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'90 DeVille '97 DeVille
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83 Posts
Discussion Starter #1
Thanks to all who have advised me in the last year regarding my idle coast-down problem. I used my multi-meter to diagnose a bad ISC motor, but before dropping any more money on this engineering turd (the ISC system, not the 4.5L engine), I hit upon a solution. I adjusted the stop screw on the ISC to yield about 550 rpm curb idle in Drive, and disconnected the ISC. In Park the drag reduction yields about 800 rpm. I've had no problem starting/ warming up the engine in this god-awful Iowa cold; all I do is hold the foot feed to about 1200 rpm for 30 seconds or so. Will it stall with AC on? I don't know and don't really care; I don't use AC much anyway. All I know is I get my optimum 70 mpg coast-down benchmark idle like I used to and about 3 more mpg.
This is still a helluva car despite that idiotic ISC.
Merry Christmas/ Happy Holidays to all!
Matt
 

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1992 Cadillac DeVille 4.9L
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203 Posts
My idle is inconsistent when I stop at a light. Sometimes when I let of the gas it will crawl forward, other times it will won't even try to move forward. Does this mean bad ISC?
 

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'90 DeVille '97 DeVille
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83 Posts
Discussion Starter #3
I guess I would do all the simple suff first: Check to see if a code is stored. Check for cracked/loose vacuum lines. Make sure the throttle body gasket is not leaking. Clean the EGR tubes and the back of the throttle blades, then perform the idle learn procedure. You should be able to do a search on these topics for more detail. Good luck!! Matt
 

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1992 Cadillac DeVille 4.9L
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203 Posts
Does the vacuum hose connecting to the intake snorkel do anything special? It's a little cracked at the connection, I don't know that it's leaking, but it could be.

How do you change the screw manually?
 

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Registered
'90 DeVille '97 DeVille
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83 Posts
Discussion Starter #5
I would replace any cracked hose/tube. The ISC screw will turn using a needle-nose vice grip as a pliers. Note: This screw does NOT regulate idle speed; it changes voltage to the throttle position sensor. I unplugged my ISC, therefore the ISC screw became my idle screw, because I "froze" my idle at that given adjustment. Matt
 
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