2008 LMM Overheat

[hansbrakob]

My 2008 LMM overheats (both engine and tranny) when pulling heavy 5th wheel trailer (15,000#) on grade or into headwind at freeway speeds (65+). Slowing down, or shifting down to 4th or 5th gear, and both temps return to normal fairly quickly.

Otherwise temps are fine.

In a previous DMax (03), overheat would activate fan (loud), and temps would return to normal.

In this truck I don't hear the fan cut in.

Does the LMM have a fan clutch which should cut in with rising heat?

[gary_lucas]

I'm sure someone else with more knowledge will chime in, but I'm 99% sure the LMM has a fan clutch which should kick in.

How badly do you overheat?

I'm asking because I had a few overheats before I got diagnosed and it caused me about $9k (CAD) in repair bills (new head gaskets).

The ultimate issue seemed to be that my air side radiator had gotten clogged up with all sorts of crap. If you haven't already do yourself a favor and take your grill off and take a look. It's really simple on my 04 (4 clips and a 10mm bolt IIRC).

There were multiple other issues that were confounding the process, including a weeping water pump and a few other more minor things.

Here's my radiator which got replaced: https://goo.gl/photos/TocR9AC9FvJUynqh8

It was almost certainly fine inside, but we were stuck in San Fran and needed to be on our way so I told them to just replace it.

Good luck!

[DmaxMaverick]

Welcome aboard!

As Gary suggests, definitely check/clean the cooling stack. If the air is blocked, the fan clutch won't engage. If it's clear, then the thermostats and/or fan clutch is the next suspect(s).

[gary_lucas]

Welcome aboard!

As Gary suggests, definitely check/clean the cooling stack. If the air is blocked, the fan clutch won't engage. If it's clear, then the thermostats and/or fan clutch is the next suspect(s).

That's interesting, I had thought that the fan clutch would kick in with / without a clogged radiator... Damn...

So that was another clue, also I had literally never heard it kick in so I had no reference for when it should / what it would sound like.

I'll have to find a big hill around here and floor it all the way up to see if my fan clutch now engages. On the way back from San Francisco I did floor it all the way up into the mountains with a 9500lb trailer but I don't think I heard it... It is however a very loud truck (less loud now that I have a muffler again). Could also be that the truck just never got hot once it had a damn radiator.

[rapidoxidationman]

Just for comparison's sake: Climbing Teton Pass regularly, my fan will kick in unless it is in the Damn Cold (below zero) category. With my foot in it and in the summer (60+ degrees) I'll make it between 1/4 and 1/3 o f the way up before the roar of the fan can be heard. Following cars or cooler temps means I could be halfway or even most of the way up before it kicks in. Sometimes it comes on right at the top, only to disengage a few hundred yards down the other side. It can be felt as well as heard; I understand it takes about 30 hp to keep those rather large blades spinning.

[More Power]

The fan-clutch utilizes a bi-metal coil mounted on the front side. It responds to the temperature of the airflow that passes through the cooling stack forward of the fan. Generally, once the airflow temp reaches approximately 165 degrees F, the coil will rotate the engage circuit within the fan-clutch, and the fan will begin rotating at near pulley speed.

So, like was mentioned earlier, anything that restricts airflow will likely affect fan-clutch operation.

Cleaning the cooling stack sounds like a good first step.