Model A FordDear Ole Mechanic;

Many years ago, my Dad would “blow the cobs out” of the family car by revving it up and pouring either water or a light oil down the carburetor.  The car seemed to run smoother and quieter after he did that.  What was he doing and would that work with today’s cars and trucks?  Thank you.

I. M. Curious   


Dear Curious;

DO NOT try it with today’s cars and trucks; although, I am not too worried, as almost none of them have carburetors any more.  I’ll start with what was going on with the OLD cars.

The piston-ring technology in old cars was not what it is today.  Engine oil could get past the piston rings and would partially burn, leaving carbon deposits in the combustion chamber and on the spark plugs.  The carbon buildup would cause spark knock or "ping."  It could even build up to the point where the piston could hit the carbon, causing a knock in the engine.  In the 1920's and 30's, it was common for mechanics to remove cylinder heads and scrape the carbon out. 

World War II saw a big improvement in piston rings, but even in the 1950's, it was not uncommon for a car’s engine to burn a quart of oil every 500 miles or so.  What your Dad was trying to do, and it sounds like he was successful, was to “blow out” some of the excess carbon buildup in the combustion chambers.  Now it took practice to get the right amount of liquid poured in at just the right speed with just the right engine RPM.  Too much liquid could fill the combustion chamber, cause a hydraulic lock and do a lot of expensive damage to the engine.

I have seen a lot of liquids used and will try to explain the idea behind using each of them.  Water was used because a little water will expand into a lot of steam quickly when heated by the burning gasoline in the combustion chamber.  It was hoped that the expanding steam would knock the carbon loose so it could go out the exhaust.  Some people would use a light oil like Automatic Transmission Fluid (ATF), kerosene, distillate or even diesel fuel.  The hope was that the excess oil would act like a solvent and soften the carbon deposits, so they would loosen and go out the exhaust.  As late as the 1960's, there were a lot of cans of “cleaner” sold to be dumped down the carburetor in hopes of clearing deposits out of the combustion chamber.

1936 Ford ConvertibleThe 1960's and 70's saw even more improvements in piston rings.  Also the 1970's brought unleaded gasoline, which burned cleaner and would not harm catalytic converters--a pollution-control device. 

That brings us to why we must be very careful of what we put into the gasoline engines built in the last 25 years.  If too much of a heavy hydrocarbon--like a light oil--is put into one of those engines, it can cause the oxygen sensor to clog up and quit working.  That will cause the engine to run poorly and turn on the check-engine light.  Also, if too much of any burnable material gets into the catalytic converte, it will overheat and melt down internally.  That can plug up the exhaust like having a couple of potatoes jammed up the tail pipe.  If it runs at all, it won’t run very well. 

In short “blowing the cobs out” may have worked with really old cars and it may have helped pre-1970 cars, but DO NOT try it on anything built in the last 25 years unless you like paying expensive repair bills.

Herr Professor Nuzanbolts