I just put on my 454 tbi and my car wants to idle at about 2500 rpm. I didnt notice any vacuum leaks, however Im using the stock 305 tbi gasket between the unit and the manifold. This leaves part of the path that runs to the IAC open, could that be my problem? Obviously I'll be ordering a gasket kit but no one around here stocks it. My tps is reading normal (7%) and Im pretty sure I wired up the IAC correctly: blu/blk -> a blu/wht -> b grn/wht -> c grn/blk -> d I just spliced the connector in instead of cutting off the old square connector, that wouldnt matter would it? Any help is appreciated.

Reprint of the wiring diag. I have lists the IAC hookups as teh folowing for the stock IAC routing.

A = BLU/WHT
B = BLU/BLK
C = GRN/WHT
D = GRN/BLK


The old gasket shouldnt matter much other then that itll hang into the bores and will interfier with the operation of the butterflies when the throttle is opened. If your just idling it then that shouldnt be a problem.

EDIT: if the gasket is still in good shape then you can jsut use an exacto knife to cut it to the larger bores. I did it with my old gasket and im still using it today.

Just out of curiosity, seeing as to how you had to splice a different connector in, do some of the larger, newer TBI's use different IACs that arnt reverse compatable with the standard ones?

I tried the wires the other way with no luck

Napa carries the gasket so I picked that up and I'll put it on, Im hoping there's a vacuum leak under the tbi or something and that's the reason. The 454 gasket is quite a bit larger, I'll take a picture of the two when I get home. On the 454 ss tbi's, the IAC is 4 pins strait up and down instead of 2x2 like on the Holley and stock 305/350 tbis. Im going to try that and cleaning the iac real well, as Shifty had mentioned to me over aim.

did you set the throttle blases properly? i had mine idle very high when the weatherpack was not connected properly, so it wasn't reseting the IAC back to park when i started it, so the IAC was stuck full open.

So I take it that even the mount that the IAC screws into is different then the standard ones? Its interesting that the footprint of the TBI is different as well. Ive never seen one of the later 454 tbis so could you take a pic?

Oh, about the wiring... Those are from a chiltons manual, which to my knowledge has teh full reprints of the gm f-body wiring manual. If the present IAC is the same internally then those should be correct, in theory.

my 94-95 454 did not come with a base gasket. the howell plate did. the gasket supplied with howell plate was inadequate in that no gasket under iac. seems it diagonally cut off that seal area. maye be a holley gasket? not sure. a gm gasket fit purfectly and sealed all the 454 TB. if i had not caught it massive vac leak. cant you visually inspect the pindle to see if going through range of movement when you short it?

Ok, just put that gasket on and it idles kinda like I thought it would, pretty rough. Needs tuning and Im going to clean out the IAC and see what happens, but it does get down to my target 675 rpm and fluttered around it. Better than 3000 rpm. We took some pictures and I'll do a tech writeup when Im done. Throttle is sticking though, makes driving interesting. TV cable needs adjusted... normal stuff I kinda expected though.

Thanks guys