Head Gasket or EGR

Hello Masters need help or advise please
KW T680 X15 640K miles year 2020, couple of months ago coolant start leaking from relief valve, pressure in the coolant, the coolant leaking was random sometimes no coolant leak for a week and sometimes once per week, hauling 43k weight water temp stays at 175 but in traffic goes 210 and than fan turn on, never went to 225F (no fan turn on between 175 to 210F)
I see bubbles in reservoir near the coolant return line and increase with engine rev, did liquid change test but came negative 2 months ago when problem starts. When coolant leaks from relief valve I catch in the empty bottle so I'm not adding any new coolant, no crazy coolant leak or adding like 1 gallon or major
Did replace water pump, thermostat, air compressor head and Cartridge but did not solve the problem , but recently I saw the black color fine stuff in the reservoir walls , looks like soot in the coolant. There is no oil in the coolant and I did not find any coolant in the oil.
I know it may be a head gasket or EGR, how to make sure if it's head or EGR.
if I have to replace unlucky head gasket than where to start , I heard a lot of expensive horrible stories.
Please guide should I fix head gasket, rebuild or sold the truck and buy another if it's worth
Thanks for your time

 

I am not sure about an X15. On a 60 Series if you remove the coolant reservoir cap, turn the Jake on, run to max RPM then let off the pedal. If it blows coolant in the air when the Jake comes on you have a bad head gasket.

 

Bing Videos

 

fan on at 210, off at 205 is normal. black fine kind of gritty but smears on your finger when you dip into the reservoir is from hoses. carbon black rubber degrades and sloughs off to collect on the reservoir tank walls. what means "relief valve"? the cap? It needs to be replaced as needed, if the reservoir fill cap they allow pressure to blow off and you'll see dried coolant on the outside of the reservoir. I don't see anything alarming in the description.

 

Attached is the video and couple of pictures showing the black stuff in the coolant. Please watch the attached video

https://youtube.com/shorts/3d3J7wvJO_U?si=0zdGpG49HYPo_1CB

 

I don't think the black stuff is from hoses because I changed all three main hoses , thermostat hose radiator top and bottom hose.
Problem is coolant leaking from relief valve and truck heats up during idling or while stuck in traffic and now black stuff shows In coolant

 

You most likely have dropped liner# 5 or 6 and have early stages of compression leakage by head gasket into cooling system.

Inframe rebuild time. Counter bores will need cut and liners shimmed back to spec.

 

Get a combustion leak test kit.

 

So unlucky if that's the case what's the best option ? , if I let mechanic open the head and find out the dropped liner and other issues at that time what option I have if I can't afford all the expenses

 

Start shopping for rebuild pricing. Make the decision ahead of time to swap the truck out or repair it. Don’t go in there with the promises of “it’s just a head gasket”. Changing the gasket will seal it for a short time and you’ll be right back in there.