I have a problem that has been going on for about 8 months. The check engine light will go on indicating a misfire on a couple of cylinders. I’ve had coil plugs changed and the problem looked at 4 or 5 times. About $500+ in. Each time the check engine light it “reset” but the same error code comes up again and in the same loop. I’m putting about 1200 miles a month on the car still. The car does start up a little choppy, but after it warms up about 2 mins it runs fine. Not sure if this is worth fixing? Should I replace the PCM? A Little lost. One mechanic told me I needed an engine rebuild.
One possibility is that coolant is getting into one or more of the cylinders. This would explain the misfire when the engine is first started after sitting overnight. I had this problem with a Ford Aerostar. The problem was a hairline crack in a cylinder head. Fortunately, the Aerostar was under warranty and Ford replaced the engine
And since this one’s out of warranty, it’s not worth fixing if that’s the case. Actually it’s probably not worth fixing at all. The Mariner is a re-badged Escape, and the Escape isn’t great now, but that generation was godawful. I’d be thinking long and hard about putting the repair money toward a different vehicle instead.
You can double check your vehicle value on Kelley Blue Book but in my area even a 2008 Mariner in good condition might only be worth 2000.00 . So I say just drive it until it dies or trade it for something else.