Engine Temp

This seems to be a recurring theme as time passes! My XJ runs at about 190-195 99% of the time. AC on combined with Florida heat and stop-and-go might raise the temp up to 200. I've never seen the other side of 200 after rebuild and new cooling system on the road. Offroad, I've seen it get up to about 205-210 flogging it through deep sand or mud.

My FSM doesn't list a normal operating temperture range for the 4.0L. I have to assume that it's the standard 195-220 that most engines use. A lot of people here seem to think that 210 is normal operating temp. Isn't this purely for pollution issues?

I am running on the assumption that as long as the engine isn't stuck in the cold loop function it's plenty warm. I've got no fouling and 20-21 MPG hwy so I'm pretty sure that my engine is warm enough

The reason I ask has to do with what I learned back at school. An engine has 2 basic power settings, best economy and best power.

Best power runs a little richer (less efficient) but runs cooler.

Best economy runs leaner but hotter.

We were instructed to operate at best power ONLY. This saved on wear and increased engine life even though it cost a little more in fuel.

Your opinions?

I like for the engine to run for longevity. This usually means a tad on the cooler side. I have a 180* thermostat and a 3-row radiator, so mine runs fairly cool now.

I pulled a sparkplug yesterday, mainly to see how the O2 sensor was working. The electrode was the prettiest light gray color I've ever seen on a plug. That means that the mixture is good, and it's running perfectly, so I'm going to leave it alone.....life is good.

"I like for the engine to run for longevity. This usually means a tad on the cooler side. "

A tad, yes, but only JUST a tad!

If the engine doesn't get up to the designed operating temperature, then the metal inside doesn't expand as much as the designers intended. The result is that the internal clearances do not get to the point the designers intended. In some cases this may not be an issue, but in other cases it could significantly affect longevity.

That's why a "tad" cool is fine, but anything more than a tad and you're making other trade-offs that you might not be aware of. So, how much is a "tad?" On an engine like the 4.0, which is designed to use a 195 °F thermostat and run right around 200 °F or so, I wouldn't even consider running anything less than a 180 °F thermostat and 185° or 190° would be better if I could find one.

You're exactly right, Dmillion. I wouldn't go below a 180* either. A 160* would be too cold, and might not get the engine into closed loop. That's one reason I pulled a plug and took a look at it. It's been about 10K since I changed the thermostat and plugs, and I wanted to see what the mixture looked like.

Since the plug color looks so good, I'm going to leave the O2 sensor in a while longer. The combustion temps must be about right too, because the plug looked so good. By the way, the plugs are Motorcraft platinums. Anyone else running these?

Reason this out, guys:

The temp sensor for the temp guage is located on the BACK of the cylinder head. That's the HOTTEST spot in the engine, basically.

The OEM thermostat is set to open at 195 degrees. There will be a small temperature differential between the thermostat at the FRONT of the engine, and the sensor at the back.

Thus, we can expect the guage to read slightly higher than 195. Somewhere between 195 and 215 is normal, depending on ambient air temp.

These temps are what the designers WANT TO SEE for the coolant temp. If it's much lower, expect emissions to go up, and fuel economy to go down. Likewise, if it's much higher, expect the Nox emissions to go up, and possible engine damage if sustained.

The key here is that the DESIGN operating temp for the engine management system is about 200 degrees F, give or take a few degrees!!!

I don't know about yours but my temp sensor is in the thermostat housing, my son's 89 4.2L YJ is in the back though.

The newest generation, the ones with OBD-II, use a single temp sensor in the thermostat housing for everything. The Renix models and the Chrysler OBD-I models have a stand-alone sensor in the back of the head for the gauge.

Changing the thermostat temperature will not affect how rich the engine runs, unless you run a 160-degree and it stays in oipen-loop mode. The ECU and O2 sensor set the fuel mix, and they're going to do it the way the engineers designed it, not the way your power mechanics teacher told you it "oughta be."

There is absolutely no reason to run anything other than the 195-degree thermostat the factory designed the engine for.

Here is a new little tid-bit I found out today.There is a factory severe duty t-stat listed in the parts list,180*. Now what is severe duty I can only guess (cops?) but a 180* stat is a factory replacement part.
Wayne

If you do that 'extreme duty' stat better reprogram the computer. As Eagle said, everything about the system is designed for specified temperatures. Had better get used to it. New engines are completely designed to run hotter. Mechanical clearances and materials used in construction have been designed for that.

All this talk about design operating temp... maybe they had to increase the design temp for emmissions? 160 is too cold IMHO but 180 is right in the ball park if you ask me. Usually an engine has an acceptable temperature range. I wish someone could find out what it is for the 4.0.

As for the fuel issue, I was saying that we run our engines cooler at the expense of burning extra fuel. The engines I am talking about burn over 1000 lbs of Jet A per hour and cost about a million bucks a piece. My "engine power teachers" are UND Aerospace and Piedmont Airlines. Both pretty qualified in their fields of engine maintenance and operations.

I,m running a 180* in my 1989 4.0 and it does just fine. The temp at the t-stat housing where I tied in my aftermarket gauge runs 180 minimum to 200. Incidentally, when I had to remove a faulty t-stat that had locked up, I ran without a thermostat a day or so and the engine oddly enough ran hotter at high RPM than with the T-stat in place. I guess the high flow rates without the restriction caused the pump to cavitate or maybe limited the residence time in the radiator-don't know but I got wide temp swings 215+ to 160.

My engine had a lot of crystallized coke deposits and crusty stuff in the valve cover and I'm convinced that it was high temps that it ran prior to the new water pump, t-stat,radiatorshroud,fan clutch, and chemical flush. I don't think 210 and 220 is good for an engine and I think that designers might sacrifice a bit of engine life in order to meet their emissions goals.

My plug color is good, but it turns out my rig had a 180* stat when I got it and I have nothing to compare to in terms of mileage. I get 13 when I run the hell out of it and 18-19 on the highway. 16 is average. My O2 sensor is due for replacement. I have the AW-4, 3.55's and 235/75 tires, stock exhaust and everything, new plugs,wires,dist. cap. 159K miles

I love the sheep on here..... baaaaaa,baaaaaa

If the XJ was designed to run at 210 or 200 or whatever then they would have put a 200 pr 210 thermostat. The fact is that the XJ hates hot temps. It loses power dramatically. I ran the 160 for awhile and I had the same gas mileage and it ran much better cooler=HP. When I put my new motor in, not due to t-stat issues, I put in a 180. I ran that til it froze up 6 months later when I put a 190 in because that's all they had in stock. The difference from the 190 to the 180 is noticeable but not by much. If I had the choice I would run a 180 again. The 160 was a bit cold for the winter months when the heat needed to be cranked to warm the inside up.

As for the tolerances thing if you think that a mere 10 or 20 degrees is going to affect it that much then you need an electric vehicle. Do you think the temps are that precise? Come on give me a break. Think about what the outdoor temps are when you are driving then tell me the whole engine runs at that temp. The advantage is that when it's that much cooler in the engine then it's that much cooler in the air intake so there's that much more power.

Mine runs at 190 until I let it sit then it goes up to 210 before the aux fan kicks in then it goes back down to 190 even in the hottest weather. It has long been known by hotrodders that a cooler t-stat= more HP.

I believe the closed loop comes from the O2 sensor reaching temp not the coolant. So it's the exhaust temps that matter unless it's a heated O2 sensor.

"Do you think the temps are that precise?"

Yes, they are. That's the whole point of the thermostat--so that even when it's only 10 degrees outside the engine will still get up to the designed operating temperature.

FatXJ said:

I believe the closed loop comes from the O2 sensor reaching temp not the coolant. So it's the exhaust temps that matter unless it's a heated O2 sensor.

Click to expand...

I don't know about the new OBD-II models, but I do know that for your Renix model it is the coolanr temperature sensor in the side of the block (not the one in the head) that controls the change from open loop to closed loo operation. It is not the O2 sensor.

FatXJ has some good points. The older XJ's performance falls off dramatically after 200. A 180 can be a better choice if a number of variables are present, such as higher ambient temps, high workloads, towing ect... Also, those with bored motors especially to 60 over should run 180 stats due to cyl wall thinness.

160 stats keep you in closed loop for sure (experience)
great power poor mileage

180 stats keep you in what i think is the best range 180-200
however cold weather and highway speeds will keep the engine on the too-cold side of things.

solution: design a variable/adjustable t-stat!