Sorry we missed any replies yet, Jagguy. Have you put in new spark plugs anytime recently? it is prudent to replace them every 20k miles or so, with the inexpensive Champion RC12YC plugs. Occasional stalling and/or stumbling is known to be caused by old plugs. If in doubt, change em out!

I'm not sure this would improve mileage, but perhaps it will to a small degree, by making combustion more efficient. 26 mpg on the highway is a good number. Also if you get a scan tool you can read the fuel trim readings which give an idea of the fueling efficiency.

Correct about the coolant temperature sensor. Your new scan tool will display the coolant temperature that the ECU sees from that sensor.

 

The ECU uses the 2 wire sensor and the cockpit temp gauge uses the 1 wire sensor next to it .

the 2 wure is the one that biases the Short Term Fuel Trim when it sees cold engine coolant in the starting phase of the engine run .

A dirty connector gives a high resistance which mimics a low temp on this thermister .

The thermister can be verified against a meter with the graph on page 78 :

http://www.jagrepair.com/images/Trai...20-%202000.pdf

The Iinlet air temp sensor is the same on page 85 and should read the same before coolant temp rise of 1700 ohms at 86 F or 30 C

 

 

Beside using the OBD - II connection port to retrieve the standard recorded failure codes ( free service from some auto parts stores ) you can use a device yourself that goes beyond the check engine light failures and that is the values like the LTFT amd the STFT . These 2 LTFT and STFT values are not sensor values per say hut are values derived by the ECU as a result of the sensor values as a final regulation ( target of 0.0 ) from the O2 sensors .So you have 3 things / failure codes , sensor values , and regulation values .This information requires purchasing a $ 12 US ELM - 327 device and some time to learn how to use it .. In the mean time you can retrieve any CEL codes for a sensor failure . The codes are on page 111 for a 1995 and on page 121 for a 1996 and above on the 801s document linked to above .To see a video on how the ELM - 327 device works see below video .

Without this device with a volt meter some of these sensor values can be obtained and referenced with the information from the Jaguar wiring guide printed your meter table of voltage or resistance values . The LTFT and STFT can not be obtaned in this way

This is the best way I can explain this iand how I understand n laymens terms and corrections welcome

One thing I failed to mention is the difference between a your meter seeing a reading in voltage or resistance vs, a ELM - 327 seeing a value . An example is the coolant sensor in post # 3 . With your meter seeing a physical thing like resistance and the ELM - 327 giving you a temperature value for that sensor . .

.

 

 

The Jaguar CEL codes are on page 111 of the 801s doc linked above .

Notice there is a list fot 95 and 96 - 97 for whatever reason .

the 8 % value us probably the STFT and it changing verifys the ECU shifts to closed loop mode ( more economical )and is a function of your ECT sensor working correctly and hitting a certain coolant temp .

The slow response may be the heater element in the sensors that bring the sensors up to operating temp before the gasses do and is powered by a fuse and the heating elements can be verified with a meter .




Note the X300 uses a different titanium based technology and work differently then normal .

Some world regions use 2 O2 sensors and others like US use 4

The O2 sensors are not used until the closed loop mode is triggered..

The O2 sensor values can be monitored with the ELM - 327

The O2 sensor connectors can get corroded and cleaned and is important that they be placed in the proper position and effects the MPG if wrong .







The O2 wires are shielded carrying sensitive intelligence signals so the shield grounds are important to be cleaned and maintained. on the firewall . Tthe 2 for this are the small multi wire terminals on the left and on the right side of the firewall .

There was a good Youtube video explaining how the O2 sensors work in engine regulation if I can find it .

 

My 95 4.0 has the same faint miss at highway speed. Not a full on miss, just a little noticeable stumble.. Not discernable on my logging tools. O2 sensor voltages are cycling as they should and within ranges that are reasonable. Not enough to make me try to fix it....but it is there. Will pay attention to coolant temps next drive. New plugs last week. Oil on two coils so also did all valve cover gaskets. Slightly improved the situation. Could be a coil I suppose , but not throwing a code.

I do get a cat efficiency code but only infrequently.

107000 miles

I'll take better notes next drive and maybe it will help. I use Torque pro. Lets you set up pages of graphed data so you can see trends in real time. You can also record and down load to review later. https://play.google.com/store/apps/d...orque&hl=en_US

 

Yic in RVA ,

With your Torque Pro you may see your EVAP system cycle at the by design highway rlike egime .

This assuming a % 100 working correctly working ignition system .

By it cycling the STFT may go too low forrotting a adiquit fuel mixture for cylinder fire off .

By the EVAP lines cycling open it may expose a leak like the shell of the charcoal canister seen in some bad environments of the world ( mine is fine )

Just a guess

Al Roethlisberger has a situation where the voltage regulator ( or alternator as a whole assembly ) fails at times after he changed it and this I believe causes the ECU reference voltage to be off until the ECU adjust or the alternator comes back on line as can be seen on his ELM - 327 device . This reference voltage being off puts the many different sensor values out of wack including the reference voltage to the different in how they operate titanium O2 sensors

I'll try to find the graph someone else noticed and marked of his drive log that shows this



You can see how when the blue line voltage goes astray the O2 sensors go astray and bottoms out on what he has stored in his battery, other sensor not on this graph can be doing the same having ill effects on his engine regulation he is feeling as a stumble under his bonnet as the British might say .

Editing