Mr.T

The problem is almost certainly insufficient cooling system efficiency, a new engine that's running good without coolant loss doesn't sound like a heat input problem, it's more likely a heat removal problem.

The ECT temperature was verified accurate by testing with the sensor in hot water with a thermometer. The A/C working great means that the condenser has sufficient air flow, and this (plus checking fan operation and shrouds) makes the odds against this being an air flow problem. You've ran it without a thermostat and that didn't help, a coolant flow problem with a new water pump is very unlikely. If it's not a problem with the ECT indication, air flow or coolant flow, the only thing left is that the efficiency of the radiator is not high enough.

So I'd bet that the radiator is the most likely culprit in this mystery despite being replaced earlier.

I'd avoid using the same brand-model as before in case there's a widespread problem. FWIW I recently replaced mine (due to a cracked tank) with an APDI (Agility) brand from NAPA (NR2957B). It fits 2007-2018 JK's and it's been working fine (same as the original) this summer, even with a low mounted winch up front and towing a 2000 pound camp trailer (not a tear-drop or pop-up) at +4K RPM climbing the grades.

speedoflife

I signed up for the forum to ask you if found a resolution to this issue. I’m chasing a similar problem and have tried replacing many parts. Thank you.

john77

No, I have not figured out the issue yet. I have had another issue come up that is taking priority right now and will address this one down the road.
But if you figure anything I would love to hear about it.

Sixty4x4

Speedoflife - it is usually one part that fails not multiple. Throwing parts at a problem gets expensive.

speedoflife

That makes sense. I don’t want to
hijack his thread. I’ll update you if I figure something out.

Jhai88

New to the forum. I came looking because i have the SAME exact issue as you and did everything you did, almost verbatim. New engine, new mishimoto radiator, thermostat, fan, literally everything.. it should be a brand new jeep, but still heats up to 240 when driving up an incline. Were you able to jump back on this? It's driving me crazy. 2017 Rubicon

john77

I am getting 2 different answers, 1 is that it needs a larger radiator (BS as I have been running for 4 years with the same Mods) and the 2nd is that there is a pin hole leak in one of the head gaskets. I just can't get it to fail a block test. I think this is an issue with these motors. I did take it to Moab and the only issue was a steep grade on the way home to which I pulled behind a semi truck and cruised at 45 MPH and made it home.
Please let me know if you have any luck with this issue.

TerraGod

Oddly enough I've been having this issue over a year at the dealership.
Water pump, thermostat(OEM after a few aftermarket didnt work),radiator (OEM), radiator hoses. Took my winch off thinking it was blocking flow.


I purchased a new engine back in 2020 because mine had multiple failures (hi/lo oil pump, timing chain tensioner, oil filter housing) and the new engine was a little more & no miles. That engine worked fine no problems, until my catalytic converter dislodged internally and caused too much pressure. I ended up purchasing another engine which kept trying to overheat before they found the head bolts were over torqued. Warranty put a new engine in & it's been in the shop for almost a year trying to diagnose it. They're pretty much stomped & don't want to work on it. They told me all the new engines they have put in others vehicles( any with the 3.6) have done the same thing so I'm thinking its a problem with these new engines & they are putting faulty engines out there.
4inch lift with 38s. They tried telling me it's the tires BUT my other engines never had any issues.