Hello everyone. First time poster here. I am currently working on a 2003 Ford Ranger Edge with around 273k miles. The engine however is a pre 1999 3.0l that was replaced because the original engine failed. I replaced the engine about 7 years and 37,000 miles ago with a remanufactured long block. I know the engine to be pre '99, but not sure what it came out of originally. Now to the reason we're all here.....
The engine has developed a cylinder #3 misfire and I just don't know where to go. It starts and runs normally and idles smoothly while cold. As it warms and gets to eventual operating temp it presents a hard misfire on cylinder #3 along with an MIL with codes P0303 and a P0316. Compression on cylinders #1 to #3 are all about 150psi regardless of temperature. Vacuum holds at a fairly constant at 17psi at idle warm and cold. I took the truck to two different shops that placed blame on low compression. Based on their recommendation I tore down the upper engine and took the right head to a machine shop. They said they found leakage on the exhaust valves and recommended replaced of the cylinder head. So that's what I did. I bought a remanufactured head and installed it on the engine. The same exact issue is there. I'm at my wits end on this thing! Here are all the other things I've done to specifically address this problem that have not worked

1. All plugs, wires and coil pack replaced
2. Swapped fuel injectors with known good cylinders
3. Swapped upstream O2 sensors from left to right
4. Replaced upper and lower intake manifold gaskets
5. Cleaned MAF sensor AND throttle body
6. Checked #3 injector circuit with noid light. Checks good
7. Checked multiple times for vacuum leaks

Thanks everyone for any advice!

 

Welcome to the forum

You might be able to find the year of the engine looking here: https://www.therangerstation.com/tec..._0-build.shtml

There were changes to the block over the years, but not an issue except for head bolt length and proper torquing of the head bolts
1991 to 1998 Ranger 3.0l used the shorter standard head bolts
Both lengths seen here: https://www.therangerstation.com/tec...3.0build14.jpg
Shorter standard bolts used these torque specs
Torque to 59 lb-ft
back off 1 turn
Torque to 37 lb-ft
Torque to 68 lb-ft

In 1999 a new block was used, and they recessed the threads for head bolts and used TTY bolts that were longer
Torque specs were change to
Torque to 37 lb-ft
back off 1 turn
Torque to 22 lb-ft
Turn 90 degrees additional
Turn 90 degrees additional a 2nd time

TTY=torque to yield, these types of bolts are made to STRETCH out when tightened(they can not be reused), it makes it easier and better for the machines that tighten head bolts at the factory, torque needs to be calibrated often when above 40ft/lb or you can break bolts or not get one tight enough, but 90deg is 90deg no calibration needed for that, easier for computer controlled wrenches to do that correctly each time for all 8 bolts(V6) on each head

So block year matters, heads were the same as far as toque specs were concerned
But in the above link you can see that the valve STEMS were changed in 1999 and the valve spring caps look much different between 1998 and 1999 so easy to spot the difference with valve covers off




The "Dollar Bill Test" is not definitive but can be tried
Google: Automotive Dollar Bill Test
Warm up engine first of course

But I would do compression test regardless
Hard misfire is almost always an exhaust valve issue, and this is not a code misfire, you can actually feel it
Compression is good or bad because its mechanical, so best to take that off the table first
So do a COLD compression test again
Remove all 6 spark plugs first
Test each cylinder and write down results
Press gas pedal down to the floor all the way when cranking engine for the test, this does TWO things, gives max air flow AND shuts off the fuel injectors

Then do a WET compression test on each cylinder
Add teaspoon of oil to each cylinder and test it again
I use a straw, dip it in a bottle of oil, put finger over top of straw, lift it out and put it in spark plug hole, move finger and oil drains into cylinder

All cylinders should show higher numbers WET, but cylinders with leaking valves numbers will not go up as high
The oil only seals the Rings better not the valves

If there are no lower cylinders especially #3(or #5 or #6), then misfire is not compression related, so 99% of what causes a misfire has been eliminated, and you can move on to spark and fuel

Firing order for 3.0l is 1 4 2 5 3 6
The Cam sensor is the "misfire detector", and its "pretty good", but can be wrong, lol
How a misfire is detected is by rotational speed, each cylinder adds to the rotational speed of the crank and cam when it fires
When there is a misfire speed is not added so rotation slows
And by calculation the cylinder that misfired can be IDed, or can it, lol, we are talking milliseconds here and if you forget to "carry the 1", well goofs can happen
A computer makes a mistake??????, I love computer controlled systems, but they can goof up once in awhile(I have had to reboot my cellphone more than once in my life, lol)
Which is why #5 and #6 are "on the table" because they come before and after #3 in the firing order
So P0303 most likely means #3 is misfiring, but its not 100%, so you need to see if #5 or #6 has a compression issue
Also check the firing order on the coil pack
[3 4]
[2 6]
[1 5]
These are the matched PAIRS of spark plugs, 3/4, 2/6 and 1/5 on the coil pack, the 5 6 4 side can get mixed up, so follow each wire from the coil pack to its cylinder, I have chased my tail more than a few times KNOWING that the firing order was correct.....................and it wasn't

You can also reverse the 3 and 4 spark plug wires at the coil pack, since they both spark at the same time this tests that the coil pack is sparking both wires the same
Coil packs can crack and one spark plug wire may not be getting full spark voltage after coil pack warms up, so swapping wires around and then getting P0304 would indicate bad coil pack

When computer detects a misfire it will switch from Sequential fuel injection to Batch fire injection
Batch fire on a V6 means computer opens 3 injectors at the same time on each 1 RPM, usually odd and even, so injectors 1, 3 and 5 open, then on the next rotation 2, 4 and 6 open, and back and forth
This keeps the intake full of air/fuel mix, like a carburetor did, so when an intake valve opens it will suck in air/fuel mix and no misfire
A bad injector can certainly cause a misfire, even with Batch fire
But totally failed injector would cause issues at startup and through warm up, since you reported "after warm up" misfire then compression or spark would be more likely