I have this 1999 extend cab ranger, it’s a 4.0 OHV, 4x4, auto. I got the truck intending to flip it but found that I really like the damn thing and have decided to keep. Thus, the heartache begins. I put an engine in it and all seemed well. It was hard starting but ran ok other than the lean codes. Found the fuel pressure at 40psi. (not good) replaced the pump and filter and got 70psi upon cycling the key. Starts well now. The truck has had a miss since I got it on the road. After doing some investigation I found that the miss only happens in the 2500-2600 RPM range. It happens in all gears and can be duplicated in park with no load on the engine. Searching the forums I have looked into all the usual culprits and just can not resolve this miss. I have done the following;

New pump and filter

New plugs (motorcraft SP-500’s)

New Motorcraft wires and coil

New IAC

Mass Air Flow cleaned, reading within specs and seems to be responding

TPS checked and is within spec and responding

Ran Techron through it 3 times

Nothing has changed the miss, I pulled wires off one at a time and ran the engine at the affected RPM the miss was always present. I know my way around an engine but I’m at my wits end with thing. The computer never shows a code, doesn’t register a misfire even when I disconnected the wires and ran it. No injector faults are shown. Passes the KOER and KOEO self tests. Any ideas would be great.



Thanks,

 

Have you checked the compression and also for any vacuum leaks?

 

Welcome to the forum

So you have an engine vibration at 2,500rpm, that you assume is a misfire
I say assume because OBD2 has not confirmed it as a misfire.

Computer will only show misfire codes if misfire reoccurs several times at specific RPMs so unplugging spark plug wires wouldn't set a misfire code unless you left them unplugged for a few drive cycles.
And just a heads up about your spark system, there are only 3 coils in the coil pack, 2 cylinders are sparked at the same time, Ford calls it a Waste Spark system, actually it is the very first gasoline engine spark system ever used, each cylinder get spark at every TDC, compression and exhaust stroke, same as your lawn mower engine, lol.
Spark plug wires are in series on the one coil, so unplugging 1 wire can effect 2 cylinders if it breaks the circuit.
Matching cylinders, at TDC at the same time, are 1 and 5, 2 and 6, 3 and 4
Which is why coil pack wiring is:
3 4
2 6
1 5
front
That 5 6 4 side still gets me now and then, 3.0l engine uses the same, very easy to mix that side up, not your problem but always double check it, then check it again, lol.


I had a 2,500rpm vibration on my 1994 4.0l, turned out it was a failing fan clutch, would get out of balance around that RPM range but ran smooth below and above that range.

Does it have the vibration when engine is cold or only after engine has warmed up that would indicate a computer or sensor issue.


If it has vibration when cold so you can test it, then I would remove fan belt, engine COLD.
Start engine and run it up to 2,500 to see if vibration reoccurs with pulleys and fan clutch not spinning.
You can run a COLD engine for 2 minutes without a water pump(or alternator), but no longer.


Misfire codes are generated two ways
Each time a coil in the coil pack fires it will generate a voltage spike back at the computer(transistor) that cut the ground to that coil, so no or low spike would set misfire code.
More common cause of misfire code is crank and cam sensor timing.
Computer gets RPM data from both of these sensors, and at any RPM it times milliseconds between each cylinder firing, if a cylinder doesn't fire then there will be a slight delay in that timing because no power was added to the crank(or cam) so it lost speed.
If it repeats then computer watches coil feedback and adjusts spark timing, and also adjusts fuel mix slightly, if problem can't be resolved then it will set a misfire code, P0301 to 306 if it can ID which V6 cylinder is misfiring, or P0316 if it can't or if multiple cylinders are misfiring