2007 Impreza wagon 2.5. Previous owner didn’t take car of it and I was thinking of using the CRC intake valve cleaner. I would love your opinions on whether to do it or not. Please explain why it’s good to do or bad to do. Thank you
I've used a lot of different induction cleaners, including CRC Intake Valve Cleaner, on many different cars and trucks. It does work, to a widely varying degree, to improve acceleration, idle, and gas mileage. Its effectiveness depends on how much deposit buildup exists, how much oil the engine burns, and how long the car has been neglected.
Though CRC Cleaner is safe and easy to use if you follow the instructions, you should be careful. One threat is to spray too much cleaner at too low of an idle, which I've seen cause harmful rod knock. If you spray the cleaner into the intake with the engine off, upon starting you could potentially hydro-lock the pistons and destroy the engine!
Another, much less serious drawback is that some of the cleaner will wash past the piston rings and get into the crankcase oil, thinning and degrading the motor oil and carrying carbon particles into the engine's oil circuit. It's therefore a good idea to change the oil and filter within 500 miles of using the product. I've had as much as 40% of the cleaner get into the crankcase, amounting to a small rise in my oil level on the dipstick. After about 100 miles of driving, I sometimes get a flickering oil light on my dashboard while stopped at red lights, which disappears once I change the oil and filter. I use a motor flush for five minutes before my shop drains the oil, which helps to remove the carbon particles.
CRC requires multiple applications if the engine is old and neglected. Carbon buildup can be extremely hard and stubborn. It helps greatly to pre-treat the engine with Marvel Mystery Oil in the gas tank, 2 oz. per gallon, for several weeks in between intake cleanings.
After successfully cleaning on an old car, there is a final problem. Carbon pack on old cylinder walls and worn piston rings has a sealing effect, and removing the carbon can ironically cause a loss of compression and an increase in oil burning. In such cases, I compensate by adding Engine Restore to the crankcase.
You don't need to clean intake valves on a port-injected gasoline engine using a dedicated valve carbon-removal solution as the valve neck and face are constantly being washed by the fuel injectors. The valve face is the only part of a valve susceptible to carbon buildup on older, indirect/port-injected engines.
It's a different story on both diesel and gasoline direct-injected engines as neither offer a constant fuel stream passing over the intake valves which would otherwise prevent the buildup of carbon deposits. This affects all DI engines without a supplementary port injection system. This is why intake valve cleaning products such as the CRC line exist.
However, cleaning the entire fuel system via a cleaning agent deposited into the fuel tank can be very effective as it will help dissolve deposits within your fuel injectors and all fuel delivery components leading up to those injectors. Products sold as "Fuel System Cleaner" or "Injector Cleaner" would be more appropriate for your Subaru if it has been neglected as it will help restore injector spray pattern, critical for an effective and complete combustion cycle.