Piston Slap: Unclogging a RAM-dom misfire diagnosis

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Hagerty Community Member hearsedriver writes:

I have a head scratcher for you. I have a “spare” 1997 Dodge truck, and what’s not to love about a 318 Mopar V-8, five-speed manual, 4×4-motivated truck for occasional use? Well, mine has a “random engine misfire” code. She runs like Jack the Bear for weeks, then stumbles, misses, and literally dies going down the road. Sometimes you barely turn the key and she’s running, other times, I crank for 15–20 seconds and nothing. After stalling when running, I’ll pull to the side of the road, mumble unkind things, turn the key and she starts.

I would get codes P0300, 0301, 0303, etc., but it changes to whichever cylinders misfire at the time. I also went through the Mopar TSBs (Technical Service Bulletins) and found the one to reroute the ignition cables to prevent “cross firing.” I have replaced TPS (new), engine position sensor (new), known-good used MAP, and distributor pickup. I ran an extra ground to the computer body. (I hate computers!) I ran for several weeks with a fuel pressure gauge connected and poking up on the hood where I could read it while going down the road (good pressure per spec), all to no avail.

Sajeev answers:

That’s certainly a cool truck, especially with a manual transmission!  So let’s get down to fixing this mighty steed. Random misfiring issues are tricky, so start with the basics. Assess the age and condition of the spark plugs, plug wires, ignition coil, battery (yes, really), and vacuum lines. All it takes is the right amount of spark plug wear (bigger gaps), weak coil output, and a vacuum line leak (causing a lean condition) to trigger a bizarre pattern of engine misfiring.

If the ignition and vacuum systems pass muster, check your fuel system. Your fuel pressure is good, but clogged injectors can still be an issue. And it could come from water-contaminated fuel, especially if you get gas from the same station (with a leaky in-ground tank), so consider a water-removing chemical like HEET for your gas tank. But let’s dig deeper still into the injectors.

There are several ways to test an injector’s spray pattern (i.e. safe way and hella unsafe way), but considering the truck’s age and that it’s sometimes hard to start, consider a set of new injectors as preventative maintenance. I’ve seen rebuilt units go for $10 or less each on eBay, but at that price you are at the mercy of the rebuilder’s quality control standards. Buyer beware, there.

There’s one final, and somewhat unlikely, reason for your misfire. We can’t expect soldered connections on a 23-year-old circuit board to be perfect. I know a local service that’ll diagnose engine computers before rebuilding it, but if you aren’t so lucky, there are places online that sell rebuilt computers and take your computer back as a core for a reasonable amount. But let’s hope you just need new injectors instead.

That’s all I got, and I assume the fuel injectors are clogged. What say you, Hagerty Community?

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