I thought we should probably have an area for discussing nothing but vacuum leak diagnostics. After checking the usual stuff (pump-down of systems and inspection of the connecting points and valves and such) there's a few things you can look at if an engine fails to respond with mileage increases.
Gasket Leaks-a smoke test will reveal many leaks through to the heads, so that's a good one to run.
One thing you can count on. With an unidentified vacuum leak, this creates an artificially lean condition, and will result in hotter temps. If you don't have a laser thermometer, GET ONE! With this, you simply run a check on the exhaust manifold after the engine has been run to normal temps.
A higher reading will point in the direction of any such leak.
There is one other area that should be checked as well. That is the injector seals. While checking these can be a bit of a pain, if you've checked everything else, then this is a likely candidate, and is the least checked point of potential failure.
If your hoses look like they're beginning to crack, then you can almost GUARANTEE these seals need to be replaced. They shrink, they swell in the prolonged presence of oil and they crack due to the temps and the UV radiation. This is worse in the warmer climates.
Any other pointers?
I'm going to try to attach the FRESHLY REVISED paper on Vacuum Testing and Some Tweaks...
