91 chevy 350 2500 4x4. 3 months ago thought thermastat was sticking because temp gauge would read 190 but then drop to 150?, then heat back up. fluid level was full. I replaced the thermastat, which did the same thing. May have gottin bad one so I replaced thermastat again( different brand ). now temp gauge only reads 150-160 and no more. fluid level still ok. Heater is just warm. Hoses to intake and heater core(supply & return) are just warm to the touch but not hot.

Hi,
Thermostats come in temperature ranges. Did you buy a 160?
You need a 180 or possibly 190.
Bob B

1st one was rated 190, 2nd one was rated 195.
Both did same thing.(Yeap spring side is down. lol)