Q: My car has begun to lose power above 65-70 mph. I’ve noticed it also bogs down when I accelerate hard, like when getting on the freeway. It starts OK and runs smoothly at normal speeds. Any idea what could be the cause?

A: There are several things that could cause this driveability symptom. Either the engine is starving for fuel, or it can’t breathe properly due to an intake or exhaust system restriction. If you had also noticed rough running or sputtering symptoms, we’d also want to take a look at the ignition system. Does your “check engine” light illuminate? If so, valuable clues await discovery via a scan-tool connection.

A restricted fuel filter is the first thing I would suspect. A dirty filter may pass enough fuel to satisfy lower-speed engine requirements, but it can’t deliver as needed when a large volume of fuel is called for. Should repeated filter problems occur, substantial tank contamination is likely, requiring draining and cleaning of the tank.

Inspecting and renewing the air filter is also a good idea, as well as checking for debris in the air intake system, which typically comprises a plastic box and hoses.

If fuel- and air-filter replacement doesn’t do the trick, diagnostic tests will be needed to check fuel pressure and volume, as well as exhaust system back pressure. If fuel pressure or volume fail to meet specifications, a failing fuel pump or restricted fuel pickup sock within the fuel tank may be the cause.

Excessive exhaust back pressure can be caused by a failed or internally disintegrated catalytic converter or an obstructed tailpipe. Typically, the engine runs acceptably at lower speeds but just can’t breathe well enough to run fast. A check of engine vacuum, comparing the readings at idle to higher speed in neutral, can point to breathing problems. Exhaust back pressure also can be directly checked with a gauge attached to a vacated oxygen sensor fitting.

In many cases, a failed catalytic converter can also be identified by smacking it several times with a rubber mallet. If a rattle is heard, this indicates breakup of the converter’s internal substrate, a ceramic honeycomb plated with precious metals. Should a failed catalytic converter be identified, one should diligently seek and correct the cause of failure — usually an engine misfire or fuel-mixture fault — in addition to renewing the converter.

Based on your symptoms, I blew past the ignition system, which is of course critical to proper engine performance. In most cases an ignition fault results in choppy or rough running or a no-start condition. Should the above suggestions fail to rectify the problem, a check for the ignition system’s available voltage and successful spark delivery to the spark plugs would certainly be a good idea.