Drive and RestoreMultiAir technology explained: how Fiat's electro-hydraulic valves work
MultiAir is one of the cleverest pieces of engine technology to come out of the Fiat group. Introduced in 2009 on the Alfa Romeo MiTo, it lets the engine control each intake valve individually, cycle by cycle, without a conventional intake camshaft acting on the valves. Here is how it works, what it replaces, and what to watch for as these engines age.
The problem MultiAir solves
A normal petrol engine controls its power with a throttle: a flap in the intake that restricts how much air gets in. At part load, which is most of the time, the engine wastes energy pulling air past that restriction. Fiat put those pumping losses at around 10% of the input energy.
The other compromise is the camshaft. A cam lobe has one fixed profile, so the intake valve opens by the same amount at the same moment whether the engine is idling or at full power. Variable cam timing helps, but only shifts the whole profile.
What MultiAir replaces
MultiAir breaks the direct mechanical link between the camshaft and the intake valves. On a MultiAir engine:
- There is no conventional intake camshaft driving the intake valves. A single overhead camshaft operates the exhaust valves directly, and also carries one extra lobe per cylinder that drives a small hydraulic pump piston instead of a valve.
- The job of controlling how much air enters each cylinder moves from the throttle to the intake valves themselves, so the engine can run with far less throttling.
How it works
The heart of the system is the actuator unit that mechanics call the “brick”, bolted on top of the cylinder head:
- The intake lobe pushes a piston that pressurises engine oil in a small high-pressure chamber.
- That chamber is connected to the intake valves through hydraulic elements.
- Between them sits a fast, normally open solenoid valve controlled by the engine computer.
- Solenoid closed: the oil is trapped, behaves like a solid link, and the valve follows the cam lobe exactly.
- Solenoid open: the oil escapes into an accumulator, the valve is no longer driven by the cam, and its spring closes it. A hydraulic brake cushions the final part of the closing so the valve doesn’t hammer its seat.
By choosing when to open the solenoid, the computer can close the intake valve early, open it late, or give it only a partial lift, every cycle and for every cylinder.
The operating modes
Fiat described several strategies:
- Full lift: the solenoid stays closed and the valve follows the full cam profile, for maximum power.
- Early intake valve closing: at low revs the valve closes before the end of the cam profile, trapping more air and improving low-speed torque.
- Partial lift: at part load the valve opens only as far as needed, replacing the throttle’s job.
- Turbulence mode: a partial opening designed to increase turbulence in the cylinder for better combustion.
- Multi-lift: combinations of the above within a single intake stroke for very low loads.
Claimed benefits
At launch Fiat claimed 10–25% lower CO2 emissions and up to 60% lower emissions of other pollutants for petrol engines, along with better power, torque and throttle response across the rev range.
Where you find it
MultiAir debuted on the 1.4 16-valve FIRE engine, naturally aspirated and turbocharged, in the Alfa Romeo MiTo in 2009. It went on to the Fiat Punto Evo, the Fiat 500 and Abarth, the two-cylinder TwinAir, and Chrysler-group engines. Schaeffler manufactures the system under the name UniAir, and won an automotive innovation award for it in 2012.
The idea took a long time to mature: Fiat researched variable valve actuation from the 1980s, abandoned electromagnetic solutions, switched to electro-hydraulic actuation in the mid-1990s and patented the system in 2002.
What goes wrong
Because the whole system runs on engine oil, oil quality and oil level are everything:
- Use exactly the oil grade and specification in the owner’s manual and change it on time. Old, diluted or wrong-spec oil changes how the hydraulics behave.
- A low-pressure fault in the brick (code P1523) or solenoid faults (such as P1041 or P1061) are typical MultiAir codes. Solenoid circuits are best tested with an oscilloscope, not just a multimeter.
- A failed fuse or relay for the actuator can make the engine crank without starting, which is easy to mistake for a broken timing belt.
- The brick, its roller followers, the oil temperature sensor and the oil supply O-ring between brick and head are replaceable. The internal pumping elements are not sold separately, so a faulty brick is usually replaced as a unit.
- Timing belt work and brick removal need the correct locking and spring-compressor tools. This is not a job for guesswork.
If you are replacing a brick or its seals, original parts from the manufacturer are the safe choice: the tolerances in this system are tight, and the engine depends on them.
Sources
- Autocar Professional: Fiat reveals new MultiAir tech (2009)
- FenderBender: Fiat Multi-Air engine technology
- Electronic Specifier: Schaeffler receives 2012 PACE Award for UniAir