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2002 SLS 1965 Deville
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Discussion Starter · #1 ·
I have put new oem plugs in and a used but known to be good coil... cleaned the throttle body 3 times... no leaky fpr... no codes being set... weird idle when i put it in park it will idle up to 2000 rpm and slowly go back down... I admit I am new to this N* and i need help. any advice will be appreciated.
 

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2002 F55 STS, 2014 Explorer XLT, F-150s
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Clean the IAC (Idle Air Control) - carefully - and its bore. Use TB cleaner. Sticks out the front side of the TB. Don't drop it - it's $165.
 

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2002 SLS 1965 Deville
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Discussion Starter · #4 ·
I also have only run premium QT/BP/Shell gas in it sense I bought it 3 months ago..Never run anything else... Not sure if that is important but its all I run in my cars.
 

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I also have only run premium QT/BP/Shell gas in it sense I bought it 3 months ago..Never run anything else... Not sure if that is important but its all I run in my cars.
Our cars are set up for 87 - regular. I run the screaming hell out of my car and it's perfectly happy on Shell 87 - either at 120 or on the Interstate. You're plain, flat throwing money away.
Once every 6 months I run a 20 oz. jug of Chevron TECHRON through it.
 

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2002 SLS 1965 Deville
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Discussion Starter · #6 ·
Our cars are set up for 87 - regular. I run the screaming hell out of my car and it's perfectly happy on Shell 87 - either at 120 or on the Interstate. You're plain, flat throwing money away.
Once every 6 months I run a 20 oz. jug of Chevron TECHRON through it.
Really??? Is this true? I figured being a "Luxury" car it wouldnt like 89 or 87... How do these cars like ethanol?... or do they? If I can save a few bucks on gas that would be awesome!!!
 

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2002 F55 STS, 2014 Explorer XLT, F-150s
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I'm in the Baltimore/DC metroplex - we have to run E10 because the County, although not required by EPA standards to run E10, "opted in" to get some Fed kickback bucks for caving in. (Of course, that program went away, so we're stuck) The car and truck seem perfectly happy, and I notice almost no gas mileage differences when we run to Maine or Florida and fuel up in non-ethanol areas. (Ethanol is an "octane" booster because it burns slower and at cooler temps than pure gasoline.) Higher octane fuels actually burn slower in order to slow flame travel to take advantage of increased ignition timing - but our electronically controlled cars won't increase that timing enough to take advantage of the higher octane. No - nobody does PCM tuning for us - yet. It ain't like '65 any more.

EDIT: Just came back and saw JimD's post - for clarification, the Owner's Manual states that if you are going to drive the car to the MAX or tow trailers, then you might consider 89 or 91/93. My long-term fuel use says that there's less than 0.4% difference in gas mileage when swapping octanes. At today's prices, you gotta get a LOT better increase than that to justify the extra cost.............
 

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'98 STS (RIP @ 206,xxx miles)
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Really??? Is this true? I figured being a "Luxury" car it wouldnt like 89 or 87... How do these cars like ethanol?... or do they? If I can save a few bucks on gas that would be awesome!!!
The Northstar refresh that occured for 2000+ model years included a fractional decrease in compression ratio and a slight redesign of the combustion chamber to control the flame front. The result GM was looking for (and obtained) was a high performance street engine that was happy with 87 octane fuel.

That being said, I can tell you that a 2000+ model will deliver "crisper" performance on 93 octane fuel. Been there done that (my name is on the title for a 2004 DTS).

My daily driver 1998 model is a different issue.

As for saving a few bucks on fuel, here is the math:
If 93 octane fuel costs 6% more than 87 octane fuel, unless you see a 6% increase in fuel mileage, you are spending more than necessary. It will require some long term MPG data to provide the answer.

Look to section 5 of your Owner's Manual for the answer to the ethanol (oxygenates) question. GM recommends their use without stating a percentage.
 

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'98 STS (RIP @ 206,xxx miles)
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Ok so back to the original subject... the missfire... help please!
Did you install ACDelco plugs?? What you said was OEM but OEM does not mean the same thing to aftermarket parts suppliers as it does to GM.

How about the possibility (however remote) that a plug porcelain was broken before or during installation?

Was there any carbon tracking on the porcelain of the plugs you removed? If so, did you replace the boots on those plug locations?

You installed a used but "known good" coil. Why?

A sure-enough missfire will result in a flashing or steady SERVICE ENGINE warning. And there will be a P0300 code. Neither of which you mentioned.

The only place you mention missfire is in the thread title. Your first post is all after the fact and mentions high idle RPM.

Help us out here with more details.
 

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2002 SLS 1965 Deville
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Discussion Starter · #11 ·
Yes ac delco plugs

i checked them all and no cracks. i only changed the plugs cuz i found a
receipt for some champion plugs in the glovebox. (turned out they were champions in the car)

didnt replace the boots but checked them all

cuz a new coil cassette is $200 lol

nope no engine codes

oh sorry lol it missfires also

what other info would u like?

I checked the fpr and no gas so its good

Checked the vacuum lines and plenum with starting fluid (no idle change)

after 1500 rpm it runs and drives fine.
 

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....cuz a new coil cassette is $200 lol

nope no engine codes
How did you determine which coil to replace?
And how do you know the used coil you installed is 'known to be good'?.

At a "cold start", what might be seen as high idle RPM is normal with 2000+ engines because of the AIR system working to get the catalytic converter up to operating temperature ASAP.

If/when you see a flashing or steady SERVICE ENGINE light, a Tech II analyzer will point to the offending cylinder(s). It will be a learning experience.

....after 1500 rpm it runs and drives fine.
Drives fine as in no missifre??
 

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08 cts 68k, 07 STS-V 15K, 97 Sts 160k
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It could be revving high due to the fact that you cleaned the TB. I am not sure and someone will correct me if I am not but I think your idle may change after a TB cleaning. After a few starts it will get use to or learn the new position of the butterfly and compensate for it. Which will decrease the high R's at start up. If you have a misfire you will have a misfire code and from what you said you don't..??
 

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Discussion Starter · #14 ·
How did you determine which coil to replace?
And how do you know the used coil you installed is 'known to be good'?.

At a "cold start", what might be seen as high idle RPM is normal with 2000+ engines because of the AIR system working to get the catalytic converter up to operating temperature ASAP.

If/when you see a flashing or steady SERVICE ENGINE light, a Tech II analyzer will point to the offending cylinder(s). It will be a learning experience.


Drives fine as in no missifre??
I drove the car the coil came out of and it ran fine. the miss was gone for a few weeks then I cleaned the tb and its back along with this idle issue.

I dont feel the miss when im driving its mainly at idle and when im stopped at a light or just stopped in gear anywhere.
 

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Discussion Starter · #15 ·
It could be revving high due to the fact that you cleaned the TB. I am not sure and someone will correct me if I am not but I think your idle may change after a TB cleaning. After a few starts it will get use to or learn the new position of the butterfly and compensate for it. Which will decrease the high R's at start up. If you have a misfire you will have a misfire code and from what you said you don't..??
i cleaned it over a week ago. Its not idleing high when it cold starts, its after its warmed up and i have driven somewhere then put it in park then it goes to 2000 rpm then slowly drops.
 

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Discussion Starter · #16 ·
Ok so I may have found the issue... I went over everything again and shot some carb cleaner around the throttle body and it idled up some so I am going to change that gasket there. This was after getting 2 codes finally p0171 p0174... Lean... I hope this gasket fixes it!
 

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2002 F55 STS, 2014 Explorer XLT, F-150s
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Ok so I may have found the issue... I went over everything again and shot some carb cleaner around the throttle body and it idled up some so I am going to change that gasket there. This was after getting 2 codes finally p0171 p0174... Lean... I hope this gasket fixes it!
Do you mean the plenum - the connector between the throttlebody and intake manifold ? The intake manifold has to come off.

There are tutorials down in Discussions; Item specific; Cadillac Technical Tips.

The plenum is a dealer-only item, about $40, P/N #12555840

Go up to the light blue bar ^^^ and click the "Albums" tab. Open mine - the red Seville. There are more intake and fuel rail pictures there.
 

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Discussion Starter · #18 ·
Do you mean the plenum - the connector between the throttlebody and intake manifold ? The intake manifold has to come off.

There are tutorials down in Discussions; Item specific; Cadillac Technical Tips.

The plenum is a dealer-only item, about $40, P/N #12555840

Go up to the light blue bar ^^^ and click the "Albums" tab. Open mine - the red Seville. There are more intake and fuel rail pictures there.
Ya I bought the black rubber clamp plenum thing lol it was $30.
 

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Discussion Starter · #19 ·
Ok I replaced the rubber clamp plenum thing and it runs like new. The old one was torn. It runs so smooth now. Thanks for everyones advice.
 
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