bkroske

Just bought a 91 LS400 and am having probs with the idle being too high.
Throttle position sensor maybe?? If I push on the lever the idle drops, but picks right
back up. Are they hard to replace? Could there be other issues typical to this car,
causing the high idle? The car has 232000 miles on it
Any help would be great !!
Thanks

LScowboyLS

first, check all three throttle cables (main, cruise, and transmission) - they all three meet near the TPS - each throttle cable should have slight play, not much play, but not taunt either.

next, search for vacuum leaks all over the car, I will post a vacuum diagram a bit later into this thread

next, you can test the TPS via the tests in the service manual, and make sure it is adjusted correctly using the procedure also detailed in the service manual (you will need a multimeter)

if TPS is adjusted right, then the issue is likely either the Idle Air Control Valve (bearing failure typically) or else failing ECU capacitors

can you also do a dump of your diagnostic codes using this procedure and post them here?