RPM Inching up with Silverado 4.8 (affects GM 5.7 and 6.??? too)

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Texasmark
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RPM Inching up with Silverado 4.8 (affects GM 5.7 and 6.??? too)

Postby Texasmark » Tue Mar 27, 2018 6:20 pm

That and overall mpg for country living including idling to open the garage doors, gate, this and that dropped from 16.6 to 15.6 within the last year. Idle crept up from 600 to around 700 (best guess). Every time you gave it a toe's worth of gas the rpms would run up over 1000 and come back down slowly. If in gear, and rolling along in idle, if you shifted into N, the engine would run up over 1000 and come back down.

I thought I had an emission baffle that was sticking because it used to not do that and now it's bugging me big time. Got on the www and asked the question. Got the answer from one of these online mechanic troubleshooting sites.

The throttle is "fly by wire"....like aircraft so there is no cable sticking. The problem is the butterfly,which they call the "throttle body", that regulates air flow into the intake manifold gets loaded up with crud and that restricts the air input. The computer senses rpms lower than spec and shoots some extra fuel into the cylinders to bring the rpms back up.

The answer stated that it needs to be cleaned and that should be done periodically...I'm at 35k miles and 7 years. Whipped out the cleaner and some swabs and towels and yepper the cylinder and tips of the butterfly were caked with a black oily mess....where'd the oil come from??????? It looks like the EGR valve is down wind of the butterfly and no place for oil to enter ahead of it.

Got-r-done and per the instructions cranked up and let her idle for several minutes for two reasons: to let the engine warm up and lower the rpms, and to let the computer adjust to the new air flow. Worked like a clock; just like new.

I just had it over at the dealer for the annual inspection and went by the service mgr and asked him if there was anything I needed to have done with it's age and mileage. Any codes showing? Nope! Nothing to fix...............next time I stop by there or I may deliberately go by on my next trip to town, he will learn that yes there is something that needs to be done from time to time.

Apparently it affects all the engines listed above and has been a problem for more than 15 model years. Now that the idle works like it should, I need to drive it for awhile and see if my MPG returns to where it was.
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Re: RPM Inching up with Silverado 4.8 (affects GM 5.7 and 6.??? too)

Postby Brute 23 » Tue Mar 27, 2018 8:07 pm

Yup. We have done the same thing to two of them over the years. I trust the www before I trust most mechanics. A good mechanic will use the www if hes smart.
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Re: RPM Inching up with Silverado 4.8 (affects GM 5.7 and 6.??? too)

Postby OleScout » Wed Mar 28, 2018 10:44 am

Had to do the same on my Lincoln TC.
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Re: RPM Inching up with Silverado 4.8 (affects GM 5.7 and 6.??? too)

Postby Texasmark » Wed Mar 28, 2018 4:09 pm

OleScout wrote:Had to do the same on my Lincoln TC.


Well good, or bad depending on how you look at it. Good that it's not GM unique. Surprised it's not part of a regular dealer routine service. Wonder how much money is puked by the non-informed consumer with this problem who doesn't get the right corrective action in a timely manner....if ever.

Still curious as to where the oily residue originates. If it weren't for that, the dust that makes it through the air filter should just blow on by and it not be a problem. Since the EGR is in the same air intake plenum, about 10" on down, maybe that's the source. Maybe when you shut down the engine after running it, there are still residual crankcase gasses present in the plenum and they just settle out the length of the plenum.

Wonder if changing the EGR valve would stop it. Don't recall seeing it in my owner's manual. I'm only at 35k miles. I think I recall earlier vehicles requiring a change at around 60 miles. I took the truck in for the state annual inspection which is primarily an emissions test and handily passed that. Must not be all that bad.
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Re: RPM Inching up with Silverado 4.8 (affects GM 5.7 and 6.??? too)

Postby ALACOWMAN » Wed Mar 28, 2018 6:19 pm

The oil your talking bout, is a varnish.. Comes from the emission system and nothing to worry about engine wise..
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Re: RPM Inching up with Silverado 4.8 (affects GM 5.7 and 6.??? too)

Postby Atimm693 » Wed Mar 28, 2018 6:33 pm

Texasmark wrote:
OleScout wrote:Had to do the same on my Lincoln TC.


Well good, or bad depending on how you look at it. Good that it's not GM unique. Surprised it's not part of a regular dealer routine service. Wonder how much money is puked by the non-informed consumer with this problem who doesn't get the right corrective action in a timely manner....if ever.

Still curious as to where the oily residue originates. If it weren't for that, the dust that makes it through the air filter should just blow on by and it not be a problem. Since the EGR is in the same air intake plenum, about 10" on down, maybe that's the source. Maybe when you shut down the engine after running it, there are still residual crankcase gasses present in the plenum and they just settle out the length of the plenum.

Wonder if changing the EGR valve would stop it. Don't recall seeing it in my owner's manual. I'm only at 35k miles. I think I recall earlier vehicles requiring a change at around 60 miles. I took the truck in for the state annual inspection which is primarily an emissions test and handily passed that. Must not be all that bad.


The PCV fills the intake with oily residue as well.

As for how it gets near the throttle plate, I have no idea, but it does. The problem affects all electronic throttle bodies.

I just bought an 06 Silverado with the 5.3, need to do the same thing, it idles kind of strange when cold.
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Re: RPM Inching up with Silverado 4.8 (affects GM 5.7 and 6.??? too)

Postby sim.-ang.king » Wed Mar 28, 2018 6:56 pm

The problem comes from the PCV valve, that's why verticals without EGR have the same problem.
Using a top end engine cleaner will remove it.
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Re: RPM Inching up with Silverado 4.8 (affects GM 5.7 and 6.??? too)

Postby Texasmark » Thu Mar 29, 2018 7:09 pm

I thought about spraying with Brake cleaner. I use it a lot as it's not as rough on painted surfaces as carb cleaner, yet with the jet blast and all it does a good job on things I want to paint or want to cleanup that are painted.

On did it do anything, yes it did. Solved both problems. Had it out today and performance was like new and mpg were back up where they belonged.

My, my, who would have guessed?????
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Re: RPM Inching up with Silverado 4.8 (affects GM 5.7 and 6.??? too)

Postby Texasmark » Sat Apr 07, 2018 6:13 pm

Update. MPG up from 15.6 to 17.2 same driving distances and habits. Did nothing but cleaned the flapper edge and surrounding tubular throttle body housing. Idle is back to where it used to be.....aah life is good.
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