NOTE: I understand this is a FORD forum, so I apologize for posting a Mercury question. However, I am not having much luck getting to the root cause of this problem, and I know there are some VERY SHARP folks on these message boards who may be able to help. THANK YOU!

2002 Mercury Grand Marquis
258k miles
No modifications - original engine / transmission

CYLINDER 8 MISFIRE (check engine light)

History:
1. Cylinder 1 2 and 8 misfire.
2. Replaced Cylinder 1 2 and 8 Coil.
3. Cylinder 1 and 8 misfire.
4. Replaced Intake Manifold (leaking; coils were very wet w/ coolant).
Also replaced ALL coils with Accel Super Coils and 8 FORD/MOTORCRAFT spark plugs. Note: Intake Manifold is Dorman.....mechanic said the fit was perfect and he noticed it was a "revised" Dorman intake.
5. Car ran GREAT....brand new....lots of power
6. Cylinder 8 misfire.
7. Replaced Cylinder 8 Coil with a new Accel Super Coil.
8. Ran GREAT, but with a slight "miss" while in drive at a stop light/stop sign.
9. Cylinder 8 misfire (tonight). (other code --- P420 Catalytic System, Bank 1).

I'm upset that I keep having a misfire in Cylinder 8. My gut tells me to replace ALL coils with genuine FORD/MOTORCRAFT coils....but the cost of all 8 is prohibiting.

I'm pretty sure I am going to replace Cylinder 8 Coil with a genuine FORD/MOTORCRAFT Coil.

Could something else be going on here?

Electrical problem? Fuel Injector? Why do I keep having a misfire in Cylinder 8, even after replacing the Coils (albeit with aftermarket coils)? The car runs GREAT for a few weeks, then the misfire.

Any help/recommendations is greatly appreciated!

 

First I have to declare a disclaimer regarding my "possibly" being one of those "VERY SHARP" people .
Anyway, one of the things we try is to swap coils with another cyl, preferably from the other bank and see if the misfire moves. Fords do not always tell the truth and many times it can be an adjacent cyl with the misfire. If you have eliminated the coils, plugs, or injector and all is OK , it may be necessary to have someone with the better scan tool to look at the misfire counts and cyl ID to be sure the problem is not coming from the PCM. Sometimes a new coil may not require the voltage to make it work properly then after some use it may break down due to low voltage that could be caused by a poor connection. Problems like this require some addl tools that are not always available to everyone.

Forgot to ask what does the plug look like when removed from the misfiring cyl, could it be oil fouled with all those miles ?

The P0420 could be the result of the misfiring cyl which may have destroyed the converter.

 

Hanky -

Thank you for your reply.

I will do as you say and move the Coil, and see if another code presents itself. My misfire is in Cylinder 8.....is there a certain cylinder # your recommend swapping it to? Cylinder 8 is located - drivers side, far back (closest to windshield).

I feel confident about the plugs.....I changed them in January of this year (that's when I was originally told I needed an Intake Manifold/Gasket) (Visible leak and plug/coil had coolant on them). They are Motocraft plugs. I am not confident with the Accel Coils (aftermarket).

I'm hoping the P0420 code is just an O2 sensor. ) Funny....in 268k miles, I never saw this code (never replaced CAT/02 sensors).

Thanks again!!!!

 

That code is related to catalyst efficiency for bank 1 and # 8 is on bank two. There is a good possibility it may have done some damage to the converter. The O2 sensor after the converter is telling you something is wrong.

 

Hanky,

Thank you for your 2nd reply.

I can understand that the cylinder 8 misfire may have affected my cat converter. However, what is causing the REPEATED cylinder 8 misfire? Do you feel it has to do with the new Accel coils not being up to par with the Ford/Motorcraft coils?

Leaning towards replacing cylinder 8 Accel coil with a new Motocraft spark plug and coil.

Thanks very much!

 

We all at some time or another have to utilize aftermarket parts and some are better than others, but with Fords I believe you have a better chance of getting the right part and usually a quality part if you stick with OEM parts. Motorcraft parts in my opinion perform better, last longer and give better service. You can purchase Motorcraft parts at many locations and not necessarily at dealerships which have high overhead.
There are so many low priced parts out there that just don't perform and are defective right out of the box. Sometimes due to circumstances we have little choice, but if possible consider staying with OEM.

 

Thanks again for your replies.

The odd thing about the aftermarket coils that are now about 3 weeks old, why is 1 (cylinder 8) gone bad, but the others apparently are still working fine? I bought all 8 coils at the same time, in the same box. I figured if 1 was bad, they would all be bad. I guess this is the mystery.

I will keep you posted. When the mood hits me, I will replace the coil with a Motorcraft coil and Motocraft spark plug. Thanks!!!

 

UPDATE

NEW CHECK ENGINE LIGHT CODE

P0358
Ignition Coil H Primary/Secondary Circuit Malfunction

 

Latest Update
04/03/15

Cleared all codes. The only came that came back on was P0420.

Replaced downstream oxygen sensor, passenger side.
Replaced cylinder 8 spark plug with motorcraft spark plug.
Replaced cylinder 8 coil with Accel coil (all 8 coils are now Accel coils).

Slight "miss" when at a stop light/stop sign in drive.

Runs great if I get down/heavy on accelerator. More problematic under light acceleration: hesitation, bucks, shakes. Easy to confuse with tranny shudder and/or front end play. Personal opinion - my "miss" and problem when accelerating is in the catalytic converter. Waiting for check engine light/code again.

Not sure if I have 2 cat converters or 4. I THINK I have a right and left converter, each having a front and back piece. If code P0420 comes back, do I replace the cat on the passenger side, front and back piece? With 260k miles, I am NOT going to buy a Ford cat converter. I would buy an aftermarket and have the exhaust cut/fitted/rigged to fit properly.

Wondering the symptoms of a bad cat converter. I cannot say I am smelling fumes or anything funny. It does not feel like I am pulling an anchor when accelerating. Its weird....when I give it full gas and "punch it", I take off like a bat out of hell and do not have a single hestiation. Its more when I am under light acceleration and when I am coasting above 60mph accelerating to 70 or 80 mpg.

My next move when the engine light comes back on (and it will, I know it), is to take the car to Fod and have them take all day and inspect and diagnose. I am going to tell them everything I have done recently and all of my sypmtoms. I want them to give me a full report and the exhaust and engine performance/diagnosis. From there I will determine my next steps, on a car with 260K miles.

Thank you!

 

Sometimes the mis fire is due to being right by the ac lines that go into the box. Condensation forms on the lnes and drips down onto the coil on that side, after a while it damages the coil, i have had that happen several times, and the after market coils go out faster then the oem ones. It may not be what this problem is, but you may check it out.