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EMRSR, Master Certified Service Manager
Category: Mercury
Satisfied Customers: 3010
Experience:  20 years with Ford
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2002 Grand Marquis: Cylinder 8 and 6 misfire..replacing..coil

Customer Question

Cylinder 8 and 6 misfire on 2002 Grand Marquis after replacing cylinder 8 coil and all the plugs.
Submitted: 6 years ago.
Category: Mercury
Expert:  EMRSR replied 6 years ago.
How are you determining it is #6 & #8, do you have a scanner or code reader?
Customer: replied 6 years ago.
OBD II scanner from Auto zone
Expert:  EMRSR replied 6 years ago.
Swap coils from 6 & 8 to 5 & 7, rescan to see if the coils missfire on 5 and 7. You can do the same on the next step with the plugs. Finally you can do that with the injectors, and by process of elimination determine your failed part.
Customer: replied 6 years ago.
Hello again, I swapped the coils and the problem stayed on 6 and 8. Do you have anymore advice or tests I can perform from my home? I can also smell the exhaust I think the car is running rich.
Expert:  EMRSR replied 6 years ago.
Did you swap plugs and injectors as well?
Customer: replied 6 years ago.

no just the coils


Expert:  EMRSR replied 6 years ago.
Do the plugs first, make sure they are not failing. That rich smell is from the missfire, unburned fuel.
Customer: replied 6 years ago.

sorry I had to step out for a minute the plugs were installed two days ago


Expert:  EMRSR replied 6 years ago.

Still move them as I had said so I can give you an answer here. 2 days or 2 years a new part can fail. We need to tighten up.


Customer: replied 6 years ago.
Do you recommend testing injector pulse from the pcm harness wiring?
Expert:  EMRSR replied 6 years ago.
Not till you do the prior, using a noid light now is skipping steps.
Customer: replied 6 years ago.
Its either the coil plug wiring or the injector connection. Which test do you suggest to perform first?
Expert:  EMRSR replied 6 years ago.
Sinc you have not worked with the injectors and this started after your first repair I would look towards the COP.
Customer: replied 6 years ago.
COP has spark on cylinder #8. Injector #8 solenoid is clicking. There is fuel and spark. The COP was replaced and ran fine. 2 days later cylinder #6 started having a misfire. Companion cylinders? Swapped COP and injectors from #'s 6&8 to 5&7. Still having problems with 6 and 8. PCM suspect? Wiring harnnsses at fault? Spark test completed and what do you suggest next?
Expert:  EMRSR replied 6 years ago.
It is rare PCM's fail, but the wiring harness we assume was fine before this started and the problem has expanded to another cylinder. I would inspect the harness and then begin questioning the possibility of a base engine concern, have you checked compression?
Customer: replied 6 years ago.
No I have not checked compression, but for a misfire it has to be spark or fuel. There is no evidence of traces of oil on the spark plug leading me to believe that there is decent compression from both cylinders. I ohmed out the injector connection to the injector and the readings were 1150 ohms. No short or open in the injector harness to cylinders 6 and 8. What test besides compression would be practial?