Increasingly Poor Engine Performance - No Codes

[alvocado]

My '02 Yukon Denali (6.0L) has been running progressively worse lately but I'm not getting any codes. I'm experiencing poor power where the truck struggles to stay at speed up grades on the highway and I'm getting significant pinging between 1000-1500 RPM. I've run 93 octane for two tanks with no change. I've also performed the following:

- New plugs, old plugs had 40k on them and still looked fine; wires replaced 40k ago

- Air filter, fuel filter and R&I'd the TBI to fully clean 15k miles ago

- Cleaned MAF

- New precat O2 sensors

 

I have 135k on the truck and suspect this is emissions/fuel related. What are the key components to check? I researched knock sensors but obviously don't want to pull the intake to simply check them. Is there a way to check the EGR valve and fuel pressure to rule out the fuel pump? I have 1300 miles of vacation driving this week and worse case, I want to make sure there isn't a part about to fail rendering it undriveable until I get back.

[[email protected]]

I think it'll be fine to drive. Doesn't sound like common fuel pump problems, although 135k on a fuel pump is stretching it a bit. To check the egr, you can simply unplug the wire on top of it and try driving it around, if it drives better, it's the egr.

[alvocado]

Has anyone found a bad EGR valve which isn't throwing a SES light or code?

[nbscanuck]

have you changed the fuel filter?

[ColoradoSkiCountry]

Time for some seafoam.

[alvocado]

The fuel filter was changed 15k miles ago and performance was already declining at that point. I also tried seafoam around the same time with no improvement.

[ColoradoSkiCountry]

How's the fuel pressure looking?

[alvocado]

I haven't checked fuel pressure yet. Is there a valve on the fuel line near the intake fuel rails where I can put a pressure gauge?

[gmtech4]

exh back pressure needs to be checked

[[email protected]]

exh back pressure needs to be checked

 

Yep, that is a classic sign of stuffed cats.

[alvocado]

exh back pressure needs to be checked

 

How do I check this?

[alvocado]

Some quick research shows two options: 1) measure vacuum or 2) measure pressure at the pre-cat O2 sensor. Measuring vaccum using the EGR valve line would be easier vs. crawling under the truck to remove the O2 sensors. What vacuum pressure is within spec? If I do put a guage in the O2 sensor port, what pressure is normal?

[ColoradoSkiCountry]

Sorry, I've never had to do it myself so hopefully someone else can tell ya.

 

I haven't checked fuel pressure yet. Is there a valve on the fuel line near the intake fuel rails where I can put a pressure gauge?

[alvocado]

Wouldn't the O2 sensors trip an error code if the exh back pressure was out of spec? I'm leaning towards fuel related issues since I'm not getting any codes. I'll get a pressure gauge and check pressure at the fuel rail. Does anyone have the normal fuel pressure psi for the 6.0L?

[gmtech4]

you need a manual