warm up time

Connected OBDII reader, and monitored radiator flow, temp, and rpm while warming up engine from 50 degree air temp and 50 degree engine temp:

Took a full 7 minutes of fairly linear change to stabilize at 195 degrees and ~720 rpm, at which point the thermostat finally did open and the radiator hose began to warm up a little (heater hose was already warm by then). Water in radiator cap opening never visibly moved at all during the entire test, including after the thermostat had opened, but since radiator hose became warm I am assuming that any coolant movement just wasn't visibly detectable.

The fact that two independent temperature measuring devices (the 195 degree thermostat, AND the Engine Coolant Temperature Sensor) both agree as to when the engine coolant is at 195 degrees, leads me to believe that my engine coolant is, indeed, taking 7 minutes to rise from 50 degrees to 195 degrees.

The radiator and coolant reservoir are both at the proper levels, and the heater hose does heat up, so the thermostat and Engine Coolant Temperature Sensor should be measuring coolant, not air.

And the PCM seems to be doing exactly what it is supposed to with regard to maintaining the idle rpm at the correct rpm level for the Engine Coolant Temperature Sensor reading that it is receiving. At 50 degrees, it starts it out at about 1150 rpm, and then gradually lowers the rpm level to ~720 rpm at 195 degrees.

So I'm still at a loss to explain why my particular 4.0L engine would take much longer to warm up and idle down than anyone else's 4.0L engine. If the problem were the opposite, and my engine was heating up too quickly, THAT I could understand (anything that restricts coolant flow could cause THAT), but in this case the only thing that I can think of that would allow my engine to warm up much slower than others' engines would be if my thermostat were stuck in the open position, which definitely does NOT seem to be the case.

If anyone out there has the time to try the above and let me know how long it takes YOUR 4.0 to reach 195 degrees and ~720 rpm (from ~50 degree starting point), I would appreciate it.

Taking seven minutes to reach full temperature is not unusual but your idle should settle after just a couple seconds.

Wow...interesting. The only explanation I can think of (which seems highly unlikely) is that my PCM's software is programmed to very slowly reduce the idle rpm's in proportion to the temperature rise, and only allow the rpm to reduce below ~800 rpm after ~5 minutes, and then below ~730 rpm after 7 minutes when normal operating temperature of ~195 degrees is achieved and the thermostat opens up. Great for emissions in one way, I suppose, but then again sitting there idling the XJ for 5-7 minutes upon each cold start before driving isn't very emission-free, either.
Are you sure that XJ users aren't just slamming their transmissions into drive at ~1000 rpm, before the rpm needle settles down below that?
This whole thing just doesn't make any sense.

Yes, people drive off on high idle all the time.
Sometimes it makes a bang because of clearances in the driveline.

A couple if easy things since you indicate you are cold.

After the Jeep has sat overnight, with the engine off (always the disclaimer), grab a blade and see if you can spin the fan. You should have light resistance, but if the fan is hard to turn, or won't turn, than can be a problem.
The common symptom is a jet engine sound when taking off.

Also, replace the T-Stat with a new 195F one. It may be stuck open.

Thermostat opens at 195F like it is supposed to, and the other commenter said that 7 minutes to go from 50F to 195F sounds about right, and so maybe all XJ's also take 7 minutes for the idle to reduce to 730 rpm? If so, that is unlike any of the probably hundreds of cars that I have driven in my lifetime...I've never had one wait until the engine is completely 100% at normal operating temperature before finally allowing the idle speed to drop to minimum.
I'm installing a momentary push-button switch on the left dash that will put 1Kohm in parallel with the Engine Coolant Temperature Sensor when pressed, to "trick" the PCM into reducing the rpm to ~730 when the button is pressed, so that I can shift into gear without feeling like I'm abusing the transmission.

At 20° my 4.0L does not take anywhere near 7 minutes to reach 195°, and once in gear, a slightly higher idle speed has no effect upon the transmission. At 20° BELOW zero it does however take about twice as long to reach 195°, but the temp gauge starts to rise within 2-4 minutes. I would say that yours has OBD sensor issues, such as a faulty Idle Air Controller, or O2 sensor issues.

The AW-4 is not the delicate oriental flower you suppose it to be. At 284,000 miles the AW-4 in my 98 has no issues, and my Cherokee has lived almost all its life in Minnesota.

My guess is that the idle speed drops when the O2 sensors heat up and the PCM changes from open loop to closed loop, usually with 1-3 minutes depending on the actual air temperatures, and long before the temp gauge reaches 195° The OBD-II is programmed to bring the O2 sensors on-line for closed loop operation ASAP.

The proper way to warm up in actual cold weather is to start the engine, put the auto transmission in gear, wait about 30-45 seconds, and drive off gently. Avoid full throttle accelerations until the temp gauge at least begins to rise. Excessive idling wastes gas, and risks contaminating the oil with unburned gas.

Tim_MN said:

At 20° my 4.0L does not take anywhere near 7 minutes to reach 195°, and once in gear, the slightly higher idle speed has no effect upon the transmission. At 20° BELOW zero it does however take about twice as long to reach 195°, but the temp gauge starts to rise within 2-4 minutes. I would say that yours has ODB sensor issues, such as a faulty Idle Air Controller, or O2 sensor issues.

The AW-4 is not the delicate oriental flower you suppose it to be. At 284,000 miles the AW-4 in my 98 has no issues, and my Cherokee has lived almost all its life in Minnesota.

My guess is that the idle speed drops when the O2 sensors heat up and the PCM changes from open loop to closed loop, usually with 1-3 minutes depending on the actual air temperatures, and long before the temp gauge reaches 195° The OBD-II is programmed to bring the O2 sensors on-line for closed loop operation ASAP.

The proper way to warm up in actual cold weather is to start the engine, put the auto transmission in gear, wait about 30-45 seconds, and drive off gently. Avoid full throttle accelerations until the temp gauge at least begins to rise. Excessive idling wastes gas, and risks contaminating the oil with unburned gas.

Click to expand...

I will try it again. I was very careful monitoring time/temp/rpm/thermostat status when I measured 7 minutes to 195F/720 rpm/thermostat just opening, but it won't hurt to try it again, based upon your answer.

As far as the O2 sensors, I can say for sure that the PCM determines idle speed based (unless A/C is on, etc.) upon the resistance between pin 2 (SENSOR SIGNAL - Tan with black stripe) and pin 1 (SENSOR GROUND - Brown with yellow stripe) of the ENGINE COOLANT TEMPERATURE SENSOR connector, which is plugged into the ENGINE COOLANT TEMPERATURE SENSOR, which is part of the thermostat housing at the front of the 4.0L engine. I say this because as soon as I put a 1Kohm resistor across those pins, in parallel with the variable resistance of the ENGINE COOLANT TEMPERATURE SENSOR, the rpm within about 2 seconds drops to ~720 rpm even when the engine is stone cold, and if you use higher resistances then it drops proportionately less.
If you unplug the connector then the PCM will record an error code for "temp reading below minimum normal", and if you put on a parallel resistor below about ~500 ohms then the PCM will record an error code for "temp reading above maximum normal".
If the PCM considers O2 inputs at all with regard to idle speeds, which I doubt based upon all of the above, it would seem that the ENGINE COOLANT TEMPERATURE is the far more dominant factor.

Yes, I am somewhat paranoid about transmission failures, I admit that. The only transmission failure that I have ever had was when my 80's 318 V8 powered car's TorqueFlite transmission suddenly at a stoplight completely lost all ability to go in forward direction, and that was because I had forgotten that the transmission was in neutral, then gave it the gas, then slammed it into Drive. The car would only go backwards after that.

Tried again today...drove to trailhead 5 miles away...parked it in the sun at 55F air temp for 4 hours...got back...temp gauge was below 100F...idled it for about 2 minutes...then drove it at about 40 mph for 3 minutes....at that point (5 minutes from start-up) the temp gauge was on the hash mark (~183F) that is just below the 210F hash mark. At 6 minutes the gauge was 1/2 of a hash mark (~196F) below the 210F hash mark.
So it is taking my 1999 XJ ~6 minutes to reach 195F even when during most of the 6 minutes it was traveling at 40 mph, and all this with what appears to be a perfectly functioning 195 degree thermostat (not stuck open).