idle and 53psi under load. let it sit engine off for a few hours and psi when first started is 55psi.. had regulator changed 8 months ago [dealer] and fuel pump a little over 2 years ago with a oem part from GM.. is it bad again? Thanks

also.. when you come out in a.m. or after it has sat over night engine rotates more than normal before it starts..unless you turn key to on position with engine off for 15 seconds then it will fire right up.. left pressure tester on it over night a few times and it hold pressure it dosent leak it.. dont know if it helps any,just another issue i noticed.

[envoyxuv]

If you have the fuel injected 5.7 liter vortec engine, you may want to take a look at the mass airflow sensor. i had one a few weeks ago that would bog down and lose power after it had run for a while, it did have a check engine light on with a lean fuel trim code though. This could be a completely different case, but the little resistors inside of the airflow sensor (very very delicate) can sometimes get enough dirt on the front of them to skew the sensor output and cause driveability problems. You could use some throttle body cleaner or electrical parts cleaner to spray off the resistors if you find excessive dirt on them, and then retest it.

[1987ss]

Originally Posted by envoyxuv

If you have the fuel injected 5.7 liter vortec engine, you may want to take a look at the mass airflow sensor. i had one a few weeks ago that would bog down and lose power after it had run for a while, it did have a check engine light on with a lean fuel trim code though. This could be a completely different case, but the little resistors inside of the airflow sensor (very very delicate) can sometimes get enough dirt on the front of them to skew the sensor output and cause driveability problems. You could use some throttle body cleaner or electrical parts cleaner to spray off the resistors if you find excessive dirt on them, and then retest it.

wouldnt the maf throw a code for the maf? if it was maf would it idel fine still?

[MT-2500]

Originally Posted by 1987ss

wouldnt the maf throw a code for the maf? if it was maf would it idel fine still?

Maf does not always set a code.

But you have a fuel pressure problem for sure.
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.

[1987ss]

key on engine off 59psi
key on engine running 52 psi
driving under load or acceleration 55-60 psi
pinch off return 87 psi
leak down goes from 60 psi to 50psi after a hour
when driving pump never fadded
sprayed out mas air flow sensor with crc electronic cleaner
.
.
went and tested it on another 98 tahoe with a brand new fuel pump same exact readings x-cept leak down was faster on his..

[MT-2500]

Originally Posted by 1987ss

key on engine off 59psi
key on engine running 52 psi
driving under load or acceleration 55-60 psi
pinch off return 87 psi
leak down goes from 60 psi to 50psi after a hour
when driving pump never fadded
sprayed out mas air flow sensor with crc electronic cleaner
.
.
went and tested it on another 98 tahoe with a brand new fuel pump same exact readings x-cept leak down was faster on his..

Key on and engine running is low.
Specs are 60/66.
Make sure your gauge is reading good.
If pressure go up to 87 return line pinched it is losing pressure at the fuel pressure regulator.
Replace fuel pressure regulator.
And retest pressure.
MT

[1987ss]

Originally Posted by MT-2500

Key on and engine running is low.
Specs are 60/66.
Make sure your gauge is reading good.
If pressure go up to 87 return line pinched it is losing pressure at the fuel pressure regulator.
Replace fuel pressure regulator.
And retest pressure.
MT

I changed it last year beacsuse i was told to and no differnce it still read the same.. i have a previous posting from about 7 months ago when it did the same thing.. i dont have 400 to have it changed again.
drove it for 3 hours stright today and no acting up... can i put the pressure tester on it and pinch off both fuel lines with needle nose vice grips and see if it drops?
i thought you said its supposed to go up when pinched off??? had a buddy pinch while i read the psi and its NOT 87 it is 83 the firt time 81 second 80 the third.. i think he was dislexic..