5.4l low power with WOT

[jeffro]

Originally I had P0171, P0174 (Bank 1 & 2 lean fuel mixture). I found the air filter damaged likely causing a dirty MAF sensor. I cleaned MAF and reset the CEL codes. Fuel trims seem better so this might have fixed this but during test driving I found it has little power at ~80% to WOT. If you step on it from a stop it will never shift out of 1st. RPM maxes out at ~4,600RPM and takes a lot of effort to get there. I don't know where the shift point should be.

I have a new fuel filter I plan to install. I have not tested fuel pressure.

I was thinking the throttle body might be sticking (caused by dirt passing the failed air filter)? I have not opened the throttle body up. I'm no mechanic but it seems if the throttle body can handle EGR then a little dirt ought to be OK though.

Currently no codes. Sometimes a bit of a engine shudder at WOT and high speeds. Spark plugs replaced ~20k miles ago. The rest of the ignition system has not been worked on.

This lower power at WOT might have been around for a while (and unrelated to the lean mixture); people who stomp on the gas don't typically drive 25 year old clapped out SUVs...

Any other ideas?

 

[jr1under]

If the throttle body intake has never been cleaned, it may look as foul as the one in the picture. Can't predict exactly what issues that will lead to but eventually there's going to be some. Removing and cleaning it is simple and inexpensive, all you need to buy is a new gasket and a can of TB cleaner.

 

[jeffro]

I lean towards cleaning the throttle body and installing the fuel filter. If that works great, otherwise I'll try to figure out a diagnostic plan.

 

[Homer]

Does it run like a bat out of hell when cold but after it heats up has no power? This could be plugged cat. Possibly damaged by running with codes for awhile or just went bad.

 

[Dustin Gebhardt]

I had a low power at WOT condition a few months ago and it finally threw a low fuel pressure code at me. I hookup up the scanner and watch the fuel pressure. It was good while idling and around town but at WOT the pressure would drop a bunch. I changed the fuel filter and it was like having a brand new engine.

 

[Bruce Mitchell]

Lean mixture is caused by either too much air or not enough fuel. Occam's Razor suggests changing the fuel filter.

 

[jeffro]

Had to wait out the bad weather before I worked on the truck. Fuel filter was changed and made a big difference. Now it will shift through gears at WOT. In my case it was both the dirty MAF under measuring, and the dirty fuel filter dropping pressure. Fuel filter was probable 3/4 of it.

At WOT the fuel trims still max out at 25%. Does this indicate there is a still an issue, or expected? I'd think the trim should stay single digits if things are running well but I'm new to this. I'll probably clean the throttle body still since I bought the gasket and the air filter was letting dirt in but the fuel pump might not be up to spec. How would you measure fuel pressure under load? I don't think this truck has a OBDII PID for that and fuel trim is near zero until the load goes up.

 

[jeffro]

 

[jr1under]

At WOT the fuel trims still max out at 25%. Does this indicate there is a still an issue, or expected? I'd think the trim should stay single digits if things are running well but I'm new to this.

I can see why changing that filter made a big improvement!

The long term fuel trim values are an average over a certain amount of trips and miles. They don't change instantly but will show steady improvement as the system adapts.