I have a 9.7:1 Comp. Ratio, 4.0L V6 in my 2008 Ranger FX4 with 289,000 km on it. About a year and a half ago, I developed an intermittent miss. I got out my handy-dandy OBDII code reader and found a code P0302. Looked it up, and cyl 2 was missing. Reset the code and went for a drive. Sure enough, engine light came back on. Same code. Figured it was possibly a plug (265,000km on original plugs at that time) strangely enough, the old plugs didn't look that bad. I had a shop replace them with OEM platinum plugs. Problem popped up again. Took it back to them and they did a compression test. Front and rear cylinders (1 & 3) were 160PSI. 2 was at 108PSI. Hmm...did a wet test, same results basically. Did a leak down test. No noise in crankcase (whew) or exhaust. Intake valve was leaking. They sprayed a can or 2 of intake carbon remover in, but it actually made it a bit worse. Haven't looked in with a camera, yet. Notice: #2 cylinder starts to fire when over 2000 RPM OR on a cold (-20°C) day until it warms up. Gas milage is still the same as always. Power on the highway is still good. Well, now I know what it is, it's time to figure out what happened.
Backstory: About 2 weeks before the miss started, I had the shop replace the fan belt tensioner, belt, idler pulley and thermostatic fan clutch, as I was getting a squeal on cold start. Lasted about 2 to 3 seconds, if that. It was a hot day in August, so I was sitting in my truck for a few mins in the back yard, idling with the A/C on. I happened to glance at the dash and the temperature guage was PINNED. I didn't smell antifreeze, so I slowly raised the RPM, listened for the fan (it took way longer than should have to sound locked up) and got the temperature back to normal. I shut off the truck, opened the hood and looked around. No coolant spraying, overflow was good, hoses were firm. Hmmm. I noticed an oily substance inside the fan shroud and then saw the thermostatic clutch was wet, too. Called the shop I just had been to earlier and he said, bring it back in, if you can get it here without overheating. I said I might be 20 or 30 mins (10 min drive), but I'll get it there. Upon arrival, he had me drive it in right away. Opened the hood, saw the oil spray and went to get the mechanic that worked on it. He looked in the trash and found the box the fan clutch had been in, right on top, nothing oily around it. It was about 1/2 soaked with oil, and we all knew the problem, but somehow, none of us (2 journeyman and 1 backyard mechanic, me) had paid attention to the OIL stain on the box when taking the new fan clutch out of it. Obviously a leaky fan clutch. Change it out and you could immediately tell the difference in the sound.
Now....could that overheating have caused #2 cylinder intake valve to warp a tiny bit? (It still had 108 psi) or was it just a coincidence? The fact that that it was a couple of weeks between the overheat issue and the #2 cylinder miss starting seemed a bit out in the left field. Any one well versed in Ford V6 4.0L engines out there with any ideas? No ticks or noises other than the usual.
I have been driving it like this ever since, as the estimate to repair was over $3000.00 cdn.

 

No, I never read of general overheating effecting intake or exhaust valves, and never seen it myself
A specific cylinder running lean/hot could damage valves, but you wouldn't see that on temp gauge, maybe hear pinging/knocking, but the 4.0l SOHC uses a knock sensor, so you wouldn't hear it

Both valves spin each time they are opened and closed to prevent hot spots on the valve and valve seat, so #2 intake could have stopped spinning on and off and got warpped

Yes, compression is a temporary thing, metal on metal is NOT a good pressure seal, lol
So the less time a cylinder has to hold pressure the better, at higher RPMs it doesn't have to hold the pressure long so you get some added power if it fires

290k km = 180k miles
In my opinion I would pull the engine and have new timing chains/tensioners installed and both heads rebuilt
Lower end is usually good for 400k miles, but a new oil pump wouldn't be out of line, and front and rear main seals

The cost will be higher but 2008 Ranger should be lasting you at least 5 more years, MINIMUM
If cost was $6,000 then over 60months(5 years) thats $100/month "car payment", and there is not many vehicles that would be reliable, and you would want, that only cost $100/month, lol

It usually takes 8 hours to remove an engine, and 10 hours to reinstall, 18hours, at $125/hour thats $2,250 labor, but it really should only be 6 and 8 so 14 hours, with air tools and lift, lol
Fully rebuilt 4.0l SOHC "crate" engine, should be just under $4,000canadian
Rebuilding heads and timing chains will be much less, maybe $2,500

 

Definitely no expert here, so a wild *** donkey guess: any possibility Injector#2 could be gummed up & sticking?

 

Sounds like a possible valve seating issue. Like Ron said, with that many miles its cheap insurance to go ahead and do the timing chain replacement and rebuilt heads. Good for another 200k+ miles.