I have a 2003 3.0 and I keep getting the same code p0304 which is misfire on cylinder 4 I have already done my spark plugs, wires and coil pack and from what I have been able to get out of what’s my code reader is that my cat is clogged up. I am wondering if I can just take the damn thing off and straight pipe with a cherry bomb glass pack it from the Y pipe back or will it continue to make my CEL stay on and flash while I’m driving??? Any advice is appreciated. Here is what my code reader is sayin.

 

Welcome to the forum

P0304 misfire #4

Won't be related to clogged exhaust or bad cat

Is there an actual misfire?
Do you feel it at idle or under load?

Does the CEL(check engine light) ever Flash?
CEL usually flashes during an under load misfire then stops flashing after you let off the gas pedal


A cylinder needs 3 things to fire
Spark, at the right time
Fuel, in the right mix with air
Compression, above 120psi

When there is a misfire code first thing to do is test compression since it is either good or bad, unlike spark or fuel which can be intermittent
With a V6 engine you should test all 6 cylinders at the same time
But you can test just the 3 on the effected bank, 4, 5 and 6 in your case

3.0l Vulcan engine should be 160psi on cold dry test
If #5 and #6 are at 155 to 165psi and #4 is at 140psi or less then you found your culprit
Then you can do a WET test to see if its a ring or valve issue

Always good to change old spark plugs for new, not a waste of money, but once that is done and misfire is still there you need to do the Compression Test or you will be wasting time and money

Did the old #4 spark plug tip look like the others?

There is a Ford TSB for the 2004-2006 Ranger 3.0l engines, seen here: https://www.therangerstation.com/for..._05-26-3.shtml
You have a 2003.................so not a stretch to check compression to see if may be your 2003 3.0l ended up with the first of these failing exhaust valve seats


You can test for clogged exhaust with a vacuum gauge, $20, still one of the best tools to diagnose gasoline engines

Clogged exhaust symptom is similar to clogged fuel filter symptom, loss of power at higher RPMs, highway speeds

 

Alright thank you I will go an get a compression tester for sure and probably the vacuum gauge as well. And yes the CEL flashes almost constantly and it hasn’t been loosing power at lower RPMS anymore after I changed the plugs, wires, and coil pack to take care of the cylinder 3 code it was giving me. It ain’t really been loosing power at all just when I’m in park or neutral and get on it once it hits about 3000rpms it will putter which is what was making me think it could be an exhaust or air problem. She still runs great tho and pulls up the summit that I have to go up everyday on my way to work and on my way back just can definitely tell that the gas mileage has gone down and just don’t run like it should be

 

Best way to test compression is, cold engine, all 6 spark plugs removed, so no compression and starter motor can spin engine faster
Screw in compression gauge to a cylinder's spark plug hole, just needs to be snug don't need to wrench it tight

Press gas pedal down to the floor and hold it down, this shuts off fuel injectors, and gives engine max air flow
Crank engine and listen for 5 "hits" of that cylinders compression stroke, can't miss it
Write down results
Release pressure on the gauge
Repeat for next cylinder

Compression test is not about "high" numbers, because every compression gauge will be a little different and vehicles altitude above sea level matters, and Battery voltage matters, cranking speed
So you end up with 6 numbers from a V6 engine
Don't use the highest or lowest numbers
Add up the 4 other numbers and then divide that by 4, that gives you the Average compression for this test

Say you end up with 156psi as the average, so 10% of that is 15.6, or 15, you add that to the average and subtract that from the average
So 156 + 15 = 171
156 - 15 = 141

171 to 141 compression numbers on this engine, this day, with these testing parameters is OK, lol

If you have a cylinder out of that range then thats a problem

WET test if there is a problem
Get a straw and a bottle of motor oil
Dip straw into the oil and put your finger over the end, pull it out and put straw into spark plug hole then move finger letting oil run into the cylinder
1/2 or 3/4 straw is enough oil for test
The oil helps seal the piston rings so they leak less air, but doesn't help seal valves
Retest compression WET
It will always go up

Using above numbers, 156 average, if cylinder was 130 and goes up to 140 WET then valves are leaking, if it goes up to 150 then it rings that are leaking



Good read here on vacuum gauge use: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine

Also one simple test to see if you have a vacuum leak is to warm up the engine and then let it idle and unplug the IAC Valves 2 wire connector
IAC Valve is what the computer uses to set engines idle, when you unplug it, it will close and idle RPMs should drop down to 500, or engine may even stall, either is GOOD
If RPMs do not drop you have a vacuum leak

 

Thank you very much for the advice I appreciate it. Hopefully have my truck running like she should be here soon. Makes me sad having her run like this