GMC Repair: Fluctuating idle, vacuum hoses, air intake system

Question
What may cause a 94 GMC 350 engine fluctuate it's idle. In park it rev's very high for about 30 seconds then drops to an extreme low idle. After the engine warms up it stays at a higher then normal idle, however when it is in drive it runs at a normal idle speed. Any ideas to trouble shoot this?

Answer
In MOST cases the fault is a VACUUM LEAK with any number of vacuum hoses or possibly a leak around the intake manifold, exhaust manifold, or the engine air intake system.

Although NOT recommended due to possible EXPLOSION, You can spray some carb cleaner around the intake and the vacuum hoses and see IF the engine speed changes.

You WILL get rpm changes when you spray near the EGR valve.


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