I've done a couple of searches and found some instances of people having bad gas mileage because of a stuck open thermostat. What other specific symptoms are there? If anyone knows of a thread I've missed please steer me in the right direction. My temp needle goes up from cold (below the "100" hash mark) but only hangs out around the next hash mark (maybe 110), not getting up to the optimal 210°F like it used to. So, my best guess is the coolant isn't staying in the block long enough to heat because it's constantly circulating. Right?

 

Ill be watching this too... Sounds like yours stays about where my girlfriends runs?
(pic looks like its warmer than it is cause of angle. Its right on that first line)

 

Does it ever reach 210? if it makes it to 210 and falls back to where you say its at, I would say you are fine cooling-wise. Do you have your e-fan running constantly? My 88 goes from about 170 to 210 all day long. I can tell when the t-stat opens and closes just by watching the gauge, but I have my e-fan running non-stop cuz I'm waiting to wire in the relay for it. It will hurt your gas mileage a little but I really wouldn't worry, unless it never reaches 210. then in that case change the t-stat.

 

I"d say if it never climbs above that mark and you don't have good heat, it's probably stuck open. I've never heard that it would effect fuel economy before just now, though. That's new to me...

 

Cold engine requires more fuel to run, its less efficient. Also, it wont come out of open loop ("warmup") mode and will keep it running rich.

 

It's only a small decrease in fuel mileage. at the very most 1-11/2 mpg.

 

hey man... if your gauge isn't working properly, the first thing to check is the temperature sender.... the 4.0 HO engines have a combination coolant temperature sensor that also feeds the gauge ...

simply ... sensor functions by providing resistance-to-ground which ecu uses to adjust fuel ... ECU reads gauge resistance then adjusts the air/fuel ratio ... when temperature sensor fails (open circuit/no continuity) the high/infinite resistance will cause the ECU to falsely read the max low temperature of -40F ... ECU then tells the injectors to enrich the fuel mixture because it thinks the temp is really cold when it really isn't ... short to ground would cause a 'no-resistance' fault causing the ECU to think the coolant air temperature was extremely hot and lean out the fuel mix.... To test, use a high impedance vom ... resistance should be less than 1k ohms with a WARM engine ... e.g. 212F would be about 185 ohms, 160F would be about 450 ohms, 100F (not warm enough) would be 1,600 ohms.... then go from there ... good luck!

 

The op has a '99 which was changed from '96. The Upstream O2 provides rich/lean info (closed loop) to the ECU well before coolant temp reaches normal range. The system will enter closed loop even if the coolant temp sensor is unplugged.

 

How well does your heater work? If it is not real warm or takes a long time to get warm than it sounds like a thermostat issue to me. If heater gets warm quick and blows hot air I would look at the gauge/sender first.

 

Sounds to me like the t-stat is stuck open. I just went through the same deal, first cold start of the day it would hit 195*, t-stat would open and allow the cold coolant to enter the engine. The t-stat would never close back up once it opended and so it would over-cool and keep the engine around 160*. New Stant Super Stat and she purrs like a kitten at 195* with no over-cooling.

Not sure what model year they changed what but the coolant temp sensor plays a role in open/closed loop. Even after the o2's warm up if the coolant temp drops below 160* it will enter open loop again and run rich. Mine was constating playing the warm up, cool down game and my mileage was around 12mpg. After the t-stat swap Im back at 14mpg which is not bad for the winter time.

 

Yeah... Mine is bad and the gauge swings between >100 and 210 sometimes. You can hear the idle change every time it drops below about 160.
When it happens you can see more condensation come out of the tailpipe too.

 

As long as the heated O2(s) is/are up to temp, the system will remain in closed loop, under normal driving conditions, even if the coolant temp is below 160. When this pic was snapped, coolant temp was 94 and system had already entered closed loop. There's just no reason for the motor to run rich/dirty based upon low coolant temp when it can (and does) run lean/clean within seconds of engine start-up, based upon info received from a heated O2.

If u unplug the sending unit wire, the coolant temp (ScanGauge) defaults to -40, the efan is turned on (safety feature) but the system still enters closed loop the moment the heated O2 "lights off" even though the ECU is receiving a signal that coolant temp is -40.

 

I think that was part of the change for 97+.

I know mine does not work that way. On cold start it relies on ECT and MAP sensor for open loop. Once the o2 warms up it helps control a/f ratio after ECT reads 170*. Below 170* it will default back to ECT and MAP sensor and return to open loop.

This is discussed in my FSM, I will get you a quote once I get home.

For the mean time, http://www.naxja.org/forum/showthread.php?t=1031434


Edit: I saw you already went over this topic on Naxja back in April. No need to argue it out again, I know when she was running cold my mileage was low. Once I got that fixed it went back to normal, that was the only change I made. That tells me it must be running rich with the over-cooling issue. Common sense.

 

Why would it have to run rich? With a proper tune, virtually any vehicle with EFI runs flawless...cold, hot or anywhere in between. The OP indicates his XJ is a '99.....that's the way his operates.

 

Well, last fall I took on the disassembly of the dash and replaced the heater core. I've had great heat ever since then. You could probably cook a hot dog in front of my vents. I don't think it changed since the thermostat might have gotten stuck, but then again last week it was 10 below here for a few days.

No, and I can usually warm it up letting it idle for 15 minutes or so. Here's a pic of where it stays, just as Defiance665's pic shows.



That's good to know, was wondering how the system worked. So different from the Renix system I'm used to. I can't say I've noticed a major difference in gas mileage, but the temperature just climbs from the far left to near that second hash mark.

The thread I found with a note to check a stuck thermostat was here:
https://www.cherokeeforum.com/f2/bad-mpg-99-xj-28434/

I'm pretty sure it's stuck and the coolant can only be marginally heated as it circulates throughout the engine and cold radiator. To be sure the sensor or gauge isn't messed up, I'll use an ohmmeter and IR thermometer tomorrow to measure the exact temp the coolant reaches. If they're in good shape, I might try heating the engine with the airflow of the radiator blocked. If I can get it hot enough and tap on the thermostat housing maybe I can dislodge it. This is just a temporary, curious attempt at a fix until I can get over to my buddy's garage tomorrow night to throw in a new thermostat. At 120K miles, I was thinking of a coolant system overhaul this summer anyway. One of these puppies: http://www.ffdynamics.com/jeepradextb1.html
I'll let you guys know what I find tomorrow, and thanks for your help!