Long time reader, first time poster.

anyways, I recently picked up a 2001 Z24, I bought it with a miss and figured it was cheap so if I had to a head gasket might be in order. The previous owner did say someone put some sugar in the gas tank as well.

Got it home and compression looked good (175-180 across the board). I also dropped the tank and cleaned it out as best I could. Finally got the CEL scanned and it popped a P0304. I did the usual spark plug/boot/coil housing replacement and it still popped back on, so I finally narrowed it down to an injector. I moved the injector and the misfire followed it.So I figured I was done, too bad it is still shaking at idle. I cant really feel the miss while I am driving, it seems to have plenty of power and pickup, just at idle and if I rev it up with my hand I can feel the miss still slightly.

All injectors when unplugged cause it to run worse. I also took the coils off and made sure all plugs were getting spark. All four had a nice blue spark. I replaced the fuel filter, it was very hard to blow through so it needed it anyways.
I did notice today that I can take the vacuum line off of the MAP sensor and it will straighten out some. Not perfect, but better. I also made sure all injectors were getting the proper signal with a noid light. So what am I missing here? I also ran it up and down the highway today for about 15 minutes and the CEL never came back on, it usually comes back on after clearing after about 5 minutes on the road. I havent left it to idle for very long yet, I'm sure I will get a P0300 code back when I do.

New parts: Coil housing, spark plugs, spark plug boots, fuel filter, one injector

I haven't checked fuel pressure yet but I cannot see any visible signs the regulator is leaking. I also installed some clear tube and it has remained dry for the past couple days.

Would the ICM cause the miss to be more pronounced at idle? I'll probably take it to get tested as well. I have to use it to take our Suburban's rotors to go get turned in the morning so hopefully tomorrow afternoon I can have some free time to dig back into it.

A little follow up. I checked fuel pressure and it is about 52 psi running and engine off. About 60 with vacuum disconnected.

Vacuum at the manifold reads about 12 inches Hg. At this level of vacuum, the MAP is showing 2.36 volts. does this sound like a correct value? Or is my vacuum not reading good?

I have sprayed just about everything imaginable with carb cleaner with no improvement. so I am doubtful I would fix a vacuum leak pulling the intake manifold off. But disconnecting the vacuum line to the MAP sensor I believe is causing it to add some more fuel, which is making me think a vacuum leak or some type of lean condition may exist. I also can disconnect the electrical connector to the MAP and it runs a little better. But the MAP does show to move steadily through the voltage range when I apply a manual vacuum to it and watch it on my voltmeter.

So just to follow up with what was wrong, the exhaust cam was off one tooth (retarded). It looks like someone replaced the water pump and for some reason didnt get the cams lined back up correctly. The WP looked very clean to have not been replaced recently. The PO never told me anything about it other than his buddy said it would be an easy fix. I also repalced the tensioner since I couldnt get it to pop back out properly. So its running great now.