On my 45k mile 1990 US spec XJ40 4.0

We changed the battery and the oil and filter. First few starts and idles it was fine. Then it starts with this nonsense.


Car was for sale and is actually now sold...... on its way to new owner on Friday, so would be kind of handy if it ran properly by then.

It behaves like a vacuum leak, but the fact that it can sometimes just go away on its own, or sometimes goes away after a restart makes me think its not a vacuum leak, more like an electrical ECU type issue after the battery change.

I've changed the battery before and didn't do anything special to recover it (apart from retraining the alarm remotes) and I never came across this issue until now. Maybe I was "lucky" previously.

No Check Engine light. No codes in the VCM.

 

Resetting by disconnecting the battery cables and shorting them out, then starting from cold and waiting for warm up then driving made it slightly better.

When it is bouncing around if I shift it into drive that pulls the idle down then back in to park and it's fine.

So I have an idle problem, not a vacuum problem. My gut feel is if I just drive the thing for a while it'll short itself out, but that's not a great story for the excited new owner.

Next thought is to clean the throttle body and intake manifold........

Any other inputs welcome at this point

 

This is a possible fix ....

Take it for a drive - the ECU has to re-learn all the settings after a "big" disconnect and you need to take it out get it warm, do some town and highway driving, maybe 5 or 10 miles and then it should be ok ...

Larry

 

Hi Larry

Thanks for the suggestion. Now that the red mist has settled and I'm thinking a bit straighter, I have had the same behavior before, albeit on other Jag models. And in those cases just driving it fixed it.

Will go put some miles on it today and see what happens

Thanks

Scott

 

Check the connection at your IAC motor. It will relearn where the piston can be but it should go away relatively quick. Seems like you may have a spotty connection if it is intermittent. Did you wash the engine compartment or anything like that since you are trying to sell it?

 

Nope, no engine bay washing.

To add more detail to my second post, and also to bolster Larrys idea, I only drove the car about 300 yards after doing the reset........ I'm thinking this is not enough so will go drive it more.

But, I will check the connector and clean it anyway. We were changing oil, swinging batteries in and out, etc so there's always a chance of something getting knocked

 

So..... We did a lot of driving with the car today and all appeared well, but then after sitting for 3 hours, at restart the idle bouncing returned.

After a few key off and ons, everything returned to normal.

I inspected and cleaned the IAC connector. All looks ok.

Puzzled.......

 

When it runs normal, if you disconnect the stepper motor is that the symptom you see?

 

Have a read of this thread Sarc, stepper motor setups for the different motors

https://www.jaguarforums.com/forum/x...e-idle-118636/

Larry

 

I had a surge and found the check valve on the evap canister was broken in half. A constant 3/8 leak...... maybe yours?

 

To close the thread, I left the battery cables connected together for 30 minutes, then drove it 20 miles and all good.

She's off too her new owner.

Thanks for the help. Down to 2 Jags........ Gosh

 

Sarc when you say you left the battery cables connected together is the battery completely disconnected at this point and you just connected the positive and negative cables together?

 

This is called a hard reset, you just have to undo the neg terminal and hold it to the positive cable. The positive terminal doesn't have to be disconnected from the battery as the battery is out of the circuit with the neg terminal off.

The idea is to drain the capacitors in the ecu of any residual charge that could prevent the ecu from forgetting some settings that you *might* want to erase.

Larry

 

Larry thank you for your detailed instructions on the hard reset I’ve looked everywhere on here for that information but I was only able to find half assed partial instructions I am trying everything I can think of to fix my surging idle on my 5.0 XF

 

Hi Mike, this info is just for pre OBD11 cars, like the XJ40. The XF is way newer and I have no idea how that ECU works, take my info with several grains of salt please or better still ask in the XF forum!

Larry

 

Hard resets work on my 2007 XK, so it works on obd2

 

Hi Mike,

I replied with a few ideas in your thread at the XF forum.

Cheers,

Don