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.

Found a mechanic. Now just need to find time to get by there.

My truck's a 2wd, so it's not suck in 4 hi. I changed the fuel filter a few months ago, but I'll throw a new one on this weekend for good measure. My next steps are: 1. Change the fuel filter again. 2. Try to find a good mechanic with a data logger/live data tool to see what's going on. If I can't find that, then: 3. Get the cats pressure tested 4. If the cats look good, change out the O2 sensors This may take a week or so, but I'll reply back as soon as I make progress. Thanks for everyone's help! On that note, do any of you guys know a good mechanic in the Austin/Round Rock, TX area?

I have 150k miles on my truck. As far as I know, the O2 sensors have never been changed. I bought the truck with 53k miles on it, and I know that I've never changed them. Would I need to change all four, or just the two upstream from the cats?

yukon_denali_05 -what was your fuel pressure doing? I checked mine 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

Will a fuel pressure gauge screw onto the schrader valve on the fuel rail? Or is there a better spot to attach the gauge?

I'll see if I can get my hands on a fuel pressure gauge. Read another post that talked about the PCM running in open loop when cold, and then closed loop once everything is heated up. And that the closed loop uses the O2 sensors, and it more susceptible to vacuum leaks. So need to get a fuel pressure gauge and vacuum gauge. Can't wait to get this fixed. I've checked/changed/cleaned so many things, that my Yukon just has to run perfect once this last mystery is fixed.

Ok, so the fuel pump definitely made a difference....but it's not fixed. 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.

Thanks guys for all the suggestions. Looks like the fuel pump may have been the culprit. I still need to put some more miles on it to tell, but so far, it's running great. I'll post again after I get through a tank of gas to check the MPG. Keeping my fingers crossed that this was it.

I've been chasing this issue for a few months, and need some help. My 2002 Yukon 4.8l started losing a lot of low end power one day. The MPG dropped to 13.5-14, and it just always feels like I'm driving into a huge headwind. For example, when I'm on the highway and let off the gas, my truck starts slowing down quickly. It just doesn't coast. I don't think it's the brakes, because it's not consistent. Here's what I've done so far: Checked the plugs Changed PCV, fuel filter and air filter Cleaned the MAF and TB Seafoam treatment through the brake booster vacuum line Checked rear brakes Getting a new fuel pump as we speak, because it wouldn't start yesterday. Could it be something like the cats getting plugged? My truck has 150k miles on it. When the problem first happened, it was really bad. Then it slowly got better. One day, something felt like it broke loose, and it ran perfect for a week. Then after getting an oil change and the diff fluid changed, it started running a little off again. Any ideas and help is appreciated. Thanks!