GMC Repair: 1986 G20 vacume leak, vacuum leaks, charcoal canister

Question
 I have a 1986 G20 with 305, it has a vacumne leak that seems to appear only when full operating temp is reached (thats when I hear the whistle) i have tried testing EGR with a vacume pump and it holds pressure for over 15 min. I have tried spraying all connections with carb cleaner, and checked with a stethascope to no avail, the noise seems to come from the front but hard to tell. I have isolated some of the circuits like heating and resivoire and the hold vacume fine. i have checked manifold vacume and it is low, I think it was a few hg lower than specified, it idles ruff and hard starting (i think choke is off a little to) and runs at a very high temp on highway the longer it runs the higher it gos unless i drive under 55 then will cool down some. I would like to take in for diagnostic but money is hard, this van is my livlyhood any help will be greatly appreciated, thanks

Answer
What mechanical work or ANY kind of work has been performed on the vehicle recently ?. As most problems that occur after a recent repair was made can be traced back to the area of where the repair was made as a vacuum line may have been knocked loose or a wire connection came loose, Look around in the area where work was performed.

You stated: "the noise seems to come from the front !.


FRONT of WHAT ?.


Most vacuum leaks that occur such as this one are usually a fault of the vacuum lines at the TVS (Thermal Vacuum Switch) located on the intake in front of the carb towards the A/C compressor. The hoses tend to develop a hole on the bottom side of the vacuum hoses and is very difficult to see.

Another possible source is the carb base gasket.



Recheck all your vacuum hoses carefully. especially the ones that go to the TVS and to the charcoal canister.


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