Without going to deep into the workings of the ignition system, if you have a weak or damaged main ignition coil, it could be cutting out spark at higher RPMs. When at higher RPM, the time between ignition pulses gets shorter, equals less time for the coils magnetism collapse and produce the spark. This along with higher cylinder pressures at high RPM and load could be your weak link where the spark is not strong enough to initiate combustion, but is sufficient at lower RPM and lower cylinder pressures.

Also keep in mind that the 5.8 in these trucks were not high RPM motors to begin with. The Torque peaks at 2800 rpm and the HP peaks at around 3800. any RPM above 4200 and you are getting into valve spring float.

I had an older 2 stroke dirt bike that would run all day strong at low RPM, but at higher RPM acted just like you describe as a rev limiter. The ignition coil for the 5.8 is cheap, they do degrade over time, so you may benefit from changing it, even if its not the root cause of your issue.

 

Just pulled my PCM and yes I found 1 bulging capacitor and 2 of the 3 have some brown wet stuff around one of the "legs", and 2 out of the 3 are only standing on 1 leg. It's also generally dirty in there and a few of the things look like they're falling over. It's looking like I should either replace the capacitors or buy a new pcm. Anyone have experience replacing these? Is it worth my time?

Rockauto has a refurbished replacement for my part number for $130. I'm thinking I should just order one.

 

The industry in general got a load of "bad" caps that were manufactured between about 1999 and 2007, leading to failure of parts made from the caps. I've had mixed success with replacing blown caps. If they're just bulging and have not yet puked their guts, you can usually just replace them and get going again. If they've puked their guts and their electrolytes have eaten away terminals, there's a good chance they've also re-etched the circuit board and damaged it. If the latter case, as you've described, you're probably better off just buying a new PCM, and avoid the hassle of repairs that may or may not work.

 

Yeah that is no good, the fluid from the caps is corrosive and can damage the motherboard so best to start shopping for a new one.

 

Any PCMs listed online for these trucks are rebuilt. The prevailing remanufacturer is Cardone.Their track record is spotty, but pretty much the only game in town.

 

Yep I ordered a cardone. It'll come in a few days and I'll post here the results once I install.

 

I put in the refurbished computer and it's still the same problem. There is no more sputtering under load or acceleration though, just that pesky 3.2k "rev limit".

The ICM attached to my distributor seems like the next step? Someone also mentioned the coil. I can get the ICM for $25 or an entire distributor cap, rotor and module for $60. The coil looks like around $10. Anything else this could be? Should I just order all of these parts and be done with it? Any further advice would be appreciated.

Thanks for all of your help so far.

 

The track record for aftermarket electronic modules like the ICM and the PIP module inside the distributor isn't great.. lots of DOA and many more don't last very long. Buying a Motorcraft part is you best option but you will pay more, I agree that the ICM is the next best guess to solve your problem. If the coil was bad the misfires would be consistent across all rpms but a failing ICM could easily produce the type of problem you have.

 

I also found a connection coming off of the same group of wires as the icm connector and the spout. It's not hooked up to anything and I'm not sure where it goes. Anyone know what this is?

 

Looks like the AC clutch connector

 

Replacing the ICM fixed the problem! It now acts the same way up around 4.5k rpm but that might be a real rev limiter.

Thanks everyone for the help!