Q: I have a 2003 Pontiac Vibe with 135,000 miles on it. This past winter it would knock for a minute or so on a cold start. A GM tech told me that it has a cold piston slap and it will go for a long time. Now I can faintly hear it this summer. Could it be anything else? Is there something I need to do so it will last two to three more years and 60,000 miles?
A: Hoping to get close to 200,000 miles out of this or any engine is a worthy goal but of course there are no guarantees, knock or no knock. The GM tech is likely correct. Piston "slap" occurs when the excess clearance between the piston skirt and the cylinder wall allows the skirt to "slap" the wall as combustion pressure drives it downward in the cylinder. As the piston warms up and expands a bit, the clearance is reduced and the noise stops. And as the tech said, this isn't particularly harmful and does not mean impending failure. The engine in my '70 Corvette with 120,000 miles on it has reminded me it has a slight piston slap every time I have started it for the past 20 years!
A mechanic's stethoscope can pinpoint the particular piston/cylinder in question. If disabling the spark for that cylinder during a cold start test, which eliminates combustion pressure that slaps the piston skirt against the cylinder wall, eliminates the knock, it's piston slap.
If it is piston slap, don't worry about the noise and just drive the car. The only "repair" would be a complete engine overhaul that would not be economically justifiable.
There's one more possibility: combustion chamber deposit interference, or CCDI. This occurs when carbon deposits build up on the top of a piston and/or the roof of the combustion chamber to the point where there is physical contact between the two on cold starts until all the components warm up and expand enough to eliminate the contact. Again, like piston slap this isn't particularly harmful, but unlike piston slap it may be easily "fixable."
A professional or DIY induction cleaning with SeaFoam or GM Top Engine Cleaner can remove the carbon build-up and eliminate the noise.
Q: I would like to better understand oil change intervals on little used vehicles. I have a '77 F-150 that I use around the ranch about 20 hours and maybe five miles per year. I completely rebuilt the engine and the oil I put in over five years ago is still honey-colored. Am I hurting the engine? Seems silly to change it every year, but is there a shelf life?
A: In this case the word "silly" is synonymous with "wasteful." I just checked the date I last changed oil and filter on the aforementioned 'Vette and it was 2009 — four years ago. In that time I've driven the car less than 2,000 miles so I guess you've reminded me it's time to change it again.
But I have no worries that I'm hurting this engine, or any other "low annual time/mileage" engines I own and operate. The oil in these engines is subject to very little fuel/combustion blow-by contamination. The only time-based deterioration is oxidation from exposure to air inside the engine.
I think you're safe, but it's probably time for an oil and filter change. Save the old oil for recycling or use in topping up oil levels on your other low-annual-time engines.