Temp erratic - temporary loss of heat - new thermostats?

[Mark Rinker]

2002 LB7 - thermostats both new this fall. Operator reports that last night while plowing (-8F) that temp gauge would occasionally drop to 160 and heater would then blow cold. He claims to have 'tapped on thermostat housing' and got heat back (although I am not sure he would know where the thermostats are on these trucks) and truck seems to be back to normal today.

Any ideas, besides the obvious (unlikely) culprit of a stuck (brand new) thermostat(s)?

[DieselMonk]

a foreign object, some jamming or binding? thermostat inserted correctly?

[Kennedy]

Air in cooling system is likely culprit. Could be head gasket/injector cup seal.

[Mark Rinker]

I like DieselMonk's idea better, JK. :)

If it is air in the system, what would be my best way to determine/diagnose this? Both trucks got new overflow tank caps this fall, so it rules that out. Operator says coolant level is full. No sign of any white smoke at exhaust.

[FJ40INTOW]

I had the same problem in my truck.
I replaced thermostats. Problem got better but temperture still varied.
Tried bleeding air out but never developed enough pressure to get air out.
Replaced cap and re-bleed air out. Works like it should.
Temp gage reaches almost halfway and stays even at 32 deg without winter cover.

Problem may have been the cap all the time. I saved the old thermostats so i could test them in a pan of water on the stove.
I have to wait until the wife is out so she don't think I'm crazy.

[Mark Rinker]

How did you 're-bleed' the system?

It took awhile for my 2001 coolant level to stabilize after the water pump was replaced, haven't had any such issues on the 2002.

[DieselMonk]

why did he change the first (stock) thermostat to start with? :confused:

[FJ40INTOW]

To bleed the air out of the system there is a bleed crew so to speak on the themostat housing located on top on engine towards the front of engine. There are 4 bolts holding housing on the engine and one bolt on top to bleed.
I did not develop enough pressure to make the upper hose hard before I replaced the cap. Once it built pressure I loosed bolt until the antifreeze flowed out.

Someone else can chime in here if they want. I am just a shadetree that aint afraid to try something.

[mark45678]

Air in cooling system is likely culprit. Could be head gasket/injector cup seal.


I have to agree , winter front with the grill closed at those temps is a must to keep heat in the motor !

Combustion air in the cooling system will stop the hot water flow to the heater in every car ! If you rev the crapout of it and the heat comes back good chance you have bigger issues !

[Mark Rinker]

1) We replaced the thermostats and glow plugs when the injectors were done because the truck had 17X,XXX miles and they were originals.

2) The winter fronts should probably be on to keep crud out of the radiator when plowing, but keeping heat with the plows on in any temps hasn't been a problem. Today was -2 or -3 and the truck had no heat issues down the highway.

This latest cold snap caught me off guard, we should probably have them on. However, it sounds like air in the system from the intermittant nature of the problem. I'll try bleeding the system and see if the symptoms return.

[JohnC]

Sounds like an air lock in the cooling system somewhere...

[jbplock]

How did you 're-bleed' the system?

Mark,

[FONT=Verdana]I haven

[Kennedy]

If you put the stats in quite some time ago all the air should have been evacuated to the surge tank by now. Hate to sound so pessimistic. It's usually the 2001's that do what you are describing. On the bright side, it seems that a gasket change takes care of it. Could also be an injector cup seal...

I put the stainless cold fronts on my trucks and unless I am on the dyno I leave them on until the slop is gone.

[Mark Rinker]

Talked to the operator tonight. I thought it happened while he was plowing last night - but it actually happened after he drove the truck about 30 miles down the highway this morning at -10 with no winter front, but with the plow on.

For now - I am chalking it up to an airflow anomoly caused by the plow, the extremely cold temps, and/or no winter front in place. The cold and the plow are the only two new factors with this truck - not the thermostats, and not the coolant.

Coolant level is normal, trip home nearly (outside temps nearly 20 degrees warmer) in the afternoon everything seemed normal.