Misfiring under hard acceleration
I recently purchased a used 2000 Ranger 4x4 w/ 4.0 OHV. It's got a rebuilt engine in it (15k miles) but 145k on the truck. Since I bought the truck, it's had this misfiring/surging problem under hard acceleration. Anytime I'd accelerate in higher gears or just when there was a lot of load on the engine, the truck would surge. I couldn't tell if it was a drivetrain or an engine problem.
Initially, I thought it might have been clutch slippage since the clutch hadn't been replaced since the truck was new and didn't engage until the pedal was nearly all the way out. I had that replaced (it definitely needed it anyway) but didn't solve the problem.
I drove the truck more and paid close attention to what was happening when the truck surged. The Tach didn't jump and the engine didn't seem to rev at all when the surges occurred, so I concluded (hoped!) that the engine misfiring was the problem.
A little research online led me to the common problem this engine has with the MAF sensor getting dirty and causing the air/fuel mixture to get thrown off. A common symptom of this is surging due to misfire and rough idle. I didn't have the rough idle problem, but the surging was definitely there.
I carefully cleaned the MAF sensor (these things are really delicate) with a can of electrical contact cleaner and canned air, checked the air filter (good), and reassembled the air intake. I also pulled a spark plug to check the condition. The first one I checked had a small crack in the insulator and was also showing some wear to the electrodes, so I ran to the store and picked up a set of Bosch Platinum2 plugs.
A short time later, all problems were gone and the truck runs like a champ. Just figured I'd pass on the news for anyone having similar problems with their Ranger.