Originally Posted by vital49
The later model 3800s don't have a paper gasket between the t-stat housing and the LIM.
One from an older 3800 will fit, however. It'* not likely that he put a paper gasket in place from an older model. I'm willing to bet he'* missing the rubber o-ring.
My 98 didn't have the paper gasket (drove me nuts trying to find one) and the 2003 doesn't have one. The service manual for 98 showed one IIRC.
I wonder if putting in the paper gasket raises the distance between the top and bottom of the recess for the rubber ring around the thermostat flange and makes it easier for coolant to bypass the gasket and the thermostat. That could cause lower temperature.
I've noticed that I had a Delco thermostat from 2002 start running colder last year than it had been when faced with cold winter air. I replaced with a different brand and new gasket (Stant). When summer came the Stant kept running above 200 so I put a new Delco in and I may ahve used the old gasket on it and kept the new gasket with the Stant in the drawer. This winter the new Delco start running colder (185-180) in cold winds.
I wonder if the gasket I used had aged and harden so that some coolant bypasses it. I put the Stant back in and I'm running 196-198.
Is this something that happens with the rubber O-rings? Or do the Delco thermostat fail and lose control of the temp?