Gray_wolf_7

This is a 2000 Chevy Express 3500 with the 5.7L. We just got it and want to use it for a trip, then resell. It ran rough when I got it, found out one of the spark plug wires was melted on the exhaust. Plugs, wires, and distributor look brand new, installer messed up with the one wire. Fixed that, and now this problem.


When the engine is first started cold everything seems normal, idles great, revs great with throttle applied. After about 1 minute, the idle continues to be great, but it will bog / hesitate at anything above half throttle. If less than half throttle is applied, it gains RPM like normal. Above half, it bogs and will pretty much die if you keep your foot down.



So far I've cleaned the MAF sensor with CRC spray, new PCV, seafoam through intake and in fuel. As I said above, all the ignition components are new, except the coil.



The last thing I checked was fuel pressure, it starts at 58-59 PSI when cold, and slowly drops to 51-52 within a few minutes and holds there. I replaced the fuel filter with no change in pressure. Spec is 60-66, but I'm seeing lots of guys online around 50 without the problem I'm having. Could this be the problem? Should I spend money on a fuel pump? Or is it a sensor?

GatorHawk24

It might be prudent to check the engine timing and cylinder compression. Incorrect timing could produce many of the issues you are experiencing. Poor compression in one or more cylinder could also cause some of your symptoms, but I'd start with engine timing.

canucklehead

Sounds like fuel volume not pressure.
Keeping your foot down is leaning it out.
Since your filter is new, I could be the injector, or pump.
The mechanical suggestions above are also a good thing to check.

mountainmanjoe

Volume affects pressure and vice versa. If you open all the taps in your house and flush the toilet, the pressure drops.

Gray_wolf_7

Thanks for the replies. Remember, there's only one issue... I don't think timing is the problem, because wouldn't it mess up with the engine cold as well? Engine runs perfect for the first minute, then runs perfect unless more than half throttle is applied.

I think you're right, it definitely sounds lean, seems like it's not getting fuel when I punch the gas. but could 10 psi of fuel pressure do that, or is the MAF or something not reporting correctly?
Also, is the injector something that can be cleaned, or only replaced?

mountainmanjoe

Ignition is advanced as RPM increase. So no, your timing could be off, and it wouldn't affect your idle/low RPM performance, and then bog when you get on the throttle.

Gray_wolf_7

Ok, but what about it revving fine when cold?

I just messed with it some more, this time the fuel pressure stayed at 58 for 3 or 4 mins, and it ran great, then the fuel pressure started dropping slowly. When it hit about 55-56, the rev started acting up. Then it continued to drop to about 52, where it bogs badly. I'm really thinking it's related to the fuel pressure. Why would the pressure go down as it warms up?

canucklehead

Is your gas tank vented and not plugged up?
Are you getting any codes?
Faulty pressure regulator.....
Faulty injector ....

StanVan

Maybe a combination of low pressure and dirty/malfunctioning injector. I'd think fuel tank vent as well, but there'd be a code for that.

Gray_wolf_7

No codes, I thought regulator, but the valve where I'm checking the pressure is before the regulator I think? I thought of venting as well, maybe the fuel system is building a vacuum, but I took the fuel cap off with no change... wouldn't that fix it?