195 is what is required. Anything lower will cause the computer to stay in warm up mode which runs rich and cause lower MPG's among other things.

 

X2

Max

 

What those guys said. The 4.0L likes to run on the warm side.

 

Thanks looks like Ill be exchanging it for the 195.

 

It does affect operation, but it won't throw a CEL on the OBDI equipped YJ models.

However, on the '97 & newer OBDII equipped models (TJ & others) it will cause a CEL code p1281 to pop up, indicating "Engine temp cold for too long" while running a thermostat with a rating of less than 195°F.

To avoid issues, I just recommend the stock 195° version.

Max

 

That sounds like the code for a thermostat that's stuck open. At what temperature does the tj go from open loop to closed loop? I can't believe that its more than 180*....if you have this figure, please let me know. I'm a yj guy, so I could be wrong but would like to know. Usually its not that high, and therefore should not trigger that code unless something else is wrong.

I'll scan mine Thursday night and let you know what temp it changes over to closed loop for the yj.

 

I just found this from another thread..not sure if its the same

http://www.jeepforum.com/forum/f310/obd-code-p1281-1426722/

"Did the engine temperature reach a minimum of 80° C (176° F)?"

With a 180* thermostat, your lowest temp will be 180*, typically running at 185*.

 

185 degrees to go into closed loop. 180 is too low because the tstat operating temp will have it coming into and going out of closed loop during its operation.

 

wow, Im shocked that its that high. Is that just the tj or also the yj? Where did you get those details?

Just found this from an XJ site...not sure if its for the 4.0

"..The pcm will go into closed loop @ about 140* if the o2 sensors are warm enough, which they usually are after about 2 min. This is a fact and can be verified by anyone that knows how to use a good scanner.

@ 160* the pcm doesn't think it needs to be in warm up mode. This is a fact and can be verified by anyone that knows how to use a good scanner...."

 

I work in a Chrysler development lab.

 

That just seems excessively high but Ill take your word for it. Was that for all amc 242's or just >OBDII?

 

Anything running an o2 & computer controlled fuel injection.

 

I shouldn't have. I can dig it up. I think the factory manual from the 87/88 tbi list the closed loop temp. on one of the charts.

 

Nothing wrong with throwing it out there. Sometimes its nice to basically say Because I know this stuff now shut your piehole in a nice way.

Yeah I am still working on those people skills. Somedays are better than others:cheers2:

 

I don't want someone to have a honest question and answer it with just because I say so. I will dig for it. You have a operating range. The hysteresis causes the temp to continue in a direction and overshoot. That means that when the tstat opens at 195 out will continue to heat until 200+ and when it cools it will drop below 190. That is why the tstat is at 195. It keeps the operating range above the closed loop set point. I will find it for you.

 

Thanks Que89yj. The only figures I am able to find are that warmup mode ends (closed loop) at either 160* or 176* (coolant temp, different sites vary)....so to have that data directly from Chrysler would be a bonus

 

Interesting. I looked and I couldnt find the direct refrence to it. I know I have used it but I cant find it for the life of me. The normal resistance on the temp sensor is 1000 ohms but that only tells you the temp is reading the tstat set temp, not the temp it goes into closed loop. The only way to tell for sure would be to hook up a drb to the diagnostic and see when it happens.
Grrrr. A simple question. LOL

 

That's fine, thanks for looking. Ill be scanning on Thursday night. My predictions are that the engine will go into closed loop around 160*. My tstat starts to open at 180*, (and as you described hysteresis will cause the coolant temp to continue to climb) and will be fully open around 190*-195*. I had initially thought it ran around 185*, but I think its probably more like 195*...which is exactly where I want to be.

 

Please post it. I know the newer system warm up is different then the old but no documentation to prove it onw way or another. Like I said, unless I can back it I shouldnt throw it out there as gospel just because of where I work.

 

Here are my results. With the scanner I was using, I could not test for open-closed loop which was disappointing...

So I noticed a few things. First thing, my jeep doesn't run hot at all. It took a little while to even get close to 180*. In the summer I generally run water/water-wetter to reduce engine temps. I guess never really completely "flushed" this mixture out from the summer. I did drain the radiator and add straight coolant. I had assumed I had a good 50/50 mix (and the crappy little coolant bubble tester tells me I am adequately protected for the winter) but I am thinking I must still have some water-wetter in there. I'm getting a rad flush next week and Ill have them put in 70/30. I am hoping this will allow the engine to get a little hotter.

While at idle I was about 185*, but during travel last night (cold and rainy) I was running at 180*. I would really like to find a PCM chart, or details on when the YJ goes into closed loop. Newer cars do have a higer closed loop setting, but I dont think this is true for a '94 OBD I. At this point I can only make an educated guess. Judging by the idle speed, the coolant temp and the fact I don't have a CEL (17* Engine coolant temperature remains below normal operating temperatures during vehicle travel) I am in closed loop.

I did scan the injector duty cycle and ignition adv, but have nothing to compare it to. It would be nice to see what these figures look like on a YJ with a 195* tstat. At least I can relate the engine temp to the placement of the needle on the gauge. The gauge picture shows 185*.

Spark adv= 14*
Injc Pulse= 2.10msec
Coolant temp= 185*
Engine rpm= 704

 

I looked and I couldnt find it. The problem is that even if it goes into closed loop at an earlier temp the calibration is setup to optmize at 195. I wish I could find an answer. The cal is so old that noone that worked on it is left.

 

The problem is that even if it goes into closed loop at an earlier temp the calibration is setup to optmize at 195.

Well, optimized for emissions..which usually means a sacrifice of slight power for better fuel economy. That's why I wish someone with a 195* could scan so I can compare ignition adv and fuel trim. Either way I'm sure its not much of a difference at that point.

 

That is optimized for the 14.7/1.

 

You are correct, stoichiometric, the "golden" AFR for best emissions. But typically maximum power is reached at AFRs ranging from 12.5 to 13.3:1.

 

I wet and got a 195 after reading this man I'm still not sure witch one to run.

 

Keep the 195* ...its oem. I'm still on the fence myself, but that's just because I don't have enough info at my hands. Ill be the guinea pig

 

I an running a 160 thermostat. In the summer the motor runs perfectly at 210. I only realize a difference in the cold winter, then the motor runs slightly cooler around 190. Either a 180 or 195 thermostat should work without any problems. Depending in your climate should determine which one.

 

I am not a cal guy but I know you also have a different cal under load, with different winter/summer blends and ethanol all effecting the O2 in the fuel there is no strait up golden fuel ratio to manimize the power or emissions. The ecu advances the timing as well as part of the calibration. The ratio is suppose to optmize fuel burn to keep emissions down. The 14.7/1 is more a dynamic thing then a fixed thing. If you are worried about a few hp difference and are willing to give up 5 mpg then you can force it into the default calibration by disconnecting the O2 sensor and leaving it in open loop, going to a wide band O2, running a variable resistor to change the air temp or coolant temp sensor readings. You can even ry and change the advance by playing with the map sensor. I know there are tuners around better qualified to answer.

 

This is all true, I did a lot of tuning when I was younger and into the import scene. The engines were OBD I, so they are sort of related... Im not really worried about the HP, I was just interested in the actual electronics and calibrations behind the theory that the 180* tstat is bad.

I used to have a 3800SC riviera that I messed around with. I was able to squeeze out a little more timing/power from a well built CAI (measured on the IAT sensor). I just don't like the blanket statements that X is bad just because. I like to see the actual readings and reasons that prove it....that's where it gets fun for me..I might have OCD

 

Lol ok. I wish I had a good answer.

 

I know I'm late to this, but couldn't resist.
The temp "switch"/sender is located on the cylinder head, at the aft one at that. It is probably the hottest place to take a reading of coolant temp, no? Also, I don't believe that the computer gets it's reading from this location, I'm pretty sure that it gets it's info from the temp "switch/sender" that is located next to the water pump. Maybe not a huge difference in temp, but I would not be surprised if there is a slight change, at least. It seems rather odd that Chrysler would do that as it would seem easy enough to have one switch provide temp information to both the computer and the gage.
To add another dimension to the discussion, I recently changed the gage temp switch and for some reason my temps are running hotter than before. I run a 185 thermostat, and with the old temp sender I noticed a cooler running motor from the stock thermostat. Over time, the original sender broke down and the gage quit working. After changing the switch/sender with TU236 from autozone, my temps are indicating much higher, about 20 degrees. I'm wondering just how accurate these switches/senders are and if they can have that much of an error...or if my thermostat has slowly gone bad?

 

There is a temp difference. If you notice the sensor input needed for the ecu is 5 volts, the gauge its 12. That it's why it's different. The resistance of the sensor determines the reading. The scale is in the book. Check the reading against it to see if it is reading correctly.

 

I found some interesting information from the Perfect EMS CalTool instructions for the YJ.

2. Compensation - Coolant Temperature and Intake Air Temperature
Coolant Temperature - These values are multiplied to the Base Fuel Table values to
richen the mixture when the engine coolant, hence the engine, is cold. Or another
way of looking at it is, it leans out the mixture as the engine warms up. Generally
speaking, the values warmer than 176deg F should be 1.00. This would mean there
is no enrichment due to the engine not being at running temperature.

It appears closed loop operation comes on (when all other conditions are met) with a coolant temp of 176*, which also corresponds with the timing voltage chart showing resistance is different when coolant temps are under 176*, 190*, etc. So I am pretty confident you will be in closed loop with a 180* tstat. Just thought I'd share