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Check this one out! I've had an elusive rough idle problem on a 1989 240 with B230F non-turbo engine. After going through all the usual suspects, I started doing some creative testing of my own. If I clamp shut the one-inch diameter hose between the intake manifold and the IAC valve (and clamp it shut close to the intake manifold) the motor will stall. But if I clamp that same exact hose shut closer to the IAC valve, the motor only drops 100 RPM. That test would make it appear that the hose is leaking air in that the computer does not know about. Inspecting the hose reveals no holes, cracks, no dry rot, no damage whatsoever. My question is could that hose be porous from age, and therefore sucking a little air in all over? Can a hose do this?
Dan
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