Engine Misses

Here is the procedure for isolating which one (or more) cylinders are causing a "misfire" or rough idle in any standard ignition driven system (not diesel).

The first thing we have to do is to find out why is is misfiring - not necessarily ignition related, but it could be. Do a compression test so we can rule out bad stuff. Write down the compression readings for all the cylinders. If one (or more) is more than 15% lower that the highest, then there is a problem with either the valves or the piston/rings. I won't go into the procedures to isolate that problem here.

Assuming it turns out OK, then we do a "power test" by grounding out one spark plug wire at a time while it's idling. Use a small thin screwdriver or something else that you can slip under the boot that covers the spark plug. Attach a grounding wire to the screwdriver and clip the other end onto a metal part of the engine. With the engine idling, slip the screwdriver under the spark plug boot to short out that plug and stop it from firing, and note the drop in RPM - do it for all 8. One of them (or more) will not affect the idle condition as much as the rest. That one will be the offending cylinder. Next, find a spark plug wire which is the same length as the offending cylinder but caused a significant drop in RPM, and swap wires. See if the problem follows the wire or stays with the cylinder. If it goes with the wire then it's time to replace the wire set. No sense in just replacing the bad one - do 'em all. If the problem stays with the cylinder then start looking for a fuel problem with that cylinder. Look for a vacuum leak around the intake manifold for that area of the engine. Check all the vacuum hoses for cracks and leaks. If it is a fuel injected system (Port Fuel Injection) then pull the injector and see if it is functioning. If it isn't squirting a fine mist then check the signal to the injector using a special injector tester. Repair or replace as necessary.

Finally, if nothing else appears to be wrong try swapping the spark plugs form one cylinder to another and see if the problem follows the spark plug. If it does then replace all the plugs. Note the condition of the failed plug and from which cylinder it originated. This may be an indication of other engine problems like oil burning in that cylinder.

Back to Brother Bob's Home Page