Over-cooling problem is driving me crazy!

[Chewy1576]

My '90 K2500 with a 350 has been over-cooling and I can't figure out why. At first, it would heat up but still blow cold air and the temp gauge would go up to above 210 then drop to 140 and fluctuate between the two. Then I noticed the drips of coolant on my passenger floor mat. I replaced the thermostat, water pump, heater core, and temp sensor.

Now it will blow hot air, but the temp still fluctuates, just not as drastically. At idle it will warm to about 150 degrees and hold. As I'm driving it will warm to about 200 then drop and fluctuate between 160 and 180. When I stop and idle it will drop back down to 150.

Does anyone have any other ideas? I can pull the radiator cap and the radiator is full of coolant and there is coolant in the reserve bottle. I know the thermostat is installed correctly and there are no leaks.


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[lha1992]

Hmm you kno sounds like you have a similar problem just as I'm having. My 89 use to run right below 210 on the gauge which obviously would be 195deg.

When re-doing the truck an what not I put a new radiator in an still warmed good. Then I did my intake gaskets because of a leak from the original gaskets an put in a oreillys Murray thermo in also 195.

Now this thing fluctuates all over the place from 150 all the way to almost 195 where it use to sit.

I figure coupled with the new radiator somethings a miss with the thermostats I'm using so I went ahead an got a Stant thermostat from Napa made in America an better quality. Have yet to put it in but will report back on how it does. I kno the stants are what these trucks came with originally.

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[lha1992]

Also I went through 2 oreillys thermostats. An still never got back to 195 would just fluctuate on the cold side. My truck also has a fairly new heater core an hoses.

My next guess that if the stant thermo doesn't help is that the fan clutch might be going bad.

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[96-1500]

Positive there is no air in the system?

 

[Chewy1576]

I'm not positive the system is free of air. I was going to pick up a new radiator cap and try to burp the system tomorrow.


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[lha1992]

Deff try burping the system. I kno mine is burped but thinking its the Murray thermostats. I worked at oreillys for a while an had people always complaining about them.

A trick I did on my 72 C20 to burp is was to jack up the front end to have the radiator be the highest point in the system to let the air out.

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[Chewy1576]

I kinda tried doing that today after I replaced the thermostat again (with a Murray actually). I will really have to get it up in the air when I try it tomorrow. I may do the fan clutch too while I'm at it.

How is the reserve bottle supposed to work? Shouldn't coolant be pulled from it to the radiator if the system gets low?


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[96-1500]

They can do some weird things if there is air in the system. Getting the air out of small block GMs is pretty easy, compared to some stuff I've worked on.

 

[Chewy1576]

I had an '87 c1500 with a TPI 305 from a Camaro I could never get to run right (overheating and fuel injection problems) but I still loved that truck! It seems like every other week I was fighting with something.


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[SkyHighColorado]

My 94 does the same exact thing. I tried burping and changed t stat to 195 as soon as the temp his 195 is drops way back down. I'm leaning for towards the fan clutch myself.