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.

 

[electro]

There is no check engine light?

Prob not the idle air control if the problem happens while driving.

Could it be the throttle position sensor?

 

[Juice75]

The idle actually seems fine. I can feel it a tiny bit in the idle, but not much. And yep, no check engine light. I wish there was one, so it would give me an idea, but no such luck.

I know it's probably not related, but all this started right after I had new front shocks put on the truck. So in the back of my mind, I keep wondering what those guys bumped, snagged, cut while putting in the shocks.

I'm going to clean the MAF and throttle body today, just because the truck has 145k miles, and I've never cleaned them. Doubt that's the problem, but I'm going to give it a shot.

Still have a feeling that it's either a vacuum leak or cats. Anytime I go from coasting to part throttle, like after turning a corner, my truck starts to go, hesitates/bogs a little, then downshifts and goes.

At this point I'm throwing darts, but something has to stick eventually. Might pick up a vacuum gauge today and see how that reads.

Anyone know the best place to hookup a vacuum gauge on these?

 

[Juice75]

Quick update. I cleaned my MAF and TB (never cleaned either before), and my truck is running like a champ now. I'm not sure if the gas mileage is back to normal, because I've been having fun driving with my foot a little.....well, a lot more into it.

 

[Juice75]

Not sure what happened, but my truck is fixed now. The MAF and TB cleaning helped, but didn't solve the problem. Then on Sunday night, I was driving home from dinner, the truck surged a little, then started running perfect again. Not sure what happened, but whatever was stuck is now 'unstuck'. Maybe it was a lifter or a valve or a piece or carbon in a bad spot. I have no idea, but just glad it's running right again.

Thanks to everyone for their input.

 

[Matt06Yukon]

Lol! Thats crazy. I wonder if a piece of the honeycomb filter came loose in the cat, blocking the flow for awhile. Its probably not "fixed" but hopefully stays away. Your making me wanna clean my MAF and TB now. My Yukon runs fine but I'm at 120k, probably pretty dirty too. Can't remember if I've cleaned it or not.

 

[Juice75]

The mystery fix didn't work. The performance degraded since my last post, and my fuel pump went out a week ago. Had the pump replaced, and that definitely helped, but didn't fix everything. Went through a tank over Thanksgiving on the highway, and only got 14mpg. I used to get about 17.5mpg on they highway.

Since the fuel pump helped, I wonder if the issue is something else in the fuel system. Maybe the fuel pressure regulator?

Before the truck heats up to normal temp, it runs perfect. Or at least that's what I've felt so far. Once it hits normal temp, it just feels a little sluggish. Like the gas pedal has more travel before anything happens. It's not a huge difference, but definitely noticeable. I haven't check the fuel pressure yet, but that's next on the list.

Is there anything emission/temp related on these things that could cause this? Or maybe a vacuum issue causing problems?

 

[Juice75]

I checked my fuel pressure today, and at idle, it stayed at 49. When I pulled the vacuum line off the regulator, it jumped up to 56, which is what I read it should do. I actually took a video of it, and the link is below. The second video is of a buzzing noise that comes from the TB when I turn the key on. Not sure if that is normal or not.

I also checked the vacuum, and at idle with the AC off, it held at 19. With the AC on, it dropped to 17. Does that sound about right?

If this rules out a vacuum leak and a fuel pressure issue (and that's a big 'if'), then here's my thinking now. The truck runs great until it reaches normal temp when the O2 sensors come into play. Could it be that one or multiple O2 sensors are incorrectly reading too lean, and causing it to run rich, making the MPG go down? At this point, I'm just guessing.

My next step is to get a back pressure test to rule out a clogged cat.

Fuel Pressure vid: http://thehawks.smugmug.com/photos/i-vZw3nBM/0/M/i-vZw3nBM-M.jpg
TB buzz vid: http://thehawks.smugmug.com/photos/i-rcLC42L/0/M/i-rcLC42L-M.jpg

 

[Juice75]

Digging this one back up from the grave because it is FINALLY SOLVED!!! The problem was a bad catalytic converter. I never had a check engine light or any codes. I heard some rattling one day, so I tapped on the driver side cat with a hammer, and it sounded like the thing was full of BBs. Replaced the cats, and my Yukon is running like a champ now.