TJ 4.0 running warm at Idle with A/C on

ShredAZ · Jul 29, 2023

I live in Phoenix, AZ. It can get 115+ here in the summer. This past summer, I noticed my '03 Rubicon started running warm at idle if it sat for 10-15 minutes with the AC near full blast.

On a typical 115+ day, the Jeep will run consistently at 208-214 as measured from the OBD port while moving or even temporarily at idle. If I let it sit at idle for 10-15 minutes, the engine will gradually build heat until it pegs out at 225 (generally doesn't get hotter than that). If I kill the AC, the temps will drop back down to 212-214 after 5 minutes or so.

Few things to note, in the past 1-4 years, I've gradually replaced the water pump, t-stat, fan clutch, and radiator all with MOPAR parts. I'm running a 60/40 water/coolant blend (since we don't drop below freezing here). I put less than 3k miles on my Jeep a year, so these components are nearly brand new.

Around the same time I started to notice the warm idle, I also found the air wasn't blowing as cold at idle and struggles to get ice cold unless set to recirc.

I'm wondering if I need my system charged. Possible compressor overload? Any other ideas before I start tearing into it?

Jerry Bransford · Jul 29, 2023

Aftermarket radiator?

ShredAZ · Jul 29, 2023

Jerry Bransford said:
Aftermarket radiator?

All MOPAR. If there was one thing I've learned from the forum, it's to only replace cooling components with MOPAR hah!

Jerry Bransford · Jul 29, 2023

ShredAZ said:
All MOPAR. If there was one thing I've learned from the forum, it's to only replace cooling components with MOPAR hah!

Maybe it just needs flushing or perhaps its exterior cooling fins are covered with dirt or mud?

ShredAZ · Jul 29, 2023

Jerry Bransford said:
Maybe it just needs flushing or perhaps its exterior cooling fins are covered with dirt or mud?

I should probably do that. I haven't performed a true flush with the garden hose yet either.

The other thought I had...is it possible it's a failing coolant temp sensor?

Daryl · Jul 29, 2023

Jerry Bransford said:
perhaps its exterior cooling fins are covered with dirt or mud?

Take a look between the radiator and a/c condenser for debris buildup.

Jerry Bransford · Jul 29, 2023

ShredAZ said:
I should probably do that. I haven't performed a true flush with the garden hose yet either.

The other thought I had...is it possible it's a failing coolant temp sensor?

I'd suspect a lot of things before a failing coolant temperature sensor.

ShredAZ · Jul 29, 2023

Jerry Bransford said:
I'd suspect a lot of things before a failing coolant temperature sensor.

I recall you live in a similar environment as me. What should I expect to see from the OBD readings on the high side? I know 205-215 is considered normal. Under extreme conditions, what should I expect to see?

Jerry Bransford · Jul 29, 2023

ShredAZ said:
I recall you live in a similar environment as me. What should I expect to see from the OBD readings on the high side? I know 205-215 is considered normal. Under extreme conditions, what should I expect to see?

I pay no attention to OBD temperature gauges, I trust the TJ's temperature gauge 100% to tell me everything I need to know and it hasn't failed me yet.

KennethS · Jul 29, 2023

This summer has been brutal. I have a thermometer in the AC center vent to monitor the AC performance. I noticed when it's 110+ out and I'm idling in traffic the vent temp goes up nearly 10 degrees. When I lightly press the throttle to bring the engine RPM up from 700 or so to 1100, the vent temp drops back. This is due to the increased air flow over the condenser from the engine cooling fan and the AC compressor running faster. I do this on my other manual transmission vehicle as well and it really helps keep the AC blowing colder.

R-134A systems just don't cool as efficiently at idle as the older R-12 systems.

ShredAZ · Jul 29, 2023

KennethS said:
This summer has been brutal. I have a thermometer in the AC center vent to monitor the AC performance. I noticed when it's 110+ out and I'm idling in traffic the vent temp goes up nearly 10 degrees. When I lightly press the throttle to bring the engine RPM up from 700 or so to 1100, the vent temp drops back. This is due to the increased air flow over the condenser from the engine cooling fan and the AC compressor running faster. I do this on my other manual transmission vehicle as well and it really helps keep the AC blowing colder.

R-134A systems just don't cool as efficiently at idle as the older R-12 systems.

Nice to know I'm not the only one dying over here

I notice the EXACT same behavior. If I hold RPM at like 2k in neutral, AC blows colder and the engine temps returns to 205-210.

Have you noticed your engine temps getting warmer at idle?

I took it up to Flag this weekend, and it ran like a top. No warm idle, etc. My rig isn't overheating. Just running warm at idle in 115 with the AC cranked. Maybe my OCD is kicking in too much...

I might just chalk this up to the unnecessarily hot summer we're having if a coolant flush and refrigerant balance doesn't set me right.

KennethS · Jul 29, 2023

I haven’t seen the engine temp go up while idling in traffic.

ShredAZ · Jul 29, 2023

Ran some testing tonight. Sat in the driveway for 30 min listening to a podcast while the Jeep idled with AC set to max (recirc). It was 106* outside.

The clock started after reaching operating temp (210):

5:00 - 212*
10:00 - 219*
15:00 - 223*
20:00 - 228*
25:00 - 230*
30:00 - 232*

Dropping the AC blower to the lowest setting and holding RPMs at 1500 brought the coolant temp back to 210* in ~1 minute.

And here's what the temp gauge looked like at 232* (as read from OBD):

@Jerry Bransford, @KennethS - is this in line with what you see your gauge at its hottest? Am I over-analyzing this?

TexasTJ2004 · Jul 29, 2023

I am getting even worse:
Had to idle with AC for almost an hour after another hour of driving.
Transmission at 188F, Intake manifold at 237F and engine at 231F.
But, I have an autozone radiator and Hayden 2771 clutch that I am going to replace with Hayden 2791 and Mopar radiator.

KennethS · Jul 30, 2023

I think you should focus on what ShredAZ said about increasing the engine speed bringing the temperature down rapidly. You said before you don’t overheat at speed, while crawling, etc, only after extended idling, so it seems to me your radiator is doing its job. Maybe I’m wrong, but if your new radiator doesn’t change anything then try increasing the engine speed while at idle.

Jezza · Jul 30, 2023

Do you have an infrared thermometer? I'm curious what the radiator temp is when the obd shows 230.

ColoJeep · Jul 30, 2023

Jezza said:
Do you have an infrared thermometer? I'm curious what the radiator temp is when the obd shows 230.

And the right side of the engine block and head. Now I am curious if the actual temp is the same as what OBD is reading.

ShredAZ · Jul 30, 2023

Jezza said:
Do you have an infrared thermometer? I'm curious what the radiator temp is when the obd shows 230.

ColoJeep said:
And the right side of the engine block and head. Now I am curious if the actual temp is the same as what OBD is reading.

I don't! I really should get one. I'll snag one off of Amazon and report back this week.

Jezza · Jul 30, 2023

ShredAZ said:
I don't! I really should get one. I'll snag one off of Amazon and report back this week.

Harbor Freight has one for 25 bucks as well. Plenty good enough for the few times it gets used.

CreepinDeth · Jul 30, 2023

Flush the system

I wouldnt leave a 20 year old vehicle idling in that kinda heat that long personally

TJ 4.0 running warm at Idle with A/C on

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