What causes throttle body fouling?

[billeee]

In looking at the induction system of modern fuel injected engines, it's not apparent where the contaminants come from to foul the throttle body. On my vehicle the only additional connection between the air filter and throttle body is the air supply for the PCV system (which should be flowing away from the intake). Is it caused by unburnt fuel vapors flowing backward after engine shutdown? If so, would direct injection engines be less susceptible to fouling?

 

[Tmoose]

Why do you think PCV flow is "away from the intake" ?

 

[VAClutch]

Positive Crankcase Ventilation System Valve, is meant to direct the blow-by gases from combustion that enter the crankcase, inevitably, pasrt the piston rings. These gases have a high amount of hidrocarbons (oil) in it, when the engine cools own, these gases condensate again into oil, and it gets everywhere. Unless you have a really worn out engine, a small amount of oil, usully a thin layer, is normal in engine intakes.

 

[billeee]

Tmoose♦

The PCV system takes air at atmospheric pressure (from the intake air duct upstream of the throttle body in my car) passes it through the crankcase, and into the intake manifold downstream of the throttle body. From there it gets burned in the cylinders. So there should not be any flow of "contaminated" air past the throttle body.

 

[berkshire]

Billee
""The PCV system takes air at atmospheric pressure (from the intake air duct upstream of the throttle body in my car) passes it through the crankcase, and into the intake manifold downstream of the throttle body. From there it gets burned in the cylinders. So there should not be any flow of "contaminated" air past the throttle body.""

This works fine when an engine is new.
When the rings start to wear, at full throttle the crankcase gasses overwhelm the system.
resulting in excess gas passing back out through the intake duct upstream of the throttle body and into the engine via the throttle body. The PCV valve is supposed to stop this, but they tend to get carbon in them and leak allowing the back flow.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[pontiacjack]

Someone needs to broach the subject of intake-gas-reversion in order to comprehensively answer this query. Hint: it's already been about a century since intake-opening event didn't occur until TDC...

 

[patprimmer]

As well as reversion, at WOT even on a new engine, there is still maximum blow by and minimal in take manifold vacuum so the PCV draws very little blow by into the manifold, so excess pressure blows out the breather into the air duct.

Also some fine particles make it through the air filter.

Regards
Pat
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[MikeHalloran]

Note also that the idle circuit, typically part of the throttle body, contains a small primitive carburetor. Hot soak will cook the fuel trapped there.

Mike Halloran
Pembroke Pines, FL, USA

 

[billeee]

"Note also that the idle circuit, typically part of the throttle body, contains a small primitive carburetor. Hot soak will cook the fuel trapped there"

I thought the idle circuit was just an air bypass around the throttle body with a small stepper motor-actuated valve to control the flow. Never heard of it having a fuel source.

 

[TheBlacksmith]

All, the IAC's I've worked on were air only, with the fuel being ptovided by the injectors as usual. However, inversion is alive and well.

 

[patprimmer]

Multi point injection rarely if ever has any fuel to the throttle body other than what is carried by reversion. Throttle body injection however, by definition injects at the throttle body or very close to it. I have seen systems with a small low speed injector and a large high speed injector, both close to the throttle body.

Regards
Pat
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[MikeHalloran]

Sorry, Brain Fade.
IAC in/on throttle body has no fuel source.

It does have a gunk source; reversion, fumes from fuel/oil cookoff farther downstream in hot soak, crankcase vent vapor, whatever.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

EGR might also be another source.

Regards
Pat
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[dgallup]

EGR is certainly a prime source on intake valve fouling, particularly on GDI engines.

On my motorcycle (3 cylinder, 1050 CC, port injected) with no PCV or EGR the throttle valves and intake valves are as shiny clean after 6 years as they were the day it was made. Rather modest cam timing on these, must be little or no reversion.

I would think most automotive engines with variable intake cam timing would also have very little reversion these days.

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[1gibson]

Oil from reusable (K&N type) filters (cone style, or OEM replacement panel) is also a big candidate when used.

 

[billeee]

Most of the replies seem to indicate that the fouling is caused by fuel, EGR, or PCV vapors from the intake manifold, which makes sense to me. That would indicate that most of the "crud" should be on the downstream face of the throttle body butterfly or on the bore of the body just downstream of the butterfly. Has anyone seen this to be the case?

 

[MikeHalloran]

Uh, yeah. The few I have cleaned were mostly presentable on the upstream side and mostly black crud on the downstream side.

Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Another source of general hydrocarbonny stuff is the carbon canister if you are still using one. It should only purge under wide throttle openings, but when it is full it is full.

Cheers

Greg Locock


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[anyoldname]

The purge function works under low load conditions when the throttle is closed and there is sufficient vacuum to pull the contents of the canister into the manifold.

This can be a problem for highly boosted/downsized engines that don't spend as much time with low manifold pressures compared to their NA equivalents, if you have a problem with this you need to have a 2nd purge connection to the intake path, up stream of the compressor.