Hamilton Journal-News

Wheels: David writes by e-mail, “You helped me a few years ago with a problem on my 2001 GMC pickup engine and it was very helpful. Now when driving in the hills of Kentucky and Tennessee, while not carrying anything, the truck slows down on grades. It runs OK here in Ohio, but on long grades or higher elevations, it feels like the truck is hitting a wall. I even used “tow mode” with a 2,000-pound trailer and it didn’t help the situation. As soon as I got to Ohio on level ground, it ran 70 mph with no trouble. It sounded like a freight train under the driver’s seat going through Tennessee and Kentucky. The warranty is out except for the engine and drive train, which was extended for 100,000 miles until 7-7-07. Dealers say they want to see and hear it do that and I can’t duplicate it here. What can I do with a private mechanic to see that this is found and fixed? Can you advise what could hold it back on this and work fine at lower grades and around here.

Halderman: It appears that you are running out of fuel under a load. I suggest that you replace the fuel filter and try again. This is a relatively inexpensive part and could solve your problem. If that does not fix it, then the fuel pump could be the cause. Other possible causes could be a dirty mass airflow sensor, a clogged air filter or a clogged catalytic converter. This last item is covered by all vehicle warranties for eight years or 80,000 miles, whichever comes first.

How could David check to see if the catalytic converter is clogged?

Halderman: Checking for a restricted (clogged) catalytic converter is best left for the professional service technician. A pressure gauge is required and it is installed in the exhaust system backpressure. To perform this test, one of the oxygen sensors needs to be removed (just for the test) and the pressure gauge installed. The results should be less than 1.25 pounds per square inch of pressure at idle speed and less than 2.50 pounds per square inch pressure with the engine operating at 2500 revolutions per minute.

If the exhaust is found to have excessive backpressure, take the vehicle to a dealer for confirmation and the possible replacement of the catalytic converter. However, catalytic converters usually do not fail unless something is wrong with the engine, such as a cylinder misfire caused by a fault in the ignition. If a misfire occurs, the catalytic converter can overheat, causing it to melt internally, which will cause excessive backpressure.