So after my motor swap in my 94 zj I noticed it never seemed to get up to temp. Ran fine, so I figured it was just how that motor ran. Was riding around in the snow yesterday and got a check engine light. One for low alternator output and one for the engine not getting up to temp. Low alt output is because I need a new belt. What would cause the engine to not heat up? Were talking like its running at 100-120. Motor is a 4.0 that has been slightly bored and has an unknown cam. Also started having a problem where after running on the hwy for 15 mins it starts skipping and bogging down. Only when doing 65+, doesnt do it when riding the backroads. Problems connected?

 

Improper thermostat? Air in the system?

 

Even when I ran my old motor with no tstat (temporarily) it still got up to temp, just slowly. I drove around for 3 hours yesterday in the snow and it never broke 120.

 

The only possible cause is the thermostat. Replace your old one with a quality OEM Mopar unit. If the thermostat is working properly, it's closed until reaching 195 degrees, and then it opens allowing water from the radiator into the engine. If you're never getting to 195 or above, it can only be due to the thermostat.

Sure, it's possible for a motor to get to normal operating temperatures without one, but I'd guess it was when it was warmer out.

 

Exactly! Without a thermo or with one stuck open, the coolant is loosing too much heat from the cold air. Put a good 195* Stant Superstat in it and try it again. The PCM needs 195* to adjust the mixture properly. Without it, you're running rich on a cold engine.

 

Thanks yall. Ill get a tstat and see what happens.