ramiles

I upgraded my 86 L98 with alum heads, cam, and upgraded TPI parts - TPIS big mouth intake, Edlebrock large tube runners and a 52mm TB. I wanted to get rid of the Cold Start Injector (CSI) and I thought I knew what to do - ordered the large runners for 89 and later model, knowing these didn’t have the CSI, and assembled these to the new big mouth manifold, a ported plenum (for 86), and a 52mm TB ( again for 86).

The engine ran fine but wouldn’t idle well. Struggled unsuccessfully data logging, changing idle RPM in the BIN, removing, checking, and changing IACs, and then manually traced the IAC air path through the TB into a bottom cavity in the plenum. Then it became clear that no idle air from the IAC body was getting to the engine (thread to earlier link here). Did some research on this group and found that there must be a different pathway for 89 and later model IAC airflow - the big clue was seeing the picture that showed the 89 plenums not having the triangular idle air opening (see pic below).

Can someone verify that putting the later model (89) runners with the early model TB and plenum does in fact block the normal IAC airflow coming from the plenum and doesn’t get to the manifold? How does it block it - is that from runner machining or gaskets or what? And, how does IAC airflow get to the manifold for 89 models – Is the TB different? Does IAC air on the later TB go direct to plenum cavity?

Appreciate your recommendations to fix my problem the easiest? Would a later model TB dump the IAC air right into my plenum and that’s all I need? Any linkage issues with the later model? Do I have to replace the runners with early models? Thanks mucho!

1989GTATransAm

iTrader: (3)

Right, you can't use an early throttle body with the 1989 or later plenum without mods. You can drill a hole in the plenum so that idle air can get to the plenum. The later 1989 throttle body has a recess on the back side so that idle air can get to the two main inlet ports.

ramiles

Thanks - in using a later model throttle body with the 86 plenum are there any fitment or linkage issues or other gotchas? Am curious why they set up the early models with the separate IAC pathway beneath the normal plenum - any negatives just porting the IAC air to the main ports?

1989GTATransAm

iTrader: (3)

In that case you should not have a problem with the IAC. You will need to use a throttle cable from a 1989 or later car. The way they attached the throttle cable was changed in the 1989 model year.

ramiles

Ordered a late model TB then went looking for a late model throttle cable and got told that it has been discontinued with non in stock. Yikes!

billydonn

"looking for a late model throttle cable and got told that it has been discontinued with non in stock. Yikes!"

Try Street & Performance, Mena, AR (www.hotrodlane.cc)
Lokar also makes a cable and I have seen them on ebay.

Pat Hall

iTrader: (31)

TPI parts.net, one of the sponsors right here on the board, has the correct GM cable available brand new. I'd avoid Lokar or any other aftermarket cables. They never seem to work right. Oh yeah, Modern Day Muscle also carries the correct cable even cheaper IIRC.

ramiles

Thanks for the update - Ordered a later model throttle body - thanks for the help on getting the cables for the linkages. Wanted to document what I found with pics incase someone else has this problem.

Note on the two pics below. The top is the later model and has the recess described in the post that dumps idle air right to the 2 ports to the main plenum body.





Before adapting cables wanted to see if I'll try to get a similar recess cut in my old version TB. Any special instructions I should make to the machine shop to make this modification?