Yukon feels like it's dragging or always going into a headwind

[Juice75]

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!

[Bad Bowtie2]

Have it scanned for codes- Just in case.

 

Try disconnecting the exhaust head-pipe from the exhaust manifolds. Then drive the SUV. If it runs A LOT better, then new CATS will probably be in order.

 

Another thing, maybe the front calipers are sticking, causing brake drag? Or even the rear e-brake (inside linnings) are dragging? You will have to pull the wheels off and look.

 

Also, you'd be surprised what a front end alignment, and/or a transmission flush/rear diff fluid change can do to MPG's....

 

Something to ponder.

 

Good luck.

Edited November 15, 2012 by Bad Bowtie2

[waumo1]

If the brakes are dragging enough to cause the issue, they will be HOT when you stop from a good run. Like hot enough you will know it just by getting putting your hand on the wheel or near it.

 

When you let off the gas and feel the rapid slow down, does anything change if you shift into neutral?

[oldmann]

I put my money on fuel pump. Hope you get a fix soon!

[00Silverado4x4]

Just as another suggestion, bearings? Could have one letting go on you.

[Juice75]

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.

[Juice75]

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.

[yukon_denali_05]

I had the same issue. I thought it was the fuel pump but before changing the fuel pump I checked fuel pressure. I changed the fuel pressure regulator and it did the trick

[TimmyG]

New plugs may help if it ran bad for a while

 

Sent from my ADR6400L using Tapatalk 2

 

 

[Juice75]

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.

[Juice75]

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

[yukon25008.1]

Sounds like a clogged cat to me, I had that with an old 4.3 mini box truck, it couldnt get out of its own way..kept downshifting even on flat roads.. I cut that cat out..And it ran like a new truck!

[Juice75]

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

[yukon_denali_05]

That sounds normal to me. Looks like fuel pressure appears to be good. I think you are on the right track with next guess being O2 sensors. How many miles on the truck and do you know if 02 sensors have been changed yet?

[Juice75]

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?