I was working on my car today (a honda with multi-point fuel injection). I pulled off the air cleaner and sprayed solvent on the throttle plate to remove the carbon build-up
as per manufacturer’s instructions.
It occurred to me…where does the carbon come from to begin with? It coats the entire inner surface of the intake manifold. Its not like the engine is back-firing or anything. Where does it come from?
It’s a byproduct of the combustion of hydroCARBONS.
Yeah, but the intake manifold is upstream of the combustion chamber. You really shouldn’t have combustion products from the cylinders flowing backwards through the intake manifold.
That said, your answer might be correct. My best guess (and it’s only that) is that it’s crud from gases recycled through the PCV system. Basically, this system takes blowby gases (combustion products that have leaked past the pistons into the crankcase), and pipes 'em into the intake system for a second go-'round.
I’ve seen systems where the PCV gases were dumped into the intake airstream before ('88.5 Escort) and after ('97 B2300) the main air cleaner.
Actually, I think I said something in my previous post that isn’t quite right. Blowby gases are not necessarily combustion products - I believe a fair amount of unburned fuel/air mix gets by the pistons, too.
Hmmm…somebody’s going to have to step in and fill in the gaping holes in my knowledge. I guess I can imagine both reactants and products getting into the crankcase, but only the unburned stuff would benefit directly from a second pass through the combustion chamber, right?
:: thinking out loud ::
I guess you’ve got to do something with the gases that make it to the crankcase, burned or not (apparently, it can contaminate the oil). You can’t really segregate them easily, so just circling it all right back through the cylinders is as good a solution as any. You really want to avoid just shoving unburned fuel out the exhaust - that’s horrible from an emissions standpoint. Sending combustion products back through isn’t all bad - Exhaust Gas Recirculation is done deliberately (from another source), although I couldn’t really say why.
:: wanders off aimlessly ::
I think I’m full of crap.
Poking through my Haynes manual, I realized that I have the flow direction wrong in the PCV system. Clean air is taken from the air intake (either after the air cleaner, or before it and through a little separate filter), and forced through the crankcase. The blowby gases are forced directly into the intake manifold, downstream of the throttle body. So much for that theory.
This should have been obvious from the start: pressure is higher (or, if you want, vacuum is less) near the air cleaner than it is in the intake manifold. This pressure difference is what drives the whole PCV process.
I really am shutting up and going home now, having muddled the question horribly with misinformation, lies, and damn lies. Another gearhead can pick up the pieces, hopefully.
The carbon was not oily, but dry. I think that rules out the Positive Crankcase Ventilation system. Maybe its from the Exhaust Gas Recirculation system. Or perhaps its an od effect of fuel somehow undergoing vaccuum refining within the manifold at part-throttle.
Sound Good?