I have a 2005 mercury grand
marquis and i've been having an issue with it misfiring when its raining or sometimes even when i use the washer or wipers. the engine will stumble once maybe twice and then it'll be fine. i replaced the plugs and two torn boots but it still happens. whats up?

 

Does the check engine light come on?If yes, scan for trouble codes and post exact numbers back here. If not, you may need to put it on a tool that can detect a cylinder misfire as it is happening. If not, have some one hook up a tool and go to mode $06 and see which cylinder or cylinders are misfiring.It will stand out by being a number a lot larger than the others.

 

I realize that I need to check for misfires, but when I hook up to any scan tool there isn't any fault codes and any scan tool I've used doesn't show misfires or a misfire log. As a mechanic I am stumped. I have heard rumours about a misfire caused by water damaging or corroding the far rear ignition coils and have heard from a reliable source that its very common, especially on F150s for sure with the 4.6 so I would imagine it would carry over to other vehicles with the same engine. I haven't seen or heard of any tsbs or bulletins on this. My dad works for the city and they have pretty much 90% F150s and the mechanics there told my dad that they replace coils on the back of the engine all the time. One of the lead techs at my work told me that to find a miss or stumble when there is no fault code or misfire log I need to mix up salt water and spray down the coils and wait for it to miss... I've also been told that if I unplug a coil I can also see or feel the miss and possibly start the misfire log.

 

to be perfectly honest its more like a buck or surge of engine power and not a miss. it feels like the engine goes flat or surges like more than one cylinder is missing, kind of like if the torque converter is slipping but i know its not the torque converter because i stepped on the brake when it happened and it didn't stop or get better. i for sure know it has something to do with moisture interrupting current somewhere, there's just no misfire count or log on our brand new mac mentor or old school snap on scan tool. anyone have any ideas?

 

Let me research the Mac Mentor. It has a misfire counter under the Mode $06 area of it. One of my techs has one and I will ask him and post it for you.

 

I thought it was odd too that every other car i've worked on lets you access the misfire counts or has them on the data display except this one. I don't know if i'm in the wrong program or what. I had a 1995 marquis that had a bad misfire and it had obdII and there were hundreds of misfires on two cylinders and no ses light, but the mentor showed misfire counts..

 

Did you go under the global OBDII link? I believe it is under there to access the Fords.

 

I'll have to try the global link. I thought i already tried that one, but i'll try it again