Temperature Gauge Fluctuation

[HaroldCallahan]

My 1988 GMC suburban's temperature gauge has been flucuating quite a bit.

It heats up to almost 210, then back down to about 170 then back up, and down.

It gets more stable the longer it is driven, but it still fluctuates. A few years ago I installed a new radiator and thermostat and it was doing it a little bit right after the install.

I figured it was probably the thermostat, but since it is more noticeable now I think I better figure out what is going on. I thought of air in the system as a possiblility also. I dont drive this vehicle much and probably put less than 10,000 miles since the new radiator and thermostat.

Any ideas ???

 

[cryptokid]

the thermostat isnt bypassing when cold.

thermostat needs to awalys bypass a little bit of water.

my dad had this problem on his diesel truck. turns out the thermostats bypass hole was plugged. a quick prick with a drillbit cured this.

he had the same symptoms as you, a temp gauge constantly going up and down.

 

[ZR2RANDO]

Almost sounds like the incoming coolant is short circuiting straight to the thermostat (like coming across the front of the engine instead of going around???). When the cool coolant hits the gauge sensor you are seeing that low temp. Then the thermostat closes and the coolant circulates and heats up and then does it again...??
Have you checked the temp at the air vents with a instant-temp stem thermometer (like for checking roast or chicken for being cooked) It would show you if the heater core was getting the same coolant that the thermostat was getting.
Any debris/rust in the coolant?
Could also be as stated above,,,you just have tons of cooling capacity and the thermostat is just not dribbling to moderate it??

 

[Chris142]

The V8 Chevy engines use an internal bypass. This is either a hose that connects the intake manifold to the water pump if it's a big block or built into the block and WP if it's a small block. This allows the coolant to circulate inside the block when the stat is closed.


Assuming that the cooling system is not low on water/coolant......
It does sound like the stat is not working correctly.....But it's not the type of engine that has a "bleeder" in the stat.

Replace the stat with a good one and not the cheapest they have. Spring for the "Superstat".

 

[cryptokid]

chris, most watercooled engines need some sort of bleeder. if its not built into the stat its gotta be somwhere else, maybe inside the engine.

my dads truck has it built into the stat in the form of a small hole. other vehicles bypass around it with a hose.

most stats are built to leak a little. otherwise the engine would constantly cycle between hot and cold.

 

[HaroldCallahan]

It is a 4 row radiator, versus a stock 3 row. SO lots of capacity.

Never overheats, in fact if anything the temperature tends towards the cool side. I have heard of a trick on these gm 5.7 motors is to drill a small hole in the flange of the thermostat. I think I will do that on the original which I think I still have and put that in there. No fluctuation before the radiator and thermostat replacement. My guess is that it is the thermostat.

 

[John D]

It definately could be the thermostat. It is not that uncommon to get one (or a few) that are bad right out of the box. The cheapest route is probably to change the thermostat with a good quality one and see what happens from there.

 

[Gary Allan]

I first go to the thermostat ..but what you describe can also be some air in the system. Unless you have a leak that sorta bypasses the coolant recovery tank (allowing air to be sucked back in) ..then I'd go with a new thermostat.