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

[deserthoundLX]

Im incredibly confused here. My 89 5.0 Lx has what appears to be a vacuum leak. i only get 6 in/hg when idling. While investigating this i found the vacuum routing to be incorrect, with the FPR being fed off the EGR Regulator Solenoid, and the EGR being fed directly off the intake vacuum tree. When i corrected the routing i get a terrible surging idle, from 500-1500 and it wants to die badly. It has a new IAC, EGR and a new throttle body is being delivered to rectify some leaks coming from there and the TPS. Could any shed some insight on why my car is idling so terrible with the correct* vacuum routing but idling much more stable when routed incorrectly? Thanks for any help in advance

 

[General karthief]

Here is a 'systems check list' that should get you straightened out:

Help me create the "Surging Idle Checklist"

Updated 05 July 2022 to add rough idle when cold and smooth idle when up to normal operating temperature. See the EGR tech information, section #15 for more help. Many of you are familiar with the "Cranks OK, but No Start Checklist for Fuel Injected Mustangs" checklist and the No Crank...

www.stangnet.com

 

[deserthoundLX]

gulator Solenoid, and t

Here is a 'systems check list' that should get you straightened out:

Help me create the "Surging Idle Checklist"

Updated 05 July 2022 to add rough idle when cold and smooth idle when up to normal operating temperature. See the EGR tech information, section #15 for more help. Many of you are familiar with the "Cranks OK, but No Start Checklist for Fuel Injected Mustangs" checklist and the No Crank...

www.stangnet.com

Thank you, ive been going through that and its definetely been helpful. I shouldve added that as well as i also recently replaced the o2 sensors to get rid of codes 41 and 91. im still not really grasping why the car would idle better with the fpr receiving no vacuum while the egr does when it should be the opposite. Thank you for your help i will keep digging into the checklist

 

[CAMTWO1070]

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