08 Silverado 4.8 horrible accelration/MPG Stumped ??

[GMguy84]

GM dealer told me my truck was a fleet special order and it did ??? this comes up in my VIN RPO on GM global connect ??

ENGINE GAS, 8 CYL, 4.8L, SFI, IRON, CYLINDER DEACTIVATION GM 4.8C W/CYL DEACTVN(LY2)

Either way im even MORE happier it doesn't have the $$AFM crap$$ at all. I never see a "8cyl/4cyl" mode in the message center like I did on my 07 Suburan 5.3

 

[clinebarger]

Pedal is hard?

 

[GMguy84]

Pedal is hard?

Ya pedal is hard and throttle response is HORRIBLE !!

Like Im towing 10K lbs ??

Dave

 

[GMguy84]

Around 80 MPH pedal to throttle response is done. I have to floor it and Tach revs up 3K just to go over a small incline on highway

Possibly Trans ???


Dave

 

[WyrTwister]

Things like make me think back to the times when the only electronics built into a car was the AM radio ..........

Back then , trouble shooting was a little simpler .

Back to the OP . Any chance the cam or valve train may be worn out ? Any loose tappet noises ?

Please report back after having the back pressure checked .

 

[hank2]

The 4.8 did not have cylinder deactivation in 2008. LY2. I believe that all the 5.3 engines did by 2008. Later, the 4.8 did get it.

 

[OilMagnate]

The 4.8 did not have cylinder deactivation in 2008. LY2. I believe that all the 5.3 engines did by 2008. Later, the 4.8 did get it.

More incorrect information. The GM 4.8L literally NEVER had AFM. It's final year was 2013, still without AFM. One of my customers owns one...

 

[Lubener]

+1 on possible bad cat.

Very possible. A bad CAT could be a "victim" of an underlying issue with the engine. I would check fuel trims and see exactly how well the engine is running and verify possibility of an exhaust restriction with a vacuum gauge. At 200k, anything is possible.

 

[supton]

I think the hard pedal is something else. Like a bad pedal. It's DBW, right? If the throttle wasn't opening all the way it'd be low on power but high on mpg, which tells me that something else is bad, like cat or some other restriction.

You've checked for dragging brakes, right?

 

[350Rocket]

Nothing went over my head and NOT Leading people to believe anything ?? Well according to GM dealer and my RPO LY2 code my 4.8 has AFM.

ENGINE GAS, 8 CYL, 4.8L, SFI, IRON, CYLINDER DEACTIVATION GM 4.8C W/CYL DEACTVN(LY2)

You want to call GM and tell them it doesn't exist ??



Dave

Must have been an option. Our 2013 4.8 at work has no AFM, just vvt. I'm not sure the engine code though, I thought it was the LR4 like my 05.

 

[GMguy84]

I think the hard pedal is something else. Like a bad pedal. It's DBW, right? If the throttle wasn't opening all the way it'd be low on power but high on mpg, which tells me that something else is bad, like cat or some other restriction.

You've checked for dragging brakes, right?

Regarding possible Exhaust issues. Gonna check it out today if possible. Had the front rotors pads done when I bought the truck 2 weeks ago those checked out fine.

It has rear drums and haven't checked them out yet. But No brake light is on either ??


Dave

 

[supton]

Dragging brake won't trip the light.

Go out for a drive, 10+ miles. Brake gently to a stop. Put your hand near to all 4 hubs--if you can get close to the rotor, all the better. Just don't touch! in case it's hot--they can get really hot! so use caution on this test. On a gentle 10 mile drive nothing should be warm. If a brake is dragging you'll know very quickly.

A dragging brake won't always cause a pull, especially not on the rear.

Eons ago I did brakes on a vehicle and the front pads were too thick. It dragged on one wheel until I took some 60 grit to the pads to get some clearance. [No idea if I had a bad caliper or bad pads--25 years ago--so who knows--but a recent brake job smells fishy to me. Sometimes calipers don't like being pushed all the way in and you may have lost one.]

 

[tiger862]

On my dad's now brothers Silverado with 4.8 it is a dog with poor fuel mileage. His was a fleet rental when dad bought it. The gearing is terrible. If you take off hard it will spin tires but over 45 mph it revs great only if floor it then runs out gear at 80 mph. It will top out at 100 if you hold it long enough. We got in hilly area in Alabama trying to follow Aunt which she had frt wheel drive Dodge sedan which I was floor most of way and it struggled. Pulled a K Car from Florida to Louisiana fuel mileage was less around 6 mpg at 70.

 

[supton]

On my dad's now brothers Silverado with 4.8 it is a dog with poor fuel mileage. His was a fleet rental when dad bought it. The gearing is terrible. If you take off hard it will spin tires but over 45 mph it revs great only if floor it then runs out gear at 80 mph. It will top out at 100 if you hold it long enough. We got in hilly area in Alabama trying to follow Aunt which she had frt wheel drive Dodge sedan which I was floor most of way and it struggled. Pulled a K Car from Florida to Louisiana fuel mileage was less around 6 mpg at 70.

Yeah, some of those fleet sales had 3.08's I think. 6 mpg at 70, it must have been pulling in 2nd gear for long runs at a time, taching high for the 4.8 to get required power. Getting some better gearing should help, although it's still a 4.8 pushing a 5k+ truck pulling whatever extra weight in hilly country--perhaps a 5.3 wouldn't do that much better.

 

[Bud_One]

Another vote for restricted cat converter .

 

[350Rocket]

The 13 4.8 at my work feels gutless compared to my 2005. They both have the same 3.42 gearing. 100k on the 13 and 200k on the 05. No codes or check engine light on either one. I wouldn't say the 13 felt like anything was wrong with it if I hadn't driven both and realized the difference. If I didn't know better I would have assumed the 05 had a 5.3 and the 13 a 4.8.

Both lack torque off the line but the 05 pulls hard above 4000rpm, while the 13 doesn't.

No engine or exhaust repairs to either one yet except a water pump on the 05.

 

[supton]

The 13 4.8 at my work feels gutless compared to my 2005. They both have the same 3.42 gearing. 100k on the 13 and 200k on the 05. No codes or check engine light on either one. I wouldn't say the 13 felt like anything was wrong with it if I hadn't driven both and realized the difference. If I didn't know better I would have assumed the 05 had a 5.3 and the 13 a 4.8.

Both lack torque off the line but the 05 pulls hard above 4000rpm, while the 13 doesn't.

No engine or exhaust repairs to either one yet except a water pump on the 05.

Torque management at work? IIRC the ECU won't let full power in 1st gear.

 

[FastLane]

4.8 is no drag racer. GM torque management is ridiculous. I can barely chirp a tire.

The only issues Ive had with the engine is an oil pan gasket leak and lower rad hose. I cant complain.

Again, the 4.8 never had AFM. It did get VVT which my 2010 has.

 

[supton]

Isn't that the point? Spinning a tire on dry pavement means it's going to be spin-fest on any other kind of surface. That sort of thing is hard on spiders, and worse if a spinning tire suddenly grabs. Plus a transmission has a limit to input torque--the allowable torque likely rises as one goes up in gears, but in first gear it's gotta be low to prevent damage, to the trans and/or rear diff.

Now maybe GM was over the top, dunno.

 

[BAJA_05]

Regarding possible Exhaust issues. Gonna check it out today if possible. Had the front rotors pads done when I bought the truck 2 weeks ago those checked out fine.

It has rear drums and haven't checked them out yet. But No brake light is on either ??


Dave

How did the truck run when you bought it 2 weeks ago? I assume you took it for a test drive? So, only since you did multiple things to it -- explained when you started thread -- items after first paragraph -- you started having issues?