dyllidan

Hi, i have a 97 Mustang GT and for the past 6 months it has been an absolute nightmare, it blew a head gasket late 2016 and i got a good used head to replace it with the same cam profile and chamber size, i reassembled the engine and it ran fine for a few months, then i started to hear i hissing sound, like high pressure air trying to flow through a small straw, it slowly got louder until it began to affect the idle rpm, it was idling as high as 1800 rpm, i checked the IAC, and all 3 other vacuum ports on the throttle body with no success in finding a leak, i decided to plug the IAC line and see if it would idle on the vacuum leak alone, it ran on the leak alone at about 900rpm and i figured that was close enough to just let it be for now, but then after a month or so it started to cut out under load, it revved fine in neutral but as soon as you put it in drive it would cut out with any throttle and then intermittently run fine for a few hours and then go back to cutting out and stuttering horribly, i smoked the intake system and no leak could be seen, i replaced the intake manifold and it changed nothing, it has new plugs and wires and i checked the fuel injector 0-rings when installing and they were in good shape, currently i have all orifices on the throttle body plugged, including the throttle blade itself, even though i can clearly see it close, and the engine runs about 900-1200 rpm surging at idle, i have no idea what else it could be other than a cracked intake runner on a cylinder head but in order for it to be idling this high on the vacuum leak alone, i think i would have seen it when i was cleaning off the deck surface before installing the new manifold, the only system i haven't touched is the EGR, the CEL has been on for years do to a bad DPFE (typical ford) and it hasn't affected it, however i do have new CEL codes that say Lean on cylinder banks 1 and 2, which makes perfect sense for a vacuum leak however i just can't seem to find it, if anyone could help me with this i would greatly appreciate it, thanks

Z28KLR

Safe to assume new gaskets when you R&R'd the intake?

Get a can of carburetor cleaner and with the engine running start giving little blasts at any potential leak areas. When/if you observe a change in idle speed while doing this then you just found your leak. Obviously carb spray is flammable so excersize appropriate caution.

dyllidan

yeah, the new manifold came with gaskets, i'll have to try the carb cleaner, thanks

Z28KLR

Did it come with gaskets, or did you get a Dorman brand replacement that has the built-in O ring gaskets?

Although (not) having a positive seal is one of the lesser experienced issues with the Dorman manifold, if you have it just be aware that it's commonly regarded as an all around pile of crap. Google "Dorman intake mustang 4.6" or something like that for more info if any of this applies to you.

Should have specified earlier, but using the carb spray method for finding vac leaks will (or should) cause idle speed to increase at any leak areas where the spray gets drawn in.

Derf00

Have you traced the vacuum lines to see where they go and the condition of the lines at the opposite end? Given the age of the car it may just be time to replace them. If an EGR diaphram is broken it could cause a hard to find leak.

Get a vacuum pump and guage and test it and the lines while you are it (if changing all the vac lines is a real PITA or you want to pinpoint the source first).