Does this engine use a TPS? I had a very similar problem on a 525efi and tracked it down to the weather pack connector for the TPS. Check all connections as well to ensure the pins are contacting and stay tight.

Yes it does. Ill check tonight.

I pulled the whole intake back off, removed the throttle body, removed the IAC sure enough it had carbon build up so I thought this has to be it. Cleaned it all up looking good as new and put it all back together with the exact same results.

When I put my finger over the air tube feedhorn the engine dies very quickly but when I put my finger over the hole in the throttle blade it idles smooth and clean at 650 700 rpm.

I can reproduce the same results everytime I cut the engine off and start it and it will idle all day at 800 but one push of the throttle and it remains at 1500 rpm until I cut the engine off.

I checked timing, sprayed around looking for a vacuum leak, looked at all connectors on the intake for something corroded or bent.

I noticed in the picture below it states not all mag engines have the throttle blade with a hole in it. Is there possibly slightly different parameters on a ecm tune for this difference?

try disconnecting throttle cable and do the rev up w/ hand and let go,,see if its a cable adjustment messing things up.

In the tune there is a target rpm, iac minium position for restart, iac reset key up etc, You adjust minimum throttle blade position manually unitil your IAC counts on a scan tool warmed up are in the 30 to 40 range. Ive had to drill holes in TBs with big cams to get enough airflow for iac to have a good range and NOT have tb cracked open so far it send tps past its minimum range, ive had to plug holes. Your tune has ALL the stock idle characteristics a 502 uses as far as iac max, reset, etc for THAT style tb but fueling PW for a gen 6 cam . The extra 2 to 3% fuel it gets in higher rpm pw bands ISNT making it idle too high. These things are usually a combination of problems when looked at in person, when you said THIS, was it diling at the 1500? " When I put my finger over the air tube feedhorn the engine dies very quickly but when I put my finger over the hole in the throttle blade it idles smooth and clean at 650 700 rpm.", If so, adjust throttle blades closed more or run a very short screw, tight fitting screw in that hole with red loctite.

I've done that as described above. However it does do it after revving like it doesn't know where ri reset the throttle.

I would bet your iac counts are near or AT zero WARM , when you rev it up, it gets into a little more timing, IAC counts go to ZERO, idle NEVER drops as NOTHING is going to make that happen. When you restart it, it gets less timing, IAC barely has control of it warm though. That would be MY guess. Does it also idle at 1500 cold if you rev it a tiny bit 15 or 30 seconds after starting it? IF it doesn't , I can tell you its DEFINITELY the IAC NOTt having control warm, IF it DOES, then IDK for sure

What is your fuel pressure at idle? Is engine hard starting when hot at all?

When I had a bad fuel pressure regulator and my idle fuel psi was quite low, one of my engines was doing something very similar. I had a very surge-y idle and often it would not drop below 1500 rpm as you stated until key off/key on/restart. If you say idles good when hole in TB blocked off, indicating engine is happy with LESS air, your idle fuel pressure may be low. Engine was also very hard starting when warm on mine though. Fixed FPR issue and that solved the idle and hard start issues I had.

It doesn't do it when cold. I can rev it and it settles back to 800 but as soon as the engine is warm it does it. If it's not having control warm what would that lean to?

Anyone know a GM part number to replace the IAC? As I understand it the IAC is the same as the car engines but I do want ot make sure I get the right one and since they are much cheaper it will definitly at least let me test it.

No its starts up at the rub of the switch everytime easily and sounds great. I havent checked fuel pressure but I will. Good thought. Havent had a problem with it before but I would also think if fuel pressure was the issue it would stay high when I start it back up. maybe when I give it throttle its changing fuel pressure. Ill have to check.