Q. My 1984 Oldsmobile Eighty Eight continues to run upon stopping and putting the gear in park. This goes on for a few seconds with it starting to huff and puff. I turn the keys back on and the motor starts to run again. It repeats each time I try to cut the engine off. Finally, I discovered that if I turn the keys off while the gear shift is in drive, it finally will stop without much more puffing. Do you have any idea what’s causing this? Could it be bad gas? Is this hazardous? I just put new spark plugs, wires, coil, cap and rotor repairs onto the car, and that was expensive enough trying to correct the problem.
— James Harrion Jr., Benicia, Calif.
A. Finally an easy one! Engine run-on or dieseling is an occasional ailment of carbureted engines and is most likely to occur during warm weather. After shut-off, the engine tries to keep running because of heat build-up in one or more combustion chambers. Under normal conditions, the air-fuel mixture is ignited by the spark plugs, and combustion stops when the ignition switch is turned off. Dieseling isn’t a problem with fuel-injected vehicles, as the electrically operated injectors stop fuel delivery upon engine shut-off. Dieseling can be harmful as it places severe stress on the crankshaft. The usual causes are faster-than-specified idle speed, carbon buildup on combustion chamber surfaces and/or a lean air-fuel ratio. What can be done to put the brakes on unwanted combustion? Start by checking-adjusting the engine idle speed to specifications. Idle speed is often increased by well-meaning folks to mask a rough idle condition, or might be fast due to a sticking throttle or choke linkage, a too-tight cruise control servo cable or a leaking vacuum hose or gasket. (See last week’s column for vacuum leak diagnosis procedures.) Vacuum leaks are also a common cause of a lean air-fuel ratio. Some carbureted engines employ an anti-dieseling solenoid, which allows normal idle speed while the engine is running, but substantially reduces throttle position after shut-off. (It must be functioning and properly adjusted.) Other vehicles utilize a fuel-cut solenoid to positively stop idle circuit fuel at shut-off. (You’ll know when this part stops working, as the engine won’t idle.) Carbon build-up can be reduced by applying a carbon-softening solvent that is sprayed or slowly poured down the carburetor throat. Driving the vehicle afterward releases an embarrassing cloud of particulate matter as the deposits are released. If the above methods fail to bring relief, it might be necessary to switch to higher-octane fuel or seek professional help for more in-depth diagnosis or a more effective decarboning process. Your in-gear shut-off method reduces idle speed enough to quell the dieseling, and should be employed until repairs are made. Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif. E-mail him at under-the-hood@juno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.
