Moisture and Misfire



My 2002 Ford Ranger (6 cylinder) got three misfires within minutes after leaving the house this morning (#4,5,and 6). There’s been 3 solid days of very dense fog and drizzle here, so I know it’s the moisture because it was fine until then. My question is, do I replace the plugs AND wires for these three, or can they be saved? I had a tune up 2 years ago, but don’t know the brand of plugs that were used…any thoughts?


Plugs and wires are good maintenance and cost very little. Why not just replace them all? If you want to cherry pick to pinch pennies, the issue is more likely with the wires than the plugs but you’ll have to buy a whole set anyway. When you do, make sure you use dialectric grease to seal up the boots. If you want to pinch even more pennies, wait for some dry weather and mist the wires with a water bottle to make sure your assumptions are correct.

Don’t leave the coil pack out of consideration either. Some clever work with some plastic and a mister bottle of water can help you sort that out.

I assume, btw, that you pulled codes P0304, P0305 and P0306. Note that these misfire codes are not ONLY caused by spark problems. Anything that messes up the normal combustion cycle will set them (fuel, air, spark compression). Spark is commonly the issue and the easiest place to start though.


Before replacing anything, try spraying the coil and plug wires down with a liberal coat of WD-40 to see if that helps any.

If that does not cure it then replace the plugs and possibly the wires if there are any signs of corrosion on either end of any of the wires. Spray with WD-40 afterwards. The WD helps to repel moisture.


Thanks for the quick input, guys. I’m not so worried about pinching pennies, and was advised not to use Bosch, btw. Ready to replace if needed, any advice on a sturdy brand for my make/model? I noted your posts on a similar thread, cigroller, and did pull the codes you listed. I’ll be sure to seal up the boots and check the coil pack, thanks. I wanted to ask you ok4450, should I wait for drier conditions before testing, and should I run the engine some first?


OEM Motorcraft spark plug wires are available at lots of different places and are not very expensive. I don’t know anything about Bosch except that I’ve used other things by Bosch without issue.

@ok4450 should correct me if I’m misreading, but I don’t think the WD40 spray was a test so much as a way to help prevent the moisture issues. It could help provide a moisture barrier and stop this problem.

I was suggesting the water misting as a test to make sure that you do know where the problem is. E.g. if you do have motorcraft wires on there and they are 2 years old, they actually should be just fine. But maybe not. But on a dry day - or generally when the truck is running ok - you could cover the coil pack with plastic and mist the wires to see if it sputters. Then you can reverse and spritz the coil pack. It’s just to make sure you know where the moisture is causing a problem.


That does make sense about the WD40, of course. I’ll try the misting test today or tomorrow after the rain moves out. Fortunately, I don’t have to drive the truck at all until next week, save one trip to the corner store perhaps. Till then, just drying it out and diagnosing…


Cigroller is correct. The WD-40 is more of a protectant against moisture than anything else. During the right atmospheric conditions moisture can creep in anywhere; including inside of a distributor cap on vehicles equipped with that. This is especially true if the weather gets dreary during or before a warm engine is shut down. The heat draws the moisture in.

There is nothing wrong with Bosch plugs if they’re used in the correct application. I think where a lot of the Bosch bad plug complaints come from is that some people buy those 3 or 4 prong Bosch plugs and use them in an application they’re not made for. When a problem surfaces the plug gets the blame and it hits the internet.

For what it’s worth, I’ve installed more Bosch plugs than I could even begin to remember (including those 4 prongs, etc) and have never had a problem with them. The 4 prongs were originally made for use in the old air-cooled VW Type 2s, or bus as they’re known.