axhoaxho

Yes, I think 'periodic' is the key...

I heard from the RS4 owners that after their dealers performed a top engine cleaning, they could find the carbon built up again in just a low few thousand miles (understandable consider the PCV system is constantly feeding back 'dirty' blow-by gas to the intake.)

I doubt dealers are willing to perform 'periodic' top engine cleaning under warranty for customers, it probably need some justifiable symptoms like a CEL due to misfire.

Don't know how much a volunteer top engine cleaning will cost us these days, but it will definitely add to the ownership cost and headache for us. In addition, it is always recommend to change oil more frequently on DFI engines (oil dilution) like every few thousand miles. And a more careful driving habbit is encouraged for DFI engines (e.g. no short start-and-stop cold-engine trips.) And so on...

Not to mention how the carbon build up issue will affect the engine after high mileages (50K, 100K, etc.) and beyond warranty period...

These DFI engine design issues add up to be some good considerations about ownership (especially long term) of a DFI engine car, especially on $100K cars like ours...

With best regards,

ADias

Like oil changing, the top engine cleanup can be a DIY. Also... as OCBen pointed out it may be that the 9A1 engine design may be less prone to this gunk buildup than the more vertical Cayenne V8 which started this thread.

I note that 'carbon buildup' has been used loosely here. It is not hard carbon crust. It is a soft oily slush.

OCBen

Nah, dude, you got it all wrong. C'mon, I'm sure you've heard of manifold vacuum and vacuum lines in an engine, right? The pressure inside the intake manifold is much, much lower than atmospheric in a normally aspirated engine. The intake stroke of the piston creates this vacuum, sucking in the air through the intake valve. Each cylinder contributes to this vacuum resulting in a steady vacuum pressure inside the intake manifold. If you could open up a port in the intake manifold and connect a fat hose to it you'd have a powerful shop vac right there, though I wouldn't recommend trying to clean your engine bay that way.

The intake valve CANNOT close while the piston is still in the intake stroke, otherwise the valve will get sucked off the cam lobe in a typical overhead cam engine as the vacuum pressure would work against the valve spring, compressing it and lifting the valve stem off the cam - not recommended.

When the piston reaches bottom dead center the vacuum buildup ceases, the intake valve closes, and the piston begins its compression stroke. Yes, it's possible (pressure-wise) to inject fuel during the compression stroke like in a diesel engine, but gasoline is much more volatile than diesel fuel and susceptible to exploding prematurely - severe detonation, engine knock - resulting in poor efficiencies.

The air as it gets sucked in is turbulent airflow, the ideal type of flow for optimum mixture of the atomized gasoline particles with the oxygen content of the air. I'm sure this engine is no different in being designed to take advantage of that.

nyca

There has to be a better solution to this then periodic manual cleaning of the intake rail, especially if the engine doesn't come with a prefabbed fitting to accept a cannister containing the "cleaner". Every car manufacturer in the world has DFI on their roadmap for engine development, there has to be a permanent solution to this problem.

PogueMoHone

^ you had better hope!

brendo

i have to wonder if this is an issue that will really cause long term problems or if it's just gaining momentum out of fear / concerns. i don't have a DFI car yet, but i will i'm sure.

hopefully there is some urban legend building here - no disrepect intended - but these concerns could just be momentum from a few isolated issues ??

ADias

Please note that we do not yet know what are the effects on the 997.2 - I initiated the thread to ask that. This potential issue would not stop me from acquiring a DFI car.

Quadcammer

If the problem is truly related to the pcv valve, then its an easy problem to fix.

Cap the intake port where the pcv went, and route the PCV valve to a catch can.

The oily mixture coming from the pcv will then end up in the can,and not in your intake.

Pretty common with high powered/hi boost performance engines of every marque.

VicL

I'm usually just a reader here on the forum. Lots of good info here.
I found this technical document on PlanetPorsche.net. It has a specific section on oil seperation and oil seperators. I have attached 2 pages but the whole document is worth the read.

Also, I have had 3 DFI engines so far, (A4 2.0T, Q7 3.6 and now 997.2S) and I have never experience these problems. The A4 had over 36K miles. I think these are a few isolated incidences. No need to worry about someting that may or may not happen. If this does materialize, I'm sure there will be TSB to address the problem.

Here is the link to the whole document. http://www.planetporsche.net/downloa...do=file&id=289

nyca

Well, the first document does indeed reference this issue and says that "effective oil separation is important" but then doesn't describe how that is done. The posts here by ADias with links to various VW/Audi boards don't seem to indicate that those cars have catch cans.

ADias

Great article!

Chris from Cali

I checked with my service department today about the fuel pump and PDK software items... He said he'd heard nothing (a tech that was nearby also heard nothing of it). My advisor told me just to stick to the 10K or 1 year service schedule and I'd be fine. He also told me that if anything did come out (TSB, update, etc.) we (owners) would hear from PCNA.

Thanks for this thread - there can never be too much awareness...

Minok

I think I didn't state things clearly enough in my post.
I agree that the pressure inside the cylinder is less than in the intake manifold, because the restrictive opening of the intake valve opening.
The point I was trying to make is the pressure is not HIGHER. Whatever the pressure is, the direct injector can easily put the fuel into the cylinder starting the instant the intake valve closes in a cylinder.


I never said that, or rather, never intended to say that.


How much piston movement upward there is while the injector deposits the mist of fuel into the sealed piston is not something I know, but the higher pressure the direct injection system runs at, the quicker than can happen. For all I know, and what I was thinking may happen, the piston moves up only fractionally during the fuel injection phase.

While gasoline is more volatile than diesel, it is not like diesel, which uses no spark plug and relies on the heating from the compression to detonate the fuel. Gasoline requires spark plugs to detonate reliably. With a higher octane rating, the detonation is much more predictable, and hence the timing can be tighter. However, I would think that there should be plenty of time for a direct injector to put the fuel in the cylinder before the piston moves up anywhere near the amount of travel that might cause an occasional premature detonation with low-grade gasoline.

OCBen

Now why on earth would you want to do that?

To extract the most power out of a combustion, and thus maximize the efficiency of the engine, the ideal gasoline/air mixture for that is a homogeneous mixture, where every atomized particle of gasoline is surrounded by equal amounts of air molecules for an even distribution of gasoline droplets throughout the combustion chamber. Each particle of gasoline is thus equidistant from each neighboring particle, and filling the void that separates them are the molecules of air. That's the ideal mixture for maximum combustion efficiency.

Now, what better means of striving for that ideal than to take advantage of the stochastic properties of turbulent airflow coming in through the intake valve? That's a freebie right there. Combine that with the super high pressure injector nozzle of this engine, which reduces the gasoline droplet size dramatically, and it's easy to see how Porsche engineers were able to extract more power out of this new engine while consuming less fuel, since most of it now gets burned and little wasted.

Here's an analogy for you. To inject the gasoline spray while the intake valve is closed is analogous to you standing in an empty room and pouring out a box full of those BB-sized foam ***** used for packing. The dispersion pattern that these tiny foam ***** make as you pour them out of the box will be roughly conical in shape, as they float down gently and disperse out over the floor. Now imagine that this room has a door that opens to the outside where a fierce wind storm is blowing and someone happens to open the door just as you're pouring out these tiny foam ***** - yep, they'll be everywhere! And that's where you want them to be if you want an even distribution of them in the room.

Since gasoline particles are heavier than air - no matter how small you make them - they need to be mixed up real good to strive for homogeneity of the mixture. And that's another function of the intake port/valve system - to create turbulent airflow for optimum mixing.

Cpa4S

i bet you they do not like **** gas that much