DIY Tune Up and Misfire Codes – Top 10 Misfire Causes

Tune ups are not required on today’s vehicles nearly as often as they were in the past. Most vehicles today, no longer have distributors (with points) and carburetors. A tune up used to consist of much more than it does today, with technology and innovative design vehicles require much less attention from us when it comes to maintenance items affecting the engine’s performance. The car’s computer is running thousands of tests everyday to insure optimum performance. If the SES (Service Engine Soon) light comes on, the computer is telling you there’s a problem. Today I cover what a tune up consists of and the most common causes of misfire codes being set or stored in your car’s computer. Also, in this article read on to find out why my friend with an old Chrysler lean burn system used to carry a big stick! 

What a Tune Up Used to Be

In cars during the 60’s and 70’s a minor tune up consisted of replacing the spark plugs, distributor cap, rotor, points and condenser. The timing and dwell would also be set if needed. Back then a major tune up would also include spark plug wires and adjusting the carburetor. Computerized cars no longer need all of this attention! Electronic ignition and fuel injection in today’s cars have almost completely banished the use of distributors. And practically the only place you’ll find a carburetor is in a dinosaur or a hot rod. Today spark plugs typically last from 60 to 100,000 miles (sometimes less if there’s a misfire). Computers control the mixture of fuel and air to the cylinders so efficiently that a car that floods today, is a rare one indeed. Don’t get me wrong there were some growing pains in the 80’s with carburetors that still needed adjustment and computer systems that never really allowed some cars to run very well from day one. I had a friend with a 1980 Chrysler equipped with a lean burn system that worked in conjunction with a feedback carburetor, it caused him many headaches. The lean burn computer was mounted on the air cleaner housing. He discovered that when the car wouldn’t run, it only took a light tap on the computer to make it work again. Instead of switching back to a conventional carburetor or relocating the computer away from the heat of the engine like many people did back then; he found that carrying a stick in the car, would allow him to stand outside, reach the stick under the hood and tap the module while he turned the ignition switch to start the engine!

Cars Today

Misfire codes (including P0301, P0302, P0303, P0304, P0305, P0306, P0307 & P0308) indicate that there is a misfire in a cylinder, the last number indicates which cylinder has the problem. If something is causing a miss in random cylinders or is affecting all of them, a P0300 misfire code will be stored in the car’s computer. It’s always smart to begin with the basics. Also consider the mileage and service history when diagnosing a misfire. Higher mile vehicles are more inclined to have mechanical issues with the engines, like low compression from worn valves or rings etc. Any accompanying codes should also be considered in case they may be related. If the spark plugs are worn (excessive gap) or the car is past due for a tune up, it may be smart to go ahead and start with spark plugs and spark plug wires and go from there.

The most common misfire causes on the cars I’ve worked on have been:

  1. Spark plugs
  2. Spark plug wires
  3. Ignition coil
  4. Fuel injector
  5. Wiring to fuel injector
  6. Timing Belt
  7. Vacuum leak or stuck open EGR
  8. Contaminated fuel or bad fuel pump
  9. Weak compression
  10. Blown head gasket

Obviously there are many different types of cars, so a service manual for the specific one that you are working on may be required to help pinpoint the misfire you’re looking for, but hopefully this will direct you to some of the most common causes of misfires in cars of today.