Will do. I think it is sealed, but it’s just turning in place with medium pressure.

I’ll look into that. The Jag’s my only car with OBD II capability so I’ve been hesitant to invest in something better, but if she’s sticking around with all these computer issues I probably should.

 

My opinion is for the O2 sensors to see them swing or dither the data transmission rate or frame time other than a USB port wire to a laptop / notebook may miss that

And O2 sensors are your area of directly picturing other than seeing a result of fuel trim numbers

See what your longer example HI - way milage is , convert to US miles / US gallons as more examples of 4.0 liter here to get contributor comparisons

The instrument cluster value calculation again makes too many assumptions like ............................

 

Well, after my 16 mile drive into work this morning my MPG has dropped, as expected. It's now at 25 mpg, which is 20.82 US mpg and still 15-20% better than before I changed the CTS and thermostat. The live fuel economy readings were promising though, they seem higher than I remember.

The average has probably dropped due to more stopping and starting on my commute than yesterday's test drive, but a live fuel reading of 28-30 mpg on level ground at constant speed seems better than it was before. More data and more miles will hopefully confirm the improvement.

Checked the CTS when I arrived at work with the engine running and there was no leakage, so that's ok. Not sure why it wasn't threading in or out, but that's not important.

 

Evening all, I drove from work to a fuel station and the onboard computer was showing 22mpg UK by the time I arrived. I filled up with fuel and reset my trip computer, then had a ten mile drive home. Unfortunately, when I got home it was only on 19mpg, which is right around what I was getting before the thermostat and the CTS. So it seems we're back to square one with that, annoyingly. No idea how it managed 28mpg yesterday but is now back down in the high teens.

I suppose next step is replacing the O2 sensors and seeing if that does anything.

Any other potential causes or things for me to check now that the thermostat and CTS have been ruled out?

 

Fueling is trimmed by:
1. TPS
2. CTS
3. O2 sensors
4. MAF...I think you've already sprayed some MAF cleaner on to the sensor?

 

Why are people so hesitant to replace O2 sensors. These things are probably nearly 30 years old. I have replaced them all on my Jags and never had any problems with emissions testing.

Maintenance simply costs money

 

TPS has recently been reset to fix my idle, CTS has been replaced, O2 sensors arrived today and are next on the list, MAF has been cleaned but not tested yet, I’ll do a test of that soon.

I have ‘new’ ones to go in, I’ll do that early next week probably. The reason I have been reluctant to replace them so far is simple; they are simply a bugger to get at!

 

but easier than spending 1000000 hours trying to diagnose something else that will inevitable do nothing as the simple solution is often the best solution.
i did my 02 sensors (here in AUS atleast we only have 2) and it made a good difference to how my car ran

 

I’m now tackling the O2 sensors. How do I remove the bolt which is holding the two wires to the bell housing? It looks like a hex key, but I can’t work out the size and can’t get a good grip. Seems to be between a 6mm and 7mm hex key? The P clip closest to the plugs at the top is also proving stubborn, any tips there?

I’ve managed to get one sensor out of the exhaust and it’s plug undone, so now just need to get it out of the car, which is proving to be a challenge.

The good news is one of the wires near the sensor was split and had wires poking through, so this probably is the problem as suspected. I’ll replace both as they’re likely the same.

This is a very annoying job. I’d like to meet the chap who designed these…

 

Well, the 'new' O2 sensors are in. Eventually I took a leaf out of the jungle warfare book I'm reading and resorted to cutting my way through the sensor wires to pull them through the bolted on P clip thing. I'll drive it now over the next few days and see if my MPG improves at all. Hopefully this has made a difference.

 

MPG after a 12 mile test drive last night and my 16 mile drive to work this morning has risen from the before figure of 22.8 to 23.7 (UK). I didn't reset the trip computer this time to see if it climbed from what was already logged in the system since I last filled up (I reset the trip computer at every fill-up).

I have about half a tank of fuel left, hopefully by the time I need to fill up my average is still higher than normal. Another emissions test on Monday, so next task before then will be looking into the exhaust leaks.

 

After more driving, MPG has come up further to 24.5, which is nice. Interestingly though, my temp gauge is now a bit different. Before the new thermo and CTS it would work its way up to just left of centre, then stay there no matter what. Now, it takes a longer time to get to the top, including stopping at two points along the way.

It will also fluctuate a bit while on the way up to temp, dropping down when coasting and then climbing back up a bit when accelerating up hill, but still below the centre 'normal' position. As I arrived at work this morning it had reached the halfway mark like it used to and seemed to be sitting there like before, but that was after 16 miles and half an hour of driving. Previously it would reach the middle position after only three miles and about 5-10 minutes.

I assume the old thermo was constantly open, letting warm water through quickly which was enough to make the gauge rise to the centre. Now if the thermo is closed longer it will read cool longer, but it seems to be cool for a long time and doesn't suddenly rise to the centre, indicating the thermostat has opened.

 

It sounds like it was constantly closed. It warmed up faster because it was serving less.

Thermostat's job is to warm up the engine faster, not the reverse.

 

I think you have air in the cooling system. Turn the heater on high for a while while driving.

 

was going to say the exact same thing.
if thermo was stuck open car would warm slower.

with the thermostat closed the water (in the engine) heats quicker as it is not fully circulating.
so the engine heats up quicker.
then the thermostat opens.
at which point the average temp of the coolant drops. as the warm liquid from the engine mixes with the cool liquid in the radiator,
you will see the car warm to thermo opening temp, thermo will open and then the temp will drop then begin to rise again.

think of a closed thermo as a closed door.
you have the coolant in two sections.
Engine, gets hot.
Radiator, stays cool.
when the car warms up and the thermo opens it then mixes and the temp equalises.

 

In leu of not being able to read the ECT sensor value on your live data reader there are cheap inferred tempature readers at Harbor Freight that you would aim at the therostat housing

You were getting a meter resistance value of less than 270 ohms ?

Too bad the seperate instrument gauge temp reading is not accurate but a rough gauge

If you pulled the ECT connector do you see a change of state on engine regulation , proves if ECT does put it in closed loop

Have to consider if the thermostat is bad out of stock , common on thermostats as not tested often enough in their production

 

That makes sense. It never overheated though, and the fans very rarely came on so it wasn't outrageously hot.

Good suggestion, I'll give that a try (in the middle of a heatwave, yay!), I'll also try letting the car come up to temp with the fill cap off so I can see if it sucks the level down. I'll add more water if necessary. I did squeeze the top hose and add water until it levelled out at the fill mark with the engine hot and running, but I might've missed a bit. Thinking about it now, I didn't have the heater on full blast when I was topping it up after replacing the thermostat so there might be some air in the heater core.

That sounds like what I'm seeing, so that's a sign all is good. Still seems to be taking a long time to come up to temperature initially though.

Haven't tested the resistance of the new sensor, but pulling it off does make the engine run differently so I think it is working. Could be a bad thermostat I suppose.

 

You can test a thermostat on a string in boiling water as 100 C

There is a calculation to change the boilnig point with added salt mass to water mass to be at 90 C to be spesific ( chemical engineering nerd stuff )

As you come up to 90 C ( just above the 88 C closed loop temp ) before that the thermostat opens in increments vs. temp that someone has done on a V - 12 thermostaet in a PDF I had on an older broke computer

So shouldn't see slam open / slam closed

But........................................by deduction since the engine regulation does change on connector removal the new sensor is reading the correct closed loop coolant temp value so both thermostat and sensor good

The fans in high mode is hard to do since the coolant cooling system / parts is robust / over capacity

The low fans mode comes on at 86 C by the indapendent 3 wire coolant temp switch on the radiator , unless the TSB has been done on the freon 2 wire pressure switch with a jumper on the 2 wire switch connector ( low fans at all times with key in run )

 

Before I left work I topped up my coolant system with the heating on full blast and let my engine come up to temperature with the cap off to keep an eye on the level, there doesn’t seem to be any air left in the system now. I will continue observing my temp and coolant level over the next few days, perhaps I have a small leak somewhere.

 

Unfortunately , the cabin heating system adds extra coolant hose connections but ads instant cabin heat

The cabin heat system as nothing to do with the main engine coolant system thermostat as it pulls coolant off the block by a hose on the rear of the engine block by a pipe down inside a cylinder sleeve

But I respect your thinking