Paul Brand: Carbon buildup may lie behind engine stumble

Q My wife has a '95 three-quarter-ton Chevy truck with the 350 V8. Seventy-five percent of the time, it almost stalls out after about three minutes of driving, seemingly just as it goes into closed loop. At 1,000 rpm it kicks back on to maintain a nice idle regardless of throttle position.

The dealer and the local shop failed to find the problem. Oddly, they still charged me handsomely, though: new oxygen sensor, EGR valve, fuel filter, thermostat, plugs, rotor, cap, air filter, even a new throttle body, including fuel regulator. None of those helped.

I'm thinking the ignition and fuel pump are OK since the engine runs OK at wide open. I understand this forces open loop on the powertrain control module (PCM). It did a similar thing last summer until I pulled the air conditioning plugs. To me it sounds like the PCM shuts off the fuel in closed loop at certain times under load. But I don't know what to do about it. Any thoughts, or am I way off base?

A I don't think you're off base, but the problem may be related more to a carbon buildup on valves, pistons and in the combustion chamber than to a specific component that has failed.

From your description, it appears the near-stall occurs just at the moment the engine management system tries to switch to "closed loop" operation, meaning the fuel-air mixture is now adjusted by the feedback from the oxygen sensor. Before reaching operating temperature and shifting to closed loop, the fuel-air mixture is determined by a warm-up program based on coolant temperature, manifold pressure, throttle position and engine rpm.

If carbon deposits in the combustion chamber are absorbing a percentage of incoming fuel, the engine may run lean and stumble a bit just as it shifts to closed loop with oxygen feedback. Once the system recognizes the lean condition based on oxygen feedback, it can adjust -- in this case, slightly richen -- the fuel-air mixture by lengthening injector pulse widths.

Have the dealer or independent shop connect a scan tool and monitor the "integrator" and "block learn" circuits to see if this is what's happening. Or, just try the Minnesota Tune-Up with SeaFoam -- or have a professional induction cleaning done -- to remove carbon deposits and see if the symptom improves.

Q I'm having the same problem as someone you responded to in 2006. I've got a 2002 Subaru Outback and the brakes freeze on cold days for the first few minutes of driving. It's pretty terrifying. In our case, the pedal is at the top and requires a huge amount of pressure to stop the car even at 2 miles per hour. So far, we have drained and bled the brake lines, replacing all fluid, replaced the master cylinder, and put new rotors and pads on the front -- the rears are in great shape.

A It's very likely that moisture is freezing in the check valve in the vacuum line between the engine and the power brake booster. Until this thaws as the engine warms up and starts allowing engine vacuum to reach the booster, the brake system in your vehicle has no power assist to help you push the pedal.

Before starting the engine, push the brake pedal very firmly a half-dozen times. Without power assist, I'm sure you'll find the pedal rock hard and very difficult to push. Now, start the engine. Push the brake pedal again -- does it still feel rock hard? Or, with the engine running, is the pedal "softer" and easier to push part way down? If it's still hard as a rock, no vacuum is reaching the booster.

Flushing, cleaning and drying the one-way check valve in the vacuum line may solve the problem. Otherwise, replace the valve.

Q I recently bought a 1990 Cadillac Sedan DeVille with electronic climate control. Now that it has gotten colder, I have noticed that the heat doesn't keep the interior warm. While the vehicle is in motion the heat will blow warm, but when I come to a stop at a red light it will blow cold, as if I had just started the car and it hadn't warmed up yet. I don't know where to start to fix this problem.