I have a 93 Jaguar XJS 4.0 (AJ6)which I am in the processes of doing a running restore and getting back to being a nice motor.
However when she has just been started , I start to drive off and it tries to stall. If I approach a roundabout and slow down to stop the engine stalls. Alternatively I drop it into Neutral and I get round it that way.
I have serviced the engine and gearbox, replaced the stepper motor and Lambda sensor. Checked the voltage is correct at the Throttle sensor.

It seems to sort it'self out when fully warm.

I am getting a bit fed up with it so any help and advice would be greatly appreciated.

Thank you

 

Firstly, welcome to the Forum.

When the Jag is sorted, please do an Intro in the New Members Area.

Quick thoughts from down here,

The TPS, mounted unde the throttle body is oil comtaminated, very common. Sometimes, they respond well to a good solvent clean out, sometimes not.

The throttle body/disc is contaminated with the Jag Black Goo. Solvent soaked rag and wipe it out until clean.

CTS, Coolant Temp Sensor is flaky, age related issue now.

 

I too would suggest a good cleaning of your system as a whole. Most times these cars have been sitting around and as in any car, the fuel gets old and gooey therefore causing blockage in the injectors and other areas as well ie. parts on the throttle body get sticky just from non use and perhaps years of dirt. I also suggest adding some additive to you gas for a few tanks full. Most cleaners that clean the injectors will do.

When people park cars and not drive to blow them out, this is so common with old cars that sat for a while. I have owned many classic cars from Rolls, Jaguar and Mercedes and I always try to buy a one way ticket then drive the car home. If far enough away, will require a few tanks is best. Cars love to be on the road, so have no fear and drive it and blow it out before buying and replacing parts. That is my suggestion. Try it. You will find it liberating!

 

Hi

Thank you for getting back to me, however I have cleaned the throttle body out and checked most of the electrical connectors are clean. The throttle butterfly was gapped to 2 thou. She has been out for several blasts with fresh fuel , however it still stalls when cold.
I will try and check the CTS and run some injector cleaner through.

Kind regards

 

It sounds to me that whatever type of cold start system the car has is not working.
Greg

 

Have you tried exchanging the ECU? If you have a cold fuel delivery problem, that's going to point in the direction of that brain.

The ECU does not take the Coolant Temp Sensor, or oxygen sensors into account for fueling until the car has been warmed up. It runs it off of a predetermined map.

Because your TPS has the proper voltage at rest, does not mean it's sending good data as it makes its swipe with the opening of the throttle. That thing is flakey and often causes strange problems.

How about the camshaft position sensor? Is the cam aligned?

Lastly, I'm sure you've checked the plugs. They are dry and gapped correctly?

 

Hi Vee
I haven't tried exchanging the ECU but I did think of trying it. I do have a modified crankshaft sensor from XJR developments but this didn't result in any poor running at that time. Haven't touched the camshaft sensor, it seemed to be running OK when I first got it, however there was a bit of popping from the exhaust , I was advised this was because the silencers had broken down so I had a new exhaust made by AJ6 engineering. All the plugs are new and gapped correctly, new leads, dizzy, rotor arm
and filters and Oils are new.
When the car is hot it runs well and sounds great, the stalling issue is starting to annoy.
I will investigate further, thank you for your advise.
Regards

 

How is she idling? Steady? At what RPM?

 

Hi Vee
Idle speed is about 850 but not so sure it is that smooth, I have replaced the stepper motor then tried doing the manual setup but the last part where it is unplugged and you start the car I am unable to set the idle, it fluctuates widely. Normally it fluctuates between 850 - 950.
I might have to invest in a new TPS when I get back. One thing I did find spurious was that on the TPS , voltage on pin 3 should be 12V but it isn't, the closed voltage is 0.58V.

 

That's close enough on the TPS, I suppose.

Did you replace the IACV (stepper motor) trying to solve this problem? I can tell you that your idle is high. You're best and cheapest option is going to be taking it to a shop that has the WDS to reset your idle adaptations. There is nothing else that will reset the saved programming. It will also reset your TPS in case it is off.

Your idle should be ~580 hot, in gear
~700 hot, in park
TPC needs to be 0.6v when closed

You should have a shop that specializes in servicing these Jags. If you could find someone with a PDU, then you'd be all set.

You can read through my many posts. I've thrown just about every part at trying to resolve an idle problem and I find that a $150 charge to let a mechanic use a computer to reset my settings solved it (almost) every time. I have the AJ16 engine though.

Now that assumes the idle is high due to bad settings. Your other symptom is a vacuum leak. High bumpy idles can lead to a vacuum leak. It might be something just loose enough that, when cold, allows air to pass, but when hot, expands and smooths out. I had an old Ford that had a similar issue, which I solved by replacing the cam over and spark plug gaskets on the engine. Have you tried spraying some carb cleaner along some of the gasket joints of the engine?

 

If problem goes away when car is hot - I would agree that this is likely 'temperature related' problem.
'cold start system' could be not up to par. Not sure how it is on your car, but on mine there is an extra injector connected to 'termotime' switch. This injector works specifically while car is cold and bumps rpm to about 1500 for me.

Also, could be bad coolant temperature sensor.

And another thing is air conditioner, when it kicks-in there is a system in place to give engine some 'gas'. Turn it off to make sure it all works without it first.

 

I don't think the engine reads the coolant temp sensor until the engine is warm and the car goes closed loop.

 

Good point, mine stalls though the moment I disconnect it even when cold. go figure...

 

Well then it certainly is working. I guess the computer tries to detect whether it's there or not, and might not use it for fueling. I have to say, I'm not sure I ever had that happen... I can't recall.

What you can try is unplug it, with the engine off, jump the two prongs on the plug, (not on the sensor) and then start the car. Jumping the two connectors on the harness will trick the engine into reading a fully warmed engine. If that makes anything better, then you've found the problem.

 

I'm not experienced with AJ6 or AJ16, yet wonder how the ecu would determine the engine is warm if not by coolant temperature sensor reading ?

 

It reads it, but does not adjust fueling according to what the Sensor is sending. (Until it reaches a certain temperature)

 

sorry been away trying to find a job so the Jag had to wait a little. As regards the jag, I found that there was no gasket of any kind for throttle body/manifold. Made a thin gasket so as not to allow for air leaks. Put everything back together then got the FF44 again, I think getting an engine diagnostic test may be the way forward. I did renew the stepper motor but when I have tried the manual way of setting the idle this doesn't seem to work.
But thinking about what I did during servicing , I drained the gearbox oil and replaced the filter and the stalling started pretty much after this, is this coincidental?? as none of the engine management was touched other than removing the ECU.