I just bought a 94 F-250 with an EFI 460, Auto trans, 4x4. 241,000 miles. The engine has a steady miss that is only slightly noticeable at idle, but becomes very pronounced between 1200 and 2000 rpm. I just did plugs, wires, cap, and rotor. Some of the plugs were the correct brown colored, and some of them were a little blacker, indicating a miss. So, I changed the plugs and wires, and it's still missing. I've got broken vac lines everywhere, but I pretty well capped them off, and I don't have any major vac leaks. I do have a CEL on, so I'm going to pull the codes here directly. Any ideas as to what this might be caused by?

 

Codes a 331 and a 33. Probably because the EGR stuff is disconnected.

 

First thing to do would be to install all new vacuum lines and get everything hooked up properly...

 

Your 1994 truck should display three digit codes. While it is possible to have two digit codes, trucks with a C6 transmission typically have two digit codes, it is rare. You will never get a mix of two and three digit codes, it's one or the other.

I interpret your 331, 33 as KOEO code 111 repeate, hence the 111 then 111 again. The 1 is the separator code between the KOEO and CM display. The next set of "3's" is Code 111 again (repeated).

Code 111 indicates System Clear (no faults).

 

I guess that's good. The CEL does occasionally come on. If I've capped all the vac lines, and don't have any vac leaks, could that really be causing my miss?

 

Quit plugging them, fix them.

 

I guess this would be a prudent time to ask the best way to go about putting it together. Does anybody have a good diagram of how the vacuum system works on it?

 

Vac diagram from my 95 with 460.

Yea while not likely reason for miss restore the vac lines rather than plug them, don't necessarily need them all and depends on what is intact on it to start with but for now replace all lines.

A miss is going to be cause by two issues spark or fuel (provided cylinder/s condition are fine otherwise).

Did you narrow it down to a cylinder/s yet?

Did you verify spark at all cylinders?

If has good spark at all cylinders it would have to be a fuel problem, zero or very little fuel to problem cylinder/s causing the miss.

Remove plug wire one cylinder at a time until you find one or more that are not participating. Or cylinder or cylinders that do have spark yet makes no difference plug wire removed.

Myself I do that working at the cap end of the plug wires rather than plug end, that way when wire is removed power to the wire is instantly "dead" unlike the plug end (helps prevent getting lit up!).

I unlock the wire pop it loose from the post so it can be easily lifted off, all the way on but not locked and to point still provide good contact to the plug then start the engine, then using a rubber coated tool (coated both ends) lift wire off post pay attention listening for any changes in how the engine runs/acts, doing so for each cylinder. Yea process takes little time.

Cylinders that are participating there will be a notable change in the way the engine sounds and acts, the rpms will drop engine will shake or shake more if already missing when plug wire removed from an active cylinder, any that are not participating there will be no notable change in rpms or in how the engine acts wire on or off.

Remove the wire then put it back on back and forth off on off on make sure yes it makes a difference or no it does not, doing so until you know which cylinder (or cylinders) are cause of the miss (probably just one but keep in mind could be more than that).

Until you know that which one it is and if its spark or fuel? unless you do already, you can't do anything about it to correct the problem.

 

I drove it a couple of miles today. It ran fine for about 2 or 3 miles, and then started missing again. My thought would be a fouled plug. That would make sense, too, since I pulled a fouled plug out of the #2 cylinder (I think it was #2) when I changed them yesterday. But what would foul a plug after that short a time of driving? Bad injector? Bad rings? I'll pull the plugs here in a little bit to confirm it.

 

Any ideas to foul a plug like that other doing a compression test? I was hoping to do a dry, then wet test and see what I can isolate.

 

Gas fouled or oil fouled?