Needs to Chill
Q: I own an '81 Corvette that has a slow air-conditioning leak. For years I just recharged the system in the spring and hoped it would cool until fall. That didn't work for me this year, so I've decided to convert the system to R-134a, rather than continue to pay $35 per pound for R-12. I would like to do this over the winter, when I don't drive the car as much.
From what I understand, the recommended retrofit procedure consists of simply evacuating the system, repairing the leak(s), adding service-port adapters, charging the system with R-134a and PAG oil, and running it as is. The final is step is to pull a vacuum on the system for 30-45 minutes to boil out any remaining moisture and air. (I know there are other methods, but I'm trying to stay with the most economical.)
My question is, when recharging the system, is there a conversion chart or formula that will tell me how much R-134a to use? Or do I use the same amount of R-134a as I would R-12?
A: The sticker on your car probably calls for 3.0 pounds of R-12. If so, the system should take 2.45 pounds of R-134a. When converting from R-12 to R-134a, multiply the given R-12 amount by 0.9 and then subtract 0.25 from that amount to get the R-134a number.
Why? R-134a has a higher pressure than R-12 at a given temperature. Therefore, you must use less R-134a in your system than you would R-12.
When performing an inexpensive conversion, I would also recommend that you remove the compressor, drain out as much mineral oil as possible, and replace it with PAG (polyalkylene glycol) oil. You also may need to adjust the clutch-cycling switch on some A/C systems.
Knock, Knock: No Joke
Q: I'm the owner of a stock '99 Corvette with 65,000 miles and an automatic transmission. I've had the car for several years now and have experienced no mechanical difficulties during that time. I only use the recommended synthetic oil, and I only drive the car during the warm months here in Ohio (May through October).
Recently, the engine developed a knock. (I'm an old gearhead from the '60s, so I know a little about engines.) It sounds like a lifter to me, but here's the odd part: When I first start the car and the engine is cold, I don't hear it. But when I back out of my driveway and add gas, the knocking becomes very prevalent. It then goes away as soon as the transmission shifts into Second gear.
When I reach the end of the block, stop, and then pull away again, there is no knock in First or any other gear. Similarly, if I let the car sit at idle and warm up to full operating temperature before driving, the knock does not occur. (I should note that the car runs great whether it knocks or not.)
My mechanic did some research and tells me that it could be a worn cam lobe. He also suggested that the synthetic oil GM requires in these cars has changed over the years, and that some of the cam-protecting additives have been removed. I'm having a hard time buying into this, but he is a mechanic I know and trust.
I would appreciate your thoughts as to why an engine with only 65,000 miles might have a worn cam. Do you have any other ideas about what might be causing the knock?
A: While it's impossible to know for sure without hearing your car's engine in person, it's unlikely that the problem is attributable to a worn cam lobe. Your mechanic is correct in that some oil formulations have changed in recent years, but this should have no bearing on your situation, which involves a synthetic lubricant and a hydraulic-roller valvetrain. (For more information on oil additives, see our "All About Oil" article elsewhere in this issue.)
A far more common problem with LS1 engines is piston slap, which is caused by excessive piston-to-bore clearance during cold operation. The good news is that this isn't a harmful sort of noise. Chevrolet has acknowledged the issue and states that while there is no fix, it will not affect the engine's performance or life expectancy.