XJ40 ( XJ81 ) 1986 - 1994

Weird surging idle after battery change

 
  #1  
Old 05-02-2019, 01:27 AM
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Default Weird surging idle after battery change

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.
 

Last edited by Sarc; 05-02-2019 at 01:40 AM.
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Old 05-02-2019, 09:22 AM
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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
 

Last edited by Sarc; 05-02-2019 at 11:15 AM.
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  #3  
Old 05-02-2019, 10:22 AM
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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
 
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Old 05-02-2019, 10:53 AM
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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
 
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Old 05-02-2019, 10:57 AM
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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?
 
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Old 05-02-2019, 11:13 AM
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Originally Posted by nunger1330 View Post
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
 
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Old 05-02-2019, 10:20 PM
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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.......
 

Last edited by Sarc; 05-02-2019 at 10:42 PM.
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Old 05-03-2019, 02:00 AM
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When it runs normal, if you disconnect the stepper motor is that the symptom you see?
 
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Old 05-03-2019, 02:25 AM
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Have a read of this thread Sarc, stepper motor setups for the different motors

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

Larry
 
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Old 05-03-2019, 04:59 PM
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I had a surge and found the check valve on the evap canister was broken in half. A constant 3/8 leak...... maybe yours?
 
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Old 05-03-2019, 05:48 PM
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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
 
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