Miss fire help

But the 1986 300 six still had a carburetor, not EFI.  So while it was EEC-IV surely the wiring is quite different between the carb'd engine and the EFI'd one?

Very true. Gary, the best indicator I can give you is the ignition system, if it is a Duraspark, it would need to be a DS-III for computer advance control. TFI was introduced for (a) hotter spark (b) more compact and (c) compatibility with computer control. If a 300 6 has a TFI distributor and a carburetor it will be an EEC-IV control system. The MCU (Mixture Control Unit) only had 24 pins, was a potted assembly and mounted underhood. Those systems had a regular DS-II distributor with only the vacuum advance being computer controlled.

1985 and 1986 LD 300s used an EEC-IV for spark and mixture control.

As for wiring, that's what the pinouts were for, to show where the differences and changes were in the wiring.

This should lead to an easier swap over I would think. I know i have the EEC-IV diagnostics port next to the battery and TFI.

So got my sister in laws misfire sorted out...
Told her I would pay for all the parts and get it straightened out. A wise man once said never assume it is one thing that is causing the problem. And this sure was the case here.

Of the 16 valves all but 2 were significantly out of tolerance. The exhaust valves were very tight across the board. Spec was 0.16-0.20mm I couldn't even get a 0.153mm feeler gauge to think about sliding in there. The valve cover was leaking a bit too so killed two birds with one stone here. Whoever had the cover off last used rtv in conjunction with the felpro blue gasket in spots around the cams. I found the leak here, as well as some RTV making its way into the head, pulled some of it out that broken loose

She had a friend replace the plugs after being told by a guy at AutoZone she needed new ones to fix the random misfire.

She didn't keep them so I didn't get to see what they looked like. But I took the new ones to check gap. Should be 0.044 " she had anywhere between 0.030" and 0.040". Regapped and reinstalled.

They never recommended wires... Seems silly, especially when you look at this obvious spark leak from the boot over Cyl 4's plug:



Got into the distributer. Found the cap to be a cheap replacement (probably car-quest like the wires). The pick ups were undersized compared to the OEM grade cap I replaced it with, a couple were even melted (not shown). The rotor contact was significantly worn and the coil was a bit corroded. Got those cleaned up and replaced.



Got that all replaced and cleaned up. Boy does it run smoothly. It definitely gave me some good practice for trouble shooting the 300Six on the van. Hopefully I can get into that this next week. Have a paper due this weekend, and a research paper the following weekend, Cal test Thursday (yay for anti derivatives) and a final next Tuesday. I'm ready for a break and to just get under the hood of something for some therapy time lol.

You sure found a bunch of problems!  But they were all either causing issues now or soon would be.  Well done!

Now, on to the van?

Gary Lewis wrote

You sure found a bunch of problems!  But they were all either causing issues now or soon would be.  Well done!

Now, on to the van?

yes, finals time so probably get to it just before Christmas. Just had my last unit test in derivative calculus yesterday. Have a paper due this weekend,  calc final next Thursday, and a research paper due the following weekend. Then freedom (less work and kids).

That gives me nightmares!  My degrees are in Math and Physics.  But I hated Calc and Diff E.  Good luck!