08 Silverado 4.8 horrible accelration/MPG Stumped ??

[GMguy84]

Scratching my head on this one guys ??

Pedal is hard and horrific acceleration when I get on highway on ramps and getting like 12 MPG'S ?? Only like 175 hwy miles half tank. Pretty sad my 01 Suburban 5.3 can smoke this like nothing


2008 Silverado 4.8 2WD

Around 200K miles

No check engine light

AFM disabled

Have used different Misc types of fuel cleaner Lucas, Chemtool B12, Chevron techron, and nothing ??

New AC/Declo platinum plugs

Intake cleaned 2X

New KN drop in filter


Wondering if its transmission related ?????? It is shifting kinda hard only when cold but then goes away when warmed up ?? Already have left over AAP clearance Castrol transmaxx and a new filter gasket for it. Mechanic suppose to " Trans pan Drop" do on Tuesday

Haven't cleaned out MAF or changed Spark Plug wires yet(still waiting from amazon)

 

[i6pwr]

Do you have a CEL on? I would have an exhaust backpressure test performed. This would confirm or rule out a partially clogged cat. Also wouldn't hurt to confirm fuel pressure.

Symptoms point to a possible clogged cat.

Since you have a K&N, I would clean your MAF just in case.

 

[joemo]

Maybe the cat. Converter .

 

[GMguy84]

Do you have a CEL on? I would have an exhaust backpressure test performed. This would confirm or rule out a partially clogged cat. Also wouldn't hurt to confirm fuel pressure.

Symptoms point to a possible clogged cat.

Since you have a K&N, I would clean your MAF just in case.

No CEL at all. It was doing this before KN filter. I do have to clean out MAF


Dave

 

[OilMagnate]

At 200K, who knows. In my experience, Techron lowers MPG slightly (~1-2%) for the tank it's used on. Not bashing Techron, I'm a huge fan. But that's probably not the case here.

Those plugs should be iridium, not platinum.

But what I'm most curious about... How did you disable AFM on this truck?

 

[GMguy84]

At 200K, who knows. In my experience, Techron lowers MPG slightly (~1-2%) for the tank it's used on. Not bashing Techron, I'm a huge fan. But that's probably not the case here.

Those plugs should be iridium, not platinum.

But what I'm most curious about... How did you disable AFM on this truck?

Previous owner did the AFM disable changed over all the top end hard parts but I have a range disabler I haven't hooked up yet , One of main reasons I bought it lol

Double platinum is acceptable and didnt really change much anyways. Techron Was just 1 bottle a week ago. I'm mainly a Chemtool B12 guy lol

Dave

 

[OilMagnate]

My question was a rhetorical one. Might want to recheck your answer. Then recheck it again...

 

[GMguy84]

My question was a rhetorical one. Might want to recheck your answer. Then recheck it again...

Ya I knew that. Just choose to ignore them and just answer your comments

21 posts and your comments are a little trollish ??

Oh and 200K miles on a LSX is nothing I have a 01 Suburban with 320K miles that runs GREAT and fast !!

Dave

 

[BrianF]

At 200k, how is the output on the oxygen sensors?

 

[OilMagnate]

Ya I knew that. Just choose to ignore them and just answer your comments

21 posts and your comments are a little trollish ??

Oh and 200K miles on a LSX is nothing I have a 01 Suburban with 320K miles that runs GREAT and fast !!

Dave

Since it still apparently went over your head, I was trying to point out that the only "troll"ing going on in this topic is how you're leading people to believe (and believe yourself) the 4.8L has AFM. That's why I said you might want to recheck your answer.

 

[GMguy84]

Since it still apparently went over your head, I was trying to point out that the only "troll"ing going on in this topic is how you're leading people to believe (and believe yourself) the 4.8L has AFM. That's why I said you might want to recheck your answer.

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

 

[GMguy84]

At 200k, how is the output on the oxygen sensors?

Have to pull out my OBD2 scanner to check that, but NO CEL at all so not really looking at that

Gonna have it checked out at local exhaust shop Monday

Dave

 

[FowVay]

Since you have no CEL I will second (or third) the suggestion to check for exhaust backpressure. Does the engine rev freely or feel labored when you rev it up? Valve timing would be another concern but if nothing has been touched on the engine such as the timing components then that shouldn't be a concern. Was the vehicle like this when you bought it? Has any major mechanical work been performed?

 

[Trav]

Scratching my head on this one guys ??

Pedal is hard and horrific acceleration when I get on highway on ramps and getting like 12 MPG'S ?? Only like 175 hwy miles half tank. Pretty sad my 01 Suburban 5.3 can smoke this like nothing


2008 Silverado 4.8 2WD

Around 200K miles

No check engine light

AFM disabled

Have used different Misc types of fuel cleaner Lucas, Chemtool B12, Chevron techron, and nothing ??

New AC/Declo platinum plugs

Intake cleaned 2X

New KN drop in filter


Wondering if its transmission related ?????? It is shifting kinda hard only when cold but then goes away when warmed up ?? Already have left over AAP clearance Castrol transmaxx and a new filter gasket for it. Mechanic suppose to " Trans pan Drop" do on Tuesday

Haven't cleaned out MAF or changed Spark Plug wires yet(still waiting from amazon)

+1 on possible bad cat.

 

[supton]

I thought this was drive by wire? How would the pedal be hard? That sounds like a separate issue from whatever the rest of the problem is.

 

[zrxkawboy]

Good suggestions on O2 sensors and exhaust back pressure...those would be at the top of my list to check.

This might be a long shot, but have you checked for codes with a scan tool? Just wondering if maybe the CEL is not working and there really is a code or two set.

 

[GMguy84]

Good suggestions on O2 sensors and exhaust back pressure...those would be at the top of my list to check.

This might be a long shot, but have you checked for codes with a scan tool? Just wondering if maybe the CEL is not working and there really is a code or two set.

My mechanic already checked with his snap on scan tool and everything working good CEL/no codes ???


Dave

 

[FastLane]

4.8 never had AFM. Remove the tune and go back to factory settings. If that doesn’t work, then Probably replace both upstream O2 sensors.

 

[FastLane]

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

Google LY2 and learn about it. It never had active fuel management. No 4.8 did.

 

[supton]

LY2 comes up as non-AFM, http://www.rpocodes.com/GM-RPO-codes/gmrpocodesGthroughM.html scroll down (or cntr-f to find quickly). Appears to have come up before Link as being somewhat confusing.