2006 cylinder 5 misfire mystery
 

Hi everyone, I just inherited my father's 06 grand marquis ls. It runs great when it's cold, starts up and idles no problem. However, once it reaches running temps it will throw a p0305 cylinder 5 misfire, only at idle. Once the code is thrown if I try to accelerate at all the motor completely shuts off. If I then restart the motor it starts up no problems and runs well at idle for about 30 seconds then throws the code again. I have replaced the spark plugs and the coil packs. I'm wondering if anyone has ever had a problem like this and if they could point me in the right direction for what to check next. Thanks for any help!

Hello, Tyler -- and welcome to the Mercury Forums.
From the description of your problem, I have one question:
Did you RE-SET the code indicator !??
After the engine throws out a code, it must be RE-SET after the repair is completed.
RE-SET your codes by disconnecting the NEGATIVE Battery Terminal for 30 minutes -- then re-connect.
When replacing Spark Plugs, use NO ANTI-SEIZE -- anti-seize often causes plugs to short across the gap.
Some Spark Plug manufacturers offer plated surfaces that won't seize -- I use the BOSCH Double Iridium, Fine Wire design,
gapped at .053 ", torqued to 15 ft.lbs. Your '06 engine takes the BOSCH #9606 Double Iridium -- Best spark plug prices and selection at RockAuto.com.
If the engine keeps throwing the same code, swap the coils and see if the problem moves -- if it does, the coil is bad.
Replace any bad coils with the stock MOTORCRAFT coils -- no need for those "Hi-Performance" rip-offs.
You din't say what brand of spark plugs that you installed -- hopefully it wasn't those E3 plugs or the super-expensive Pulstars,
which go to prove that P.T. Barnum was correct -- "There's a sucker born every minute!".
Anyways, let us know how you make out with those codes, and good luck !

I used the bosch DI plugs aplugs used TRQ coil packs. It had a cylinder 5 misfire before replacing all of the plugs and packs and it still throws the same code. On the highway the car runs perfectly, but as soon as I have to stop at a light for more than 30 seconds it throws the code. And yes I have re-set the car every time.