Cyrk:
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From reading posts on this forum, my guess would be that a vacuum leak - and/or poor seal from the intake manifold gaskets/o-rings may be the culprit.
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This may be especially true if you notice this when the engine is very cold (think Space Shuttle - Challenger).
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I had (actually still have) a Ford Explorer with the SOHC engine - this engine sufferred from a similar problem. New intake manifold o-rings cured the problem.
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t2p