I am the happy owner of a Mitsubishi Carisma 1.8 GDI LX from 1999.
During my ownership I have been acquainted with the GDI engine, and it's few, but fatal flaws.
I was able to get my engine running smooth, with high mileage, and no ticking/tappets from the valves.
When I first got my car, it was running odd. Pretty stable idle, but many judders, shakes, and just NOT smooth. The mileage was poor, and it didn't pass emission test.
The cause of these problems I wound, was carbon deposits in the inlet, and on the valves. My first view down the throttlebody:
I've read my valves looked quite like this (called an elephantfoot):
I have gone through the procedure of cleaning the crap off the inlet and valves, and got interested in why It was even there in the first place.
I can explain the cleaning process if anyone likes, but for now, I'll explain what is the cause of the problem, and how to correct the design flaw.
There are two systems on the engine, that are the combined cause:
On the inlet manifold, there are connected two pipes from the valve covers. It dumps blowby pressure, to the inlet... if it weren't there, the pressure would build up in the valvecover,
and oil system, and push out oil of all the seals. It's on all cars. The problem with that, is that it leaves oil fumes through the pipes, which seat in the inlet like a tiny film of stickyness.
To decrease NOx emissions, a small amount of exhaust gas is recirculated through the inlet. That keeps the burn temperature and NOx emissions down. The problem with that, is,
that small unburned particles enter the inlet through this system, and it really like to settle in the oil film the PCV system is responsible for.
In short, the PCV system deposits oil to the inlet and EGR deposits carbon in that oil. Oil, carbon, oil, carbon, oil, carbon.... through the years, that's allot of gunk.
It's actually a problem on most cars... the problem was just solved by accident. In normal MPI car's the injectors are seated in the inletmanifold,
spraying it with disolving/cleaning gasoline... keeping it neat and clean longer, and because the injectors in a GDI is seated directly in the combustion chamber,
it doesn't clean the valves and inlet.
Get the gunk cleaned out.
Put universal gasolinefilters on the pipes on the PCV system. There are two. I can provide images... That'll catch the oil.. and then the EGR cant deposit crap.
On the -1999 cars... not the facelifted ones, you can remove the seal from the EGR valve, and put in one that pugs the hole completely... the computer won't find out...
it's a different case with the facelifted version though. And it'll pollute more, so I would suggest the first solution.