Cadillac Repair: 1989 Cadillac 4.5L engine, throttle return spring, vacuum leak

Question
Hey Bill, for some strange reason, the throttle does not spring back immediately when I release the gas peddle. It also will cruise at 25-30 KPH by itself. The electronic idle control has been replaced but no improvement. Watching the idle control, the piston doesn't retract quickly or totally at times. Could there be a vacuum leak? Could I have low compression in one cylinder that the control is preventing the engine from stalling? I am at my wits end on this one. I seem to always be braking as the engine revs don't come down quick enough. Please give me some possibilities to check for. One mechanic I talked to said it is "normal" for 4.5L to act this way. Can't be!!

Answer
Hi Garry, Time to talk to a Caddy tech. because you are right this idle speed is too high and not normal. The first possability is that the throttle return spring is weak and not allowing for the propper closeing pressure. If you have too strong a spring then the ISC motor can be damaged. Assumeing the spring was good then there is a procedure that needs to be done and that usually gets the idle to normal. If procedures don't make sense let me know and I can go into further detail.
1) Warm the engine until the thermostat opens
2) Make sure the timing is no higher than 10BTDC.
3) Remove the air cleaner housing and plug the vacuum hose.
4) Ground the generator terminal
5) retract the ISC motor and adjust idle to 500rpm
6) with key on engine off adjust tps to 0 degrees on scan tool or .50 volts
7) with key on engine off move the ISC motor to the max extension and then turn the screw until tps is 10.5-11.5 degrees or 1.05-1.15 volts as read with a scan tool
8) retract ISC motor and turn off key. Remove all test equipment and reassemble the air cleaner etc. Start up the engine. It should return to normal idle.

If this procedure doesn't result in a normal idle or you don't have the tools to make the checks and adjustments then you will need to take it to a shop. Let me know what happens. If need be we can go to the chat room and discuss this further. Just let me know a time after 7pm cst. Bill