I noticed a couple of weeks ago when I'm trucking along and the heater is on the temp gauge will rise suddenly and then go back down just the same. I turn off the heater and it's solid as a rock right where it should be??? I turn the heater back on and it starts doing it again.

I checked the thermostat by waiting for the truck to warm up and feeling the upper hose and it seems to be opening as it should. I have recently changed over to an electrical fan, could that have something to do with it? I'm stuck and maybe overlooking something, have any of you guys had this happen to you?

 

Anyone??? I am going insane trying to figure this one out.

 

Well.. it's either the t-stat, rad cap, or maybe the guages wire grounding out on something.

I'm not sure about your motor.. but most have an internal by-pass that flows through the heater core all the time. When you have the heat on.. it basicly opens the t-stat less because the heat disipates through the heater core/cabin. This mode of operation tends to only slightly open the t-stat. Maybe yours is sticky and opening suddenly?

I'd suspect it's the t-stat and/or rad cap considering the year of your truck.

Rich

 

Rich,

Thanks for the info. I suspected the t-stat but it was replaced about 15,000 miles ago so I dismissed it. It's a pretty cheap part so I'll give it a shot.

Since dealing with this problem I discovered that the truck is using antifreeze also???

 

If you installed an e-fan recently than make sure that the thermostat on it is set properly. It seems like it might be set too high, which might cause your coolant temperature to rise until the fan turns on and cools the radiator. It also might be a bad radiator cap. When you say that your truck is using cooland do you mean that it is loosing it. This would mean that you either have a coolant leak somewhere or a blown head gasket, either way I would keep a very close eye on it.

 

I checked the fan turn on and it seems fine. I don't think it's got anything to do with it anyways because it happens while I'm driving down the road. For instance: I was cruising down the road one cool morning doing about 50mph or so, and the temperature rose suddenly and then back down just as fast??? It only happens when I have the HVAC turned on. It could be heat, A/C, or just the vent; but it only happens when I have something dealing with the ventilations system turned on. When I drive around with everything turned off it's steady as a rock. Any one have a clue as to what this could be?