I've noticed a rough idle when it's cold out for the first 30 seconds or so after starting. I have to wait until this clears up before driving or it really stutters.

Yesterday I noticed bogging & hesitation while at highway speeds while slowly accelerating or going uphill. I checked OBDII codes and got P0304 cylinder 4 misfire. I swapped a new plug into 4 and this morning it had no improvement. Then I pulled codes again and got P0303 so I swapped cylinder 3 plug. This solved the bogging issues but the plug in #3 has a green coating on the end of it. Any idea what would cause this?? Coolant leaking from the head? I replaced the lower intake manifold gasket 3.5 years (30,000 miles) ago. Truck currently has 197,500 on it.

I noticed my coolant was low but I just replaced the radiator, cap, hoses, water pump, and gaskets a few months (3,000) miles ago and I don't think I've checked the level since then.

The plugs were changed roughly 14,000 miles ago.

Here is the green #3 plug...


Here is the #4 plug...

 

Looks like you may have a head gasket leak in #3

Plug looks steam cleaned compared to #4, which happens when coolant leaks into a cylinder via blown head gasket cylinder ring.

Have you noticed any unusual up and down movement of engine temp gauge?
More white exhaust "smoke" at first startup?

Or overflow tank being full to the top just after shutting down a warmed up engine?


These are all signs of cooling system to cylinder leak


Why did you replace all those cooling system parts 3,000 miles ago?

Was there an over heating problem at that time?

 

Thanks for the reply, RonD. I added my answers to your questions below. I bought a compression tester so I plan on checking the compression on each cylinder and putting new spark plugs in all 6 spots. Is there any other diagnostics I can do to verify a head gasket leak? Should I buy one of these "radiator pressure tester kits"? Is this something that should be fixed immediately or is it okay to drive on it for a while?

Radiator Pressure Tester Kit



Have you noticed any unusual up and down movement of engine temp gauge?
- I haven't been watching the gauge. I have an OBDii display I can plug in so I will start monitoring the temperature.

More white exhaust "smoke" at first startup?
- Yes... there is definitely white smoke at startup.

Or overflow tank being full to the top just after shutting down a warmed up engine?
- I haven't been checking. I will start to today.

Why did you replace all those cooling system parts 3,000 miles ago?
- I had a radiator leak between the metal part and the plastic part of the radiator. I figured since I had the fan off and radiator off I may as well replace the water pump since it is right there. I also replaced the front main seal, harmonic balancer, fan clutch, belt tensioner, belt idler pulley, and fuel pump at the same time.

Was there an over heating problem at that time?
- Nope. As far as I know it's never overheated since I've owned it starting at 120,000 miles.

 

Compression tester is for testing overall wear on the rings and valves, and possibly bad rings or valve if 1 cylinder is 10-15% below average.

Head gasket leak(or cracked head) might show slightly lower pressure but it wouldn't be definitive that there was a bad gasket or crack.
This is because cranking speed doesn't generate much pressure.

general rule is you should expect 18 x compression ratio for pressure.
4.0l OHV has ratio of 9.0:1
18 x 9 = 162

So if all spark plugs are removed(to get good crank speed)
And battery and start motor are good(for the same reason)
You should see 160psi from each cylinder.

I have 350k on my 4.0l, it averages 157psi

To get an average on V6 you test all cylinders, make sure you get at least 4 "hits" of compression stroke on each, you will hear it.
Once you have all 6 pressures written down, take out the highest and lowest
Add the other 4 together and then divide by 4, and that will be your average.

Say you get 155psi average, 10% is 15.5psi
If you have a cylinder with 140psi then I would be concerned.

Compression gauges are not calibrated very well, lol.
So if you are getting 140's as average do not be too concerned it is just the gauge or slow crank speed.


search for Glove Test at this site, I have posted how to do it many times, free and easy test for leaking head gasket.