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One for the EFI gurus


Ivan
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Quick question. My 1991 V8i Discovery starts and runs fine. However, I have not been able to set the tickover rpm correctly. When it first starts it ticks over at about 1100 to 1200 rpm. Once warm it drops to about 750 rpm (which is what it should be). However, after driving for a short while it's back up to 1100 to 1200 rpm. As a quick test I disconnected the plug to the AFM and it did not make any difference at all. Does this mean my AFM is shot ? I have tried adjusting the idle using the alan bolt on top of the lpenum chanber which does drop it a little but I can't get it any lower.

TIA

Ivan

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Very often that is a classic example of a vacuum (air) leak on the inlet………… such as a split or porous pipe, or a leaking gasket, ………. use WD40 to detect the leak with the engine running.

If all is well, then suspect the idle air valve stepper motor at the rear of the plenum ……. They fill up with sh!t and stick, however, yours appears to be working OK as its behavior from cold is correct.

Other than a slight air bypass feature, your AFM has very little effect on idle .......... for the AFM to be faulty, then it normally effects the whole rev range. Check the flexi hose from the AFM to the plenum for leaks as this will give the same symptoms (un-metered air)

:)

Ian

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If all is well, then suspect the idle air valve stepper motor at the rear of the plenum ……. They fill up with sh!t and stick, however, yours appears to be working OK as its behavior from cold is correct.

Ian

Mine, with similar symptoms (but more extreme - would rev to 2500 rpm) was the idle air valve stepper motor. I think that when you come off throttle it opens up (to smooth the switch from on-power to off-power to reduce apparent transmission slop) and it then gets stuck resulting in the high idle. Switching the engine off would sometimes re-set it. I tried cleaning it multiple times then I got a 2nd hand unit that was the same - neither worked, then finally bit the bullet (they are over £60!) and got a new one which fixed it straight away. Best bet would be to try someone else's since they are easy to change...

Rog

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Thanks to all who replied. Problem was the stepper motor. When I took it out I tried to move the littel plunger and it was sticking badly. Gave it a clean up in some brake cleaner and it now moves freely. Car seems to run a bit smoother as well. P*ss*d off the neighbours when I took it for a run down the high street at full throttle. :P

Thnx for the help and info.

Ivan

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