After replacing ignition coil and plug for #3 cylinder, my mechanic found heavy carbon buildup on all valves. The car only has 108K miles. He is replacing the #3 fuel injector and egr valve along with cleaning the valves.
He plans on cleaning using WD40 and a brush.
Is this a common problem?
Thanks.
Make model year engine?
On some models of engines it is a common problem, on some it is not.
I am guessing he is using WD40 brand carb&throttle body cleaner, not WD40.
On the valves?
Get the hell out of there!
Tester
You might want to switch to Top Tier gas if you don’t already use that.
Might be WD-40 Machine and Engine and degreaser foam, I don’t know if it is effective for cleaning valves.
In the past Lexus used General Motors Upper Engine Cleaner to clean valves on problematic direct injection engines, requires a two hour soak followed by an oil change.
Found carbon on valves. Does that mean the head(s) are off or did he use a bore scope?
You might elaborate on the reason behind all of this as it does come across as a bit odd.
Unless WD has come out with something I’m not familiar with I’ve never heard of anyone using WD 40 to clean up anything; especially carbon deposits.
See my comment, WD makes a carb cleaner under their brand label, not the WD40 we all know and love, right next to our duct tape.
2009 mazda cx7 2.3 liter engine
I saw the carbon buildup using a flashlight.
Huh??? How? Were the heads off?
Me thinks Sludge not carbon.
Is there an engine performance problem? Is it logically traceable to a problem with worn or leaking valves? A vacuum gauge test and a compression test can help diagnose these.
You can’t see the actual sealing parts of the valves and valve seats without taking off the cylinder head. That’s very invasive and expensive work and unless there’s a performance problem that warrants it, a big useless project.
Yes, my mileage dropped from 20 to 18 mpg. The carbon buildup was quite visible. 
The intake manifold was off. I know what intake valves look like.
Could be.
He is using CRC GDI IVD Intake Valve & Turbo Cleaner and a brass brush.
My mechanic showed me the hose going to the PCV valve. It was constricted.
Like air was having difficulty getting through.
This has the turbo engine, right? Maybe the oil seal is leaking on the turbo. How much oil do you use?
This engine also has direct injection, so it might be more subject to carbon buildup. You might visit some CX7 forums, see of other owners are dealing with this, and how.
Direct injection means the fuel gets injected directly into the cylinders after the valves instead of into the intake air stream ahead of the valves. The fuel never gets a chance to clean the intake valves on its way into the engine. These types of engines commonly have carbon buildup on the intake valves that must be cleaned up to restore performance.
No amount of Top Tier gas or cleaners in the fuel tank will do anything to prevent or fix this problem. The fix is either removing the intake manifold and removing the deposits with a media blast, chemical sprays and manual scrub, or a chemical spray into the intake stream while the engine is running, soak to loosen and hard drive to blow the deposits out the exhaust.
The intake treatment could have been done much earlier (maybe every 35k miles) to melt the little bit of buildup off rather than waiting to 100K with a performance fall-off.
But you also need to make sure the PCV system, the turbo seals and everything else is working properly to avoid having a large buildup again in short order.