Coolant Temp Sensor

I need to replace the CTS on my '96 4.0. I'm chasing crummy MPGs (just like every other thread recently!), and thanks to some resistance values from Alldata, I've determined that it's toast. In addition to the CTS and the O2 sensor, are there any other little items which may keep this engine from attaining closed loop operation? Recent tune-up, air filter, fuel filter, O2 sensor, even a new distributor! This car never got the mileage that my '92 can get, but lately, whoa....
Also, what sort of sealant wants to go on the CTS? Is Teflon tape OK? Thanks for listening.

No sealant, CTS needs to ground on the block.

heyhar said:

In addition to the CTS and the O2 sensor, are there any other little items which may keep this engine from attaining closed loop operation?

Click to expand...

The manifold air pressure (MAP) and air charge temperature (ACT or MAT) are used to analyze pressure and temperature, which cumulatively tell the computer the density of the air in the intake chamber, which is then use to calculate the amount of fuel to install. If either of them are off kilter then the computer will subsequently apply the wrong amount of fuel.

If you can buy an original factory service manual, there is a whole chapter on fuel injection that discusses all of the inputs and provides some diagnostics.

Tim_MN said:

No sealant, CTS needs to ground on the block.

Click to expand...

not even some teflon tape?

nope

the cts is a two wire thermistor-it just changes resistance with tempature; why would it matter if the threads were insulated from the tstat housing by teflon tape/paste? i've always used a dab of teflon paste, never had an issue.

It's the gauge sender that's one-wire and grounds through the engine. The CTS is two-wire and provides its own circuit.

that's what i thought, all the talk of "no tape on the cts threads" just didn't make any sense. now, on a one wire variable sensor, obviously it needs a good ground.

ehall said:

It's the gauge sender that's one-wire and grounds through the engine. The CTS is two-wire and provides its own circuit.

Click to expand...

are wa talkign about the coolant sensor in the thermostat housing or on the head?
the one on the thermo housing can have sealant, but not the one in the head (for the gauge)
the IAT sensor should have no sealant as well?

anyone?
also i was going to clean those sensors
i was going to pull then and spray with electronics cleaner, and then wipe them off, or use a brass wire brush. good idea? or bad?

The pins and sockets are tiny and fragile...None of these things are prohibitively expensive. All the items that can keep you in closed loop operation can be renewed for around $100 or so. About the cost of less than two tanks of gas at today's prices. I know we're never going to get 30MPG, but I at least want to wring out of each gallon the most I can. When I get to the point where the monthly nut on a 40MPG sh!tbox is less than filling the XJs, I'll have some deciding to do...

heyhar said:

I need to replace the CTS on my '96 4.0. I'm chasing crummy MPGs (just like every other thread recently!), and thanks to some resistance values from Alldata, I've determined that it's toast. In addition to the CTS and the O2 sensor, are there any other little items which may keep this engine from attaining closed loop operation? Recent tune-up, air filter, fuel filter, O2 sensor, even a new distributor! This car never got the mileage that my '92 can get, but lately, whoa....
Also, what sort of sealant wants to go on the CTS? Is Teflon tape OK? Thanks for listening.

Click to expand...

96 OBDI ... or 96 OBDII ????

In either ... Closed Loop only occurs in idle or cruise modes.

As the FSM states .... The heated O2 sensors are fitted to enable Closed Loop operation A.S.A.P.

OBDII XJ goes Closed Loop by about the time the coolant temp reaches about 65*f ... a few minutes after startup ... when the heated O2 sensors have reached their operating temp.

There also appears to be a coolant temp requirement of at least 20*f when the vehicle is mobile ... otherwise the trouble code P0125 / code 17 ( Closed Loop Temp not reached/engine too cold ) .... is triggered.

OBDII accepts a coolant temp of 160* as a warm up cycle temp for "trip" definition purposes - according to the FSM ... but thats obviously seperate to Closed Loop operation when a scan gauge shows Closed Loop happening nearly 100*f sooner.

And I cant imagine OBDI systems utilising heated O2 sensors to attain Closed Loop operation quickly ... being much different to the OBDII PCM temperature parameters.

Priority for Closed Loop status would appear to be:
Correctly operating O2 sensors ...
CTS with correct resistance values.

I replaced the O2 sensor last Saturday, even though the four-year-old sensor in there had less than 60k miles, and when tested according to figures found on was close. The Heep ate a tank of gas about as quickly as usual. So, there are other parts to the puzzle. Next up, the Coolant Temp Sensor. While cold, about 50 degrees, the resistance is in the middle of the acceptable range. Drove the car enough to get the temp up to operating, shut off engine, attach clips to the pins in the plug, and instead of dropping, the resistance actually increased! Now I'm stoked. A $20 piece stands between me and gas mileage. It's only been two days, but on a 50 mile round trip today, the gas gauge barely moved. Of course, only time will tell, and I'm running out of stuff to change.

And, yes, I used Teflon tape, as the brass plug in the aluminum water outlet held to the iron block by steel screws was probably generating enough electricity through chemical action of dissimilar metals to power itself anyway.

sweet, can you list those resistance values, or point me in the direction to find them?
thanks

any results?

http://i266.photobucket.com/albums/ii244/AHhub/forum stuff/Resistancetable.png

I found this somewhere around here, and saved it. I'm not certain, but I believe this is for the OBDII sensors.

This is the resistance table from my 91 FSM, it applies to both the coolant temperature sensor and the air charge temperature sensor.

Without plagiarizing anyone's copyrighted data, or spending all night typing, I'll give you some ballpark numbers. At about 50*F (which is where I started), across the two terminals, engine off, you should have a resistance of 17,990-21,810 Ohms. I drove around a while, getting up to operating temp (190-195), and you need to see 860-970 Ohms. Mine actually climbed the other way, indicating that the CTS was bad. It's been replaced, and although I don't have hard numbers to share, I can say that I'm back down to about $10 a day in gas. Given that the commute is 60 miles round trip, and gas as I write this is a little over $3.50 a gallon, I'm looking at better than 20 MPG, more than this chariot has ever given me! And, since it takes a while to come up to full potential any time you change something like this, I'm only hoping for the best. Of course, your results may vary. And, whatever you do, DO NOT remove the CTS plug to test it while the engine is running. You WILL throw a code! Don't ask how I know!! Good luck, and report your findings.....


Edit: Looks like I've been beaten to the punch. I should have answered sooner, but second shift is beating me up....

My CTS shows infinite resistence. Total open... Gonna go get one tomorrow, and then see if i can pass the CA SMOG TEST in Folsom, CA.