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