Okay, too funny and a little frustrating. For some reason my search engine (MSN) today pulled up an OSO thread from a few years ago that Eddie Young answered this very question. Why did my search the other day not ? I dunno. suks,

here is the thread: 500EFI Throttle position sensor

So, the 500EFI uses TPS / Throttle Position Sensor:
AcDelco #213-895
GM #17106681

Mercruiser # 849903

700 SCi
565 EFI
1075 SCI
700 SCI
525 EFI
600 SCi
500 EFI
850/1025/1075/1200 SCI
SCi662 SCI

Haha no the googler screwed me up with the 894 AC delco part number. See google would have bitten you on this one.. Whatever walker 200 - 1047 was referenced to but I could not get a reference to cross for AC delco. You know I have to mess you

swap TPS from engines and run to see if issue follows or stays. You need to scan the engine regardless. Could be IAC issue in the mix or a voltage issue.

Not looking at your set up here nor the procharger set up but just throwing this out there.

By pass valve issue ?

Hey BUP, hopefully yo can take some elbowing back. LOl. Hahahaha

Of course and for me sticks and stones may break my bones but words and the finger will never hurt me nor take away from what I do. haha as well. Its all good man.

Looks like you have a fan for life there .

Ill go back to say --- swap out tps then possible IAC from the good engine one at a time to see what follows or not. And add in a scan diagnostic along with voltage readings.

NEXT If applicable ??? do not know here but will add the --- By pass valve in the mix for a possible issue ? just a dart throw at it with this

UPDATE - The issue was in fact the TPS. Put a new one in and problem solved. In case anyone is looking, the correct part number for a 500EFI is AC # 17106682. $26 at your local auto parts store vs. $175 thru Merc.