A family member is claiming their 4cyl engine on a 2000 Chevy S10 Extreme LS is misfiring. However we're not seeing a check engine light. Wouldn't that automatically trip the check engine light for sure?

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    I'm not sure, but I believe it depends on the type of misfire and the system which is observing it. A misfire due to a non-firing injector is different from a dead coil which is different from a grounded spark plug (where energy leaks directly to ground instead of jumping the gap) ... I'm sure there are other misfire types I'm not mentioning. The other thing to think about here is ... is it actually a misfire? There could be other reasons it seems like a misfire, but it isn't. Jan 8 at 21:53

A misfire is detected primarily by the crankshaft sensor. It will actually detect a speed variation caused by a weak contribution from the weak cylinder. The computer knows which cylinder is misfiring by the crank location. A fault code is triggered when a misfire is detected multiple times. Many times a misfire is detected before it is noticed by the driver. Misfires can also be tracked using a scan tool.


My car misfired for years with audible ping, sometimes quite badly, without any check engine light (CEL). It wasn't until I soaked the engine with Seafoam for several days that the dashboard light suddenly appeared. As it turns out, there was a leak around the PCV valve all that time. Now, no more light and the car runs great.

I had the same phenomenon with a Dodge Ram 2500 pickup truck. It hesitated, guzzled gas, and generally ran poorly with no check engine light. I sprayed Marvel Mystery Oil through the intake at idle and let it sit for twenty minutes. When I restarted, a CEL immediately appeared and persisted. In that case, it was corrected by replacing the gas cap.

Carbon buildup on an old car masks a check engine light, due to the fact that oxygen sensors measure the health of combustion only indirectly. Carbon absorbs fuel during the compression stroke of a cylinder, then releases excess fuel during the power and exhaust strokes. Carbon on intake valves retards the proper vaporization of atomized, micro droplets of fuel. The result is uneven combustion -- lean misfire during the initial, critical portion of the power stroke, followed by excess, dirty flame at the end of the power stroke and the exhaust stroke. Oxygen sensors might ordinarily detect a lack of oxygen under such circumstances, but if an air leak is added to the mix, the computer compensates with positive fuel trim, maintaining acceptable oxygen parameters. No CEL light is triggered.

The driver, however, definitely notices something amiss. Poor fuel economy is pronounced. All the while, diagnostic computer data (OBD II) may not appear terribly abnormal.

I see your car is model year 2000. Assuming high mileage or city driving, you likely have the same frustration I did. You cannot rely on a CEL light!

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