01 3500hd 8.1l

[24v]

Hey guys, first post here. Hoping I can get some help, as I am ready to pull my hair out.

The truck is a 2001 Chevy c3500hd

8.1l 2wd auto tranny

 

Here is the issue.

Under wot the truck is lacking power, and surges a little. I have HPTuners so I plugged that in to monitor things. When I hit WOT the 02's continue to switch between 700mv, and 100mv's. About 1/2 way through the pull they will stabilize a little over 900mv, then they gradually decrease down to high 700's. At this point, they go back to switching. When the 02's start switching at wot I get some knock retard. I have seen as much as 4 degrees so far. To me, driving the truck, it feels very similiar to plugged cats. Before logging, we pulled the cats, and checked them, and they are clear. Any advice on this would be great. I have a decent knowledge of OBDII, but I am lost at this point. It could very well be due to frustration.

Thanx in advance for any help/advice you have to offer.

[24v]

Ok, a little more observation. I hooked up a fuel pressure gauge, and hre is what I found.

Key on, engine off - 53 psi (did not drop at all when left for a minute or so.

Engine running at idle - 45psi

engine running, driving under a light load - 55psi

Now, under WOT, the fuel pressure drops down to 50 immediately, and gradually decreases ending up at 30psi right before the shift. After the s**t, it is back up to 50psi, and then gradually drops down to 30 before the next shift.

I now think this may be a fuel pump, or regulator issue. However, the 02 readings still confuse me. Why is it going back to switching while at WOT? Also, I have no idea if these pressures are good, but it seems to me like way too much drop.

[24v]

Bump for help.

[chevroletmb]

i agree 30psi seems low.

when was the last time you put a fuel filter on it?

it's alot cheeper thana fuel pump

[GM-Tech]

Key on, engine off - 53 psi (did not drop at all when left for a minute or so.

 

NO good. Pressure should be 55-62 psi. I know a couple of psi difference may not seem important, but trust me, on these vehicle it is very important. Also, some of your other readings, like when it goes to 30 psi, is really way out of spec.

 

Also, pressure should go up when you step on the throttle, not down. Sounds like a bad fuel pump for sure.