Alright, hopefully my last cooling related question. Thanks again everyone for all the help so far.

'96 ZJ 4.0, as usual.

I replaced my radiator, coolant pump, and thermostat. Flushed out everything while I was at it. Even got one of those annoying 8 mm square engine coolant drain plugs out and flushed a lot of sediment out from there that I couldn't get in my pretty thorough previous flush.

Problem now is that I have very low heater output. Here's what info I have:

• Heater seems to have good initial performance which quickly drops off, which to me indicates poor flow through heater core
• I don't know how to measure this, but coolant flow through radiator seems low as well
• As of just before installing, there were no noticeable restrictions in the block, heater core, hoses, or radiator.
• Upper radiator hose eventually heats up, lower stays cool. It is below freezing out though
• No hose swelling
• Pressure tested coolant system and cap to 18 psi. No leaks.
• Very little pressure buildup by warming up engine, but still holds pressure when I manually apply it
• Pressure drops a few psi while revving the engine
• Seems to be repeated air buildup in the upper radiator hose. Maybe air trapped in the system somewhere?
• No indications of combustion leak
• Temperature stays under 210
• Belt is not slipping, and I have a correct rotation direction pump
• Thermostat is 195° and tested good before installation
• I've driven ~150 miles since all the work was completed

My best guess is air trapped somewhere which is causing low coolant flow rates, but I would expect overheating in that case. I'm lost! Any thoughts?

 

A couple more heating/cooling cycles seems to have done it! Guess there was still some air trapped down there after all. Might try jacking up the front next time I fill it to help prevent that.

 

There is one other thing that might be a problem and I never thought about this until my heater core sprang a leak and had to be replaced. When we took the old core out, half of the fins were clogged with debris, leaves, seeds and other crap, blocking airflow thru the core. Since replacing the core, there's enough heat to make your ***** sweat!

 

Oh ya, I already discovered that when my AC started leaking into the front passenger flow area haha. Pretty much had a small ecosystem in the heater core/AC area!

 

Ha,ha,ha! Yeah, that about explains it! I expected to see bugs!