Cylinder 8 Misfire (again)
 

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!

THANK YOU!

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!

DJR ,
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.

I have a '99 Grand Marquis, and I have the same problem. I've replaced all my plugs and my coils but still a miss fire. Big thing (I think) there is oil all over my cylinder 8 spark plug.

Misfires - Plugs or Injectors ?
 

After changing all the spark plugs and the coils, and the mis-fire persists -- swap the fuel injector of the mis-firing cylinder with another cylinder's fuel injector.
If the problem moves, the injector is faulty.
A whole set of eight (8) reconditioned/flow-matched injectors can be had for 100-150 dollars -- shop around.
Premium-quality Iridium spark plugs give best longevity, performance, and fuel mileage.
Your Coil-On-Plug engine requires the Bosch #9606 Double Iridium design, gapped at .054 in.
ALWAYS check the spark plug gap !!
Torque spark plugs to 180 inch/pounds. Use NO anti-seize !! Bosch spark plugs are plated, so no anti-seize is necessary.
RockAuto.com for the best prices on Bosch Iridium plugs -- and other parts, too !
https://cimg2.ibsrv.net/gimg/www.mer...00842f6831.jpg
'95 MERC Engine Bay

His use of non Bosch plugs is not causing his misfire. IF they are so good, Ford would use them as OEM from the start.

Cylinder Mis-Fire
 

NOBODY, including myself, has said that the OP's problem was caused by the use of non-Bosch spark plugs. Your obsession with downgrading Bosch products is comical, if not downright silly -- not to mention your ever-present surliness.
The Bosch plugs that I use and recommend for the '92 - '97 Grand Marquis is the Bosch #9605. The double-iridium design was not available when The 95 MERC was built, so Mr. Ford couldn't have put them in, even if he wanted to.
The Motorcraft plug that came in the car was a "SINGLE-Platinum" design, and the Factory Shop Manual says that these should be replaced with a DOUBLE-Platinum plug at first plug change. NO Grand Marquis were fitted with double-iridium plugs from the factory -- ever.
Along came the Bosch Double IRIDIUMs, and the multiple features of the double-iridiums were sufficient to promote the up-grade.
The Motorcraft plugs are made in Ohio -- where you live. Coincidence??
The Motorcraft plugs are a fine product, just like a Timex watch -- tells time cheaply and reliably.
The Bosch Iridium is more like a Rolex -- and I prefer a Rolex.

https://cimg6.ibsrv.net/gimg/www.mer...2af9a3256a.jpg
Sleeping Beauty, Rolling Thunder

You said "Your Coil-On-Plug engine requires the Bosch #9606 Double Iridium" like that is the only plug that will work. Your pushing of Bosch is ridiculous and sounds like a sales commission is involved. I have yet to see anything proving your claims other than Bosch sales information. I never seen anyone on any forum being as one sided as you.. In addition to pushing Bosch products, you clutter this forum with worthless pics of your G.M. The pics are irrelevent to solving anyones questions. It's almost as worthless as all the smutt that is on this forum.

Let's be clear -- It's not MY fault that YOU insist on making up your own interpretations of my words. IF you have a problem understanding my words AS WRITTEN, the civil response is to ask for clarification. Instead, you rant on and on, seemingly determined to degrade Bosch products for no apparent reason other than to bad-mouth ME.
However, once again, I will attempt to explain my words in a manner which you may be able to comprehend.
YES. I said:
"Your C-O-P engine requires the Bosch #9606 " -- as opposed to the '92-'97 (with plug wires), which requires the Bosch #9605. The "requires" applies to the part number comparison, calling attention to the difference between the early and later engines.
Does either of those engines "require" the exclusive use of Bosch spark plugs ?? OF COURSE NOT !! And I never said that they did, despite your twisted implications.
Don't like my pictures !?? Well, that's just too, too bad -- looks like they got lots of up-votes from other visitors, so perhaps YOURS is the odd perspective. It's understandable, what with the jealousy and all, you've made it so obvious.
But hey, you could always buy a camera and take some pix of YOUR Grand Marquis, showing details of the modifications that YOU made, and the part numbers that YOU used in the build. Like I did.
https://cimg9.ibsrv.net/gimg/www.mer...4240963aaf.jpg
Walker H-Pipe, MagnaFlow #18134 Glasspacs; Walker Tailpipes