Rick's Free Auto Repair Advice

Fix Misfire problems

Fix Misfire problems

Many misfire problems are caused by worn spark plugs, leaking spark plug wires, or bad ignition coils. So you should always start your diagnosis with the basics. In this case, it’s probably easier and cheaper to replace the plugs and wires since they’re cheap and you’ll have to remove those parts to check them. If your vehicle has multiple coil packs or coil-on-plug (COP) coils, switch them around to see if the misfire code moves with the switch. If so, you’ve nailed the problem. If you have distributor ignition and you’ve already replaced the plugs and wires, it’s almost impossible for the ignition coil to cause a misfire on a single cylinder.

If you’ve done all those swaps and still have the problem, DON’T start throwing expensive parts at the problem. Your next step is to perform a compression test.

A bent, carboned, or burned valve can cause a misfire code for a single or multiple cylinders. A dry compression test tests the valves, a wet test (injecting oil into the cylinder before cranking) tells you the condition of the rings. However, at low cranking speeds a bent, carboned, or burned valve can seat well enough to throw you off course. So do a “snap” compression test as well. That’s done on a running engine.

Start by doing a dry cranking compression test and record the readings for each cylinder. Then remove the valve core from your compression gauge, ground the spark plug wire and install the gauge in the suspect cylinder. Start the engine and record the highest reading while at idle. It’s usually about 50% of the cranking pressure–that’s normal. Then snap the throttle open quickly to wide open throttle (this is really a quick snap procedure–don’t let the engine rev up too much). Record the highest compression reading. It should be about 80% of the dry cranking reading. If it’s a lot less, especially if it’s just one or two cylinders that read low, it’s an indication of a valve problem.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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