Q: We have a 2009 Chevy Cobalt with standard transmission. The "Check engine" light has been coming on and the dealer has diagnosed the problem as carbon buildup and low compression in cylinder No. 4. The car had 116,000 miles on it when this occurred. My research tells me carbon buildup should create high compression.

A: Not necessarily. If the carbon buildup is on the piston rings or ring grooves, it can prevent the rings from sealing properly, thus creating low compression. It's worth trying to "de-carbonize" the engine with GM's Top Engine Cleaner or SeaFoam Motor Treatment. I've had some success by removing the spark plugs from a warm engine and pouring a couple of ounces of SeaFoam directly into the cylinders and letting it sit for several hours. Disable the ignition and injection before cranking the engine to expel any excess additive before reinstalling the plugs and starting the engine.

If this doesn't help, listening to where air pressure is escaping during a cylinder leakdown test — throttle body (intake valve), tailpipe (exhaust valve), oil filler or PCV port (piston rings/cylinder) — might pinpoint the problem.

Q: I have a 1990 Honda Civic with 110,000 miles. I don't drive it every day, but when I do it almost always starts to run rough after 15 to 30 minutes and it does not have the same power to accelerate as it does when it is running normally. If I am stopping and going a lot, it gets progressively worse till it won't stay running. If I turn it off for a few minutes the problem goes away; then the cycle repeats. It is worse in hot weather.

I thought it might be vapor lock, but I have opened the gas cap to test this and the problem remained. There is a diagnostic light under the passenger's side, and if I read it correctly, it gives Code 1 — the Oxygen Sensor/Primary. Do you have any ideas what to check or repair?

A: First off, opening the fuel filler cap is not an adequate check for vapor lock. By definition, vapor lock is literally "fuel percolation," where under-hood temperatures reach high enough levels to actually boil the ready fuel supply, causing a loss of fuel pressure. You could have some type of evaporative emission system issue that is not allowing the fuel tank to vent properly, starving the engine of fuel.

Another potential cause could be some type of exhaust or catalytic-converter restriction causing excess exhaust back pressure, literally choking the engine of power. A glowing red exhaust manifold when the loss of power occurs would be a good indication of this.

With regard to the Code 1, according to my ALLDATA automotive database, this identifies an oxygen sensor circuit problem — either a wiring/connection issue or a failed/tired O2 sensor that's driving the fuel-air mixture extremely rich or lean.

Another relatively simple cause for a rich fuel-air mixture and the resulting driveability issues would be a leaky diaphragm in the fuel-pressure regulator, allowing excess fuel to be drawn into the engine.

Q: After my 1953 Ford with a flathead V8 is not run for a while, the crankcase gets contaminated with gasoline. What is causing this?

A: The only way raw fuel can leak into an engine crankcase while the engine is not running is through a leaky carburetor float system or, on an engine that utilizes a cam-driven mechanical fuel pump, through a leaky fuel pump diaphragm.

I assume your Ford is relatively stock and relatively "mature," meaning carburetor and fuel-pump components are not only older (original?) but sensitive to today's alcohol-blended fuels.

Fixes include bypassing the mechanical fuel pump to install a low-pressure electric pump and fuel-pressure regulator or installing mechanical fuel pump "hardened" for today's fuels, and rebuilding the carburetor with upgraded float, needle and seat.