Cold-start knock isn't normal, but it's common

  • Article by: PAUL BRAND , Star Tribune
  • Updated: September 1, 2010 - 4:45 PM

Q My 2002 Chevy Tahoe, with the 5.3-liter V8 engine and 93,000 miles on it, makes a rapping noise just after a cold start. After a few seconds, it disappears, and the engine is the quietest-running and most powerful truck engine I've ever owned.

We do no towing or hard driving, change oil and filter about every 5,000 to 6,000 miles or when the "change oil" light appears and do lots of preventive maintenance on the vehicle. This is the second GM truck with the same engine I've owned, and they both made the noise. A mechanic told me the noise is normal for this engine.

A Normal? No. Somewhat common? Yes. My Alldata automotive database pulled up GM service bulletin 01-06-01-028A from March 2003, describing a cold-start knock that fades after a few seconds caused by carbon buildup on the piston and cylinder wall. The bulletin does not recommend trying to repair it and says it doesn't hurt the engine's performance or shorten its life -- but decarbonizing the intake system and combustion chambers with a product like SeaFoam can't hurt, and might help.

Q The windshield wipers don't work on my '97 Ford F-150. Each time I replace the fuse and try the wipers, they will oscillate a couple of times and then blow the fuse again. I've taken the truck to several mechanics, the dealer and an auto electric shop with no luck. Do you have any suggestions?

A Ford outlines a simple test of the wiper motor in their service literature. With the wiper motor disconnected from the wiper linkage, amperage to the low-speed terminal should be 3.5 amperes or less, and 5.5 amperes or less from the high-speed terminal. If amperage draw is higher than that, the problem is the wiper motor itself. If amperage draw is within specs, check for mechanical binding in any and all of the wiper linkage.

Q My daughter has a 2003 Ford Focus with aluminum rims. They leak air, and I'm not sure what the fix is. I've heard you shouldn't use the "fix-leak" stuff in a spray can, but I'm not sure why. Can anything be done to these rims to keep them from leaking air?

A Air leaks with alloy wheels are fairly common, particularly with the second or third set of tires mounted on the rims. It's often caused by corrosion around the bead area where the tire touches the rim. Sometimes the alloy wheel itself is porous enough to allow a slow air loss.

And you've heard correctly, the tire "stuff" in an aerosol can should not be considered a repair. This material is just a stop-gap measure to get your vehicle to a service station or tire shop for a proper tire repair or replacement. Some of these products use a flammable propellant, so make sure the shop is informed before they attempt a repair.

Most tire shops can stop the air loss by thoroughly cleaning the bead surface of the rim and applying a special sealant to the inside of the rim.

Q I set the air-conditioning unit on my 1999 Honda CR-V to the coldest temperature, but the air blowing from the vents varies from cold to warm. I had the unit recharged, but the problem continues.

A If the air blows warmer as you accelerate, this could be normal. The engine management system disengages the A/C compressor when the engine is under heavy load. Poor engine performance could magnify this.

A diagnostic test might identify a problem with the air mix control motor or the evaporator temperature sensor.

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