Start With a Scan to Locate a Misfire

Dear Car Talk:

I have a 1997 Mazda B2300 pickup. It has a brand-new fuel pump, new clutch and new coil packs.

Sometimes while I'm driving, it will buck a little bit, like there's a misfire, and then the bucking will stop. Also, when I'm going uphill, it will lose power and lose speed. When I get to the top of the hill, it goes back to normal speed again. I also smell gas sometimes. At least the check engine light has not come on. I'm lost. Any ideas? -- Ken

Add "broken check engine light" to your list of things to fix, Ken.

When you turn the key to the run position, just before you crank the engine, all of the dashboard warning lights should come on as part of a self-test. If you don't see the check engine light then, it's not working. And I suspect it's not.

But even though the light is not on, your computer may still be storing a code. Or multiple codes. So try scanning it. It may tell you which cylinder is misfiring. A misfire could explain everything. It would cause bucking, low power on hills, and I suppose, under rare circumstances, it might even create a smell of gasoline if enough gas goes into a cylinder but doesn't get combusted because there's no spark. Or you might just have a separate fuel leak. Your truck is a quarter-century-old, remember.

The things that typically cause misfires are components of the secondary ignition. You've already replaced the coil pack, but it could be a bad spark plug, a bad plug wire or a faulty crank sensor. The crank sensor tells the car's computer which plugs to fire. The part costs about $20 and is easy to change.

So I'd start with a scan and see if that helps you narrow down the location of the misfire. Then, I'd try the crank sensor, then plugs, then wires. Then, a new truck. Not necessarily in that order. Good luck, Ken.


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