Engine (Help needed) Engine will only idle with incorrect vacuum routing, huge surging idle when routed correct?

If youre running the stock EGR chances are the internal diaphragm is busted and leaking air plus another two trouble spots are the PCV valve and the vacuum booster for the brakes....

I found out that as PCV valves age they get sticky and before they get sticky they get so beatup inside the restrictor hole it allows alot more air than normal aswell as sucks alot more oil too and just cleaning one may not be optimal and cause high idle conditions..

Plug up all the vac ports and then do a vac test using a vacuum gauge....

You may find you have leaks around the throttlebody, the IAC , the plenum or even the lower intake manifold or some bad and burnt valves.............


Vacuum is also dictated by the distributor setting.....................When at exactly 10 BTDC the vacuum should be around -23 inches of mercury on a healthy engine and should bottom out near zero instantly when the TB blades are cracked so your TB or IAC valve maybe open too far too...

You may find your balancer slipped and giving you the wrong reading causing your low vacuum issue and BTW at -6 inches of vacuum the FPR shouldnt even be sucking enough to change anything so I suspect that the internal diaphragm inside the FPR is cracked and leaking when under vacuum causing your engine to run rich from the extra fuel being drawn in....

This in turn would cause a need of advancing the timing and opening up the TB blades more causing your low vacuum reading issue and idle fluxuations when FPR vacuum hose is in-place then removed..

Good Luck