LML Coolant Problems...

[Jpsnods]

I have a 2012 LML that has been deleted. I pull a 10K pound 5th wheel with it. For the past 2 years or so I have been having coolant issues with it. It first started when I was on a trip pulling the camper and the low coolant light came on about 300 miles into our trip. I pulled over and looked under the hood and one of the rubber caps the mechanic put on when it was deleted had ruptured and was leaking coolant from there. I bought a replacement rubber cap and on the drive home the same thing happened. I bought an actual coolant hose cap for the line and that fixed the problem for about a year. Now when I pull the camper I get the low coolant light as soon as I slow. Meaning, if I drive for say 200 miles and pull over to get fuel or use the restroom, as soon as it returns to idle I get a low coolant warning. If at that point I add coolant I will constantly be chasing low coolant the rest of the day. I will add it and it will puke it out the overflow.
I've taken it to a few different mechanics to get some different opinions. Some say 100% head gasket, others say no. I have had the thermostats replaced (no change) and I put on a new reservoir cap (no change). I also had to replace the radiator due to a pinhole leak right near the upper hose (no change). I have had the chemical test on the coolant done twice and both times came back as no head gasket leak. The upper rad hose does get stiff when towing but after an hour or so it goes back to normal. There is no coolant in the oil and no white smoke out of the exhaust.
Can the increase in pressure alone point to head gasket? Why does it not become a problem until it returns to idle?
Not sure what to do next.
Any suggestions would be appreciated.

 

[Ron Nielson]

Where is the coolant level sensor in your truck? In the oolant tank, I'd bet. Might that sensor be bad?

 

[2004LB7]

I do know the LML suffers from faulty coolant level sensors

 

[Jpsnods]

The sensor is in the tank but the level is low when the sensor says it is. It doesn't seem like it's giving me a false reading. Unless it takes into account for the changes in levels that the coolant actually goes through and it's not reporting that correctly. Maybe changing out the tank and sensor may be my last step before surrendering to the head gasket.

 

[2004LB7]

If you drive around with the coolant cap loose, does it do the same thing?

 

[Jpsnods]

I have not tried that yet but I can. I just messaged my mechanic to make sure he did a pressure test to see if the pressure rose. The problem is, it only seems to do it under load so not sure an idle pressure test would be accurate.

 

[NC-smokinlmm]

Lml has crap head gaskets. I say it’s the gaskets since it only does it under load.

 

[036.6turbo]

Mine was showing the same symptoms as yours with one exception. My upper rad hose stayed firm after the truck cooled down. I drove it that way for quite some time. (years)

My coolant tank became discolored as well, from the exhaust being present.

Couple weeks ago, it pushed coolant out onto the floor of the garage after being parked. At that point I took it out of service and replaced the head gaskets.

Keep in mind the tests meant to analyze the coolant for combustion gases are not real reliable on diesel engines.