2000 Express Van Loss of Power

[GarageFiend]

I have a 2000 Chevy 2500 Series Express Van, 5.7L Auto w/overdrive. It's got about 65,000 miles on it.

It experiences a lack of power, especially after a warm restart. It seems like it can't get enough gas, as when the gas is pressed, it doesn't want to go and proceeds to backfire out the intake manifold. After 20 or 30 seconds the van will continue to clear out and go under slightly less than normal power.

When starting cold, it has this less than normal power, but it doesn't experience the 20-30 second no-go period.

I've done a complete ignition tuneup, changed the filters (fuel, air, PCV Valve), and a fuel injection cleaning (not all of this in an attempt to fix this particular problem, but just so you know what's been done thus far). There has been no change in performance after any of this.

There is no check engine light and there are no computer trouble codes. Fuel pressure reads specification at all points, and no other readings are abnormal.

If you've got anything to go on, it's much appreciated. I'm stumped.

[DFBonnett]

I wonder if the cat could be restricted? Can you get under and bang on it with the side of your fist or a rubber hammer and see if it rattles?

[MT-2500]

Check engine vacuum at idle and at 2K and post back reading?

What is your fuel pressure running?
Maf sensor reading?

[GarageFiend]

There doesn't seem to be any restriction in the cat. As for the readings requested:

Vacuum:
----Idling: 20in
----Neutral, 2k RPM: 21in
----In gear, at 1200 RPM (max it can achieve when "doing its thing"): 0in
Fuel Pressure:
----Key on, engine off: 63psi
----Engine running: 54psi (stable)
Maf Sensor:
----Idling: 6.5 g/sec
----Neutral, 2k RPM: 18.3 g/sec
----In gear, at 1200 RPM (see above): 40g/sec

With any luck, that's more than you were asking for, but I figured I'd get as many readings as possible to ensure that.

[MT-2500]

Vacuum is good no cat blockage but the fuel pressure is way low.
Specs 60/66 on fuel pressure.

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.
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.
Post back fuel pressure readings.
MT

[GarageFiend]

I apologize for not getting back to you. Various tasks have kept me busy, and on the van front various leads did the same.

The dealership said they dealt with the problem a lot, and had a kit to repair it. Got the kit, installed it, no go.

Went through the diagnostic tree step-by-step (some of the steps being "If you suspect a fuel delivery problem (yes) then you should replace the fuel pump," without a test for if the fuel pump was faulty). In the end, replaced the fuel pump, no go.

Eventually, on basically a complete guess, replaced the MAF Sensor and, poof, it works again. Only difference in readings between this sensor and the old is a 2 or 3 g/sec difference when quickly brought up to 2k RPMs.

*Shrug* Sometimes the guesses work, sometimes they don't. Thanks for all the thought you put into this, though!

[MT-2500]

Thanks for posting back how it went and glad you got it going good.
10-4 on.
*Shrug* Sometimes the guesses work, sometimes they don't.
It usually takes proper testing in the end to get to the main problem.
MT