my 87 IROC was originally a 305TPI and I rebuilt a 350 and installed it in the car. The motor is not drastically built, just a couple upgrades:

24# injectors
083 heads opened up to 2.02/1.60
AFPR
Comp Cam TPI Cam 264/274 .480/.480
Hedman headers
Catco converter
Flowmaster 3" catback
Hypertech low temp fan switch
Hypertech thermomaster chip for an 87 350TPI

With the Hypertech chip, before and after setting the timing the idle was bouncing around between 500 - 1250. It would occasionally stall out. When I give it some gas it smooths right out and its extremely responsive.

With the stock 305TPI chip it idles decent, idle is between 600-800. When I give it some gas it does nothing for a split second and then responds. Not nearly as good as with the hypertech chip.

I set the TPS at .54v and verified smooth increase up to WOT at 4v. I also set the idle as close to 450 (idle surging) with the iac unplugged and plugged it back in.

Right now I can not find any vacuum leaks so I am a little stumped on this one. The entire TPI setup is the same as what was on the 305 previously with the exception of the injectors. When the 305 was in the car it idled fine and never surged at idle.

I also verified it was not a ECM problem as I have two other `165 ECM's which I tried and the problem stayed constant throughout.

Does anyone have any ideas? This one has got me kinda puzzled. Aside from a custom burnt chip I am not sure which direction to go in right now. Could the IAC be on its way out?

 

Any opinions on this one?

 

Adjust fuel pressure while the car is running then get a custom chip.

 

I had a vacuum line of the TB and it acted like that . But I would also recomend a proper tune .