Desarial

I normally run my jeep with a 195 thermostat in it, this keeps it between 190 and 200 most of the time. but i know a lot of people like to run hotter or cooler. I read in the FSM that if the engine doesn't get to 160 or above the ECU will run in a closed loop, dumping fuel and ruining gas mileage. so, with that obvious boundary set, what is good for your engine, a little cooler(170-190) or a little hotter(200-210). I see a lot of people saying that if it's not near 210 the emissions will get worse, but I happen to remember that the reason they included the egr in early models was to prevent detonation on the cylinder walls. detonation being cause by a hot cylinder from my understanding. input, debates, questions? I feel like this will end up like a back pressure thread.

djb383

All EFI motors run in open loop 'til the O2 heats up and closes the loop. I not so sure about "dumping" fuel....why would it dump fuel?......my experience with a properly tuned computer controlled EFI motor is.....they run/perform flawlessly from a freezing cold winter start and run even better once the O2 sensor(s) "light off/get hot". I'm not sure about OBD-I but on OBD-II, if u don't get a CEL light/code for "too cold coolant temp too long" all is good.

Coolant temps and combustion chamber temps are 2 totally different beasts and a couple thousand degrees apart from each other. If one is running at least a 180 stat, all cooling system components are functioning properly and the motor is properly maintained/tuned, again, all should be good.

hankthetank

The ecu doesn't get input from just the o2 sensor(s)...the coolant temp sensor determines when the motor is at or near operating temp, thereby switching loop function. O2 sensors are heated so they give accurate readings quicker.

hankthetank

...not quite.

djb383

Ok, virtually all OBD-II motors and most OBD-I motors run in open loop 'til the O2 lights off and virtually all OBD-II motors enter closed loop way before so called operating temp is reached. Some heated O2s light off in as little as 10 seconds after cold start and close the loop. Plug a scan tool in and watch the loop close very quickly after cold start.

Desarial

ok, so I got the open/closed loop terms backwards. either way, it does cause the engine to dump fuel till it's up to temperature, this is an attempt to get it up to temp. and the open/closed choice is made by the temp sensors. the o2 sensors just work better when warm, the same can be said of every piece of electronics. not "hot" but warm. still, is running the jeep nearer to 160 a good idea? nobody has really touched base on the actual questions here. also, my opinions are based off of the Renix system. I'm not sure weather or not o2 sensors are functional exhaust gas sensors or not.

cruiser54

They never "dump" fuel. They just run off a pre-programmed fuel map.
Renix O2 sensors kick in quite quickly BTW.

On cold start the CTS, IAT, and of course the MAP are major players.

Only use a 83501426 stat from the dealer if you want good stable temps from your 4.0.

djb383

Running rich causes a motor to stay cool longer, lean makes it heat up quicker. What info do u have that "it dumps fuel"? The slowest process during cold engine start-up is coolant warm-up but why would the motor have to run rich and dirty for +/-10 min, while coolant warms up, when it can run lean and clean and flawless while coolant is warming up. An O2 needs to be about 650F to work and when they're electrically heated, they start to work with-in seconds of cold start.

The "loop" consists of:

1. The ECU
2. The motor and it's sensors
3. The O2 sensor(s)

Look at it like a triangle.....in open loop the motor sensors are signaling back and forth ECU/motor, motor/ECU and the motor runs on pre-programmed parameters (a/f ratios, etc). As soon as the O2 reaches +/-650F, it starts signaling the ECU, thus closing the loop. Does the motor need some heat to run most efficiently?......yes. The engineers determined/set the MINIMUM efficient heat at 160F.

cruiser54

One of the big benefits of fuel injection is that it DOESN'T dump fuel like carbs with chokes used to. That's also the reason fuel injected engines live so much longer than carbed engines. No washing of cylinders on start up.

djb383

^^^^^^^+1000.

Desarial

maybe I'm mincing info with a carb'd system. Still, I'm sure in the 88 FSM it clarifies that the change in loop is determined by the temp sensor, not the O2 sensor. I'm working on finding out where it says that now.

djb383

In a "closed loop engine management system", the coolant temp sensor is working/sending info to the ECU the moment the key is turned on.....the coolant temp sensor is in the "loop", just like the MAP/MAF sensor, IAT sensor, IAC sensor, etc. They're all in the loop (the open loop) including the O2 sensor BUT, the O2 sensor relies upon heat, about 650F, before it start working. When the last sensor (the O2 sensor) starts working, the loop "closes". There are a number of operating conditions, even on a fully warmed up motor, that will cause the "loop" to switch back to open from closed.....1 condition is WOT.....WOT is a "open loop" operating condition.....even if the motor is warmed up.

cruiser54

Absolutely correct and well stated.

DFlintstone

So your good Desarial, you just got your loops backwards is all! Any functioning thermostat will be fully open before it overheats. I think you get better atomization and economy there about the designed running temp is why I run a 195.

Exhaust gas Recirculation keeps combustion temps lower, more out of the range where Nitrogen Oxide's form. Guess that would help with pre-detonation as well, use to just move the timing for that!

If you take Cruisers advice and go with the OEM T-stat, you will never have to wonder later if the thing is working like it should or not.