Gotta major problem!!
 

I have a 90gt and I have a problem with the engine/i don't know.
When the car is running it with all the sudden the idle will drop and when I give it gas it will miss out horribly. It started in May when we got back from WFC. I raced the car and it was fine but the day after, it acted up. It usually does it after the car is warmed up or if I let it idle too long. I 've tried well at least it seems like everything. The fuel pressure regulator, control module, ACT sensor, TPS, O2 sensors,different coil,fuel filter,fuel injection wiring harness,fuel pump relay etc.. The strange thing is that when the fuel pump primes, there will be a hissing sound under the hood. My buddies cars don't do that, we are dum bfounded on what to do next. I'd appreciate any advice you guys might have. Oh, also put new short block,in thinking maybe it was losing comp. but that wasn't it , that was an expensive expirement.

engine woes...
 

You have probably checked this, but have you looked at ALL of the vacuum lines to make sure that they are in tact and not cracked or loose? Especially those attached to the upper intake manifold. Look over the vacuum tree as well to make sure everything is hooked up well there and the tree itself is not cracked or broken. Just a thought.

I think it's the "X" cam, not that the actual cam itself is bad, just how it likes to run.

Really need a more detailed picture of your car's parts to tell.

Get yourself a code tester and run the cylinder balance test. It'll give you some indication if maybe you've got a bad injector. Also, I had a FPR go bad once that hissed terribly because the fuel was making it past the diaphram and actually being sucked through the vacuum line and into the intake. Check to see if you've got raw fuel in the vacuum line. Might also check your vacuum at idle.

I say the TFI module, my car did this about a yr ago, when it ran kinda hot, stock 195 stat, it would start missing out horribly, out of nowhere and die?

I still dont know what that was, but im sure it was the tfi, or coil, which i just put in an accell coil, but it happended once more, when it cooled it ran fine, and i heard the hissing noise, like it was trying to breath through the maf, hasnt done it since i been running it cooler.

SAME HERE MAN
 

Hey man,
just thought that id let ya know that my car does the exact sane thing. I have a 95 gt and it does exactly what you are describing. i dont quite know what to do either. i first thought it might be the cats...so i had a flowmaster system put on it and the cats cut off....no help though...then i thougt it might be the plugs. I changed them out to autolite platinum plugs...no luck there eithe. I dont know what it is. please let me know what u find out.

Got pretty much the same problem (88GT), car will start idling low and even cut off - and spitting and sputtering missing. My son and I don't know a lot about these engines with all the electronics and stuff. We took it over to our local Mustang specialty shop to look at. These guys are really good and are stright shooters as well!