I just reread this and it gave me an idea. If you shoot the IR temp gun at the temp sensor it's in the metal that the coolant is cooling and the sensor is in the coolant that is cooling it.... They shouldn't read the same the IR gun will most likely have a much higher reading from the metal. A more accurate place for the IR gun would be the intake hose to the radiator. The only heat in it is from the coolant that has been through the engine and actual it's heat.

 

Does the exhaust smell "funny" at all?

 

Home in a couple days I'll report back on exhaust smell and readings from each spot with IR gun

 

I'm back home and starting some diag on it. First check of the new radiator cap, holds 7 psi. Checked my dad's truck with the same cap (5.4l) and it holds 0. But I just did 3000 miles in that truck with no issues. I found t that's the issue but the one cap.in stock at the parts store held 12psi so they are bringing in more today. I'll get the best one and try it.

I guess after that, isn't there a dye you can use to see if exhaust gas is in the coolant?

 

Yes there is


You can also let the car idle and see if the coolest bubbles at the coolant expansion tank without the radiator cap on.

 

That will be my next test. It has not bubbled at all or blown any coolant out

 

Put on the best cap I could find - 12psi. Drive it 60 miles round trip on the freeway with coolnat temps at ~215-220 just cruising along, cylinder head temps at 250 or so once hot. Got it home and opened as soon as possible with coolant at 200 it did not trigger the block tested to go off and change to yellow.

Going to let the coolant pressure tester sit on it for an hour and see if it loses any pressure.

Fired it up for the first time in Two weeks this am. No coolant loss, no white/yellow smoke and oil/water still no mixing.

Getting really stumped.

 

Pressure tester was down to 5psi after 20 minutes... Still no idea where its going

 

Was the Engine still warm when you did the pressure test?

 

Yep still plenty warm

 

You want to do the pressure test on it with a cold engine, The cooling of the engine will condense the pressure and won't give an accurate pressure reading. It's normal for an engine to go from the maximum pressure of the cap hot to zero psi cold.

 

Totally forgot that! I'll do it in the am

 

I know from experience that when your truck is broken and you're worried sick about it is the hardest time to think of little things like that. I'm slowly learning that the best tactic to diagnostics is to step back and have a cold drink of some sort and set that worry aside and just look at the facts and think about how the truck works.

 

Are the caps not rated to about 16psi? Should be, also sounds as if your dads truck is ready for a new cap also. Like I said it took about 6-7 tries to find a cap for my truck at a couple of stores. Most caps are imported from israel or someplace so look at the required psi and find one.

 

Yep 16psi. Tried 5 and best was 12.5psi holding so I put that on. No difference.

Block test came up negative for completion gases. Pressure held but lost about 5psi over an hour, no leaks and after I started it without any white smoke.

Running out of ideas here....