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Q: I have a well-maintained 1998 Toyota 4Runner Limited with 239,000 miles. The truck has been awesome, no more than normal maintenance issues. The problem is one of the lifters is knocking. I hear it in the cabin and can isolate the noise from the back of the driver's side valve cover. Mechanics want to rebuild the engine of course, but I tried Lucas fuel additive, which made the knock go away. Is this an issue I need to be concerned about? I love having no car payments, so I am willing to put a little money into it.

A: There's an interesting contrast in your question. A fuel injector additive should not affect a lifter knock. A noisy lifter is a caused by mechanical wear and/or a lack of oil pressure pumping the hydraulic lifter up to eliminate the clearance between lifter and valve train.

So what else could be causing the noise? Injector cleaner would tend to help reduce noise from CCDI — combustion chamber deposit interference. This knocking sound is due to carbon buildup on the piston crown and the combustion chamber roof causing a very slight physical interference between piston and chamber roof at start-up. It usually only lasts until heat is generated in the components, expanding the clearance slightly and eliminating the non-harmful knock. Injector cleaner would also help reduce the audible clicking noise from a noisy fuel injector. Either way, periodic use of a decarbonizing/injector cleaning fuel additive is good maintenance.

If the knock is from a lifter, you could try this; with the engine warm, remove the valve cover on that side, pour Sea Foam directly onto the rocker arm/lifter. Add half the can to the oil. Let it sit overnight and see if this treatment cleans the lifter's hydraulics enough to restore oil pressure and stop the noise.

Q: I have a 2002 Acura MDX with 90,000 miles on it. The car is in good shape and runs great. However, between around 1,300 to 1,500 rpm a very loud "fluttering" noise comes from the engine compartment. It sounds like sticking a piece of cardboard into a fan. The sound stops as soon as I either increase or decrease rpm. It doesn't seem to impact performance and has been going on for a long time. Suggestions range from loose engine mounts, a loose heat shield, expensive transmission problem, etc., but no one, including dealers, seems to know for sure what causes it. Any suggestion?

A: The simplest and most common explanation for this noise is a loose heat shield on the catalytic converter. But other possibilities include an air induction/vacuum leak, a driveshaft/bearing issue or a problem with the IMRC (intake manifold runner control) and IAB (intake air bypass) systems.

These systems regulate intake airflow and manifold air routing based on engine load. Since the noise occurs during maximum engine vacuum, the IAB valve or IMRC actuators may be fluttering. A multimeter (voltage/resistance) can be used to test the circuitry and resistance values on the system's modules and actuators.

Q: I have a 2002 Saturn. For a few weeks now, the "service engine soon" light has been on. My owner's manual says to reinstall the gas cap, which I tried several times with no effect. I understand from the book that it may be an emission control system malfunction, which sounds very serious. Since Saturn has gone out of business, should I take my car to a service provider to check the vehicle?

A: Yes, take your vehicle to a GM dealer or independent shop. They will connect a scan tool to the diagnostic link on your Saturn and determine which specific DTC fault code triggered the light. There are hundreds of DTCs that can be recorded and downloaded for diagnostics. You won't know whether it's minor or serious until you check. If the vehicle runs well, it's more likely not a serious issue.