XJ6 & XJ12 Series I, II & III 1968-1992

'85 XJ6 won't idle - sometimes

 
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Old 07-15-2016, 12:06 AM
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Default '85 XJ6 won't idle - sometimes

Greetings folks,
You were very helpful in getting Esme, my '85 VDP, to stop backfiring and pass smog last year. After I went through pretty much everything last year she was running like a new car, started immediately, idled slow and smooth, got 16 mpg. This lasted from May until February. She pretty much didn't get driven in March due to bad weather. The next time I drove her, in early April, she had a new problem: She still starts easily when cold. After she warms up for a few minutes the idle speed starts to surge up and down hundreds of RPM. Eventually the "down" rpms get so low that she'll completely stall out. She can easily be started again, but can only be kept running by opening the throttle a bit. If, instead of letting her sit at idle, you drive her, she runs fine. Even waiting at stoplights is no problem, but if you have to sit at idle for more than about two minutes she starts the same behaviour and will ultimately stall. The gas mileage seems to have dropped from 16 mpg to 15.


I have tested the coolant temperature switch; seems fine. Fuel pressure regulator seems fine. I tried grounding the O2 sensor- no difference. Removing any vacuum hose increases the idle speed a couple hundred rpm and thus masks the problem.


The fact that she will drive fine, including stopping at lights, seems the most confusing part of this. Any ideas?


Thanks again,


-mB
 
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Old 07-15-2016, 12:21 AM
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I'd begin by cleaning the throttle body and checking/setting the throttle blade to the correct .002" gap.

I worked on an '85 Series III years ago with an almost identical symptom. ^^ is what fixed it.

Worth a try



Cheers
DD
 
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Old 07-15-2016, 07:28 AM
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Idle hunting: check that all the air ducting are well seated and clamps are tight but not excessively, start at the front 'cornet' all the way back to the aluminum elbow at the air intake.
 
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Old 07-16-2016, 01:48 PM
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Thanks, Doug and Jose.

I tried those, but they didn't work. Also tried bypassing ballast resistor, didn't help. As a last resort I turned up the idle speed. That worked, though I didn't test it thoroughly, i.e. get out of the driveway. It still isn't very fast- 800 RPM in park, 600 RPM in gear. It idles smoothly in both cases. Can it be that simple? Remember it ran great from June-February. I notice the idle screw turns so easily you can't feel any resistance, so maybe it jiggled itself in. Also I have a wild theory that the cold air valve wasn't closing all the way but after using the car several months it freed up and now closes all the way. effectively lowering the idle. Or there's a problem and I just masked it by turning up the idle. I'll probably find out in the next few weeks.

-mB
 
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Old 07-16-2016, 02:00 PM
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Originally Posted by Mike Beda View Post
Thanks, Doug and Jose.

I tried those, but they didn't work. Also tried bypassing ballast resistor, didn't help. As a last resort I turned up the idle speed. That worked, though I didn't test it thoroughly, i.e. get out of the driveway. It still isn't very fast- 800 RPM in park, 600 RPM in gear. It idles smoothly in both cases. Can it be that simple?


Possibly. The idle must've been quite low before if you presently have 800/600

But, yes, you might be masking another problem or a stack-up of problems.

Sometimes you get one that can just barely sustain life at idle .....with no single and/or easily visible identifiable fault. I attribute this to general degradation of all involved parts and systems. Slightly leaking injector seals, worn AFM and distributor, perhaps a torque converter dragging more than some others, valve adjustments getting out of whack, etc

You can pursue these when/if time and inspiration allows. Or, if happy with the present result....leave well enough alone

Cheers
DD
 
 

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