Hmmwv overheating...sort of...

[RustyM923]

1993 m998 with a rebuild in '07. Engine is a GEP 6.5.

Temp shows 240 (after a proper warm up.) Fan comes on. Less than a minute later, the fan shuts off. Then about 2 minutes or 3 minutes later, the fan comes on again. Tsu for fan seems to be sensing the heat and also the cooldown from the fan cycle.

Coolant flushed. Small hoses and coolant tank nipples clean and clear. Running straight water until resolved.

Grounds are good, harness appears unmolested. TDM unplug sets fan on...clutch working as it should.

New 6.5 specific t-stat ( looked identical to the one that came out, had the same jiggler valves in it, also tested on the stove top, both were open when water was boiling)

I checked the temps with non- contact therometer....results:

Cap off....Coolant tank about 188 to 197. Radiator hottest part (drivers top), 200 to 205. No boilover with straight water and cap is off...no pressure.

Changed temp sending sensor to a new one (that was fun). Now reads 235 to 240. Fan coming on does not indicate a temp drop of more than a few degrees (hard to tell).

The gauge looks brand new (no over spray from 07 repaint, no scratches. ..definitely new).

Next step, a new gauge.

Anything else I'm missing?

(I've read the TM and the relevant portions of the 280-20.)

 

[SCSG-G4]

Sounds like the gauge and the sensor are not matched properly. Check what the resistance is on the gauge at various temps and do the same on the sensor. Could be a bad wire or connection raising the resistance value that the gauge is reading. YMMV and someone who knows more will chime in shortly.

 

[RustyM923]

I thought it's only the newest (LED indicator) gauge that requires the Grey boot sensor. This is the standard gauge. Maybe because it's new, it may require the new sensor.

I have a new LED indicator gauge, I'll slip that in and see if it changes anything.

If the pure water coolant (no cap) isn't boiling out at 235, I'm guessing it's still a gage issue. The truck was most recently serviced in July 2013, based on paperwork that was in it (NG truck, 40th BSB)

 

[DatGuyC]

What's the part number for the led gauge?

 

[RustyM923]

What's the part number for the led gauge?

I'll check when back home tonight. The one that's in there is just a new "normal" style one. I have an LED laying around. I'll post a photo of both of them in a few hours.

 

[jimbo66348]

In GM trucks, the coolant system was upgraded in 1996 to a larger water pump and dual thermostats. This set up made a difference in the pickups, and there is a kit to convert to a duramax fan and clutch, as well. The 6.5's in trucks, always ran hot. The engineers determined that the radiator had enough cooling, so they didn't make it any bigger. This was in the commercial line. I have no idea what was done in the military line.

 

[TedG]

Check/change cap on overfill bottle. Might solve it. Grey boot sending unit is for led gauge only.

 

[RustyM923]

I've checked the cap, it went to about 15psi before releasing.

Would having the cap off have any bearing on the actual cooling? My understanding is that the pressure only raises the boiling point of the liquid.

 

[jimbo66348]

Like I said. I don't know squat about the military versions, but the coolant won't circulate in a civilian version of a 6.5 without the cap installed. They only circulate under pressure. I had problems with mine, overheating till the day it died.

 

[RustyM923]

Current gauge

 

[RustyM923]

Spare gauge...Haven't tried this one yet. Sending unit is new and black. Replaced it yesterday

 

[DatGuyC]

Thanks for the picture, I just got one off ebay. Are you sure that the proper sending unit for that gauge is black, from what I've seen its supposed to have a gray boot.

http://g503.com/forums/viewtopic.php?f=63&t=243754

Comparing the gray boot sender with the black boot
http://g503.com/forums/viewtopic.php?f=63&t=141303&p=1057125&hilit=sender#p1057125

Edit:

Looks like the part number for the gray sender is 12480699 from kascar or 6015561 from mac motors. NSN 6685-01-534-2536.

 

[RustyM923]

I'm using the black boot (standard ) sender with the standard gauge.


Just finished another test. Fan turned on about 230-235. Stayed on 50 seconds. Temp didn't come down at all on the gauge.

Shut the engine off and then restarted immediately. Temp was 215. How can it go down with the engine off? Makes no sense.

Maybe it's the gauge after all.