Frank: Yeah, my brother used to drive a ‘74 Chevy Mosquito Fogger. You’re absolutely right, Frank. It’s unusual, and even somewhat offensive now to get stuck behind a car that’s blowing smoke out the tailpipe. Why is that? I give credit to three major developments. The Clean Air Act, Japanese cars, and better oil technology.
Let’s start with the Clean Air Act of 1970. Forced to clean up their tailpipe emissions, car makers phased out carburetors. Carburetors were crude devices. They weren’t much better than pouring gasoline into the cylinders from a boot. When excess gasoline went into the cylinders, it couldn’t be combusted completely, and came out the tailpipe as black smoke (and smog-producing hydrocarbons).
Over the next decade or so, fuel injection replaced carburetors. Fuel injection measures the fuel much more precisely, and when combined with computer controls and instantaneous feedback from sensors, it really solved that “excess fuel” problem.
The second reason for smoke was poor manufacturing tolerances. Tolerances are the spaces between parts — also known as “slop.” The more slop, the poorer the manufacturing. Japanese companies, particularly Honda and Toyota in the ‘70s and ‘80s, revolutionized engine manufacturing, with much tighter tolerances. That made their engines more reliable and longer lasting. It also left less slop between the pistons and the cylinder walls.
When there is slop between the pistons and cylinders, lots of oil remains on the cylinder walls between piston strokes and gets burned up with the gasoline. That oil also comes out the tail pipe, as blue gray smoke.
In fact, my late brother used to pull his ‘74 Mosquito Fogger into a gas station and ask the attendant to “fill the oil and check the gas.” But once the Japanese jumped ahead in quality, the rest of the world knew they had to catch up, so eventually, all cars ended up with better tolerances.
Finally, oil used to be a lot thicker. You may remember 50 weight oil, or 10W40. That thicker oil was harder for the rings to scrape off the cylinder walls, so it was more likely to get combusted and turned into smoke.
Now, we have oils that are not only lighter, but better at lubricating, too. And because they’re lighter and thinner, less of that oil adheres to the cylinder walls, meaning less gets burned up and sent out the tailpipe as smoke.
Now, if you write again in 10 years and ask me the same question, I’ll probably add the rise of electric vehicles to my answer. You’ll never see any smoke coming from an electric vehicle. Unless the battery catches fire.
Dear Car Talk: I have a Honda CR-V with all-wheel-drive. I’m curious about how frequently I have to change the rear differential oil. The automatic maintenance reminder that pops up on my dashboard has recommended this service three times in 60,000 miles.
That’s obviously not as often as engine oil, yet the differential stays cool, has no combustion gases mixing in, and I never go off road (other than to the Trader Joe’s parking lot, your joke).
Is this a profit center for the dealer? There is indeed a good lake nearby for boating. — Bill
Bill: If you had a different car, Bill, I would suspect that your dealer has a boat payment coming due. But Honda CR-Vs do require unusually frequent rear differential fluid changes.
I don’t know why — probably an engineering flaw of some kind — but the differential fluid in CR-Vs does degrade faster than in other cars we work on. And eventually, when it degrades enough, it creates a “chattering” sensation when you make sharp turns — as if one wheel is grabbing, which it is.
So far, we haven’t seen any permanent damage done by this. As soon we change the differential fluid, the chattering goes away and appears to stay away. But I suspect it’s better to change the fluid before the chattering starts.
How often should you change it? Every 20,000 miles, like your service light suggests? I don’t have the research (or the automotive physics lab) to answer that question precisely. But the CR-Vs we see in the shop that are chattering generally have 30,000-60,000 miles on them.
So, do I think you’ll be visited in your sleep by the ghost of “differential replacements future” if you did it every 30,000 miles instead of every 20,000? Probably not.
But if you like to be meticulous about your maintenance, and plan to keep the CR-V forever, then change the fluid when it’s recommended. It certainly can’t hurt.
Got a question about cars? Write to Ray in care of King Features, 628 Virginia Drive, Orlando, FL 32803, or email by visiting the Car Talk website at www.cartalk.com.
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