A very good way to test for leaks but you must be careful for sparks or real hot exh manifold igniting it is to take carb cleaner or brake cleaner in areosol cans and ,with the engine on and idling now at 1000rpm approx, aim short bursts of spray at critical areas like the vacuum system, egr valve diaphram, intake plennum gaskets above and below the fuel rail, along the throttle body area and visually search the vacuum lines that run over to feed the interior heating controls, some have vacuum pods to move doors that direct heat and air flow. Listen at idle, is there a faint his inside the truck under the dash, behind or near the glove box or heat controls, has your interior control failed to the defrost position regardless of where you move the lever? Go back out and follow the line for the EVAP system canistor purge valve above your steering box and frame under the hood near the battery- it is a gold colored solenoid with 2 wires with a tougher line that leads up under the Tbody from the battery area. Under the Tbody the line has an elbow and a short piece of chwaper rubber hose connecting it to the Tbody for vacuum supply and inducing fuel vapour into the engine at light cruise. That short piece of hose rots out all the time and is hard to notice, takes a good leak from that to set light typically as ot is a ported vacuum leak.
You have checed that all the caps are on the vacuum tree at the rear of the intake plennum, drivers side? They get spat off regularly buy a inor minor backfire or burp from and incompleted keystart when they get soft and aged.
Best is to test everything you can with the spray, when you find a leak, your rpm will jack up even more, briefly until that shot of spray is consumed through the leak point. The spray becomes the hydrocarbons that the unmetered air moving through the leakpoint needs to combust and add more power to the engine. This can be repeated over and over to pinpoint a leak but be wary of excess spray build up and ignition. This is a dangerous trade and some testing may be more risky that other tests. A test like i described above is a great test but requires the caution of any dangerous activity involving moving parts and combustible materials with ignition sources so be smart, go slow and triple check.
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