Yukon's not running right...need help

[Juice75]

I'm calling on all the experts for some troubleshooting help.

About a month ago, my '02 Yukon 4.8l started running strange. Here's a list of symptoms:

It's lacking low end power below 2500 rpm
Gas mileage dropped from 16 to 13-14mpg
It surges a little under light acceleration
When I give it gas, it starts to go then hesitates a little, and then downshifts

And all of this seems to be worse after the engine heats up to normal temp.

I've done the regular stuff: checked the plugs, changed the air filter, pcv, fuel filter, and ran through a few tanks with the Lucas fuel cleaner. None of this seemed to help. Other than a Black Bear tune, everything else is stock.

Could it be an O2 sensor that's on it's way out, but hasn't triggered the Service Engine light yet?

I'm at a loss, so looking for advice.

Thanks!

 

[Sasquatch]

It sounds like you could have clogged catalytic convertors.

 

[Juice75]

Is there any way to test a catalytic converter to be sure that's it before spending a couple hundred on new ones?

 

[Sasquatch]

I had no idea so i turned to google.

http://www.brakeandfrontend.com/issue/article.aspx?contentid=38643

 

[sethsperlik]

I was having this problem and I found it was a vacuum leak for the vacuum brakes.

 

[Juice75]

I've wondered if it was a vacuum leak. How did you locate the leak?

I can tell it wants to run right. The truck will surge a little to how it's supposed to run. Now that it's not 100 degrees outside, it's running a little better, but not the same. Having the AC on magnifies the problem a ton. I know the AC draws a little power, but it's like flipping it on and off in a little Hyundai.

 

[PwtrSS_OakCliff]

Hey Juice,

I would also think it will be a clog on the converters. I had this happen to my Camaro and honestly man what I did is drill a hole in both of them and the motor woke back up. I would leave this to a last resort.

It's almost like trying to go for a jog and breathing through a straw.

 

[Juice75]

I just keep hoping it's something smaller/cheaper. One other thing I forgot to mention is that on the highway, when I let off the gas, my truck starts slowing down faster than it used to. Almost like it doesn't want to coast much. All of this has gotten a little better over time, but it's still there.

When my truck first started running bad, if I was going down the highway and let off the gas, it felt like I was on the brakes.

 

[erikrichard]

Have you jacked up each corner and spun the wheel? could be a stuck emergency brake.

 

[Juice75]

I spun the front, but not the back. It doesn't pull to either side, so both wheels would be stuck. I'll check that in the morning.