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1992 Fleetwood S&S Hearse, 1993 Buick Roadmaster
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567 Posts
Discussion Starter #1
Hello all. I will preface this by saying that my car probably needs a tune-up, I have no check engine codes, I recently cleaned my throttle body, I have a new air filter properly installed, I'm running the recommended "premium gas." I rodded out the upper part of the EGR tubes, through the throttle butterfly, best I could. I did not remove the EGR and do anything on that end. The rod didn't go very deep.

My issue (1992 Fleetwood 4.9) is only when coasting at higher speeds, i.e. when letting off the gas at 45 mph or higher. I'm coasting to slow down, and it feels like the car is very gently misfiring or surging. It's not real bad, but I know that sometimes it's better to try to fix it early, if it's doable.

Another thing I have not done yet is the fuel filter. And I'd imagine most of you would tell me to complete the EGR cleaning, change the fuel filter, and see what happens. However, I went ahead and asked the question, because this could just be one of those quirks that these cars do, like when I had an earlier 3.8 (1978 Olds motor I think) and they were known for sort of an eratic idle. Anyways, any ideas would be appreciated, even if it's just that I need to finish what I started.
 

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1992 DeVille
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2,339 Posts
Could be an ISC motor issue. I have a thread on here that goes into great detail about calibration of the minimum air screw, TPS, and ISC motor. Just run a search for isc and it'll come up. I'll post the link here in a bit too.

The ISC motor will catch the throttle hammer and regulate idle and if the car maintains acceleration after you let off the gas, this is probably your issue

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http://www.cadillacforums.com/forums/showthread.php?t=620194

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You put a new cap and rotor, cool, plugs and wires?
 

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1992 Fleetwood S&S Hearse, 1993 Buick Roadmaster
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567 Posts
Discussion Starter #3
Not yet. I've got the wires, but hurt my back at work. I probably won't be able to install until next week. By then I should have cap and rotor. I pulled a plug and it looked good, will pull and check more though. The ISC makes sense now that you say it, so I'll probably go ahead and do that too.
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
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19,273 Posts
Not yet. I've got the wires, but hurt my back at work. I probably won't be able to install until next week. By then I should have cap and rotor. I pulled a plug and it looked good, will pull and check more though. The ISC makes sense now that you say it, so I'll probably go ahead and do that too.
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check the fuel pressure at the fuel rail -
 

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1992 DeVille
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2,339 Posts
My money is a new cap and rotor should fix you up. Could be fuel related but you'd probably see other issues.

The cap and rotor could be really old and have build up. Cheap easy fixes that can make a world of difference
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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86,785 Posts
I'd imagine most of you would tell me to complete the EGR cleaning, change the fuel filter
It ain't the fuel filter. If it was, you have trouble getting up to speed (higher fuel demand) more so than coasting down (lower fuel demand).
 

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Registered
1992 Fleetwood S&S Hearse, 1993 Buick Roadmaster
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567 Posts
Discussion Starter #8
The ISC motor will catch the throttle hammer and regulate idle and if the car maintains acceleration after you let off the gas, this is probably your issue.
After driving it today, paying extra attention for anything wrong or out of the ordinary, I believe this is my issue - a slow ISC. It fits the symptoms very well. I do still plan on putting plug wires, cap, rotor button, and fuel filter on the car within the coming week or two.

Will the ISC cause the gas pedal to depress itself slightly?

I should probably also mention that this is all since I cleaned the throttle body, replaced air filter, and started running premium (the manual says to.) And it hasn't made any detrimental effect - just the opposite actually. My mpg is steadily climbing - only by fractions, but I'm up from 15.5 average to 17.2!
 

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1992 DeVille
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2,339 Posts
If it does cause it to depress it won't be anything to speak of really. You won't be able to feel it. Take for instance when mine failed it was stuck holding the throttle hammer but the travel of the pedal felt totally normal.

Tune up and cleaning should help mpg for sure. Also how you drive =)

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You're puppy is heavier than my sedan so that extra weight will affect you as well but if you take off from stops real easy it shouldn't be too bad
 
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