Car Doctor Q&A: Rough Running Colorado May Have Carbon Issues

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If your engine’s running rough, you may have an issue with carbon buildup. Car Doctor John Paul has a few answers before you go spending big money.

Q: I have 2007 Chevrolet Colorado and it is running rough. The rough running is worse when the engine is cold and in fact sometimes it spins over for an extra long time before it does start. So far I have replaced the spark plugs and all five ignition coils. I also had someone checked the fuel pressure and flow and that seemed normal. There are no codes other than an occasional misfire code. Any suggestions, Car Doctor?

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A: If the engine is mechanically sound, the rough running and hard starting could be a result of excessive carbon building up in the combustion chamber and on the back of the valves. This carbon acts like a sponge and absorbs fuel and if thick enough can hold a valve open causing a misfire.

General Motors makes a very good carbon cleaner. The procedure is simple: spray the cleaner through the throttle body with the engine running at about 1500 RPM and then shut of the engine and let it rest for about two hours. This will soften the carbon.

After the engine sits, add a cleaner to the tank.  use AC Delco’s Upper Engine and Fuel Injector Cleaner, or something like Techron and drive the car for an extended period of time. If this fixes the problem use Top Tier ( to help remove any other carbon buildup.

Over the years I have seen cases that the carbon is so bad the engine needs to be cleaned with a sand-blast type tool using ground up walnut shells or in very severe cases the engine needs to be disassembled. Try the additive first and see what happens, if you are not sure what to do contact a professional.

John Paul is public affairs manager for AAA Southern New England. A certified mechanic, Paul tests dozens of new cars each year and also hosts a radio show on AM 950 and 550.