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Old 03-21-2015, 08:39 PM
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Default Cylinder 8 Misfire (again)

2002 Mercury Grand Marquis
258k miles

No modifications - original engine / transmission

CYLINDER 8 MISFIRE (check engine light)

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!

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Old 03-30-2015, 08:29 AM
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Ignition Coil H Primary/Secondary Circuit Malfunction
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Old 04-05-2015, 08:50 PM
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Latest Update

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!
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Old 10-08-2018, 10:50 AM
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i know this thread is 3 plus years old but you might be able to help . i have a 2000 GM with with the same symptoms as yours and did the same fixes as you with the exception of the coils . did you find out the problem ??

thanks ,
armond martin
saucier , ms.
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