Toyota Repair: 92 4runner erratic idle and missing, vacuum hoses, intake hoses

Question
hi i have a 92 4runner with 3.0. problem(s) when you first start it, it idle up and down for about a min then idles pretty smooth but it missing. engine is shaking and if you rev it up sometimes it doesn't drop below 1500 rpm. now what i have noticed if you take the oil cap off while running the engine idle goes really low like its going to stall? is this maybe a vacume leak, intake leak? im lost would a head gasket do this? water pump was replaced do to over heating problems. doesnt overheat anymore. any suggestions? thanks a whole lot Patrick

Answer
Yes it could be a headgasket problem as these engines are prone to have that problem especially after being overheated, I'd suggest having the headgaskets checked by the toyota dealer or a reputable repair shop, the test can be done either with a checmical block test or a more accurate method is to use the emission control test machine to check for excessive hydrocarbon(HC) emissions at the radiator opening. If that test is good then I would have the engine compression tested, if the compression is ok then other factors can be considered, possible vacuum leak on the intake side, look for any cracked intake hoses or vacuum hoses off, spark plugs, distributor cap, rotor. It's up to you how to approach this, what I suggested may seem backwards but it would eliminate a major problem before putting any parts on that may not solve the problem.