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1998 Deville d'Elegance
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Discussion Starter #1
I have a 1998 DeVille d'Elegance and at 87,000 miles it began misfiring. The code returned by the computer is P0300.
One of the things I'd like to check is for an intake manifold leak.
I have seen several videos on how to take apart the Northstar intake manifold but none for the 1998 DeVille.
They generally show a rubber boot connecting the manifold to the throttle body that cracks and lets air in.
I do not see such boot on my 1998.
Can anyone tell me where I should look for potential leaks there? Is the sole means of repair a new manifold?
This and any other piece of advice would be greatly appreciated.
Thanks. :)
 

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2001 Seville STS, 1990 Seville (RIP), 1972 Sedan Deville
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26,323 Posts
The rubber coupling is the plenum duct. Pre '00 engines don't have one.

With no lean codes, I wouldn't suspect a vacuum leak. Are the plugs and wires original?
 

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98 DeVille, 97 DeVille d'Elegance
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7,345 Posts
You first need to find out what cylinder is missing with an obdii reader. You have either a bad plug, wire or coil.
 

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1998 Deville d'Elegance
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20 Posts
Discussion Starter #4
Re: 1998 DeVille air intake manifold.

@MoistCabbage: Yes, the plugs and wires are the originals. I plan to change those. And I will check the coils as suggested by rodnok01. I will report here what I find. Thank you both. :)
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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69,813 Posts
Re: 1998 DeVille air intake manifold.

Use this to make sure the proper coil tower is connected to the proper plug. Remove the coils and ICM, clean the coil base contacts and the coils themselves. Use only the correct AC Delco #41-950 Platinum spark plugs and the expensive AC Delco wireset - Rock Auto. Make sure the ICM base ground cable is connected.

More in Tech Tips. Google "cadillac forums 1998 northstar spark plug replacement" or something close. Expect some oil on one or more plug bodies - normal.
 

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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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19,842 Posts
Re: 1998 DeVille air intake manifold.

the P0300 is a generic misfire code -
this is one code our on-board scanner falls short on -

while it WILL store the code for specific cylinders misfiring -
P0301 = #1 - P0302 = #2 - P0303 = #3 - etc -
it will NOT display them - dumb! -

go to any big auto parts store -
they will scan the computer for free -

once you discover WHICH cylinder is misbehaving -
diagnosis becomes much easier -
 

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1998 Deville d'Elegance
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20 Posts
Discussion Starter #7
Re: 1998 DeVille air intake manifold.

Good suggestion basscat, I will do that.
@submariner409: Very helpful diagram, thank you. Do you know what resistance range is acceptable for a coil? Is anything in between 1 and infinity (and other than those two values) acceptable?
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Re: 1998 DeVille air intake manifold.

I don't have the primary/secondary coil resistance values for a pre-2000 waste spark system.
 

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1998 DeVille
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186 Posts
Re: 1998 DeVille air intake manifold.

Blueblood;

I just finished chasing down a P0300 code. Found out I had 3 plugs misfiring and 2 of them seriously. I have a 1998 DeVille with 189,000
miles and original coils. No idea about the plugs so, they went on the list along with wires. I'll spare you all of the details but, ended up
replacing the coils, spark plugs, and wires. All items suggested here. I knew coils were original but, had no idea about the plugs/wires.
After replacing those items, I have my car back! When I removed the #1 spark plug, the ceramic insulator fell about in my hands when it
was removed from the spark plug socket! A few of the others didn't look too much better! Not stuttering at the traffic lights, no
stumbling on acceleration, just nice smooth power!
 

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White Diamond '03 DHS (with DTS floor shift)
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87,179 Posts
Re: 1998 DeVille air intake manifold.

You might start with a good TB cleaning. Believe it or not, a dirty TB can cause a misfire (P0300).

A manifold leak will usually return a lean fuel trim code, P0171 and/or P1074.
 

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1998 Deville d'Elegance
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Discussion Starter #11 (Edited)
Re: 1998 DeVille air intake manifold.

Good for you sparkydm ! I hope my trouble will be similar to yours. :)

@Ranger: pardon my ignorance, but what is a TB?

Anyway, I did some research and I have found the range of values for the coils:

-The primary should be about 0.7 Ohm + or - 0.2 Ohm;
-The secondary winding should be 7.5k Ohms + or - 2.5k Ohms

What is important is not so much the individual values but the ratio between them.
For example, if a coil has DC resistance of say only 0.5 Ohm on the primary winding and a resistance of, say 7,000 Ohms on the secondary, the ratio is then 1 to 14,000. For a 12 volts system, the DC current applied at the primary would produce (12 x 14,000 =) 168,000 volts at the secondary. Surely plenty to fire a spark plug (similarly, you would need to apply 168,000 volts at the secondary to get 12 volts at the primary.)
 

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White Diamond '03 DHS (with DTS floor shift)
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87,179 Posts
Re: 1998 DeVille air intake manifold.

Please study the sticky post ^^^ on the main threads page - "Acronyms".

TB = Throttle Body
There is a cleaning tutorial in Tech Tips if you need it.
 

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1998 Deville d'Elegance
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Discussion Starter #13
Re: 1998 DeVille air intake manifold.

Well, I replaced all 8 spark plugs and PCV valve. Couldn't find any air intake leak.
Did an oil change too since it was almost time. Water came out (about a glassful) first followed by the oil. The car ran very smoothly for a few minutes then developed the rough idle again with the P0300 Current code.
I noticed white smoke coming out of the pipes at night and a puddle of fluid on the ground when I raised the front end of the car (no doubt it came out of the tail pipes.)

So, it's a far more serious problem than I had originally thought.

Thank you to all that have tried to help.
 

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1998 Deville
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181 Posts
Re: 1998 DeVille air intake manifold.

One thing...I found out that the plastic engine cover on top of the N* has four long studs that it sits on and held in place with four nuts. what I found out is that these are actual intake bolts that are torqued. I had to replace one of these studs and did not know this so I just threaded it tight and went on. Well I developed and intake leak at that very spot. Just something to check.
 

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2001 Seville STS, 1990 Seville (RIP), 1972 Sedan Deville
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26,323 Posts
That's '93/'94 only. After that it was varying styles of plastic cover, covering the actual intake manifold.
 

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1998 Deville d'Elegance
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Discussion Starter #16 (Edited)
Re: 1998 DeVille air intake manifold.

Update:
Got my 98 DeVille back this weekend. Fixed and purring like a kitten!
I found this nice gentleman near Macon, Georgia who does just that: Fixing Northstar engine head gaskets.
He is very reasonable and knows what he is doing.
I highly recommend him to all those who have an overheating Northstar.
If you do not live too far, he will pick up the car at your residence. If you live a little farther, he will also pick it up for a nominal fee (to pay for his gas, mainly.)
For me, he drove 640 miles one way to pick it up, same to return to base, then another 1,280 miles round trip to deliver it, for a total of 640 X 4 = 2,560 miles. All this travel for an extra $200.00. No other commercial service I know of will do that for that price.
To top it all, he even detailed the car (for free) before delivering it!
He also gives you a two year warranty on the repair.
The car works great and I feel it's the best thing I have done for my Caddy short of boycotting General Motors.

He can be contacted at northstarcadillacman65 at yahoo dot com.

Let me know if you need more info.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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69,813 Posts
Re: 1998 DeVille air intake manifold.

I deleted your duplicate "here's where to get it fixed" post in Northstar - Would you like this Northstar engine thread moved to ......... Northstar ?
 
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