Did a radiator flush on my "new" jeep last weekend because the coolant was opaque and rust colored. Prior to flushing it, the heat worked and running temp sat a little low, around 150. Since we flushed it, it is reason very low temps (maybe 120 max), and the heat no longer works. We allowed the engine to idle for about 10 min after flushing via the lower coolant hose, which we also replaced. We did not squeeze the hoses during this time. Any idea if the problem is a clogged heater core, open thermostat or air in the system, or do you have an easy way for me to diagnose it?
It is a 96 xj with the 4.0 straight 6, if this helps.

 

Pop the thermostat cover and just replace it with a MOPAR one. Might as well replace the upper radiator and heater hoses while you are there.

 

IF it has a thermostat, it's probably stuck wide open (one of those goofy "fail safe" t-stats maybe).

After refilling with coolant, let the motor idle idle with the rad cap off , keeping the rad full while it's idling. Install a new rad cap when coolant/air "burp" out of the rad. Check coolant level next several days motor COLD. Both heater hoses should be very hot to touch.

 

Those fail safe t-stats are a pain for sure.

An old trick for hard to bleed systems is to drill a small hole in the thermostat so the air can burp through. YMMV.

 

I think I'm going to replace the Tstat and radiator cap and backflush the heater core, just to cover all my bases. I have noticed some steam/coolant coming out at my cap.

 

When you flushed the cooling system before did you flush the heater core separately or all at the same time? If all at the same time that could be you problem why you aren't getting heat like you said you did before. Do like you said and flush the heater core in both directions several time. Also like the others have said your t-stat may be stuck in the open position. Let us know what you find and how it's working after you take care of those things.

 

If the motor is only reaching 120-150°f then the thermostat is open. One, along with many reasons engines have thermostats is an engine needs to run at a certain degree to operate correctly and efficiently. At 120° the oil is still thick and cold and won't flow through and lubricate properly

 

Oh and ALWAYS flush the heater core by itself and un connect it if flushing the engine

 

Turns out the previous owner decided that the thermostat was unnecessary. Installed a new T-stat, flushed the heater core and the reservoir hose. I have heat and proper operating temp again!

 

I'd watch it carefully. That happened to me on my old '99 Ranger--heat would go cold suddenly, eventually got a CEL for engine running too cool. here there was no thermostat. Put one in, and viola...blown head and gasket rears its ugly head. The PO took the thermo out to mask the problem.

Luckily it was under warranty!

 

I know that they replaced the head gasket within the last year. Maybe they'd taken the T-Stat out around that time? Idk, I'll keep an eye on it. What should I be looking for?