Kind of a strange cooling system issue

Hey all got a hopefully minor issue for you all,
My 87 k20 tbi never warms up been doing this for the last year or so only warms up to 120f or so. I assumed the gauge was broken but ive recently confirmed it works and ive developed a new more concerning issue.

So when all this started i installed a new aftermarket radiator and thermostat my truck runs at 120f almost all the time, now its doing a new trick where all of a sudden maybe once per day itll rise to 210, then rapidly drop back to 120 if i step on the gas. Im worried im gonna crack the block or blow a head gasket with that rapid cooling.

Im guessing junk thermostat right out of the box? Im going to change it this weekend but wanted you guys 2¢

P.s seems this guy had a similar issue:
Thread 'Odd Cooling System issue' https://www.gmsquarebody.com/threads/odd-cooling-system-issue.45714/ but he never posted what he found (nevermind i see he did post want he found) still appreciate any input you guys have

I would start with a thermostat….

Put the thermostat and a thermometer in your least favorite pot, fill with water then turn up the heat, check what temp it opens.
You will also confirm if a watched pot never boils.

Are you running a reverse poppet thermostat? When they fail they stick open and send cooler temps. So coolant flow is constant and temps do not get a chance to build properly. Also some create this style of thermostats intentionally by drilling a 1/8” hole in a stock thermostat to mimic a reverse poppet style of thermostat so it never slams shut on ya and cause overheating.

Ditto on the thermostat theory

Are you low on coolant,is your fan belt slipping? Look in the radiator to be sure,and check your belt. Stepping on the gas makes it cool down down,makes me think you aren't moving enough coolant or maybe even not enough air* fan clutch) at low rpm. Thermostat I'd expect it to stay hot as you reved the engine. What work have you done recently to the truck.

Ricko1966 said:

Are you low on coolant,is your fan belt slipping? Look in the radiator to be sure,and check your belt. Stepping on the gas makes it cool down down,makes me think you aren't moving enough coolant or maybe even not enough air* fan clutch) at low rpm. Thermostat I'd expect it to stay hot as you reved the engine. What work have you done recently to the truck.

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Not low on coolant and to be clear this will happen once per day maybe, 99% of the time its stuck at 120f and its not much fun in the winter my heat doesnt get much past luke warm.

As for fan airflow issue i was running without the radiator shroud but i reinstalled it and didnt make a difference. Fan seems to pull plenty of air the issue is its usually too cool

Sad Sack said:

Are you running a reverse poppet thermostat? When they fail they stick open and send cooler temps. So coolant flow is constant and temps do not get a chance to build properly. Also some create this style of thermostats intentionally by drilling a 1/8” hole in a stock thermostat to mimic a reverse poppet style of thermostat so it never slams shut on ya and cause overheating.

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I got it off amazon that was probably mistake #1 but other then that dunno much about it. Was supposed to be for a squarebody

I suggest trying a Robert Shaw high flow thermostat from Summit. I have been using the 195 degree RS high flow thermostat for years and it has been very reliable.

One additional note. I recently replaced it with a new one because my engine was not reaching full temperature of about 195. I found during troubleshooting that the old thermostat (maybe 8-10 years old) was sticking slightly at the top and leaving a sliver of flow when it should be fully closed. Temps usually ran about 180-185 degrees when fully warmed up. A new 195 degree thermostat fixed the problem.

As for constantly too cold I'd guess you don't have the correct fitting for your heater supply. The correct heater supply fitting has an internal restrictor the hole is about 5/16ths. The wrong fitting allows to much water bypass so seriously delays warm up. Try pinching your heater supply mostly closed with vise grips so if things improve. I can't imagine it's bypassing enough to randomly close the thermostat.But I never thought Bruce Jenner would,oh you know.

Went and grabbed the best one autozone had (autozone sucks) ill install it this weekend and we will see

Squarescare said:

i was running without the radiator shroud

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NEVER do this!

Did you complete a system flush when you replaced the radiator?

Try pinching the heater supply hose most of the way shut with vise grips,not completely shut just restricted. See if both problems go away. I thought I already posted this but saw it was still sitting here waiting for me to hit post a day or 2 later.

Snoots said:

NEVER do this!

Did you complete a system flush when you replaced the radiator?

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Ya i did, shoulda kept the old one though the new aftermarket one i bought does quite fit right. Old one was just ugly.

And The new thermostat fixed the surging, but my temp never goes above 150°. My next step is I'm going to double check the restrictor and the heater core and hook up an external temperature gauge so I actually know for sure that it's not heating up Even though I did that with a heat gun not really the same thing need one in the coolant

Our 85 never gets warm. Thermostat works, heater core is not full of Dexcool “mud” anymore (father in law used orange coolant once instead of green). New water pump and radiator hoses, new radiator, full system flush times 3 to get rid of afore mentioned Dexcool mud…

No heat. No defrost. Fan only works on high.

I’d lay dollars to donuts OP has the same issue, same cause(s)… Firewall full of debris (leaves, pine needles) from a broken firewall top cover and a heater box full of leaves, mouse hair and “rodent rice”… Open up the heater box inside the cab, vacuum it out, add a 195* thermostat and maybe sacrifice a chicken to Marie Laveaux just in case…