still got warranty. told by many workshop to do walnut blasting due to gdi after high mileage

Modern GDI don't have this problem already or no where as severe as the Peugeot / VW GDIs of old. I drive a relatively newer Peugeot, 160k mileage with no walnut blasting done. If you really want to know whether there is severe carbon build-up, get the workshop to open up the valve cover and look at the valves, pistons etc.

Open up the engine top can see already whether walnut blasting is required or not, you can decide after looking at the carbon build-up. Anyway, nowadays its not too expensive do this, below RM1k I think.

Hi, anyone know where to get this service at a reasonable price in Penang?

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before..after 350k..cannot see anything

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after blasting

Then don't do it yet.

Carbon build up is due to cars EGR system. It recirculates exhaust gas back into the intake at low load for the engine to run cooler and have cleaner emissions. EGR is not new, even on my old 1989 gallant  has EGR but I blocked it with a blocking plate. The PCV system also may channel oil fumes back into the intake and can cause intake fouling. Turbo GDI engines generally will have higher carbon build up as compared to N/A engines.

With no more port injection, the intake valves don't get cleaned hence the carbon build up. Toyota does is correctly with their own D4S engine with both port and direct injection.

So if you drive a lot at light loads then you will get more carbon build up on the intake as the EGR typically functions at lower engine loads.

Only way to check is to remove the intake manifold.

If your carbon built up issue is not severe, sometimes you can just clean it with carb cleaner. A diy job if you are handy.

I guess you've never seen a clogged up egr before.

At low rpm low load, and thus low intake velocity or in the case of GDI, improper burn, the combustion charge will turn to soot and gets re circulated through your EGR. That's what the EGR is supposed to do. Generally, GDI goes to lean burn up to 20:1 so soot is less of a problem but it does happen.

Also all engines will burn a little oil through the rings. Some more, some less.

I already mentioned the secondary cause which is the PCV.