2007 sierra classic down on power and fuel economy.

[littleblazer]

Hey guys, as you remember I had the issue with the tune on the sierra and basically bought a pcm from a junker. Scan numbers and everything correct and it's from a 2500hd. But ever since that swap the truck hasn't been... how do I put it... the same. Power is down, fuel economy dropped by about 1 mpg and shift points don't seem right. I had the scanner on it and one of the upstream o2 sensors is more of a box wave then a sine wave. Being the originals with about 160k on them would it be a safe bet that it could cause the loss of fuel economy? Motor mounts also seem to be on their way out...

6.0l vortec

 

[Sparky]

If it still has original O2s with that many miles I'd say it is a safe bet and not a waste to replace them. Good chance they are getting lazy being that old, and lazy sensors can screw with performance and mileage.

 

[Mooseman]

Agreed. If that doesn't help, maybe take it to a dealer or someone with a Tech 2 and get it re-flashed with the correct file. There may be a difference between yours and the junker you got. There may have been an update that possibly might not have been applied to the junker.

 

[littleblazer]

Well the guy it was bought from reflashed it for the vin and options.

 

[littleblazer]

So I'm seeing 2 different parts, one being professional the other not Idk I think the regular should be fine.
Professional p/n: 2132956
Regular: 2131161

 

[littleblazer]

Just thought I would update. It got worse after I replaced the O2 sensors. We're looking at a solid 9 in town. My next step is to reset the fuel trims and clean the throttle body and see if that helps. It does seem to run better now though.

 

[Sparky]

I assume you've checked operating temperature right? Sounds almost like a cold stat like how the GMT360 platform MPG goes down the crapper when running cold.

Speedometer correct? I'm wondering if it has the right gearing programmed into it.

 

[littleblazer]

It's off by what it's always been. Tire size is one over oem. Temp holds at 194, same it's been all its life. Over a 300 mile trip the distance was off by about 20 miles iirc.

 

[Blackwater]

Might want to check your MAF and MAP sensors as well.

 

[littleblazer]

I think I'm just gonna drive around with the tech 2 this weekend. Look at the fuel trims and all that. Maybe check fuel pressure. It is strange that both the tb and his truck started this at about the same time though. And the alternator is defiently an issue. I don't remember if it was here or another thread but when you have it floored for more then a few seconds the voltage drops and the headlights dim severely. Let off the pedal some and they come up. As you're going it looses power but if you refloor it after backing off its back up on power. Which I am half expecting to be part of the issue. We'll see.

 

[Blackwater]

Have you figured it out yet?

 

[littleblazer]

I forgot about this.
No I have not yet. The power seems to be back but the fuel economy is bad still and the alternator not charging when accelerating is getting worse seemingly. Can't afford to play with parts at the moment since he's bust with work. Once it slows down we will though.

 

[Blackwater]

ok. I have a classic 2007 GMC Sierra. My fuel econ went down but I have knock sensor number 2 out. pain to change compared to my 93 GMC.

 

[Mounce]

ok. I have a classic 2007 GMC Sierra. My fuel econ went down but I have knock sensor number 2 out. pain to change compared to my 93 GMC.

When you get around to replacing them, use ACDelco only. If not, you'll have a bad time by doing the job twice.

 

[littleblazer]

ok. I have a classic 2007 GMC Sierra. My fuel econ went down but I have knock sensor number 2 out. pain to change compared to my 93 GMC.

So you have a cel I'd imagine. There was a tsb floating around when replacing them to build up a wall of sillicon around them I think...
Edit:TSB #02-06-04-023A
This link is what I was referring to just in case.
http://www.autorepairinstructions.com/?tag=02-06-04-023a

 

[Blackwater]

That will help thank you

 

[Mounce]

He's not MAY03LT, but he's pretty good. Been watching him for a while.