Sorry this is a long one. Ok I’ve been dealing with misfire/intake popping on my 93 mustang for almost a year now and I think I’ve finally narrowed the issue down. I was finally able to put a vacuum gauge on the car. It only reads 6 inches at idle. The engine is a 302 with afr 165 heads and Comp Cams NX274hr cam. I talked to Comp Cams and they said I should be around 12 to 15 inches with this cam. Engine codes for the car show 41/91. Please believe me when I say I’ve been down the 41/91 diagnosing. I’ve replaced the o2 sensors (they were actually bad).
I also replaced a weak fuel pump and a bad a FPR that would not hold fuel pressure. I verified the orange ground fire is attached to the intake manifold. Here is the tricky part, for the most part she holds steady vacuum at 6 inches but occasionally the vac will drift up to 10 to 12 inches and the idle will rise from the normal 800 rpm to 1100 rpm and hold. When this happens the car drives great. No misses, no pops or anything but when the idle lowers to the normal 800 rpm, the problems resurface. The problems (intake popping/misfires) only occurs in drive when increasing load. It revs fine in neutral and does not pop. I’ve tried to close the TB outlet and close the PCV vac hose to try the get the car to stall but it remains running.
Am I 100% on intake gaskets guys? When I spray water, brake cleaner around the lower there is no change in idle. I guess the leak must be somewhere between the runners. When the problems started I did notice my coolant levels were dropping slowly, so I re-torqued the lower because I noticed some of the bolts were loose and the leakage stopped but the gaskets but must have already been compromised at that point. I even pulled and re-dropped the distributor at its set exactly at 10 degrees BTDC. Could I have valve(s) leakage or stuck open? Any suggestions will be appreciated.
I also replaced a weak fuel pump and a bad a FPR that would not hold fuel pressure. I verified the orange ground fire is attached to the intake manifold. Here is the tricky part, for the most part she holds steady vacuum at 6 inches but occasionally the vac will drift up to 10 to 12 inches and the idle will rise from the normal 800 rpm to 1100 rpm and hold. When this happens the car drives great. No misses, no pops or anything but when the idle lowers to the normal 800 rpm, the problems resurface. The problems (intake popping/misfires) only occurs in drive when increasing load. It revs fine in neutral and does not pop. I’ve tried to close the TB outlet and close the PCV vac hose to try the get the car to stall but it remains running.
Am I 100% on intake gaskets guys? When I spray water, brake cleaner around the lower there is no change in idle. I guess the leak must be somewhere between the runners. When the problems started I did notice my coolant levels were dropping slowly, so I re-torqued the lower because I noticed some of the bolts were loose and the leakage stopped but the gaskets but must have already been compromised at that point. I even pulled and re-dropped the distributor at its set exactly at 10 degrees BTDC. Could I have valve(s) leakage or stuck open? Any suggestions will be appreciated.