Sooo, we fixed my throttle position sensor, and the car ran okay for a bit, but then it went back into limp mode. The VSC gave me error code 14 which indictes the coolant system isn't operating in range. We did a coolant flush as I had spoken to the previous owner and he said he'd never changed it. While that definitely need to be done, it didn't resolve the issue. My service manual says next to test the coolant temp sensor, and then the thermostat.

Being that I don't have an ancient Jaguar computer to read live data, I ran the engine at idle until it warmed up, which didn't take too long, so I imagine the thermostat should be fine.

I haven't tested the resistance on the sensor or the signal voltage yet, but I'm here asking if anybody has experienced this or a similar problem and has any pointers. I'm always open to learn, especially when t's about something I know nothing about.

 

I uploaded a copy of S91_AJ6 4.0 Engine Management.pdf to the internet so it will be on this site somewhere and GUS will have it on his site.

The file is over 13mb so too large to post here.

All you ever wanted to know about the engine management.

bob

 

I've actually already seen this data and flow chart from Alldata at my school. And I have the Hayne's manual. I guess I'm looking for practical advice on working with this system in this car. It seems easy enough to work with, especially compared to my '88 VW Cabriolet, where the thermostat is buried behind the steering pump and a/c compressor, inside the water pump. But if there are any pitfalls to avoid...

 

Hi Ijgabloo,

You can download the document Bob mentioned at the link below. See the data on pdf page 80 and the diagnostic flow chart on page 81:

Jaguar AJ6 4.0L Engine Management System OBD I/Diagnostic Guide Publication S-91

Testing the coolant temperature sensor (CTS) and its electrical harness is pretty simple. Personally, I recommend replacing the CTS because a new one is inexpensive and your old one is no doubt tired even if it seems to still be functioning. Given that the CTS signal is a primary input for the ECM's fueling determinations, it's worth replacing every several years.

It is also worth replacing your coolant thermostat as part of a good tune-up on these engines. If the thermostat sticks open, it will not allow the engine to reach full operating temperature, prompting the ECM to continue to apply cold-start fuel enrichment even after the engine is hot, This leads to rich running and poor fuel economy.

Cheers,

Don

 

I think I'll get a new one too Don, haven't swapped it out since I got the old bus.

Good tip!

Larry

 

So I replaced the CTS this week but to my surprise got a check engine (FF14) almost right away and even had a hard time getting the car started, cough, splutter and rough idle....popped open the hood and replugged the CTS - and got a "click" this time as the plug went home ..it didn't do that the first time

Cleared the code and all went back to normal, nice and smooth again. Went on a test "economy run", about 20k each way, some stop and go city then decent road (but not freeway speeds) avg 60-70 kmh - best economy turned in at 12.2 L/100k before coming back into town, avg now at 12.8 L/100k.

Ok by me, although I was driving very gingerly not roaring about as usual

Larry

 

additional ... several false starts later ...every cold start was a no go ..much cranking to get her going and having to rev her up after she did finally catch ..decided to put the old temp sensor back in -

For a while I thought my fuel pump was on the fritz but weirdly, car ran fine once warmed up ...the new sensor just wasn't telling the ECU to richen things up (put the choke on!) for cold starts

Looks like I bought a defective part ..Will return it.

Larry

 

Well, the wheels of progress turn slowly here, and I've now got the CTS in hand and I'm ready to install it. But It did occur to me while I was at work, that it might be the coolant level sensor is fouled. I looked it up and it costs a good bit more than the temp sensor. I haven't pulled mine out yet to check it, but is it prone to fail on these cars ('92 XJ6 Sovereign)? The sensors I saw pictures of all looked kinda lame.

 

FF14 definitely indicates that the CTS signal is not in operating range, so go ahead and replace yours. It's easy. And while you're there, clean the electrical connector to the coolant temperature sender (the one-wire device mounted next to the CTS on the thermostat housing) and check the wiring harness for both devices for cracked or broken insulation.

Regarding the coolant level sensor, I recommend leaving it alone unless you get a low coolant warning that won't extinguish with the reservoir full. The sensors get brittle with age and just breathing on them can break them.

Cheers,

Don