I've just picked up a XJ-S convertible v12 when I went to view it the car was running and had been for a while as the temperature was normal. Now I've got the car home it is almost impossible to start from cold. Sometimes it will catch and tick over but if you touch the throttle it dies. If I eventually get it to catch and let it warm up it is almost running as normal. I'm new to this engine any ideas where to start
thanks

 

My first thought is the coolant temp sender. That's the biggest influence of fuelling on a V12. It's on the left side thermostat housing, near the front of the engine. Make sure the plug is seated and making good contact, sometimes the contacts can get pushed back into the connector housing and not make contact with the sender.

The senders are reverse resistance, they should be about 3000 ohms at 20C, dropping to about 200 ohms at 90C. If you unplug the wire it tells the computer the engine is full cold, see if that makes starting from cold any easier.

 

Unplugging the CTS delivers infinite resistance and the ecu will enrich. Jumping the CTS contacts in its loom connector will tell the ECU that the engine is hot and the ECU will not enrich the mixture. This may also resolve the issue. What year XJS? Do you have cold start injectors?

 

Firstly welcome to the forums. Don't forget to visit the new member area and introduce yourself.

Before we can assist you we need to know more details about your car. It's a good idea to add this to your signature.

Model year
And if its 1989 is it Lucas or Marelli ignition, this can be identified by the following
Lucas - has a module on the LH inlet manifold
Marelli - has 2 orange modules on the radiator top support.

Lucas or Marelli is important as the reason your car dies could be different with the 2 ignition systems.

 

Agreed, good plan

On a pre-facelift, at least, if you unplug the CTS the engine will NEVER start.
Greg

 

I had a Series III V12 saloon start with the CTS unplugged. Mind you, it was about -35C at the time...

 

One should always be securely within the igloo at these temps. And the Jag should be wintering in Florida.

turnermr,

If it will run decently once started you can eliminate a lot of possibilities. Spark, compression and fuel OK! So it's the cold start condition itself that's the issue. How cold is it?

This will be immensely informative. XJS Engine Performance manual

I'm betting on throttle adjustment and TPS settings being a bit off with a bad CTS.

 

hi thanks for the responses. The car is 1988 and it has a Lucas module bolted to the L/H inlet manifold so I'm presuming its a Lucas system. I didn't get much time to today but unplugging the CTS made no difference in fact, apart from the odd pot, it wouldn't fire up at all. All the connectors on appear to be ok .The temperature here is 16 so nothing extreme . Any ideas thanks

 

Silly question, ATS and CTS plugs not mixed up are they?

 

yea they seem to be fitted correctly

 

quick update , I've looped the pins on the CTS cable and it fire up ok although a bit lumpy to start. I've ordered a new CTS so will see how it goes thanks

 

When it starts, what is the cold idle rev reading.

I am thinking the AAV may be in need of some TLC.

The V12 get all gooey around the throttle discs. It is "normal", and it reeks havoc with idle, mainly hot when the AAV is closed, BUT, if your AAV is jammed shut (most are), then the engine is running on the bleed air at the discs, and that goo will reduce that bleed to almost zero. Solvent soaked rag, wipe is out until they be clean, repeat annually.

TPS out of range for idle fuel map/s, and going flaky in the "just off idle" section of the wiper area inside it, will cause what you got.

Inside the distributor cap, dead centre, is a carbon contact on a spring. They wear away, some burn up, some just disappear (aliens in that one), and that has an awful effect on the starting ability, cold mainly. I've had 3 with that sucker gone, one was mine, and it took ages to find.

 

i've replaced the CTS and and I've now got it running although a but lumpy when cold. Grant has suggested I look at the AAV , where do I find this?
thanks

 

Left side water rail, at the back of the engine. There will be a hose coming from the back of the left side air cleaner housing to it, and another hose going to the crossover pipe at the back of the engine.

There is a bolt on the side of it, that's the air bypass adjustment, use that to adjust idle speed. The AAV itself is an alloy housing.

 

Here ya go.

AAV rebuild procedure.doc

Enjoy.

 

Have you cleaned the throttle discs yet???????, if not DO SO.

 

I've cleaned the throttle discs and but this doesn't seem to have made much difference. After changing the CTS it now starts but the tick over is erratic when cold sitting between 200 - 800 if you touch the throttle sometime it will just die. when you go to start again 50% of the time it wont catch until I put the throttle to the bottom it then sits at 800 press the throttle and it will either die or will increase but a bit lumpy. When the temperature gauge moves approx 1/4 it is ok revs ok and starts ok. I've stuck my thumb in the AAV pipe in the air filter housing (when warm and running ok) there is plenty of suction and the revs hardly change when I block it off. At this stage i'm assuming it's not the AAV at fault? I don't think its the electrics as its ok when warm. Is there anywhere else I should be looking? or should I take the AAV apart?

 

Blocking that pipe with your thumb when warmed up should drop the revs a lot, almost to a stall out.

Sounds like a vac leak, and a large one.

On top of that AAV is a 90deg elbow hose, THEY SPLIT, and that will leak huge amounts of vac.

The 1" balance pipe has a hose connector at each end, THEY SPLIT, and also leak.

The brake booster has a diaphragm inside and THEY SPLIT and leak vac, soooooo disconnect the vac hose TO that booster, cap the spigot on the RH imlet manifold, try again. DO NOT DRIVE WITHOUT THAT BOOSTER RE-CONNECTED.

The 2 inlet manifold gaskets "settle out" and leak vac, so an 13mm spanner and tighten the 24 nuts, NOT stupid tight, just firm them up.

Rough running can also be the spark plugs fouled from all the "trying to start", and sometimes they clear themselves, mostly, they do not.

The fickle throttle is also mostly TPS related. The attached spells it out, but if its "clear as mud", ask, and I will assist some more.

Adjusting the HE TPS.doc

There is more, but that will keep you occupied for an hour or 3.

 

Its seems to settle down once it has some temperature and revs ok , is it still worth looking at the TPS? No obvious vac leaks so far but I'm still looking

 

update
I can't find any obvious vac leaks. Grant, I'm trying to check the brake booster as suggested and I cant find the spigot on the R/H manifold , there is a pipe that goes from the brake booster to what looks like the bottom of the master cylinder and that's it ?
all the other pipes look ok , I've tightened all the manifold nuts they all took an easy 1/4 turn
I've also checked the TPS this was set at .27 on the red wire and 4.9 on the green I've managed (after some time) to get it to .34 and this has made a real difference to smooth running , very noticeable.


However I've still got the problem with cold start. I've done some more testing from cold it will fire up sit a 200 and slowly increase to 800-900 if you touch the throttle before it gets to 800 it usually just dies and to restart you have to put your foot on the floor to get it to fire up again. If I leave it sitting at 800-900 until it warms up everything seems fine especially after the TPS adjustment


I did another check on the AAV and its only dropping 200 with my thumb in the hole


Do you think I am still looking at a vac leak?
many thanks