300c spark plug analysis, rough idle, mediorce economy

2007 Chrysler 300c. 21,000 miles, fresh oil, new plugs installed, old plugs marked and saved. The spark plugs from the NON-MDS cylinders (2,3,5,8)look suspicious as compared to the MDS cylinders (1,4,6,7). The spark plugs from the MDS cylinders have a consistent, light brown haze on the porcelain tip. The NON-MDS cylinders are mostly white with an obvious black spot. What could this mean?

MDS is the system that turns off the cylinders when not needed, correct? Since some cylinders have plugs that fire all the time and others have plugs that “rest” and don’t fire constantly you can expect some difference in the appearence. Compare the plugs that fire all the time to each other, and the plugs that rest with each other. You can’t compare the plugs that fire and the ones that rest to each other. They are operating in different enviornments.

The plugs resting still have air going through the cylinders, it is just that the gas is not being injected. Some cars open all the valves to allow the pistons to pump air, and others keep the valves closed so the air is compressed and then pushes the piston down when the compression is released on the down stroke. Regardless of the system employeed the plugs are working in different conditions.

As for fuel economy, you didn’t give any specifics on what kind of mpg you are getting and under what conditions. Bottom line is the 300c is a big car, and a heavy car. You have an 8 cylinder motor made to give you strong acceleration. Shutting of 1/2 of the pistons isn’t going to change the basic physics much. You have a big mass to move and a big motor to move it quickly. That adds up to mediocre mpg. If you are getting over 20 mpg on your expressway trips you are doing OK. Around town you are going to get something like 15 mpg if you live in a cold weather climate.

Thank you for the reply. Per the service manuals, the 300c keeps the valves closed in MDS mode. My primary concern is rough idle ( worse when cold ) and if the spark plug pattern of the always on/running cylinders (2,3,5,7) indicates something wrong. My experience (age 60+) in looking at used spark plugs (from an engine that is running smooth/proper) is seeing a light brown haze on well used plugs on all cylinders. Since the NON-MDS cylinders are carrying the load when the other cylinders are loafing, I am thinking the timing or fuel mixture is a bit off when the engine is in MDS mode. Do you know of any references/sites to proper spark plug analysis/patterns for the Chrysler hemi with MDS?

I don’t know of specific plug analysis for the hemi with MDS. At idle all cylinders should be firing and the MDS should not come into play until coasting at higher rpm’s.

I don’t think the plugs are the problem, unless one is bad and it would be different than at least 4 of the others. There maybe a bad coil on one or two plugs causing some misfire at idle, are these coil on plug type coils?

If so, there may not be enough misfires to trigger a code. Sometimes the rubber boots and springs that run the current from the coil to the top of the plug deteriorate and really don’t look bad, but have carbon tracings which can “ground” the current and cause a misfire.

If you can order or buy a set of boots from NAPA, install a new set and see if it helps. I have coil on plug issues with my T’bird and the boots deteriorate every 30K miles from heat and whatever.

First, I would check for codes with the “Key Dance”. Simply turn the key to acc and then to on (not start) very rapidly 3 times ending on “on”. This will display any trouble codes stored in the Computer on the EVIC ending with a display of “Done”.

The plugs are always firing even with the MDS on so expect a variation in color.

Conditions for MDS to turn on/off are vehicle load and speed,Throttle Position, Oil Viscosity and Airation, (USE 5W20 or 0W20 oil only!), temperature variation between Cylinders and Engine Temp. The Transmission must also be in “D” or the MDS will not function.

A good sight for more info or questions will be http://www.300cforums.com/ or http://www.lxforums.com/.

Thanks again for the pointers. I will keep digging.
The coil and boots (notice the plural) are one piece. Two plugs per cylinder. The boots/extensions smell a bit strange. When you mentioned carbon tracking, the odd smell of the boots came to mind.
As to mediocre mileage. It has been about 20 overall. I have only had the car about 4000 miles so road mileage is still negatively affected by enthusiasm.
I will check for trouble codes again.
The local dealer suggested a $190 de-carbonizing/fuel injector cleaning. I did not want to pony up to that solution yet. Warranty is still good until June.

Thank you for the response. The Key Dance did not produce any codes, just the Done. I did clean the carbon deposits off the throttle body. Took it off and wiped the gunch off with acetone. Now that I have seen the gunch the EGR system provides to the intake system, I may remove/clean the egr valve and/or remove/inspect and/or clean the injectors. I have very little experience with injectors. All the vehicles I have owned with FI did just fine with changing fuel filters and the occasional can of Berrymans.
I have posted a similar question on 300cforums. No response there yet.
My interpretation from your comments is: The engine should not feel like it is skipping/missing at atartup(in neutral) and/or at stop lights (in drive at idle).

Most 300c drivers would be deliriously happy with 20 mpg.

Just to let you know, My 06 5.7L Charger also sometimes (but very rarely) stumbles at idle/cold starts. That condition was very pronounced with the original Champion Copper Cores but after my 30,000 mile plug change using Autolite Iridium Enhanced Plugs, the engine smoothed out. I average 19 mpg on my car.

(The Brand Change was due to my experience with defective Champion Plugs due to Exhaust blow by from bad crimps on the Metal Shell of the plug. I also like to keep my plugs “gap” longer than the Copper Cores short 30,000 mile range.)

Rule of thumb for the Hemi, due to the way the Coils discharge the two plugs,(one plug discharges from the center to side electrode, the other plug discharges in reverse), you have to use Double Platinums or Iridium for extended change intervals. If not, plug changes need to be done every 30,000 miles.


Thanks again for the additional comments. The comment about exhaust blow by on the spark plug crimps makes sense. The champions I removed had/have what looks like blow by on the insulators above the metal “crimp”. The stuff wipes off but it sure looked suspicious.
After visiting the chrysler dealer and getting his offer of a $190 decarbonize service, I elected to try a 3M product on my own. It included a run through the intake system can and a 16 ounce can of stuff for the tank. Took a trip out to Ruidoso (from austin texas area). After first tank, idle was smooth(er). Trip back without AC on (It was cool outside) the highway fuel consumption (cruising at 75) was ~25 mpg. I am going to inspect the spark plugs again now that they have operated in MDS mode for a while. If they have tracks of blow by, I will go for a different brand.