Intake build-up

[obbop]

2004 Silverado. 4.8 liter V8 engine. Still under warranty.

Idle a bit rough and a couple instances of hesitation upon acceleration from a stop.

Work completed at dealer, asked about cause.

Service writer points at intake between air filter and engine.

This engine uses direct-into-cylinder fuel injection, one injector per cylinder from what I can see and have read.

The spot pointed at is some sort of butterfly-type valve with an electronic sensor connected to it.... perhaps feeding info to the computer about airflow amount.

Anyway, told that there was carbon buildup upon the valve keeping valve from fully closing and perhaps slowing valve movement.

Arguing, let along asking penetrating questions, is generally useless with service writers and heaven forbid I be allowed to question the mechanic.

So..... how in the heck would "carbon" build up in a spot where only outside air appears to flow.

Am I being bee essed?

Pondered about this, accessed internal data banks from past experience as well as applying general knowledge of that related to internal combusting.

Just doesn't make sense to me.

Can anybody enlighten Old Coot regarding supposed carbon build up on an engine component that appears to only be subjected to outside air entering engine? No PCV or anything of the sort visible at the component's location.

Any info and assorted lambasts and wry humor reveled in.

Yo.

 

[Laminar Lou]

Not carbon, but there could be dirt ingress. Shows that you know more than the service writer, but that doesn't surprise me. Didn't you tell him that you are the mighty Obbop? Silly mortals...

Depending on how the PCV valve works, it could also be oil that came back in through the intake. Not sure the setup on your vehicle though.

 

[T-Keith]

I'm not sure if I totally understand all of your story, but yes carbon build up is possible. It usually occurs in the TB and IAC, which can cause a rough idle. Perhaps though he was referring to the MAF sensor, which does benefit from cleaning, but not from carbon

 

[Laminar Lou]

I think he is saying that how can there be carbon build up in the intake if fuel never enters the intake? Since it is direct injection (fuel is directly squirted into the combustion chamber) there really isn't a source of carbon that is passing through that area.

I've got the same setup on my Honda Odyssey.

 

[JTK]

Quote:

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Any info and assorted lambasts and wry humor reveled in.

Yo.

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Can you point to the part?

http://media.gm.com/division/2004_prodinfo/powertrain/truck/index.html


These are throttle by wire engines. The 4.2L I6 used in trailblazers have the same setup and the throttle bodies do tend to get dirty on the 4.2L. Not sure if it's inadequate air filtration or ocassional puffs of crancase vapors from the breather that does it.

Joel

 

[Doc8404]

i'd say dirt, not carbon.

 

[Merkava_4]

Quote:

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The spot pointed at is some sort of butterfly-type valve with an electronic sensor connected to it.... perhaps feeding info to the computer about airflow amount.

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That sounds like the throttle body. The throttle body lets air into the engine when you step on the gas pedal. Over a period of time, what dirt gets passed the air filter, can settle in the throttle body and on its butterfly valve. This can cause a disruption in airflow to the engine of which will effect the idle quality.

Taking the throttle body off for cleaning is routine maintenance that's done every 50K miles or so. It's a pretty easy job that do-it-yourselfers can do.

 

[obbop]

The service writer critter read from the mechanic's notes and proclaimed it was carbon build-up.

That's why I asked.

I could understand dirt or debris from the atmosphere but am unable to determine any possible source for the carbon build-up told to be the cause.

Makes me wonder as to the mechanic's diagnostic ability.

At a future date an official Coot-brand rant regarding the past 34 months of ongoing service-under-warranty problems will be posted.

GM corporate phoned the shanty and a discussion ensued.

I was conversing with a corporate droid trained in customer service with a definite bias towards serving her corporate master.

Coot left the proverbial ball in their proverbial court.

Will be interesting if they re-contact me and what their decision wil be.

Coot threatened war if my needs, wants and desires are not met.

This affair may become interesting but Coot promises to not use this venue as a battleground.

Carry on.

 

[ThirdeYe]

A lot of the time I have to question the people at the Acura dealership as well. It's good to not just be spoonfed everything that they say and accept it as fact. I remember I had trouble with my front bumper from a slight impact that bent the bumper a bit, thinking it was just missing a part or something cracked, they told me I needed about $340 worth of parts, and when I inspected the list of parts more closely, half of the parts on there I /knew/ were fine and one of them I installed myself just a month before. I took it to another local bodyshop and he said the list was B.S. (in a nicer term though) and that none of that would even help, with the design of the bumper on my car the best fix would be to use a self-drilling screw to reconnect the bumper to the inner fender.

 

[rat]

Mr. Coot sir,
I work in parts at a GM dealer and have seen lots of this. If I can be of any assistance in your battle, please let me know.

 

[Merkava_4]

Quote:

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This engine uses direct-into-cylinder fuel injection, one injector per cylinder from what I can see and have read.

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I can't figure out whether the service writer was talking about carbon build up in the intake manifold or the throttle body. And with a direct fuel injection set up where there is nothing but air travelling through the intake manifold, I can't see how carbon could get in there.

Anyway. . . if the dealer is saying they want to charge you for taking the intake manifold off and cleaning the carbon out of it, that's going to cost you a lot of money.

 

[Dave H]

Does the crankcase breather enter the airstream before the throttle body? On my Maxima it does. I get a black oily residue buildup on my TB butterfly from this. I clean it every 12-15,000 miles or so just because it is there. The first time I cleaned it, the car had 55,000 miles on it, and the thing was filthy. Made a drastic improvement to my idle quality. So now it is just one of those things I do as preventative maintenance.

Dave

 

[g20ooh]

Quote:

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2004 Silverado. 4.8 liter V8 engine. Still under warranty.

Idle a bit rough and a couple instances of hesitation upon acceleration from a stop.

Work completed at dealer, asked about cause.

Service writer points at intake between air filter and engine.

This engine uses direct-into-cylinder fuel injection, one injector per cylinder from what I can see and have read. I "think" what this refers to on this engine is an injector that is mounted "closer" and on a "better angle" to the intake valve - some designs of which have the injector mounted "in" the head - but still discharging from "outside" the intake valve . Has to do with how the injector is "aimed" at the CC-valve opening - (and intake valve) ie "aimed directly into the chamber as opposed to mounted in the chamber" .
(This is not the "other" kind of direct injection with the injector mounted in the head and discharging directly into the chamber - w/o "going thru" the intake valve opening) . In regards to your original question this design factor is of "smaller consquence" anyway .

The spot pointed at is some sort of butterfly-type valve with an electronic sensor connected to it.... perhaps feeding info to the computer about airflow amount. (Electronic TB as others have indicated ).

Anyway, told that there was carbon buildup upon the valve keeping valve from fully closing and perhaps slowing valve movement. YEP . 'Perfectly normal" in the imperfect world we live in - on any vehicle .

Arguing, let along asking penetrating questions, is generally useless with service writers and heaven forbid I be allowed to question the mechanic.

So..... how in the heck would "carbon" build up in a spot where only outside air appears to flow.
Easy to understand once the incorrect although "intuitive" assumption is removed .
Air does not flow in one direction(all the time) in your intake manifold "system" - although the overall direction or "net" flow certainly does .
There is a tremendous amount of mixing occurring in your plenium/intake manifold system - caused primarily by the high pressure reverse flow "waves" coming out of your intake ports which have been created by the closing of the intake valves .
A useful model here would be that when an intake valve is open in a sense , a "column" of rapidly moving air heading into the combustion chamber is created .
When the intake valve closes a "portion" of that column can no longer "go forward" so it "backs up" into the intake system ie a "reversionary"( reverse flow) pressure pulse is transmitted to the intake system - whose most important effect here is to create turbulence and "mixing" which in turn moves some/all the "air" in the intake system " around" the system .( Remember too , 8 Intake Valves and how fast things are changing in there . Also the details and strenght of effect are rpm , load and throttle etc dependent).
Amongst other things , your PCV Gases/Vapors with their contaminants are being "moved and distributed" as well . There are other sources of contamination and other "effects" - some of which get somewhat esoteric but one which is not would be your evaporative emissions control system - when it "dumps" fuel vapor into your intake system .
The description of "carbon" could be tightened up I suppose to "hydrocarbon based combustion by-products , hot oil "vapors" fuel vapor , (and water/water vapor) films and deposits that inevitably "bake and oxidize" on the throttle body - but that doesn't roll off the tongue if you see what I mean .
Lots of factors in "throttlebody build up" ;
design , environmental , usage ,age ,mileage and condition cover most of it .
Your operating cycle(s) ,choice of oil and choice of fuel are the ones you control that effect things . On perfectly healthy engines TB cleaning typically can be needed or of value ( not necc. the same thing) anywhere from say , 6mo. to 6yrs depending on everything else .
Don't know your details - but it is an 04 and you are basically in the middle of that . Low mileage useage does not necc make this less likely - many times makes it worse - #starts , amount of cold cycle operation , stop and go driving etc , etc more important , generally .
In three of the locations I spend time in buying "kosticoreilli" fuel and the like dirties 'em up real quick as due certain oils in hot climates .

Am I being bee essed? Not in general , but dealerships sometimes sell services early . Given your symptom list "sounds" legit .

Pondered about this, accessed internal data banks from past experience as well as applying general knowledge of that related to internal combusting.

Just doesn't make sense to me.

Can anybody enlighten Old Coot regarding supposed carbon build up on an engine component that appears to only be subjected to outside air entering engine? No PCV or anything of the sort visible at the component's location.

Any info and assorted lambasts and wry humor reveled in.

Yo.

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There are some finer points to all this but these are the main ones . One more I guess - the "dirt" that even a good airfilter lets by finds these sticky films etc attractive for sure - but again , on a healthy system this is not the "main" thing .

BTW, sonically tuning all these pressure waves in some sense is SOP for all engines - you have to to make sure one cylinder doesn't have it "too differently" than another "too often,too long" .
You can also use the "management" of these pulses to create a type of natural "supercharging effect" ;
AKA "Ram tuning " - of which there are no finer mass produced examples than Chrysler's various versions from the early 1960s .(Of particular note the "hottest" 413/426 {Wedge} packages) .
Used slightly differently and with some other design considerations its part of what made a slant six of any type run "special" .
Doesn't sound "wrong" at all from here - but I'm not there so must qualify - assuming this is the "problem" .
Hope this helps .

 

[g20ooh]

One other thing . On V8s with carburators ala typical usa style , the fuel/air mix "tended" to "wash" this stuff into the chamber . That said , throttlebodies (remember , mounted under your discharge nozzles) still would get dirty - maybe/sometimes not as fast nor with an as "obvious effect" - but it still was happening .
Back then , lots of things were different in regards to this service requirement - most were more favorable to the older situation - all except what happens to the fuel in the intake manifold when you shut the engine down and go into hot soak mode - so the exact nature and mix of deposits has changed over time .
This "mixing problem situation" is part of why so many sensors etc get dirty as well - and why even 07 OBD2 SYSTEMS have so much "misperformance tolerance" built into them .

 

[obbop]

Wow!!!!!!!!!!!!!!!!!

 

[mechtech2]

Possibly PCV goop is causing the build up in the intake. When it gets hot, it can congeal and maybe harden.

 

[shoz123]

I agree about the PCV blowby. It will turn to hard carbon if not cleaned out. Builds up closest to the head and lessens as you go toward the TB. But will build up at all points.

 

[The_Jeff]

What if you turn the truck off, a couple intake valves will be open.

Oily fuely fumes can then backtrack to these throttle butterflies as the vehicle bakes in its own heat.

Give it enough cycles and all the butterflies will get it provided the engine stops randomly.

 

[mechtech2]

That too!