Q I have a 2005 Ford Explorer V6 with 75,000 miles. The engine knocks when first started in cold conditions. As it warms up, the knocking dissipates. I tried using synthetic oil, but it hasn't helped. I am very concerned.
A Next time you start the engine and hear the knocking noise, open the hood and try to identify where the sound is coming from. A mechanic's stethoscope -- or a long screwdriver or wooden dowel rod -- could help pinpoint the sound. If it's originating in the lower part of the engine, it might be a crankshaft or connecting rod bearing. If the noise is coming from the middle of the engine near the cylinders, it might be a piston slap or piston pin. If it's coming from the valve cover area on both sides of the V6, it may be an oil pressure or hydraulic lifter issue.
But two other possibilities might be more likely. Combustion chamber deposit interference is a knocking sound during warmup that is caused by carbon deposits on pistons, valves and the combustion chamber actually causing a very light physical contact between components when the engine is started cold. As the engine warms up and parts expand just a bit, the clearances increase fractionally and the contact -- and noise -- stops. A professional induction cleaning/decarbonizing process may resolve the issue. Even a do-it-yourself "Minnesota tune-up" with SeaFoam can help.
If the noise is originating from the front of the left-hand cylinder head area, my Alldata automotive database zeroed in on Ford service bulletin 99-26-5 from December 1999 that identifies the possible cause as oil leaking from the timing chain tensioner after shutdown. A revised tensioner kit was introduced to eliminate this noise.
Q I have a '95 GMC Suburban with 150,000 miles. It runs very well, but the motor seems to knock/ping/miss at high speeds, especially while loaded with gear. I checked timing and found it to be ahead by about 4 or 5 degrees. The distributor hold-down bolt appears to be stuck; it won't loosen with a wrench or socket. Can I try heat to the bolt?
A If the distributor hold-down bolt is that tight, how could the timing have changed? To check the timing, you must disable the electronic spark control (ESC) by grounding the test terminal to the ground terminal in the assembly line diagnostic link under the dash. This will put the ESC into the base timing setting of zero degrees of advance. If the ESC is not disabled, you're seeing computer-controlled timing advance with the engine running, which is normal.
Other causes include low fuel pressure, too much exhaust gas recirculation, carbon buildup in the combustion chambers or weak compression on one or more cylinders.
Motoring notes As a follow-up to last week's column on long-term parking, K.D. Jones sent me a note with several more winter storage tips. I added my notes in parentheses.
