Troubleshoot my troubleshooting RE:poor fuel mileage

I have been trying to improve my poor fuel mileage (12-14 mpg) on my stock 95 Country. I've replaced the cracked header with a non cracked one, replaced the original looking fuel filter with a new one, full tune up with plugs, wires, rotor, cap and air filter, new O2 sensor, throttle body and IAC fully cleaned (carb dipped)

Visually inspected all vacuum lines and Checked for vacuum leaks with an unlit propane torch on low and found none. I'm currently building a smoke tester but the vehicle exhibts no other vacuum leak symptoms other than poor fuel mileage.

Today I tested the coolant temp sensor (in the thermostat housing) and the intake air temp sensor and bounced my readings off of the chart below. Temp readings were taken with an IR heat gun on the thermostat housing and the sensor bodies themselves (so may be off a little one way or another but should be pretty close) and OHM readings were pulled with a digital FLUKE multimeter.

Thermostat housing sensor
212 degrees shows 960 ohms. Based on the chart this is feeding the ECU with lower than actual engine coolant temps.

Intake air temp sensor measured 150 degrees and read 2,300 ohms which again going off the chart shows to be feeding incorrect data to the ECU showing lower than actual intake temps.

I'm going to call the parts department tomorrow since I'd prefer to have Mopar sensors (they are pretty good on parts pricing) and see if 1)they can still get the sensors and 2)how much they cost but before I drop the money on these can anyone pick apart my troubleshooting other than not testing them in a pot of hot water vs using an IR gun while mounted to the Jeep?

212 farenheit 185 ohms
160 farenheit 450 ohms
100 farenheit 1600 ohms
70 farenheit 3400 ohms
40 farenheit 7500 ohms
20 farenheit 13,500 ohms
0 farenheit 25,000 ohms
-40 farenheit 100,700 ohms

Click to expand...

As far as I can find the above values apply to both the coolant temp sensor and the intake air temp sensor


Cross posted on Cherokeeforum and JeepForum

Your chart doesn't match the 1995 FSM. This chart from http://www.fixjeeps.com/coolant-temperature-sensor.html matches what my FSM says.

I couldn't find any chart in my FSM. I may be looking in the wrong chapters. I checked the cooling system chapter and the gauges chapter with no luck.

So going off that my sensors are a little out of range but not by a huge amount. Couple of hundred ohms.

The formatting got all buggered up. Lets try this. (or just go to the link)

Temperature Resistance (Ohms)
C F Min Max
-40 -40 291,490 381,710
-20 -4 85,850 108,390
-10 14 49,250 61,430
0 32 29,330 35,990
10 50 17,990 21,810
20 68 11,370 13,610
25 77 9,120 10,880
30 86 7,370 8,750
40 104 4,900 5,750
50 122 3,330 3,880
60 140 2,310 2,670
70 158 1,630 1,870
80 176 1,170 1,340
90 194 860 970
100 212 640 720
110 230 480 540
120 248 370 410

devildog0 said:

I couldn't find any chart in my FSM. I may be looking in the wrong chapters. I checked the cooling system chapter and the gauges chapter with no luck.

So going off that my sensors are a little out of range but not by a huge amount. Couple of hundred ohms.

Click to expand...

I found it on Page 14-46.

Found it didn't think to look in the fuel system section.

The relay is energized when coolant temperature
is above 103°C (217°F).

Click to expand...

Came across the above quote and I did notice I had no fan activation when the tstat housing and temp sensor was reading over 220 degrees on the IR Thermometer.

So i let everything cool to ambient temps and checked again and at this point the readings are close enough both hot and cold to what the FSM says (different from what I originally posted see pic below) that I'm considering my coolant and temp sensors good to go. Even using the IR heat gun they're within specs or within a few hundred ohms tops.

I have went back to inspecting all the vacuum and evap lines closer. I found a patch I did on the vacuum line going from the fuel regulator to the intake manifold to be very loose and leaking. That has now been fixed. I ran my small propane torch (unlit) over all the junctions and lines I could find and got no response from the engine at any time but I'll still be again visually inspecting all the lines, and try to get my homemade smoke tester done today and run a smoke test tomorrow.

You replaced the header but how is the rest of the exhaust? A partially plugged cat or muffler will kill mpg.

I'm in sort of the same case. 30% worse MPG than I'd expect and a slightly bouncy idle, with a new (Jasper) motor.. I've run through most of the usual engine control components (IAC, O2 sensor). I still have the original exhaust and cat on my 98. Is there any way to know if I have a blockage without taking it to a shop? I don't have any rattles that would suggest a broken cat for example.

asymptonic said:

I'm in sort of the same case. 30% worse MPG than I'd expect and a slightly bouncy idle, with a new (Jasper) motor.. I've run through most of the usual engine control components (IAC, O2 sensor). I still have the original exhaust and cat on my 98. Is there any way to know if I have a blockage without taking it to a shop? I don't have any rattles that would suggest a broken cat for example.

Click to expand...

There is a test you can do with a vacuum gauge, I would have to do a search for the exact procedure. In a nutshell, you put the gauge on the intake manifold to monitor intake vacuum. With the engine running in a steady state, the manifold vacuum should remain steady. If the manifold vacuum drops, the exhaust is suspect.

Edit: Here you go!