88 GMC C2500 TBI pressure regulator malfunction

[Rick Hamp Sr]

Installed an adjustable pressure regulator 88 GMC 5.7 two years ago and recently the truck threw a code 34 MAP sensor and started running real bad at idle. Replaced sensor and no change. Noticed one injector wasn’t spraying great so replaced both injectors with nos oem stock Delco 55 lb injectors. Still no change in idle. Here is what I discovered after putting in a new rebuild gasket kit including a new OEM  regulator diaphragm. Still bad idle and fuel pressure showing 12 psi on the installed inline fuel gauge. EGR valve tested ok pcv the same. No intake vacuumed leaks. Sitting at 20 cfm.  But here is what I did after turning up the fuel pressure on the adjustable regulator but the gauge showed no increase in pressure UNTIL I tapped on the regulator housing with the engine running and the fuel pressure jumped up Immediately to 17 psi and the truck smoothed right out at idle. Strange indeed. So can anyone tell me how this could of happened and is there any internal valve above the diaphragm that effects the fuel pressure from the top of the TBI housing internally ? Anyone??? Thanks