• Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Temprature & enriched Fueling in OBD1 Jeeps.

R

Rainman

NAXJA Forum User
Location
Sydney, Australia
Hey guys was wondering if someone could help.
THere is a roomer that using 175*F thermostats causes the OBD1 computers to not reach the correct operating temperature and therefor does not go into closed loop mode. Causing it to run rich and give poor fuel economy.
Talking OBD1 computers here, specifically '94&'95
Any Ideas?
 
1. Computer does not reach operating temperature, the engine does.

2. Running a 175 degree thermostat will simply DELAY the engine reaching its correct operating temperature--wasting fuel in the process. Many people believe--WRONGLY--that running a lower temperature thermostat will help with an overheating issue.
 
Do OBD-I XJ's have a heated O2 sensor (3-4 wires) like the OBD-II systems? A scanner may tell you if/when the OBD-I system enters "closed loop".

OBD-II systems, year models '96 & up, (heated O2s) go into "closed loop" within a few seconds of cold engine start up because the O2 sensor quickly reaches "light off temp", provides lean/rich info to the ECU, thus the loop closes. Has virtually nothing to do with coolant temp, again OBD-II. A t-stat controls the minimum coolant temp, not the max. Installed a 180F t-stat in our '98.......runs like a top, mid-teens mpg city, low 20s mpg hwy.

click
 
From 2000 FSM:
"When engine has reached operating temperature,
the PCM will begin monitoring O2S sensor
input. The system will then leave the warm-up mode​

and go into closed loop operation."
 
winterbeater said:
From 2000 FSM:
"When engine has reached operating temperature,
the PCM will begin monitoring O2S sensor
input. The system will then leave the warm-up mode​

and go into closed loop operation."
Click to expand...

......WOW! The ECU is sending the "closed loop" signal to the ScanGauge, on our '98 XJ, well before normal operating temp has been reached, as clearly indicated in the pic.
 
Last edited:
X2 with Winterbeater.

1996 FSM same exact same words --"When engine has reached operating temperature..... goes into closed loop...."
 
winterbeater said:
Can't see much in the pic. What temp is it going to closed loop?
Click to expand...

Left click on the pic. Went into closed loop 38 seconds after cold start (27F ambient, IIRC). Coolant temp had risen to 94F by the time I snapped the pic, which was well after going into closed loop.
 
Last edited:
If you will click on the pic TO ENLARGE IT, you'll see it's dated 12/13/2009. Outside temp was 27F that morning.

How many statements in a FSM get rolled from one year to the next, to the next, to the next, without being properly edited. Stop and think about.......back in the days of carburetors with automatic chokes, the autochoke shut off/disengaged well before the engine was up to normal operating temp. With modern ECUs, multipoint EFI, heated O2 sensor(s), etc., etc., why would the system have to wait 5-10 minutes to enter closed loop to run lean and clean when it can do it (and does) within a few seconds after cold start-up? Again, I'm talking OBD-II, not OBD-I and sorry if I hijacked the thread.
 
Last edited:
joe_peters said:
1. Computer does not reach operating temperature, the engine does.

2. Running a 175 degree thermostat will simply DELAY the engine reaching its correct operating temperature--wasting fuel in the process. Many people believe--WRONGLY--that running a lower temperature thermostat will help with an overheating issue.
Click to expand...
You're certainly right that it won't help if you have an overheating issue, but if the cooling system is good, a 175 degree thermostat should keep the engine at 175 and stop the engine from reaching a higher temperature. Once upon a time, that was a pretty normal operating temp for many cars, and thermostats were set accordingly.
 
Matthew Currie said:
You're certainly right that it won't help if you have an overheating issue, but if the cooling system is good, a 175 degree thermostat should keep the engine at 175 and stop the engine from reaching a higher temperature. Once upon a time, that was a pretty normal operating temp for many cars, and thermostats were set accordingly.
Click to expand...

The t-stat only controls minimum operating temp, not max.
 
djb383 said:
The t-stat only controls minimum operating temp, not max.
Click to expand...
True enough, but it also determines the normal running temp if the cooling system is designed well and working correctly. The operating temperature should cycle around the nominal value of the thermostat. If a wide open thermostat does not bring temperature back down to its setting in normal conditions, the cooling system is inadequate and it will overheat no matter what thermostat is used. If the thermostat did not control running temperature there would be little need for one at all.
 
Matthew Currie said:
True enough, but it also determines the normal running temp if the cooling system is designed well and working correctly. The operating temperature should cycle around the nominal value of the thermostat. If a wide open thermostat does not bring temperature back down to its setting in normal conditions, the cooling system is inadequate and it will overheat no matter what thermostat is used. If the thermostat did not control running temperature there would be little need for one at all.
Click to expand...

The coolant temp will cycle around the t-stat temp during the cool/cold fall/winter months but during the spring/summer warm/hot months, the t-stat will usually be wide open under most hot driving conditions. When the t-stat is open, it's the efficiency of the rad, air flow through the rad, max coolant flow through the block/head/rad, etc., etc., that determine normal running temp. I believe normal operating temp is up to 217F, the temp at which the auxiliary elec fan is engaged.

Most XJ owners say operating temp is 210F with a 195F t-stat. At 210F, a 195F t-stat is wide open. I know this because I've tested a bunch of them over the years. Once the t-stat is open, there's nothing else it can do.....only when it's closed is it doing something and that's controlling minimum operating temp. Max operating temp is controlled by the rest of the cooling system and driving conditions.
 
The thermostat adjusts the system temp by changing the amount of opening (flow rate). As the coolant temperature passing by tries to go up or down, the thermostat opens or closes to maintain a constant temp as long as there is system cooling capacity to control it. Too cold = 'stat is closed, too hot = 'stat is opened max, and the system goes into overheat. Within range the thermostat holds temperature very close to constant.

:cheers:
 
Fuel enrichment during warm-up is primarily affected by the coolant sensor input to the ECM. An old hot-rod trick is to put a fixed resistor in place of the temp sensor to fool the ECM into believing the engine has not reached operating temp, and therefore will continue to enrich-en the fuel mixture--good for WOT bashes, but causes engine to run too rich in between "bashes".
Three things are required for "closed loop" operation. Time from start (timer internal to ECM), minimum engine temp (as sensed using the coolant sensor input), and a fluctuating signal from the O2 sensor (ECM determines that the signal is varying above/below .450 vdc).
This scheme is basically the same for Renix, OBD-I, and OBD-II systems.
The ECM is designed (programmed) to assume the engine is running at 190*f; running at a cooler temp will result in engine efficiencies, as determined by the designer.
 
Last edited:
............and that's why designers have changed from using coolant temp sensors to using heated O2 sensors for quite some time now.......to get the engine running lean and clean within a few seconds of cold start-up. Like I posted previously, OBD-II vehicles including the XJ enter "closed loop" within a few seconds of cold engine start-up because the O2 sensor has reached it's operating temp and is providing the necessary rich/lean air/fuel mix info to the ECU, thus the loop closes. In the pic I posted earlier in this thread, it can clearly be seen on the ScanGauge (click the pic to enlarge) that the system has entered closed loop and coolant temp is well below operating temp.

I've used the ScanGauge on numerous OBD-II vehicles with the same result.......closed loop is entered within a few seconds of cold engine start-up - well before coolant temp reaches normal. Again, why would an engine have to run rich and dirty for 5-10 minutes after start-up when it can (and does) run lean and clean within a few seconds after start-up???????????
 
Last edited:
djb383 said:
............and that's why designers have changed from using coolant temp sensors to using heated O2 sensors for quite some time now.......to get the engine running lean and clean within a few seconds of cold start-up. Like I posted previously, OBD-II vehicles including the XJ enter "closed loop" within a few seconds of cold engine start-up because the O2 sensor has reached it's operating temp and is providing the necessary rich/lean air/fuel mix info to the ECU, thus the loop closes. In the pic I posted earlier in this thread, it can clearly be seen on the ScanGauge (click the pic to enlarge) that the system has entered closed loop and coolant temp is well below operating temp.

I've used the ScanGauge on numerous OBD-II vehicles with the same result.......closed loop is entered within a few seconds of cold engine start-up - well before coolant temp reaches normal. Again, why would an engine have to run rich and dirty for 5-10 minutes after start-up when it can (and does) run lean and clean within a few seconds after start-up???????????
Click to expand...

There are eight different modes of operation. There are two different types of operation: open loop and closed loop.

1. Mode One: ignition switch on--open loop.
2. Mode Two: engine start-up (crank)--open loop.
3. Mode Three: engine warm-up--open loop.
4. Mode Four: engine idle (at normal operating temperature)--closed loop.
5. Mode Five: cruise (at normal operating temperature)--closed loop.
6. Mode Six: deceleration--open loop.
7. Mode Seven: WOT--open loop.
8. Mode Eight: ignition switch off--open loop.

Note that of eight modes the ECU is only in closed loop for two of them: idle and cruise, and ONLY when the engine is at normal operating temperature.
 
You must log in or register to reply here.

Similar threads

2
1984 - 2001 specs
Replies
4
Views
2K
2001JeepXJNS
2
Ecomike
Catalytic converter size and MPG, Miles per gallon
2
Replies
20
Views
2K
89Daytona
8
sidriptide
COY HILL in Ma is CLOSED!!!
Replies
0
Views
605
sidriptide
sidriptide
sidriptide
COY HILL in Ma is CLOSED!!!
2
Replies
39
Views
4K
XJ Beater
X
goodburbon
code OBDII OBD check engine
2 3
Replies
40
Views
21K
Hylander
H
Back
Top