I have a 2005 mercury grand marquis with 55k on it and I have a misfire when I am driving on the freeway and it is raining heavily or if I wipe a big load of water off of the windshield it will have a really bad misfire almost like the engine temporarily 'stalls' for a split second and then it will run just fine, but sometimes it will do it a few times, but it always runs just fine except for the miss.
About 10k ago I took all the coils off and found a couple cracked boots so I replaced them and I replaced all the plugs when I had them off and I put dielectric grease in all the connectors and on both ends of the boots, but I still have the same miss. I have never found any codes stored in any module and for some reason I can't get a misfire count graph on our brand new mac mentor or our old school brick scan tool to determine which cylinder is misfiring to pinpoint it...
I saw in alldata that ford had issued a bulletin for misfires and proper diagnosis of bad coils on the 4.6 and 5.4 engines with c.o.p. but I don't have access to a ford rotunda oscilloscope that can "stress test" the coils because I am a mechanic at a GM dealership so ford special tools are hard to come by.
My dad had mentioned something about the 4.6 and 5.4 engines on the trucks that they buy at his work (they almost always buy ford 1/2 and 3/4 ton trucks) how the #4 and 8 (or maybe #3 and 7) coils go bad from shorting due to water getting on them constantly and I thought this could be similar to my problem. He said that it happens a lot and I should replace my coils, but I don't want to just dump money into it and buy 8 new coils if that isn't going to for sure fix the problem.
Any Ideas or suggestions? Thanks!

 

the 4.6L like what you have has issues with moisture getting down the spark plug boot. and many times if it isnt moisture, the plug boot goes bad and is cracking causing it to arc some and misfire. main reason i like the design of the 5.0L i have under the hoods of the cars here at home

 

If you can hook any scope to the coils you can use the engine to stress the coils. Once you get the pattern, snap accelerate and if the coil is weak, it will fall out. Or, place in gear and hold the brake and accelerate until the engine starts missing. You may have to do it once for each coil to see which ones are dropping out. Your Dad is correct. Also look closely at the intake nipple on the passenger side rear of the intake. These will seep and cause coolant to run into #8 plug well. If this happens, only repair is intake manifold replacement. Good luck.

 

i don't have any loss of coolant so that rules that one out. is there a way to check the coils by testing resistance or is this kind of one of those things that you have to make the thing miss to find it? i can't seem to find a misfire counter in the mentor so do i have to make it misfire badly enough to turn it on or am i in the wrong program?

 

The misfire counter should be in the tool. Are you going into Mode6? Should be under 53 I think

 

I'm not sure which one I was in, all I know is that if it is in there I missed it. I used the same one on my 95 marquis 2 years ago and it showed misfires on the data display. The data display on my 2005 doesn't show misfires at all. It's kind of frustrating.

 

Are you going under Global OBDII? Then go under Mode $06. Then look for either 53 or 56. See which one gies you 8 or 10 options. This gives each cylinder total misfires. You can not see it loive. It has to be off with key on to see how many misfires each cylinder has had.