Is there a procedure for testing whether a coil needs replacing or not? The vehicle is a 2002 Mercury Grand Marquis with a V8 engine.
Thanks.

 

if the car is sitting lower than it should, that could indicate need, but coils should basically be free of major rust and cracks. If you can see any collapsed coils then you need to replace it. If the car is just bouncy, then the shocks need replacing.

 

Sorry, I should have been more specific, I was asking about the ignition coils that mount to the spark plugs.

 

ah... in that case, you'll need a multimeter and the spec for testing them. I don't have that since my books stop at 1998.

 

they test them for free at auto zone. maybe other auto part stores in your area

 

Thanks, that's good to know. I changed all the spark plugs and checked the EGR system, which looked good, no crud build up. The error code was misfire on cyl. 3, so far it seems the new plugs have cured it. The car belongs to a friend, He's happy with it.

Now he wants to have the transmission serviced and change over to synthetic ATF, any suggestions on which one?

 

no clue. I only use the stock recommended fluid (or the current revision of said fluid). A good rebuilder would know which fluid to use. Finding a good rebuilder may be difficult though.

 

Okay,quick step by step of how to test a coil without an oscilloscope.
(Autozone can't do it,at least ours can't-and resistance check will rarely show the fault.)

First,have your codes checked at Autozone.
For this exercise we will assume there is an obvious misfire and code.

Let's say code comes up as a misfire on cylinder 3 (P0303).

First,disconnect the battery. This is a step in the process,NOT a suggestion.

Second,remove the #3 ignition coil using a 7mm socket.
Next remove an ignition coil on the opposite bank of the engine,just pick one. We'll use cyl 6 because it's easy to get too.

Swap the ignition coils,so #6 goes to #3 and vice versa.

Now reconnect the battery,start the car.

Go for a drive down the road and find a good "test runway".

You will need to accelerate above 65mph,I do 70mph just because.
After accelerating to 70mph,let off the gas and do not touch the brake or gas until the car has decelerated to BELOW 35mph.

Do this 3 times. (if your CEL comes on after the 1st time,no need to do 3 times).

*I usually push the overdrive cancel switch,as the car will slow down quicker with the lower gearing. This will shorten your needed runway.*


If it is a totally dead coil,normally you will still have the misfire this whole time. Regardless,after doing this three time's your Misfire Monitor will be relearned and ready to diagnose misfires.

In a perfect world,the car will immediately flag a misfire and throw the light. Sometimes misfire will simply "go away". Won't get into that now.

Now you simply need to go have your codes checked again.
If the coil was failed,it should now read a misfire on cyl 6.
You did remember to pull the battery cable so the codes cleared right?

This is just a quick walkthrough of an easy coil check. We basically do the same thing at the dealer except we use a power balance to look at the misfire instead of codes. (or stress test the coil on a scope).

ALWAYS change the plug under a bad coil when you change the coil.
Always use antiseize and dielectric.
Always use a motorcraft plug. they are more expensive (4.99AZ/$8 dealer) but i've seen nothing but trouble using anything else.
DO NOT use BOSCH plugs,they are terrible terrible design-one look at the electrode compared to another plug and you will see that. (not to mention all the ground straps shroud the plug.)



Hope this helps,this is how I tell people to diagnose possible misfires due to coils at AZ.

 

I'll second the no Bosch plugs. Though I have no trouble with Autolite plugs. I did like the Autolite platinum in the rice burner I had (same car I found out about Bosch with... that car burned up the Bosch plugs in about 30K miles. The electrode was worn down well below the insulator. Traded the car off with the autolites still in it... with no issues after 60K miles.)

yeah, I'm used to the old stuff. Didn't know you needed an o-scope to diag the newer coils, though the AZ diag procedure makes perfect sense.

 

I'll second the autolite platinums,I use them myself and love em.
I cut em back and index them in my Yukon,and they take a beating.
My yukon runs extremely high timing on a mix of avgas and 101 octane,not to mention an occasional 100 shot of nitrous.
No issues yet on it,or my wifes impala or my fiero or anything else.

Fords are finicky,they'll either work or they won't.

And take another look at a bosch plug,you didn't wear the electrode past the insulator. The electrode on Bosch plugs are even with the insulator,and don't protrude past the ceramic even.
They are just trash,should be sold as a misfire in a box.