Weird Cooling/thermostat issue..
Thread Starter
Burning Brakes
Joined: Feb 2020
Posts: 904
Likes: 569
From: Central NJ
Weird Cooling/thermostat issue..
Hi all,
This is an issue, not really a problem. The engine is a Blueprint 355, 325HP, newer aluminum radiator-2Row, the heater valve works, it runs @200 degrees by the gauge, summer or winter, traffic or highway. It's always been cold blooded, the oil temp is also at 200. I decided to change the coolant and not knowing it's history I changed the thermostat-195. It then started slowly oscillating between 200 and 215-20, up and down, like the thermostat was opening and closing. I tried a 180,160, Hi-flow195, drilled a steam hole, there is no air in the system. I finally put an autozone195 in it with a drilled hole and the oscillation stopped for the most part. That was two years ago and that thermostat just went bad, replaced it with a fail safe 195 and the oscillation is back, I've had a lot of cars and I've never seen one do this... BTW, the system is full, no air, reservoir works...
So, has anyone experienced this or even seen this? Or, how to eliminate it? It never gets hot enough to be a problem but it makes me look at the gauge more.
often...
Thanks,
60
This is an issue, not really a problem. The engine is a Blueprint 355, 325HP, newer aluminum radiator-2Row, the heater valve works, it runs @200 degrees by the gauge, summer or winter, traffic or highway. It's always been cold blooded, the oil temp is also at 200. I decided to change the coolant and not knowing it's history I changed the thermostat-195. It then started slowly oscillating between 200 and 215-20, up and down, like the thermostat was opening and closing. I tried a 180,160, Hi-flow195, drilled a steam hole, there is no air in the system. I finally put an autozone195 in it with a drilled hole and the oscillation stopped for the most part. That was two years ago and that thermostat just went bad, replaced it with a fail safe 195 and the oscillation is back, I've had a lot of cars and I've never seen one do this... BTW, the system is full, no air, reservoir works...
So, has anyone experienced this or even seen this? Or, how to eliminate it? It never gets hot enough to be a problem but it makes me look at the gauge more.
often...
Thanks,
60
Last edited by 1860army; Nov 4, 2025 at 07:23 AM.
Melting Slicks
Joined: May 2021
Posts: 2,274
Likes: 1,321
From: Fleming Island, FL
I'm not familiar with the BP 355/325hp engine. But if it's based off of the L-31 Vortec it probably doesn't have coolant bypass passages drilled into it. Not having a coolant bypass can be the cause of the temperature swings.Drilling a hole in the thermostat helps but you probably drilled one hole. Guys drill four to six 3/16 holes in the thermostat to get around it. There is a bypass kit available with the fittings to go from the intake manifold to the water pump also. By the way I run a 180 degree thermostat.
Melting Slicks
Joined: Jul 2006
Posts: 2,291
Likes: 493
From: Salmon Arm, BC
Out of curiosity, have you purged the heating system of air.....in other words, selected the heat on, move the temp selector to full hot, and then brought the engine up to operating temperature.
I wonder if that would make the difference.
I wonder if that would make the difference.
Thread Starter
Burning Brakes
Joined: Feb 2020
Posts: 904
Likes: 569
From: Central NJ
Out of curiosity, have you purged the heating system of air.....in other words, selected the heat on, move the temp selector to full hot, and then brought the engine up to operating temperature.
I wonder if that would make the difference.
I wonder if that would make the difference.
60
Team Owner
Joined: Jun 2001
Posts: 23,375
Likes: 1,067
From: Virginia USA
First have you tired it without a thermostat? Also, whenever I fill my Vette with coolant I jack the front of the car up as high as I can and fill the system at the thermostat housing hole. I then install the thermostat and upper rad hose fitting. Before attaching the rad hose (it's already attached at the radiator) I turn the upper rad hose and fill it up then quickly install it on the thermostat housing. Since the thermostat hole is the highest point of the system doing it this way ensure there are no air bubble to purge after lowering it back down. The last time I did this I actually overfilled it and took about an hour with a turkey baster at the surge tank sucking out the excess. No air bubble though. I've been doing it this way since I had a similar issue. One minute I'd look down and temp was 190ish the next it would be 250!!!. That was an air bubble.
Thread Starter
Burning Brakes
Joined: Feb 2020
Posts: 904
Likes: 569
From: Central NJ
skids,
This is an older BP engine, 2017, not sure what block they used. This thing is the opposite of most of these cars, it runs cold, I tried one of those hi flow 195s and in winter it never reached 200 degrees, the heater did not keep me warm and the oil temp stayed well below 200. I did try a 180 a couple years ago but forget why I didn't use it and I normally save everything but I'm preparing to move so if it is not nailed down it goes... so the thermostats from 2 years ago are in a land fill... I'm going to stop at NAPA and try a 180 again, one with that jiggle valve. I did only drill one hole and I position it at 12 o clock, the highest point to aid in letting any air out. What I think is happening is the car warms up and the thermostat opens, then the cold water comes in and hits it so the thermostat closes until the water is hot again raising the temp a little until it reopens, if I had more holes it would mitigate that but probably take a lot longer to actually warm up in the first place. The biggest problem is me, training has me scanning an instrument panel often so when I see a gauge that normally runs at the 10 or 11 O clock position move up to towards 12 it jumps out at me... Yes I'm **** some times.. lol
60
This is an older BP engine, 2017, not sure what block they used. This thing is the opposite of most of these cars, it runs cold, I tried one of those hi flow 195s and in winter it never reached 200 degrees, the heater did not keep me warm and the oil temp stayed well below 200. I did try a 180 a couple years ago but forget why I didn't use it and I normally save everything but I'm preparing to move so if it is not nailed down it goes... so the thermostats from 2 years ago are in a land fill... I'm going to stop at NAPA and try a 180 again, one with that jiggle valve. I did only drill one hole and I position it at 12 o clock, the highest point to aid in letting any air out. What I think is happening is the car warms up and the thermostat opens, then the cold water comes in and hits it so the thermostat closes until the water is hot again raising the temp a little until it reopens, if I had more holes it would mitigate that but probably take a lot longer to actually warm up in the first place. The biggest problem is me, training has me scanning an instrument panel often so when I see a gauge that normally runs at the 10 or 11 O clock position move up to towards 12 it jumps out at me... Yes I'm **** some times.. lol
60
Thread Starter
Burning Brakes
Joined: Feb 2020
Posts: 904
Likes: 569
From: Central NJ
First have you tired it without a thermostat?
Thanks for the response,
60
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Drifting
Joined: Jun 2025
Posts: 1,728
Likes: 600
RE: L31 aka Vortec block
skids,
This is an older BP engine, 2017, not sure what block they used. This thing is the opposite of most of these cars, it runs cold, I tried one of those hi flow 195s and in winter it never reached 200 degrees, the heater did not keep me warm and the oil temp stayed well below 200. I did try a 180 a couple years ago but forget why I didn't use it and I normally save everything but I'm preparing to move so if it is not nailed down it goes... so the thermostats from 2 years ago are in a land fill... I'm going to stop at NAPA and try a 180 again, one with that jiggle valve. I did only drill one hole and I position it at 12 o clock, the highest point to aid in letting any air out. What I think is happening is the car warms up and the thermostat opens, then the cold water comes in and hits it so the thermostat closes until the water is hot again raising the temp a little until it reopens, if I had more holes it would mitigate that but probably take a lot longer to actually warm up in the first place. The biggest problem is me, training has me scanning an instrument panel often so when I see a gauge that normally runs at the 10 or 11 O clock position move up to towards 12 it jumps out at me... Yes I'm **** some times.. lol
60
This is an older BP engine, 2017, not sure what block they used. This thing is the opposite of most of these cars, it runs cold, I tried one of those hi flow 195s and in winter it never reached 200 degrees, the heater did not keep me warm and the oil temp stayed well below 200. I did try a 180 a couple years ago but forget why I didn't use it and I normally save everything but I'm preparing to move so if it is not nailed down it goes... so the thermostats from 2 years ago are in a land fill... I'm going to stop at NAPA and try a 180 again, one with that jiggle valve. I did only drill one hole and I position it at 12 o clock, the highest point to aid in letting any air out. What I think is happening is the car warms up and the thermostat opens, then the cold water comes in and hits it so the thermostat closes until the water is hot again raising the temp a little until it reopens, if I had more holes it would mitigate that but probably take a lot longer to actually warm up in the first place. The biggest problem is me, training has me scanning an instrument panel often so when I see a gauge that normally runs at the 10 or 11 O clock position move up to towards 12 it jumps out at me... Yes I'm **** some times.. lol
60
If this 2017 BPE motor uses a typical remanufactured GM RPO L31 Vortec block of that era, it will bear a raised Casting Number 10243880. It will also bear a raised "880" in several locations, including on side of block. Unless You are inverted while viewing side of block, that "880" on side will appear upside down. Those who cobble together 880 blocks and Vortec head or Non-Vortec heads, with God knows whose water pump; and even God dunno whose intake; that often results in wonky cooling performance. OE Vortec L31 use a different type T'stat which has a disk valve incorporated at its foot which has a Seat cast & machined into OE L31 lower intake manifold. Also, OE L31 employs a "bypass" system Very much like a Mark IV BBC does. A short bypass 90 degree hose between front of OE lower intake and top of water pump.
