Is that correlation really correct? The exhaust pipe soot is stuff escaping the engine. The intake valve gunk is stuff entering the engine to be combusted.

 

Interesting thread here - BMW 335i DFI - recommends not using oil past 4-5k miles.

 

I guess this graphic from Christophorus would help the discussion.

The intake valve opens to let in pure air, which gets mixed with the gasoline spray in the combustion chamber. I see nothing inherent in the design that would lead to unusual gunk buildup. In fact, with standard fuel injection the injector is spraying just upstream of the intake valve where it would be more susceptible to gunk buildup.

Sounds to me like someone is trying to cut corners using cheap regular gas instead of the recommended premium fuel. Just a guess.

 

I doubt the owner/driver is cutting corners. Sadly gas in the US is a mixed bag, rarely of good quality here in the Northeast (and no just going to a Shell doesn't solve the problem, I know plenty of Shell stations that feel shady).

 

That would be true if there is no PCV through the intake. Is it not the case? It sure is on the Cayenne (as the 1st post shows), Audi and BMW DFI engines.

The Cayenne valve pic gunk is not cheap gas. If there was pure air through the intake valve, whatever gas would not make any difference. There is no gas, cheap or top tier flowing through that valve. Cheap gas makes a difference in SFI engines as it usually lacks enough detergents, but not here.

 

I had not heard about an issue of DFI high pressure fuel delivery meaning fuel contaminates the oil, but the thread about the audis in post #12 is very interesting. As for that 335 article, the problems with heat with that motor are well known, so its not surprising its cooking the oil.

Hell, even GM produces a DFI engine, I wonder how well its working.

 

I initially thought that because of the pressure differential of this high pressure injector and the lower pressure of the intake port that there would be some fuel being pushed out over the intake valve. But once the fuel leaves the injector it's at the ambient pressure of the combustion chamber. The super high pressure of the injector is solely to better atomize the fuel.

 

Interesting thought. If true, good gas with good detergents would cool and clean the intake valves. We need to know if the 9A1 engine has a PCV system and is prone to the gunk buildup. Cayennes we know are.

 

Just read that entire post #12 link. This is a serious issue.

 

It is on Audis and BMWs, not necessarily on Porsches. Perhaps (i) more frequent oil changes, (ii) avoiding short trips and (iii) a regular intake cleaner service will take care of this.

 

First, I would say that if there is a crankcase ventilation system, we do not have pure air coming to the intake valve. Actually, we would have oily air coming to the intake valve. Whether or not this is the issue, I am not sure but the intake valve may be at lower temp which could attract residue? Also, with a number of folks reporting soot in the exhaust ( my BMW 535 with DI also has a sooty exhaust) I am sure there is a significant amount of soot entering the crankcase oil ( just as in a Diesel). It is possible that port injection may offer a sort of solvent cleaning of the intake valves for those types of systems. Finally, with Diesel engines, it has been a concern at times that there might be insufficient lubrication of the intake valves and that the only source would be the oil vapor from the crankcase.

These are just some thoughts from things read in the past on engine design issues. Bye the way, it seems that gasoline engine designs and Diesel engine designs are converging somewhat.

 

I started this thread for discussion and information. I also talked to many contacts in the Porsche world and here is a summary of what I gathered:

The DFI engine brings a set of issues to deal with. These are just there and they are not necessarily game-stoppers. Understanding them is important. The following are procedures that should be followed w/ these engines:

1 - Avoid short trips. Always allow proper oil operating temp and temp cycle.

2 - Change oil more frequently - perhaps 5-6k miles. (15k oil changes were insane anyway)

3 - Do a 15k miles (or yearly) intake clean service using an appropriate intake cleaner product.

Hopefully, abiding by these rules, these engines will last and be trouble free.

 

Well, I think the convergence, such as it may be, has pretty much reached its limit.

Otto cycle engines will always use a spark generator, and Diesel cycle engines will always have higher thermal efficiencies than an Otto cycle engine. But automotive engineers are doing a good job of increasing the efficiency of the gasoline engine, as evidenced by this new engine from Porsche. But the thermodynamic cycles of the two engines are completely different.

http://hyperphysics.phy-astr.gsu.edu...mo/diesel.html

 

I don't know, I think VW/Audi is going to have to fix this. With the internet now, manufacturers can't "bury" an issue like this and just deal with it by fixing it when owners complain (under warranty).

Mercedes has been criticized recently for not moving to DFI quickly enough - maybe this is why.

 

Excellent reference link.