Recurring P0304 code, and flashing CEL when idling

[Carolina Ranger]

Hi, Everybody!

I was given a very clean 2002 Ranger Edge, extended cab. 3.0 Vulcan, 5R44E auto trans. It had just under 108,000 miles on it when I bought it. It sat undriven for several years, not long before it was given to me. It passed to a friend of mine, who drove it for a few months as a grocery getter/around town truck. My 2002 F150 had 360,000 miles on it at the time, so he offered me the Ranger. I jumped at the opportunity for some improved MPG. The truck is completely rust free/dent free, and the interior, while plain Jane base level, is is great shape for a 20+ year old vehicle.

It ran well for the most part, but needed a great deal of brakes and suspension work. I did all that work myself. (Upper/lower ball joints, sway bar links, rotors, pads, calipers up front, drums, shoes and brake hardware in the back, and 4 new shocks). It came with ZERO maintenance records, except a recent oil change and replacement water pump, new oil pan and timing cover gasket.

I checked the plugs, and I don't think they had ever been changed...the gap on several plugs was nearly TWICE the recommended gap, but it ran smoothly, and highway MPG was right at 22mpg.

SO, it got new Motorcraft plugs and wires.

Not terribly long afterward, I started getting an intermittent P0304 code. Erase the code...Pulled the plug, checked gap, and even changed the plug wire. Swapped plug with another, and 0304 code showed up again. I have tried Sea Foam in tank, Sea Foam thru the intake, etc...all to no avail. After this code showed up, MPG has been around 20.

I had the cash, so I changed the coil (which was an aftermarket item) with a Motorcraft. The issue continued to show up. If I stop at a light for long, the CEL starts flashing. If I hold the idle up a bit, or start driving, the light stops flashing.

I am a fairly decent shade tree wrench, IF I know what is wrong. My diagnostic skills, not so much.

I sure could use a hand in the diagnosis department! Thanks in advance, everybody!

Recap on what I have done so far:

New Motorcraft coil,
New Motorcraft plugs
Motorcraft wires
fuel filter

 

[RonD]

Welcome to TRS

To not waste time I would do a cold engine compression test next
Remove all spark plugs first, for best cranking speed
Test compression in each cylinder, write it down
3.0l should run 160+psi, 9.3:1CR

But actual PSI is not what you are after, you want to see if #4 cylinder is different than the others average
More than 10% difference usually means exhaust valve issue
If there is a valve issue doing anything else would be a waste of time and money
So take valve issue off the table FIRST

Just to make sure
Coil pack spark plug wiring is done in PAIRS because coil pack only has 3 coils inside
3/4
2/6
1/5
These are the 3 Pairs, this has to do with how the engine is Balanced

2/6 are always on the center coil in the pack
3/4 and 1/5 are at the ends

Ford V6 cylinder numbers are
3 6
2 5
1 4
Front

Coil pack wiring "looks the same" for 1 2 3, but matched Pairs makes the other side 5 6 4

 

[Carolina Ranger]

Thank you, Ron! I'll see about getting a compression tester next week.