: P0308 #8 Misfire issue... Replaced a lot, still have issue



buddman04
07-02-14, 11:03 AM
OK... Here is my issue... on my 2000 Cadillac Deville with a 4.6L Nothstar, I only have one code and it is P0308, which is a #8 Misfire. I have dug into the issue and removed the Coil Pack and saw the #8 coil appeared a little burnt and tip was rusted. So I continued to remove the plug boot, and spark plug. I found that my plug was toast and broken. Has me scared.
This is a pic of the plug
227962

So now I cleaned out the cylinder the best i could with a magnet. I did get some shavings. I then replaced with a new Spark Plug, new boot, and went to the junk yard and grabbed a good coil pack. I am still getting P0308 error. Hoping the #8 is not damaged.
Here is an issue though.. when I clear the codes with the code reader, the car runs great for about a minute and then it starts to miss and the light/code returns. Why would it run with out the miss for that one minute? :banghead:

Thanks ahead of time for any input you can share!

Submariner409
07-02-14, 01:01 PM
Time for injector and compression checks............... and new AC Delco #41-987 plugs (made by DENSO in Japan).

rodnok01
07-02-14, 01:09 PM
Those are not ac plugs for sure but haven't seen an engine eat a plug that way in long time. Pull that plug again too. That cylinder running hot and lean to do that.

buddman04
07-03-14, 03:29 PM
OK, Pulled plug and looked fine, compression was at 122... As I replaced the boot, I saw the SPRING was missing. I replaced spring from extra boot and reset codes. It is working now and not throwing codes... It appears I dodged a bullet.
You mentioned checking the injector. How do I do I do that?
Once I am set and know I'm not in for a rebuild, I am getting new plugs for the entire car. :cool2:

Ranger
07-03-14, 04:16 PM
Use a stethoscope and listen to the injector. It should have the same rhythmic ticking as all the others.

Submariner409
07-03-14, 05:58 PM
OK, Pulled plug and looked fine, compression was at 122... As I replaced the boot, I saw the SPRING was missing. I replaced spring from extra boot and reset codes.

That missing spring is what messed up everything. Someone had little or no idea what they were doing at the last spark plug change.

More info in my albums - the Community tab ^^^, Albums, the two with the red Seville on the cover, or click my username, View Profile, scroll down, Albums.

Use this when you get to new plugs - SOON.

Mark C
07-06-14, 08:16 AM
How can a plug that is not firing, melt like that? That might have been a previous owners solution to a lean running cylinder that was eating plugs. If your injector it working check that plug after a week or so to see if its got any signs of being in a lean cylinder.

rodnok01
07-06-14, 12:21 PM
How can a plug that is not firing, melt like that? That might have been a previous owners solution to a lean running cylinder that was eating plugs. If your injector it working check that plug after a week or so to see if its got any signs of being in a lean cylinder.

A lean cylinder will eat plugs when it gets too hot, valves can be damaged and pistons overheated. Once the cylinder is really hot plug gets ate then it stops firing.

Ranger
07-06-14, 08:40 PM
If the combustion chamber got hot enough to melt plugs, I suspect new plugs are the least of your problems.

Submariner409
07-07-14, 03:30 PM
Fortunately (or not) in a spark plug wiring, insulator, carbon tracking, or preignition situation the spark plug is the first to suffer. Most of these times the plug will stop firing/trying to fire and the cylinder dies (power-wise). That irritates the owner who then may do something about it.

Sometimes the plug self-destructs as in the pictures, maybe leaving bits and pieces of metal and diamond hard ceramic in the cylinder which almost always causes mechanical damage. Stuck valves, broken piston rings and scored cylinder walls are the most common.

In this thread's situation I do not like the 122 psi compression figure - a Northstar should easily show 140+. (Remember - when doing compression tests - engine warm, NO plugs installed, ignition disabled and throttle blocked wide open. Roll the engine 3 times for each cylinder test.)

Low cylinder (greater than 10% variation from average) ? Pour in a teaspoon of engine oil. Try again. Compression raises ? Stuck or faulty piston rings. Little or no compression change ? Faulty valve(s).

Many years ago, in one of the shops I worked in part time, a Jaguar mechanic (British gent) gave me advice on how to detect severe preignition/detonation damage: The piston top and spark plug will look as if "The mice have been at it". Look closely at the threaded shell tip of that plug.

buddman04
07-08-14, 09:27 AM
Thanks for all the comments! I was shocked that the car fired and ran without the entire front back coil off as I tested the compression... I did not rev it and will do so soon. I will also try to oil checks.. that totally makes since and thanks for the heads up!
So far its been a week and I have had no codes! But the first 2 days it would just stall after being ran for about 5-10 minutes. That has stopped as well.
Stay Tuned.

Submariner409
07-08-14, 12:26 PM
You always test compression with the ignition disabled and the throttle blocked wide open.

Even at cranking speed with the throttle closed atmospheric pressure cannot fill the cylinder enough to give an accurate (air) compression reading. Think about it - at throttle closed idle speeds the engine ingests practically NO air/fuel mixture - as the throttle is opened, atmospheric pressure pushes more air/fuel into the cylinders and combustion pressures (power) increases. (In a fuel injection engine the systems keep fuel/air mixture in proper ratio as the throttle is opened.)