An alternative way to burn the carbon out of cylinders.

Dear Car Talk

Dear Car Talk | Sep 01, 2000

Dear Tom and Ray:

My mechanic buddy told me about an alternate way to "burn the carbon out" of the cylinders. Rather than risk a ticket flooring it on the highway, he suggested I trickle a stream of water down the carburetor intake while revving the engine at about 1,500 to 2,000 rpm. He demonstrated this to me on his 1977 Mercury Cougar, which still runs great. I'm not sure about this. Since fluids don't compress much, won't this blow off the heads? My other non-mechanic buddies say he's nuts. Could you settle this? -- Mike

RAY: He's not nuts, Mike. This is an old trick that actually works. We use it all the time in the garage on old, carbureted heaps that ping due to carbon buildup.

TOM: As you pour in the water, it immediately turns to steam in the combustion chambers. The steam seems to help break up the carbon, which then gets kicked out the tailpipe. And it seems to improve the performance of old cars with lots of carbon on their valves and pistons.

RAY: And the reason it doesn't blow the heads off is because we're literally talking about a TRICKLE of water. It's a small enough amount that the engine is able to instantly turn it to steam as it enters the cylinders. If you stand over the carburetor with your garden hose, you certainly will hydro-lock the engine and ruin it.

TOM: What you want is about the same volume of water that you'd drink from a glass. If you drank an 8-ounce glass of water, it would take you about 30 seconds, right? Well, with the engine revved up, that's the speed at which you want to pour water into the carburetor.

RAY: Warning: Don't use the "drinking" measure for any other type of engine cleaner. My brother's teeth have never recovered from the "Gumout" incident.

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One