Petrol engines have a throttle plate - a butterfly valve in the throttle body, so the inlet manifold is at less than outside air pressure - there's a vacuum in it - when the throttle is at less than fully open. So this tends to suck oil from down the valve stems, - the inlet valve is hot - hence the oil carbonises on the back of the inlet valve when there is no petrol spray to wash it off.

Diesel engines don't have a throttle plate, so the inlet manifold and ports don't have a vacuum in them.

Having said all that - a well worn diesel will sometimes show carbon on the back of the inlet valve from oil from the valve stems and guides.

The latest European spec VAG EA888 petrol (1.8 & 2.0 tsi) has both direct and port fuel injection - this should reduce carbon buildup

Another issue with Petrol direct injection is the creation of particulates - with port injection this used to be classed as negligable. The main reason for the port/direct injection is to reduce this although during this year you will see more petrol cars made with a Gasoline Particulate Filter (Euro 6C)

That design of dual injection is a good compromise to cover all the bases. I think Toyota are also now doing the same with at least some of their newer cars - definitely an Oriental make, in addition to VAG. The downsides of the original DI systems were one of the reasons I didn't change from my port-injected car a couple of years ago. Let's hope other manufacturers follow suit.

I agree with all of the comments to one degree or another. The often overlooked area is that the high compression of these new direct injection engines, added to the lighter spring tension on piston rings, means higher crankcase pressures . The oil is getting hotter, more convection of oil (oil vapour) and this oil vapour is passing through the PCV valve into the intake. The deposits in this area often have an oily sheen as they are a combination of combustion chamber deposits and oil.

It does mean that standard servicing is no longer going to be enough for these types of engines. They will need periodic cleaning of the injectors (as they are in direct contact with the combustion gases and, because of the nature of the lighter rings, cleaning behind the rings using a flush (it will need a very high quality flush to do that).

An earlier comment was about piston and cylinder wear. More often it is not wear but the rings have lost their abililty (because of oil deposits in the ring seat) to centre and flex, and deliver the sealing effect they need. Cleaning them properly restores compression and inhibits blow-by. This additional servicing will slow down DI deposits.

It is highly likely that intake valve cleaning will be necessary in 40,-80,000 miles on many vehicles depending on driving style

Isn't that indirect injection? Direct injection is surely into the combustion chamber.

Port injection for petrol engines is spraying fuel down the inlet port onto the back of the inlet valve(s).

Gasoline direct injection (GDI) is injecting fuel directly into the combustion chamber of a petrol engine.

Indeed, that's my point, traditional port injection petrols create a cooling effect for the inlet charge and the inlet valve, surely that must make it less likely for oil mist deposits to bake on - assuming there's a meaningful enough temperature differential between the two?

The question is about direct injection petrol engines- so no petrol hitting the back of the inlet valves unlike the indirect petrol injection like K-Jetronic etc

.www.youtube.com/watch?v=a4fJAfXYxWk

I'd have thought that the injection patterns were similar, if not in timing, for diesel and petrol.

My gut feeling is the softness of the carbon is mainly affected by the more oily nature of diesel fuels,.

Edited by Glaikit Wee Scunner {P} on 25/02/2019 at 17:49

"As to why diesels have less of a problem - maybe it's temperature? Diesels to tend to run cooler than petrols, or at least non-turbo ones did."

They don't necessarily run cooler - as far as direct injection turbo diesel motors go, it's just that they are more efficient, in other words they convert more of the thermal energy in the fuel into mechanical work- and less combustion heat rejection to coolant, lube oil and exhaust gas than petrol motors.

We don't hear so much about it these days despite there being many more GDI engines, so perhaps it is less of a problem than it used to be.

As to why diesels have less of a problem - maybe it's temperature? Diesels to tend to run cooler than petrols, or at least non-turbo ones did.

A related question is whether using superfuels such as V-Power will reduce the deposit build up. As the fuel isn't washing the inlet ports/valves, little benefit there - but maybe there is better combustion/less crud produced, and less unburnt fuel from cold running in the oil?

No such thing as 'better combustion' between standard Ron95 and superUL. The difference mainly is about ignition timing (knock resistance) and for the branded fuels, the extra cleaning agents included, which on a port-injected car will help clear off carbon deposits (though how much and how quickly is not in the public domain) to some degree. A purely GDI engine will barely get any improvement from using superUL in this regard.

John Cadogan did a good YT video on this subject. Some purely GDI engines seem more prone to issues that others.