Rob_XJ_OR

Hey,

I bought a nice, unmolested 96 XJ with 10 years of service records a few weeks ago. After about 200 miles, I was averaging about 18.5 MPG mixed city/highway. I hit 23 MPG on a drive that was mostly highway driving, and I was super happy.

After swapping out the dummy gauges, replacing the oil pressure sending unit and the coolent temp sending unit, I all of a sudden am averaging about 9 mpg...

The drop basically 100% coincided with me swapping in the coolent temp sending unit, so I know I did something...

The one thing I can think of is initially I used a non-oem temp sending unit, and it was reading very high, as part of diagnosing it, I unplugged the coolent temp sensor (that is connected to the t-stat housing, read it on some forum somewhere) and tried to start the XJ. It just cranked forever and didn’t start. I plugged it back in and replaced the sending unit with an OEM one and the temp reading are normal.

Do you think I killed the temp sensor? Or maybe I should unplug the battery for 5-10 minutes so the computer can reset? I suppose I should try unplugging the battery to see what happens, but I am sitting at my desk at work, so I wanted to see if anyone else had any suggestions.

Cane

the temps sending unit (rear of the block) only controls the dash gauge, the coolant temp sensor is what sends data to the computer and could maybe affect mileage. so just to make sure, the sensor you're talking about is the one in the rear of the block right?

EZEARL

He can access his OBD and check for codes.

Couldn't hurt to put this up:
https://www.cherokeeforum.com/f2/jee...di-obdii-3341/

Rob_XJ_OR

Ya, I replaced the temp sending unit at the back of the engine, but when I was troubleshooting the high temp reading on the gauge, I unplugged the temp sensor at the front of the engine, so I "messed with" both.

I also pulled codes and got nothing. Thinking about it, I noticed a new whistle-like noise at certain speeds... wondering if I cracked a vacuum line when installing the temp sending unit at the back of the engine bay. That temp sending unit at the back of the engine bay is a pain to get to, so perhaps I was a bit too rough when trying to remove/replace it.

I unplugged the battery and unplugged/replugged the temp sensor in the parking lot at work. Will check vacuum lines tonight or tomorrow. Annoyed as this thing was running like a CHAMP, and I screwed it up.. ha. I suppose I'll find a solve eventually.

SteveMongr

The CTS, coolant temperature sensor located in thermostat housing, sends important signals to computer regarding the pulse width of injectors. When not functioning properly, jeep can run rich and gas can be smelled from exhaust.

When you reset the computer, jeep will 're-learn' parameters according to your driving style. Recommended to drive as you normally would during this learning process.

Rob_XJ_OR

Thanks, I haven't noticed any fuel smells, but I'll stand next to the tailpipe while idling to see if it smells like gas. I pretty much drive it like a grandma, so I'll continue to do that. Also going to check for cracked/broken vacuum lines. Thanks everyone for the suggestions... Hope I can solve this or resetting the battery magically fixed it.

Cane

the coolant temp sensors also grounds itself in the thermostat housing, so make sure there is no thread sealing tape that could affect the ground.

jordan96xj

The coolant temp sensor is used for a ton of engine management decisions on the 96. If it is providing unusual readings, the system may even be remaining in open-loop (which would definitely cause mpg degradation, because the mixture would not be adjusted appropriately).

I don't have a temp chart for the CTS on the 96. But I have one on my phone for renix era XJs. It shows that ohms across the cts pins at 212f (operating temperature) should be down near 185 ohms.

I would take a multimeter set to ohms, warm up the engine fully (210f or above). Shut down the vehicle, and then go pull the connector off of the CTS (in the thermostat housing) and take a reading across the 2 CTS pins. Post the result. It should be relatively low. But someone here probably can provide insight once they know the value you are seeing.