Hi, my younger brother owns a 98 ranger with the Vulcan 3.0L in it, and we have hit a dead end trying to solve a misfire problem. This truck has just over 150,000 miles on it, and runs excellent 99% of the time, it isn't until the engine is pushed to high RPM that the misfire occurs (we're not taking it to the rev-limiter, we've never run it that hard). The truck won't throw a code, and we have no idea what the cause of the miss could be.

Here's the deal: When driving the truck it will not miss at all unless the engine is pushed to 3000 RPM. When it hits 3000 RPM EXACTLY, there is a very noticeable stutter in the engine and it takes a very long time for it to reach 3500 RPM under this condition. It makes it hard to drive because he needs to shift at a higher RPM when pulling out into traffic or going uphill. Anything that drops RPM (shifting/letting off throttle) clears this up, but it will misfire every time its is pushed to 3000 RPM. My brother also tells me that it runs worse in the morning when its cold outside. We have tried everything and have come no closer to solving this issue, so I'm hoping that somebody out there knows more about this particular problem than we do.

What we've done to fix this: (These have all been replaced within the last three years)
-New Plugs (less than a week old)
-New Plug Wires (less than a week old)
-New Throttle Positioning sensor
-New Idle Air Control Valve
-New EGR Valve
-New Synchronizer and Camshaft position sensor (the synchronizer is not loose, it was the first thing we checked)
-Mass Air Flow sensor was cleaned
-Numerous vacuum leaks were corrected via smoke test
-Coil pack is in good shape

As a last result we took this truck to a mechanic who put it on a dyno with the engine hooked up to a diagnostic machine. He found that the engine was suffering from constant lean misfires, and discovered that the rubber intake tube from the airbox to the upper intake manifold was cracked 2/3 of the way around the diameter, and was letting an excessive amount of air into the intake manifold. He replaced this intake tube and the misfire cleared up for two days and came back. It isn't as severe as it was before, but it still affects the truck's drivability.

Sounds fuel related. Change your fuel filter (If you haven't recently). Then check your fuel pressure regulator. Might have a weak fuel pump.

Or, I know MAF sensors can go "kinda" bad and give false readings. Is it worse in high humidity?

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Changing the fuel filter is probably where we'll start, it's only a couple years old but that's one of the regular maintenance things that we haven't kept up with. As for the fuel pump, that's less than a year old. We live in the desert so I can't speak much to the effect humidity has on how the truck runs, but my brother says it runs noticeably worse when its cold in the morning. After the truck warms up it returns to its normal misfire pattern.

I'd suspect vacuum leak in the intake system or a seal on a major airflow artery of sorts.

My other question is that you mention a list of parts that were replaced and new. What were those parts? Were they all OEM Motorcraft plugs and wires or local auto-parts pieces? And it isnt unheard of to have a bad wire or part in general bad right out of a box either.

Have you tried isolating as to which cylinder(s) this misfire occurs in?

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Most of the parts that we replaced came recommended by O'Reilly Auto Parts and AutoZone, so I don't think that they were OEM Ford parts. As for the plugs and wires, the set that is on there is not the Motorcraft plugs and wires; however the previous set of plugs and wires were motorcraft, but this misfire was still present even with these on there.

As of right now we don't know which cylinder this misfire is taking place in. When we got the truck back from our mechanic he told us that they were still reading "a slight hiccup" on the dyno, but never specified which cylinder it was taking place in. He suggested new plugs and wires as a fix, but when we drove it home from his shop it ran absolutely perfect, and two days later it was running just as bad as it was before. It was at least another week before we actually got around to changing the plugs and wires in hopes of fixing it, but it never cleared up.

Two things, you say it has no codes. Did you check with a scanner or just going by the check engine light not coming on. Pending codes dont turn the light on, and the lights burn out...

Secondly you say it happens EXACTLY at 3,000 rpm. That is a clue because 3k is the park/neutral rev limit. So a bad range sensor on an auto or neutral switch on a manual could cause the truck to think it's still in park/neutral and your hitting the rev limiter. Which cuts the ignition out and feels like a horrible miss.

We did run a code reader on the truck and nothing came back. The truck is a 5 speed, so I'll do some poking around and see if that neutral switch might be the culprit. To go along with that switch, I have noticed that when you're coming to a stop and take the truck out of gear it takes it longer than it should before the rpm's return to an idle level (It'll stick a little over 2000) Could this be related to a bad neutral switch or is it just a random quirk that the truck might have?

Maybe... the neutral/clutch switch tells the IAC valve what to do (it should hold around 1500 until a complete stop) so if the truck is getting a slow, or no signal from the clutch/neutral switch it could effect the coasting idle.

As a simple test, pull the fuel pump relay and try to crank the truck with it in gear and your foot off the clutch. If it so much as twitches youve got a definitive issue there.