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Originally Posted by dr_lee29
Can a vacuum test also diagnose a faulty LMG?
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A vacuum test will not diagnose a vacuum, oil, or coolant leak. A vacuum test is good for checking the overall condition of an engine, or for diagnosing a restricted exhaust system.
If you suspect an internal vacuum leak, (underside of lower intake manifold gasket): Remove the PCV valve from the grommet in the rocker arm cover, and inject a small amount of propane through the PCV grommet while the engine is idling. If RPM increases, chances are the intake gasket is leaking vacuum. Reinstall the PCV valve before turning off the engine so the PCV system can purge any propane that may still be in the crankcase.
If you suspect an internal coolant leak, an easy check is to let the engine set for 10 to 12 hours. Coolant is thinner and heavier than oil, so if there is coolant in the oil, it will settle to the lowest point, (the drain plug), and the coolant will drain out before the oil. Very slowly loosen, don't remove, the oil drain plug. If you see coolant... most likely the lower intake gaskets are leaking.