Yet another silly question: are there two sensors for the coolant temp? One for the gauge and one for the injection system? I ask as a, my temp gauge reads very low but this could be down to dodgy gauges which I believe I have, but also b, when it starts from cold, it continues to idle quite fast for ages but if you turn it off and re- start it while the engine is idling fast (about 1100 - 1200 rpm) then the idle is down at 750 straight away.

Also, where are they!?

Sorry for the stupid questions but I really do appreciate your help

 

The coolant sensor for the ECU is on B bank (LHS, UK passenger side) thermostat housing and has a plug on it like an injector plug.
The gauge sensor is on the opposite side thermostat housing and just has a push on Lucar-style plug; neither will affect the idle speed.

The idle speed may be a sticking Auxiliary Air Valve (search AAV for tons of stuff about it on this forum). It may just as likely be sticky/badly adjusted throttle butterflies or throttle linkages or capstan or throttle cable or cruise control cable. Take a good look at all these. please do not worry about asking questions! We all had to start, and we all had loads of help from the knowledgeable.

 

Thank you I am assuming (fatal I know!) that the ECU raises the idle speed upon a cold start? At least, mine does, up to about 1500 rpm on first starting. It does come down, very slowly but only to about 1100. To get it to idle correctly, turn it off and re-start and the idle is a rock steady 750 and no amount of blipping or aggressive driving affects the idle speed once past that first glitch.

When I had all the throttle stuff off to change the dreaded oil pressure switches, I checked and double checked all the linkages/throtle stops etc and everything looked fine and certainly seemed to be operating smoothly.

 

The ECU does not control the idle speed. The AAV does and the throttle butterfly gap, only. When you turn off the engine, something is reducing the amount of air getting into the engine, that before you turned off the engine was not reducing. This can only be a leak, a sticking throttle or an AAV not functioning as it should. This is not, repeat not, an electrical problem, it is caused by an air intake anomaly that is getting changed when you switch on and off. This problem could of course be in the many little pipes and gizmos that are attached to the vacuum capsule system.

 

The coolant sensor on the LH bank may "affect" engine idle as it tells the ECU to enrich the mixture for a cold start and then backs off the enrichment for when it is warm (Well that is my understanding anyway?)

Craig

 

A cold engine likes the richer mixture so the enrichment will change the quality of the idle but it won't increase the idle speed....at least not over and above the base idle settings. That is, if your normal warm idle is (let's say) 750 RPM, the cold enrichment, while making the engine run better, won't increase idle speed above 750 RPM. Only more air, or more ignition advance, will do that, as already mentioned.

Greg mentioned the umpteen little gizmos that control ignition timing. I'd have to review the system....it's a bit Rube Goldberg....to get my head around it as there are a few variations. I only understand it while I'm reading about it. But for your car, as I recall, it includes thermo-vacuum switches and a timer to control a couple electric/vacuum solenoids. So there's some possibility of an electrical issue....but I wouldn't get my hopes up. The system is designed to remove vacuum advance when cold....which would lower the idle speed, not increase it.

Weird problem. Mulling required.

Cheers
DD

 

I always have 'weird' problems, nothing in my life is simple! I was reading about the AAV last night and it sounds possible that mine may be sticking or something? But it seems odd that turning it off and then re-starting the old lass cures the problem and she is then fine for the rest of the day! I have ordered an injection sensor (coolant) and a gauge one today so while I don't think this will cure either issue, it 'may' help and will be another thing off the list. Oh, and a pair of new rad caps as the ones on my car are 33 years old - original, perhaps I should keep them as antiques?

 

Why not have a friend sit in the car and when it is warmed up and running too fast, ask them to switch off while you look carefully at the engine. You might see something move, a finger on the AAV might feel something give, etc etc.
If all else fails, then you may be able to devise a set of tests to isolate what it is. If you post again if you cannot find anything, I am sure the guys can come up with a set of tests to at least isolate the component/system that is at fault.

 

I ordered sensors and rad caps from David Manners yesterday at 4:30 in the afternoon, they arrived at 10:30 this morning! I thought that was pretty good? Changed the caps and they feel rather different when you put them on: the old ones almost turn by themselves (not quite but very loose) whereas the new ones actually require effort to screw down, I think after 33 years, the old ones had 'had it'. I shall do the sensors sometime over the weekend, even went and got a selection of spanners ready... of course, I won't have the one I require!

With regard to the gauges: all the work on the cooling system seems to have achieved 'something'. The temp gauge now moves from the bottom to about a third of the way to the 'N' mark and the idle is rock steady at 700 rpm - I know it should be 750 but that's close enough for now! I checked the actual voltage at the battery with a multi-meter, it was 12.2 before I started it (been messing around with the car with the door open and various starts/stops etc so the battery was a little down) and went to 13.8 as soon as it started. Later on, running it was 14 volts but the gauge never climbed from the top edge of the 'red' mark - off charge. I think all my gauges read low except the oil pressure which reads high! I shall pull the gauges and do a thorough clean of all the earths and feeds etc and see if that makes a difference.

Actually drove it though, it is nice though there is a slight pulse on the brake pedal but other than that it seems great! Steering is light and a bit dead but the engine and gearbox are both smooth as silk. Went and bought new front speaker grilles as somonekicked the drivers off when the car was serviced and the lack was/is annoying me!

Any ideas on the gauges?

 

If you have not already done so, download Kirbert Palm's book, free from Jaglovers, and read it.
http://www.jag-lovers.org/xj-s/book/XJS_help.pdf
You have a Lucas ignition car, by the way. This book has loads of info in it, and it has a very good bit on how to clean up the instrument contacts, which are very indirect and need careful work. This will probably fix your voltmeter, fuel gauge etc. It worked for me, anyway!
Manners very good and helpful guys there, I have always found.

 

You're on the right track.

Flakey gauges are common on these cars. Carefully cleaning all associated contacts and connections usually solves the problem. Adding a redundant ground is a good idea while you're at it.

Cheers
DD

 

To clarify, the high-idle weirdness is gone? And, what work was done to the cooling system other than replacing the caps?

Cheers
DD

 

I changed the thermostats as well. I have a new sensor to fit as well (2 actually) but at the moment, we seem to be winning! It won't last, I've had these cars before I also cleaned out the boot of the stuff the last owner (of 21 years!) gave me - two old side cushions for the roof, two 'fancy' number plates a car cover and 7 books on Jaguar and the XJ-S in particular. So when I am bored, I have plenty to read

Also just took off the drivers door card to change speaker grills (one is damaged after a clumsy mechanic kicked the cover off) and of course, I now need a load of really long really thin screws and nuts but the black 'Kenwood' grills will look a lot better than the fancy Sony Xplod ones on there before. Also gives me a chance to see why the passenger mirror doesn't work. A nice thing to see: no rust inside the door at all! I have sprayed a load of Waxoyl in there especially round the bottom just in case and also, to make it nice and dirty when I put it all back together! It is all these little, fiddly jobs that take so much time but oh so satisfying to do yourself.

 

If the high idle problem is still there, and this is just an idea but something to try, take a look at this thread:
https://www.jaguarforums.com/forum/xjs-x27-32/anti-stall-mod-85-xjs-90175/
The item pictured in the first post is there to provide a faster idle if the aircon compressor is running. Disconnect its wires from their connectors (or remove it and cover the inlet end) and see if that alters anything.

 

Thank you, the link doesn't work but the original post was only a few away from this so even I could find it!

Sounds interesting.possible so I shall have a look if the new sensors have no effect. I am ashamed to admit I haven't done anything to the car for a few days as I decided to be sleep poor but MotoGP and F1 rich over the weekend I should be back to some level of normality within a few days - I hope!