1989 New Yorker fluctuating idle

1989 New Yorker
3.0
A/T

SYMPTOMS: starts fine, idles normal for about 30-45 seconds...then begins to stumble-searching for a constant idle speed. it stumbles for a few seconds, then recovers. it continues to do this for about 3 minutes, then idles smoother, but surges when put in gear.

PARTS: it was in need of a tune up anyway, so we installed new plugs, wires, cap, etc.

then we installed a new TPS hoping to cure the idle. NO LUCK
then we installed a new IAC(idle air controller). NO LUCK
checked all vacuum lines-OK
when I unplug the IAC, there is no change at all.


we ran a 1/2 can seafoam through-no change
removed, cleaned, and checked throttle blade on throttle body.


I am just not sure what to do next...

suggestions?


thanks.