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The only place for a coil spring is up Zebedee's arse
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I've built a 4.6/3.9 hybrid V8 the engine starts runs no leaks But idle is up & down I've sprained cleaner on joints for leaks--- none found ---- could it be stepper motor?

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when you say erratic idle do you mean it revs up and then comes back or dose it miss and fart about and tries to recover it self.

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Take the Idle Valve out and thoroughly clean it inside & clean the connector..

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mad-as wrote:

when you say erratic idle do you mean it revs up and then comes back or dose it miss and fart about and tries to recover it self.

It just rises and falls at idle

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What kind of fuel injection system?

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It might be useful if you explain and state the facts.

What have you built?
What was the base engine?
What engine management system?
Which injectors?
On petrol or lpg?
What style of ignition?
What type of fuel pump and pressure regulator?
Engine management ancillaries new or second hand?

Forgive me, but your original question is like ... I washed my clothes and they won't dry, why?

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it sounds like a vacuum issue , in other words there is an air leak. the rise and fall of rpm is generally the engine leaning out due to too much air and not enough fuel. use a spray bottle with water and spray everything that could be an air inlet , eg throttle body shaft , injector seals around manifold and any think that is under vacuum. you are listening for a whistle as it sucks in the water or it stops reving up. also check all rubber hoses for cracks.

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I found the problem ---- had tiny split in vac pipe to the distributor ---- used WD 40 spray to find it And I cleaned the idle valve up then put a drop of bike oil in the sprung shaft --- cured