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

 

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.

 

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.

 

omf, Thanks and yes I have run the heater numerous times to make sure there is no air, also it's been getting cold here. It does not noticeably effect the oscillation or overall temperature when on. Another thing, the system is really clean, I drained it into clear containers and there was no sediment.

​​​​​​​60

 

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.

 

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

 

I have not run the car with no thermostat but the one that just broke stuck in the open position. The car took forever to reach close to 200 degrees, 15 or more miles and there was no oscillation once it got there. I'm pretty sure I have no air, radiator is full when I open the cap, the reservoir works as advertised, heater works. I've read about jacking the car up and burping the air out but I don't get it. I've had a boat load of various SBCs over the years with no issues, fill it up, run it with the heat on, cap off and once the thermostat opens the level will go down some as the air escapes. Fill it again, replace the cap, add some to the reservoir and run it through a couple heat cycles and recheck... Is there something about the Vette that makes them more problematic, not doubting anyone just don't understand...

Thanks for the response,
60

 

try changing the temperature sending unit,when they go bad they do weird things

 

Seems more recent BPE sbc are built on proprietary engine block that's cast to Norris' spec at West German foundry Eisenwerk Bruhl. That company's trademark logo appears as a raised "EB".
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.