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Discussion Starter · #21 ·
To what extent do diesel engine intake valves suffer from the same carbon build up as gasoline Direct Engines?

2 Answers

Profile photo for Richard Neal
Richard Neal

, former Fleet Mechanic -Explosive Ordnance Tech -Machinist at Lockheed Martin (1986-2016)
Answered May 5, 2019

Gasoline Direct Injection (GDI) engines are MORE susceptible than Diesel engines to carbon deposits forming on the backs and stems of intake valves. The reason for this is that gasoline engines run with the intakes system under vacuum most of the time, where Diesel engines do not. This is inclined to draw traces of oil through the valve guide where it burns on the stem and back of the hot valve. Since, with GDI, there is no wet fuel in the intake port, fuel detergents aren’t present to dissolve the build-up. Intake valve deposits haven’t, historically, been problematic for Diesel engines.
Interesting observation. I guess it's better to actually let the engine rev and don't let it sit there idling, l definitely agree that it is another way engine oil could end up on the valve.
Now we have a new theory. Oil being drawn past the valve stem seals. Worn seals and guides perhaps, but in tolerance guides and seals? No way.

The people advocating these modifications need to decide what the problem is. Is it fuel vapors? Carbon from blowby gases? Oil mist from crankcase vapors? Oil leaking past valve stem seals?

If upstream fuel delivery is the only preventative measure for this problem I have sad news for you. Intake valve deposit issues have been around since before Henry Ford's time. I have seen severe cases caused by internal coolant leaks, high oil consumption, heavily worn valve guides to name some. Fuel "washing" the back of the valves didn't save them.

Significant levels of deposits require a significant source of contamination. Some (and I have no clue what percentage "some" is) 2.4L GDI engines had such a source of contamination apparently.
Thinking it's a combination of all these potential sources that cause the buildup? So there is probably no way to completely eliminate carbon buildup .some people using the catch can were getting about 2 ounces of oil and 2 ounces of water over a 2 month period. So it's not very much from the upper PVC system.
I gotta say, allot of people say that hole in the back of the intake manifold on the 2.4 is a pcv orifice and they have to clean up dried carbon or drill it out. If it is a pcv orifice then why is the purge valve hooked to the front of it and how the heck is there no oil sludge to be found along with the dried carbon? I still say the only real pcv on the engine is on the valve cover to the intake. The carbon deposits in the "pcv" hole is coming from the intake valves and is directly routed to the middle two intake runners from the purge valve.
There is actually two pvc systems on the 2.4 direct injection engines
One only functions during idle and during deacceleration (high vacuum conditions) it is the one you mentioned in the intake manifold. Both systems are simply a small hole, not like a PVC valve on engines that have a orifice, plug and a spring. The one in the rocker cover functions when the vacuum in the intake is low enough for the negative pressure in the breather cover to start pulling the blow by through it instead of the internal pvc system. When the switch occurs is about 2000 rpm. If ether plugs you run the risk of blowing the rear main crankcase seal. Especially when you are above the 2000 rpms because blow by production occurs more when the engine is at higher rpms. If you have excessive blow by, and some of these engines have excessive blow by. Because around 2014 gm changed the rings to rings with higher tension. The older vehicles they replaced the rings if you could prove that you have a engine that has this issue. Ultimately if you have alot of oil consumption it's because the excessive blow by carrying engine oil vapors into the intake and it is burned in the cylinders, it's also responsible for the intake valves to have carbon build up faster than it should. The high burning units need to have the intake removed and the valves cleaned via scraping and walnut shell blasting as early as 80,000km .if you notice poor acceleration poor gas mileage, this is usually the culprit. But if any one of the PVC systems plug you could pop the rear main seal. The vehicles that are subject to cold weather conditions are more susceptible because the blow by also carries water vapor from the combustion process and it can freeze the small pvc hole plugging it. You can not make the hole bigger, because the hole works like having a air leak and the computer has been set up to account for the blow by gases if you enlarge it and you end up opening it too much the computer will turn on the service light and send a cel code indicating a lean condition.
Actually the PVC orifice does not connect to the purge valve port at all. There are internal passages going from the PCV orifice, ending in each of the intake runners. Look at the attached photo, I've passed a piece of wire through the holes... Try blowing some compressed air through the purge valve port and you'll see that is is no connection between the purge valve port and the PCV orifice and it's outlets.

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