Troubleshooting engine problems can be a difficult task if you are not familiar with engines. Sometimes problems can be a combination of smaller problems or one problem normally not associated with the specific problem. If your car has not had a tune up in a while it’s always a good place to start. We will start with a generalized analysis and then try to narrow our search. If the car runs rough, jerks or surges as you press the gas more it may be an ignition problem. It may still feel smooth at low speeds or when pressing lightly on the gas. Start with the spark plug wires. Replace them if it’s been awhile (couple years or a lot of miles) or look for any arcing coming off of them. It is easiest to look for arcing at night time away from lights. Remember that a tiny little glow is all it takes to cause problems. Next remove your spark plugs and replace them if they look worn on the electrode corners. Also check or replace the distributor cap and rotor, all of these are part of an engine tune up.
If the jerking or stumbling problem is still there or is worse at lower engine speeds check for vacuum leaks. Vacuum leaks can easily be found with a $10.00 stethoscope using the hose to listen for the high pitched squeal of air leaking. The leak can occur between the throttle body/carburetor and the intake manifold. Most times the leak is simply a rubber vacuum hose that has become brittle, pinched or usually cracked. If any hoses look bad replace them. The o2 sensor or egr valve can also cause these problems and would show up with the engine light. Either one can cause the fuel mixture to be incorrect and could run poor all the time. If the car surges at idle, in or out of gear, the iac or idle air control valve is probably sticking or the electronics not working. If the car only jerks in gear and you have gone through the other items, it could also be the lockup converter in the transmission. If lockup converter stays locked up the car will usually not want to stop and continue to want to move. Lock up converters are only found on automatic transmission cars and there is a usually wire that can be removed to disable it from working. Any transmission can slip and make the car jerk. A catalytic converter can also plug up and cause the car to lose power and run rough. If the exhaust has any leaks in it the o2 sensor will read incorrectly and run poorly. If the car will not start use the timing light to see if the engine has spark. If the engine has spark, disconnect the fuel line going to a carb then spin the engine to see if fuel is flowing out. On a fuel injected car there should be a valve on the fuel rail that looks like a tire fill valve. Turn the key forward and press in the tiny pin in the middle of the valve. The fuel will spray out if the fuel pump is working.