Fuel pressure test results (need some help here)

My truck is a 2000 Silverado Z71 4.8L with 90k miles on it. It's had all factory service done on time including a tranny flush last year.

I have been experiencing the following symptoms for months:

Lack of power - feels like I'm towing something going up a hill. Sometimes it's bad, sometimes not so bad.
stumbling idle - not horrible, but it's there.
surging - most noticeable at cruising speeds (55-65) and going up hills.

I went and rented a professional fuel pressure test kit from Autozone and here is what I found:

Key on - engine off - 55psi and holds steady
Engine idling - 49psi holds steady
Turn engine off and let sit for 5 mins - Pressure slowly climbs to 55psi and holds

Some other things I tried with odd results:

Slowly increase engine rpm's to 2000 - pressure immediately jumps to 56psi, then drops to 47psi and holds steady. This is a fast thing - up, down, hold at 47 all within the first second or two of touching the throttle.

Removing the vacuum line to the fuel pressure regulator with engine running causes the pressure to go to 58psi and hold steady there.

Over the months this has been plaguing me I have had 2 different mechanics look at it and neither could solve the problem. They did do some things to try and fix the issue though and they were:

Replace fuel filter
Replace spark plugs
Replace fuel pressure regulator (autozone part, not GM)
Chemical injector clean (the kind where the truck runs on the chemical for about 20 mins)

The specs for my truck according the the test kit are 55-62psi so I'm guessing 49psi is not good. Before I spend $500 to have the pump replaced I'd love it if someone could confirm this sounds like the issu and not something else like the regulator.

Thanks for your time:biggrin:

I found two for around $230, see http://www.autopartswarehouse.com

You might beable to buy a pump with low hrs/mileage at a Salvage yard.

I bought a brand new fuel pump for $45 off Ebay, looks exactly the same as the $300 Delphi I bought at NAPA before it busted 1 month after the 12 month warranty, don't waste your money!

Bert
http://tinypic.com/ei2g51

I appreciate the replies, however I was really hoping someone with more knowledge than I would chime in with "Yes, it sounds like the fuel pump for sure" or "No, it could be a faulty regulator" or maybe some other things for me to check.

Money is not the issue. I don't mind spending the money on good parts and having someone do it right. I just want to be as sure as I can that it's the fuel pump before I drop $500 (part and labor) only to find out it was the $80 regulator that I can replace myself (or some other issue).

I did buy some MAF cleaner today and tried that. No luck.

I do want to add this truck has never thrown any codes at all. This is why the 2 mechanics that looked at it came up with the "It's fine" answer I'm sure.

*edit* After spending an hour or so reading through a bunch of old posts here it seems that the pressure climbing after I shut the engine off is not a normal thing. So what does this tell us? Also, from what I could gather when I unhooked the regulator vacuum line and the pressure went up and held at 58psi that indicates the fuel pump is good? Is that correct?

Anyone? I'm hanging on to the tester kit in case someone thinks I need to do some further testing.

I am not an expert and I don't claim to be one. I am basically a back yard mechanic and worked my way through college as a mechanic.

I have the factory manuals for 2000, and if you read the section on fuel pressure, 52 may actually be the low end of the spec. On my 2000 Yukon, at idle it shows 50 on my gauge and it runs fine with no issues. Yes the Vortec L31 requires 60 PSI to run right, but I don't think that is the issue with the LM7.

If you remove and plug the vacuum line to the fuel pressure regulator, how does it drive?
If the fuel pressue is 58 and you still have an issue, I don't think the fuel pressure is the issue.

Have you driven with the fuel pressure gauge on it to see what the fuel pressure is when the stumble happens?

What is the pressure if you pinch off the return line (only do this for a few seconds)?

I hate to see you replace the fuel pump when it may not be the issue (although it is a possibility).

Was a back pressure test performed?

Did the 2 previous shops use a scan tool to see what the engine sensor were reporting when the stumble happened?

Has the throttle body been cleaned? When mine starts to idle rough, it usually means it is time to clean the throttle body.

Was a smoke test performed to see if there is a vacuum leak at the intake gaskets?

Is the MAF clean?

I am not sure if I helped you or not; however, my SUV runs fines with a fuel pressure of 50 at idle.

//2000CAYukon

To answer a few of your questions:

I didn't drive it with the vacuum disconnected. I will try that if it won't hurt anything.

The shops did use a scan tool. One was actually my buddy that works for a GM shop here in town and we drove around with the tool connected together. There were no codes, nothing out of spec, and no misfires showed.

I haven't tried cleaning the throttle body other than cleaning around the plate when I start getting the sticky throttle deal (done that twice). What is the best way to do that w/o messing anything up?

MAF is clean.

No backpressure or smoke tests have been done.

Thanks for the reply!

Fuel pressure measurements are always taken when the engine is cold with the ignition in the run position, the engine off, and the fuel pump running. This keeps the injectors closed, and the fuel pressure regulator at its highest regulating pressure. 55psi to 62psi is spec and you've got 55psi. Take into consideration, error of the Autozone "professional" tester, you're probably good to go. Leakdown should not drop below 50psi for 3 to 5 minutes after the fuel pump shuts off. The fact that it didn't drop most likely indicates leakdown is acceptable. The reason your fuel pressure increased is because the engine was warm which causes the fuel in the fuel rail to expand and increase in pressure. GM does not publish any "engine running" fuel pressure specs because there are far too many variables involved. Removing the vacuum hose from the regulator while the engine was running increased the fuel pressure above the reading with the engine off because with the engine running, the voltage is higher at the fuel pump. From the tests you've run, the fuel delivery system is not the problem, save your money on a pump and regulator. ;)

the fuel pressure of 49 psi is too low...on these forums there are many posts describing in great detail how to properly test this fuel pressure issue...

with no vacuum on the regulator you will get no regulation high pressure.. with the engine idling most find the pressure to fall in the spec of 53-62 psi....

what needs to be done is drive with the fuel pressure guage connected and see what happens at high power demand...

also low fuel pressure will not throw codes that show this fault...
your drivability issues are very much indicating a fuel delivery problem...

don't forget low voltage /poor grounds at pump are a common issue as well...in addition to the fuel pump relay...

when you replace the fuel pump use the delphi pump , be careful as there are chinese clones out there....also replace the relay...

Ok I went and drove it with the guage taped to the windshield and here is what I found:

Idle: 49psi
Driving around town: 52-54psi
Accelerating: 56ish
Going up a long steep hill where it bogs down: 60psi

The pressure never went below 48 even when I stepped on it hard. It actually shot up to 56 or so and held there the whole time I stayed on it.

The hill I go up everyday to work is where I really notice problems. I can be crusing at 50+ mph and just starting up the hill it bogs right down. I can put my foot all the way down to the point where it's about to shift into passing gear and the engine doesn't change at all. It's like there is nothing for several inches of pedal. I have to shift into passing gear then it goes ok but wants to shift right out. When it does it's back to bogging.

Also, I took it to a tranny shop today and had them do a full check on the tranny including road test and they said the tranny is solid and hitting all the shift points perfectly.

So what do you guys think based on this new info?

See post #8

Gotcha. Any idea where to go from here? I was thinking about calling the local muffler shop and having them do a backflow test next. Does that sound like a logical next step or should I do something else?

You can try removing the O2 sensor in front of the converter and drive it. That will relieve some backpressure and if that's the problem, you'll notice a difference. It may not totally clear it up, but you should notice a difference if the exhaust is restricted.

Fuel pressure sounds good. Have a diagnostic smoke check done, there's a good chance that the lower intake gaskets are leaking. Have the test done with the engine cold, the leaks show up much better because the gaskets expand and seal the holes when they get hot.

I agree ^^^^ another good idea!

Ok I went and drove it with the guage taped to the windshield and here is what I found:

Idle: 49psi
Driving around town: 52-54psi
Accelerating: 56ish
Going up a long steep hill where it bogs down: 60psi

The pressure never went below 48 even when I stepped on it hard. It actually shot up to 56 or so and held there the whole time I stayed on it.

The hill I go up everyday to work is where I really notice problems. I can be crusing at 50+ mph and just starting up the hill it bogs right down. I can put my foot all the way down to the point where it's about to shift into passing gear and the engine doesn't change at all. It's like there is nothing for several inches of pedal. I have to shift into passing gear then it goes ok but wants to shift right out. When it does it's back to bogging.

Also, I took it to a tranny shop today and had them do a full check on the tranny including road test and they said the tranny is solid and hitting all the shift points perfectly.

So what do you guys think based on this new info?

the testing of fuel pressure when operating the vehicle is the correct test to be sure...the lower than normal pressures at idle may mean that the voltage generated by the alternator are low ,,since at idle you will get lower amps out from the alternator at idle..normally..

check the voltage at idle should be 13.5-14.5 volts dc..
if lower than 13 volts this will need an alternator check..

loss of power on acceleration / up hills etc may mean a restricted exhuast.. this can be checked by removal of the upstream O2 sensors and leaving these holes open ,,,so exhaust can escape..if the problem lessens greatly , then you know it's the exhaust thats at issue..

normally this will throw dtc's miss fires etc...

Ok, I like the "yank the O2 sensors" idea much better than the $45 I was going to pay for a back pressure test. I will pull them tomorrow and report back.

This won't hurt anything or cause it to run bad?

And hey guys - you have been a tremendous help already. I really appreciate everyone taking the time to try and help me out.

Ok, I like the "yank the O2 sensors" idea much better than the $45 I was going to pay for a back pressure test. I will pull them tomorrow and report back.

This won't hurt anything or cause it to run bad?

And hey guys - you have been a tremendous help already. I really appreciate everyone taking the time to try and help me out.


just be sure the wires are not going to be damaged,, they will melt on the exhaust pipes...

remember your going for a test run ...not going to operate all day with them removed...it will be loud ..but it should reveal if it is the cat converter or possibly the muffler..droping the exhaust will reveal all this if it is the reason of your power loss up hills....

You can borrow a compression guage tool at the local Auto parts store for free!

Do a google ''youtube catalytic converter back pressure''

I use the upstream HO2 sensor holes for the short!

Compression guage tool? Checking compression wouldn't tell anything about exhaust would it?

Nope.

Compression guage tool? Checking compression wouldn't tell anything about exhaust would it?

checking the compression would be a good check after the exhaust is checked for restriction...

question is IS YOUR ENGINE IN GOOD RUNNING CONDITION ?

does it use oil beteen oil changes , smokes out the exhaust , hard to start, idles ruff unevenly, uses a lot more fuel , noisy...this would indicate a worn low compression engine....worn rings. valve springs, piston bore.

also main bearings worn ....

It runs good other than the slight idle stumble I feel constantly. It only has ~90k on it and has been well taken care of. No smoke, no noise, no codes, 17+mpg, nothing pointing to anything "major"....at least I don't think so.

A plugged catalytic converter will choke your engine out!

It runs good other than the slight idle stumble I feel constantly. It only has ~90k on it and has been well taken care of. No smoke, no noise, no codes, 17+mpg, nothing pointing to anything "major"....at least I don't think so.

if thats the case then compression should be fine..it would take a very very worn out engine to loose power with your issue..

I am not sure if this is the answer for you or not but I have a 99 with 156000 miles on it and it was experiencing almost identical symptoms as your truck. I thought for sure it was the fuel pump as well, but turns out it was a stopped up cat and leaking intake gasket. Had the Intake Manifold Gasket and one of the Catalytic Converters replaced on it and it is running like new again.