I was wondering if the MAU (brick) is replaced, will that create and restore the compression back to the cylinder that is low? Given that there is no damage to the piston or ring.
From what I've been able to deduce is that when one of the hydraulic/electrical lifters goes bad, it leaves a valve partially open, leading to no compression on that cylinder. The hydraulic/electrical lifters in the MAU allow the engine's CPU the ability to control the lift and duration of the intake valves, regardless of the cam lobe specs during rotation for maximum torque at said RPM if required. This is what gives the MAV engine it's low-end torque, but it leaves those MAUs susceptible to carbon build-up failure.
After replacing, it seems that it takes a while for a partially stuck open valve to work again. It's not stuck fully open, just enough that the return springs can't overcome the seized valve in the guide until it heats up and releases. For me, I was afraid to run it after the replacement and still having a dead cylinder. Finally, I said what the ****, run it until it blows up. If it's a bent valve, I'll have to pull and possible replace the engine anyway. Once I let the engine run and heat up properly, the valve popped closed, and the cylinder started firing normally with full power.
The main reason is carbon buildup in the oil. With a turbo, this happens much quicker than a normally aspirated engine. The car had been setting for the two last years, with short local trips, usually once every several months. Oil was changed on milage, not time, which was a mistake. MAU failed after the car was suddenly put back on the road daily for about a month without the oil being changed. The oil changed was solid black. Everything I remove from the MAU was coated with carbon.
I read that when the MAU fails, you'll get air puffing out the intake air duct off the throttle body. That's where I was at after a full tune-up. If the issue is a bent valve, from a bad timing belt jumping teeth, it could be either an intake or exhaust valve. If you're getting puffing out of the exhaust pipe, than you have a bent exhaust valve, which will be caused by the jumped timing belt.
If the timing belt has not jumped any teeth, then the open intake valve is caused by a MAU failure.
With all that being said, I believe for MAU longevity, the engine oil needs to be changed on a strict schedule, especially if running a turbo, to prevent carbon buildup in the oil and causing MAU issues. Fiat also recommends a special oil for the MAV engines that's supposed to prevent carbon buildup. It probably just boils down to using a high quality oil and faithfully changing the oil following Fiat's recommend oil change schedule.
There are several articles that go in depth on the workings of the MAU and the advantages of infinite positioning of the intake valves obtained by use of the hydraulic/electrical solenoids.
Have a good day.
