: What does a missfire feel like?

11-05-08, 03:37 PM
I been noticing more and more a rougher idle and at times the RPMS drop to 500 and it almost stalls. at 750 RPMS the car idles very rough and vibrations can be felt threw the car as if some things hitting a rough spot (if that makes any sense) I used a impact wrench to pull the front 4 plugs and I discovered that 2 of the plugs were missing there platinum electrode tips (not on the electrode its self but on the metal that floats above it) There gap was .060 due to the missing metal. I re gaped the 2 plugs to .050 and cleaned the oil and white flaking gunk off the plugs and started her up and the staling/vibrations are still the same...

I'm not sure if this is related to the plugs or the wires or maybe some thing else? :hmm: How can I determine if its a mis?

P.S. I know I been asking allot of questions I just feel some things very wrong here with the idle and I already cleaned the THB replaced the PVC gromits, check and repaired air leaks and replaced the air filter and cleaned the MAF. At this point I'm running out of ideas :nono:

Edit: The plugs them self don't look that bad


11-05-08, 04:09 PM
I'd check for codes first and then you could go from there. I believe the code for misfire is PCM P0300.

11-05-08, 04:13 PM
are all misfires recorded? Or only when there abundant? :hmm:

11-05-08, 04:16 PM
A random miss has the uneasy feeling of the car slightly "jerking", your gas pedal foot may feel a slight single tap here and there, the car may tell you, by a slight uneven sound from the engine.

That spark plug is shot. Regapping will do nothing. Impact wrench ???? :banghead: NOT!!! That's an aluminum cylinder head. One screwup and it's toast. You are in the market for new plugs and wires.

Go to www.rockauto.com (http://www.rockauto.com) and pick up a set of plug wires and plugs (ACDelco #41-950, Professional Platinum, gap preset to .050"). No other cutesy plugs......OEM only.

Torque, without antiseize compound, to 13 ft/lb. COLD.

Be VERY sure the plug wires are connected as shown. Click and scroll in.

11-05-08, 04:17 PM
I know at one point my car was vibrating and idling badly, and the Service Engine Soon light was flashing on the display. I don't know if yours does that, but when I check for codes the PCM was stored as Current. I know the book says the plugs and wires can go to 100k, but I'd get them replaced. After all, the car is 10 years old.

11-05-08, 04:26 PM
Ok thanks for your help guys its greatly appreciate! I got 2 codes 1 is a current issue I been having that I doubt any one here can fix :cookoo:

B2477 - Reverse Lamp Relay Shorted to Ground ( every time I replace the fuse and reverse it blows and I cant find the short so I am giving up )

B1771 - Cassette tape slow (major issues here :D)

Thats all I got so I'm going to assume its not a miss just a very rough and shaky idle. I'll do the plugs when I can scrounge up $56 dollars (Fing expensive!) :suspect: The wires will have to wait I'm far to broke I been helping a friend with her newborn and let me tell you diapers arnt cheap!!!

11-05-08, 11:49 PM
are all misfires recorded? Or only when there abundant? :hmm:

A light miss will not set a code.

11-06-08, 02:27 AM
so I could very well be having missfires.. Well time will tell I update this thread in a few days wit the results and if the idle issues remain maybe you guys can figure it out :)

11-15-08, 04:31 PM
Do I have a oil leak?

Ok I pulled the plugs and installed new ones. Now onto my findings :nono:

7 out of 8 plugs were worn out but pretty clean. One however has a massive White chunk of something on it! The electrode is burnt black and the isolator is covered in white/grey debre and the gap around the electrode and the prong is filled in with this gunk! :bonkers:


The accumulation of deposits on the firing end is influenced by oil leakage, fuel quality and the engine's operating duration

Since only 1 of my plugs have these "Deposites" is it safe to asume I have a oil leak? Would that be a head issue?

11-15-08, 05:01 PM
Sounds more like oil than coolant, but hard to tell without seeing it.

11-15-08, 05:34 PM
That's not oil deposits and also doesn't look like coolant either. Those plugs (first and most recent posts) look like they're way past their prime. How many miles on them ??

Save up for the new wires and don't worry about the plugs or leaks. The plug with the piece of ? in the gap is fouled from a persistent miss. It's an ACDelco 41-950 plug so I wonder how old it (they) are.

The color on those plugs is typical of unleaded gas.

Sorry that I keep editing this......lay all 8 plugs out on a dark surface and upload a digital shot of them.

11-15-08, 06:02 PM
will do! The cars a 98 manufactured late 97 and has 60570 miles on it. Not many for a plug rated for 100,000K :confused: She started up right away and the idle is much more consistent and smother also the rough spot is almost completely gone now! Theres still a bit of a power surge from time to time while idling but its no where near as bad as it used to be. The Wires them self have some worn areas (still not threw the rubber jacket tho) and developed a white powder on the coatings. If I didn't know any better I'd say it was from a consistent arc from one wire to another but that cant be if the coatings intact so maybe its just baby powder :alchi:

11-15-08, 06:09 PM
Best way to check wires is to remove the "beauty cover" if any and take a hard look at the wiring with the engine running in the dark. Arcs and sparks ?? New wires, ACDelco, check Lindsay Cadillac ------> or RockAuto.

You just did the plugs.........are you happy with the wires as-is ??? (Be very sure you save post #4...the diagrams are critical.)

11-15-08, 06:12 PM
well that depends if I still have a slight miss in that cylinder. I had no idea it was that bad to begin with so theres a chance the cause of the miss is still here and that would rob me of performance and more importantly gas mileage so as it stands right now I wont be happy till I figure out what caused the miss. I'll take a water bottle and mist the wires in the dark and get back to this thread tomorrow... Its pouring out and it was bad enough doing the plugs in a thunderstorm :thepan:

11-15-08, 09:52 PM
If you replace the plugs (done) and wires and have a smooth-running engine, you will never know what gremlin caused the miss/fouled plug in the first place. For all any of us know, the plug has a piece of burned gasket sealer in the gap. Stranger things have happened. Ever seen what a 7/16" nut in a cylinder will do to the top of a piston and cylinder head ?? Don't sweat the small stuff.

Drive the car and enjoy it for what it is: A large, heavy, moderately fast Cadillac sport coupe. Nothing more, nothing less.

11-15-08, 09:57 PM
http://img151.imageshack.us/img151/6419/1002624sf4.th.jpg (http://img151.imageshack.us/my.php?image=1002624sf4.jpg)http://img151.imageshack.us/images/thpix.gif (http://g.imageshack.us/thpix.php)

http://img376.imageshack.us/img376/4140/1002632nq5.th.jpg (http://img376.imageshack.us/my.php?image=1002632nq5.jpg)http://img376.imageshack.us/images/thpix.gif (http://g.imageshack.us/thpix.php)

http://img503.imageshack.us/img503/7584/1002635zv8.th.jpg (http://img503.imageshack.us/my.php?image=1002635zv8.jpg)http://img503.imageshack.us/images/thpix.gif (http://g.imageshack.us/thpix.php)

Thing is I'm a compulsive tinker/fixer I wont stop till I squeeze every HP out of this car. So if any things even slightly off like the idle I just can't help myself!

11-15-08, 10:37 PM
OK, good. Those plugs are at the end of useful life. The tips, ceramics, and electrodes are completely normal color for unleaded gasoline, except for the one with the chunk of whatever in the gap, and that's the reason for the dark deposits: the misfire allowed the plug to cool down and not cook off the fouling on the shell near the threads.

Again, you'll probably never know what the particle in the gap is.......replace the wires, make sure they're hooked up correctly, and go about other tuning ideas...........

(See the black deposits on the threaded shells ? That's from idling the engine before you pulled the plugs. Ideally, plugs should be read immediately after a WOT run with the engine shut off and declutched at maximum rpm in order not to foul the plug with cold hydrocarbons. :rolleyes: )

11-15-08, 10:50 PM
quick question about the MAF, Mine doesn't have any wires in it theres a plastic brace in the middle its around 3/4 of a inch wide I assume the sensors are inside it where I cant see. Other MAFS I seen pics of have simple sensors and bare wires and do not contain the plastic brace mine has...

Is it after market or stock? Does any one here have any stock pics?

11-16-08, 08:53 AM
The plastic brace is probably an airfoil-shaped support for the heated resistors. Your MAF is bolted to the throttlebody, right? Does it have a honeycomb flow straightener (looks like a screen, but isn't, by a long shot) ? The pics are of the later MAF which is inserted in the intake line near the filter box and held by s/s clamps.

In these (stock and Jet aftermarket - forget it - snake oil) the airfoil brace is evident and the IAT sensor - Intake Air Temp - is the small black thermistor on the outer edge. Your IAT may be in the intake pipe, closer to the filter case. The 3 tiny heated resistors, between the thicker support rods, are what generates the flow signals to the PCM to add airflow to the fuel/air calculations at everything from idle to WOT.

11-16-08, 02:02 PM
Sub, I think his IAT sensor is either in the air filter box or the duct between the box and the MAF. GM didn't wise up and combine them til around 2000 I think.

11-16-08, 03:24 PM
Yeah, caught up with myself in the middle of typing that, which is why I stuck in "Your IAT may be in....".

Checking it out, it's curious that RockAuto lists a separate IAT sensor for the 2002 Sevilles.............

11-16-08, 09:01 PM
Hmm, that's strange. I could swear my '02 SLS had it in the MAF sensor.

11-16-08, 09:13 PM
IT IS in the MAF body........the little black widget in the side, just inside the 5-pin connector. The RockAuto list is incorrect for the 02 + MAF applications.

11-16-08, 09:38 PM
Ahh, OK now I understand what you where saying.

11-17-08, 11:33 AM
mine doesn't look like any of those :duck: I'll grab a pic today if I have time Its a bit beat up tho...

Oh yea I drove around all day yesterday and logged 170 miles and I average 19.7 MPG! Before the plugs I was in the high 17's...

11-17-08, 12:54 PM
Sort of like this ?

11-17-08, 02:09 PM
yea but the internals i believe are different.:o

I Just got a CEL so here we go!!!

P0404 - Exhaust Gas Recirculation (EGR) Open Position Performance
C1255 - EBCM Internal Malfunction

So can I fix this by shaking a large stick at it or will there be $$$ involved? :suspense:

I tried to remove the EGR a few weeks ago for a cleaning but theres 2 metal fuel lines running next to it with a braket that attaches to the EGR bolt. I removed the nut and tried for the life of me to move the braket out of the way but failed the fuel lines wont budge...


If you have had the fuel rail recall done, then you have a stainless steel fuel rail. There is no flex in it like the nylon rail. You must remove the rail and injectors.

Ok questions, I have 2 Steel rails and a Rubber/Plastic/nylon so I have "3" Rails all marked with caution fuel on them... Whats going on here? Secondly removing the rails is not a option how do I bend them out of the way to remove that braket? I got tin snips I might try to cut it off if not a file or hack saw blade... Either way I need it gone to clean the EGR valve...