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2001 Seville SLS
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6 Posts
Discussion Starter #1
This is my brothers car that I am working on. 4.6 NorthStar with a Y engine VIN code. It was throwing a p0300 misfire code. I removed both coils and found oil sitting in the spark plug wells of all cylinders, one of which was especially bad. Replaced front and rear valve covers including spark plug well seals, replaced all 8 plugs. Checked the springs in the boots for continuity, cleaned all corrosion off of all electrical components, and lubed all connections with dielectric grease.

All of this made no difference, so checked it out with a better scanner.

At idle, cylinder 3 is apparently misfiring about 180 times per cycle while cylinder 4 is misfiring about 1/10th that rate. At higher rpms (1800ish) cylinder 3 is misfiring 180-200 times per cycle, and there are 0 misfires from other cylinders.

My questions:
1) Is the cylinder 4 misfire likely due to cam-lag from the cylinder 3 misfire, since it is next in the firing order?

2) Is this likely a malfunctioning coil (that is my best guess)?

3) Is cylinder 3 in the rear bank (the black coil nearest the passenger compartment)?

Thanks for your time and advice!
~Roger
 

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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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19,250 Posts
Re: 2001 Seville SLS p0300 code

This is my brothers car that I am working on. 4.6 NorthStar with a Y engine VIN code. It was throwing a p0300 misfire code. I removed both coils and found oil sitting in the spark plug wells of all cylinders, one of which was especially bad. Replaced front and rear valve covers including spark plug well seals, replaced all 8 plugs. Checked the springs in the boots for continuity, cleaned all corrosion off of all electrical components, and lubed all connections with dielectric grease.

All of this made no difference, so checked it out with a better scanner.

At idle, cylinder 3 is apparently misfiring about 180 times per cycle while cylinder 4 is misfiring about 1/10th that rate. At higher rpms (1800ish) cylinder 3 is misfiring 180-200 times per cycle, and there are 0 misfires from other cylinders.

My questions:
1) Is the cylinder 4 misfire likely due to cam-lag from the cylinder 3 misfire, since it is next in the firing order?

2) Is this likely a malfunctioning coil (that is my best guess)?

3) Is cylinder 3 in the rear bank (the black coil nearest the passenger compartment)?

Thanks for your time and advice!
~Roger
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the P0300 code is a generic misfire code -
this is one category our on-board scanner falls short -
it does NOT display WHICH cylinder/s is/are misfiring -
even though the computer DOES store that information -

your hand-held scanner SHOULD display a specific cylinder code -

P0301 is a misfire in cylinder #1 -
P0302 is cylinder #2 -
P0303 is #3 - etc -

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

Is cylinder 3 in the rear bank
(the black coil nearest the passenger compartment)?
YES -

1-3-5-7

2-4-6-8

FRONT

the boots are a known problem -
sometimes you can see a crack/split in the rubber -

sometimes you can see "carbon tracking" -
the path the spark follows rather than firing the spark plug -

most common problem is a bad coil -

you can swap coils -
they ARE identical - except for color -

if you got a P0303 code - and the coil IS the problem -
the code should change to P0306 - when you swap coils -
 

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Administrator
2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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68,404 Posts
Oil in the plug wells is common - the cam cover O-ring seals weep over time. Not usually an ignition problem - oil is an insulator.

The ICM in each cassette L-leg is removable. There's another seal and multi-pin connector in there. Moisture sometimes causes problems.

If a cassette high tension terminal looks black, rusty or has cracked plastic around the terminal(s) then the cassette itself is suspect. Junkyards or RockAuto.
 

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2001 Seville SLS
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6 Posts
Discussion Starter #4
Thanks for the help!

I used a SnapOn Modis to check for the codes. It was not throwing any P030X codes beyond the P0300, so I found it was cylinder 3 by the live feed from the car to the Modis. I carefully cleaned and inspected each boot when I changed the gaskets/plugs, and they were all in good condition (looks like they may have been changed during the last tune-up). Looks like it is the rear bank coil then. Thanks again.
~Roger
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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86,781 Posts
My guess would be a bad coil, which means changing the coil cassette (all 4 coils).

Can't say that I have ever heard of the boots being a known problem (or ever being the cause of a misfire for that matter).
 

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2001 Seville SLS
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6 Posts
Discussion Starter #6
I replaced the rear bank coil, the engine runs super smooth now with no misfire codes. Got the part online from AM for $91 as opposed to $544 from the dealership. Thanks to everyone who gave advice!
 
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