LS2 Thermostat/coolant issue

I've installed a used LS2 (2005 GTO) in my 57 2dr hardtop. I installed a new water pump and thermostat prior to installation and initial engine run in the vehicle.
Upon startup, I was hoping to top off the coolant and ensure no leaks, etc. However, the thermostat did not appear to open, the upper and lower hoses stayed cool. I was concerned when the engine temp started to get over 200 and climbing so I shut it down.
I pulled the thermostat and ran the standard thermostat test in a pan of heated water and a thermometer. The thermostat is an 86 C (187 F) model and opened around 190 F and was fully opened around 195 to 200 F.
An inspection of the thermostat and water pump fitup showed that the thermostat bypass flange actually made contact with the water pump bypass opening when I held it up to the water pump housing (I actually had to compress the bypass spring to get the thermostat to fit into position in the water pump), effectively shutting the coolant bypass off (at room temp). I would have expected there to be a clearance between the bypass flange and the bypass opening when coolant temps below thermostat activation temperatures.
I am guessing that this is why I'm not seeing proper thermostat operation. Not only is the bypass closed off on a "cool" engine but could the extra bypass flange spring pressure against the thermostat actuator keep the thermostat from opening at the proper temperature?
I double checked the thermostat part # and also checked the dimensions provided by Rock Auto with my thermostat and it appears to be correct for my engine. That would appear to leave the water pump as the culprit if my assumption, bypass should be open at room temp, is correct.
I'm wondering if I was sold the wrong water pump? Is anyone aware of differences in LS water pumps where the thermostat bypass depths are different for different engines?
I could change out the water pump or find the correct thermostat to go with this water pump if my assumptions are correct.
Hope this all makes sense and really hoping someone may have some information to help me resolve this issue.
Thanks in advance, Mike

I do not know the answer to your question. However, I do know that there are different water pumps and thermostats for LS1, LS2, and LS3, and that the thermostat needs to match the water pump. I got into this a bit when I tried to put a swivel thermostat neck on an LS3. The only one I could find was an Australian one that was actually for an LS2 (for a Holden which the GTO was copied from), though they claimed it was LS3. I solved my problem by machining on the thermostat neck, did not have to touch the LS3 thermostat or water pump.

So your hunch there may be worth pursuing in more detail.

"I was concerned when the engine temp started to get over 200 and climbing so I shut it down."
Modified stock harness?
New temp sensor?
Headers?
How are you measuring this 200* temp?

I read the engine temp off the ECU via a laptop through the OBD.
I also have a Dakota Digital dash setup with a temp sensor located at the top of the radiator, this sensor never registered a temperature above 100 F since the thermostat did not open.
This also concerned me as the radiator fan is controlled by the Dakota Digital system as well as I couldn’t see the high engine coolant temperature on the gauge.
I’m thinking I’ll move the sensor to the passenger side head location. That won’t will help with a stuck closed thermostat, but at least my gauge will read the correct engine temperature to which I can monitor while driving.
I do have the DD module to read the ECU to the dash via the ODB port, but couldn’t get that to work very well so reverted to the redundant sensors.
Anyway, still need to understand my thermostat/water pump issue and find a thermostat which fits this water pump properly as I believe there should be clearance between the thermostat bypass shutoff flange and the water pump bypass port at temps below the thermostat activation temp.

"Anyway, still need to understand my thermostat/water pump issue and find a thermostat which fits this water pump properly as I believe there should be clearance between the thermostat bypass shutoff flange and the water pump bypass port at temps below the thermostat activation temp".
I've put a considerable number of these together. The bypass spring is supposed to be compressed at cold temp.

Attachment 12048

Very interesting! So this really peaks my curiosity. How does the bypass work? Does the flow/pressure created by the water pump cause the thermostat bypass spring to compress and allow coolant to bypass?

So I'm back to the drawing board on why my thermostat didn't appear to open when the engine coolant got hot. It doesn't seem to make sense that I would see temps reaching 210 and above at the head (where the factory temp sensor is located) and yet no flow through the radiator. As I stated previously, the thermostat seemed to open properly at the prescribed temperature for the stat when tested in a heated pan of water.

Anyone have ideas for further testing to find root cause?

U have the heater ports blocked off?

I do not have the heater core in place but I did put hoses on the ports with a 3/8" bypass between them. I did notice the hoses and bypass warming as the engine warmed up.
I also noticed the steam line got hot (it is routed to the highest part of the radiator) and that seemed to be the only source of hot coolant getting to the radiator.
I'm still struggling with the bypass closed in cold coolant and how the bypass is activated. My thermostat has two springs, one on the bypass and one for the main thermostat. There is a fair amount of pressure required to move the bypass. I guess I had understood the bypass is open until the thermostat opens and then modulates with the thermostat.
I ran it again and let the temp (read at the factory sensor location in the head) push close to 220 F and still no coolant flow through the radiator except for the steam line. I did not like running up the temp to that level and do not want to try again until I figure this out. Hope I didn't damage anything.
Could I have an air lock? If so, would the best place to bleed off the air be at the heater ports?
Adding pic's of the thermostat and heater bypass.
Attachment 12050
Attachment 12051

Thanks, Mike

LS engine.