Cooling Problem 95 S-10

I'm working on a friends 1995 S-10 4.3 Vortec. Here's a little background. He had the intake and head gaskets done a while back and the guy who did that had a problem with it leaking out of a hole that a bolt threads into. So they ran Blue Devil in it. Solved the leak but then it started to overheat. I looked in the radiator and it was pretty gunky. I guess they misread and left the blue devil in the cooling system. So it's been in their for I would say at least 2 months.

He asked me to replace the thermostat and I did. I emptied the coolant and put in some new. The coolant looked pretty clean. Well now the vehicle doesnt get up to operating temp. It heats up to 150F. I put in a OEM Temp 195 Stat (Duralast brand). The heater blows semi warm air while cruising around town and it heats a little warmer while on the highway. When you then the heat on the engine temp lowers to i would say around 140-145 and if you turn the heat off it reaches to around 150-155.

So im not sure if that thermostat is stuck open or if the problem lays with the heater core being clogged. My take is the heater core is clogged but im just looking for some other opinions on this and what I could do for I dont have to replace this thermostat again!:feedback:

The thermostat is stuck open this usually will give the 155deg f..or the coolant is bypassing the thermostat somehow..installed wrong/size etc..

most low heat problems are pluged heater core..when at the correct operating temp..

normally the use of a thread sealant will stop the leaking bolt problem....

Run the vehicle until its hot. Feel both upper and lower radiator hose if their both hot the thermostat is fine. Feel the two heater hoses if they are hot their is a problem with the blend door. If the hoses are cold pull both hoses and put a water hose on one leave the other one disconnected. You should have a full stream of water if plugged pull the heater core.