Coil going bad... how to tell which one?

Honestly....I'd start over.

Unplug one coil at a time. Slightly powerbrake the car to induce the miss. Plug the coil back in and go onto the next one. Which ever coil you unplug that doesn't make the car run worse is gonna be where you start looking. At that point its going to be a bad coil, bad boot, or a dirty plug. Worse case it could even be a plugged/dirty injector.

I wouldn't start moving onto fuel pressure issues and what not until you've completely ruled out the miss as being only one cylinder. Then, if you believe its more then one cylinder, move onto fuel or air. Always spark first when you're dealing with a miss.
 
I don't know what effect it would have on COP coils, but back in the day, we just pulled individual wires and held them close to the engine. If the wire sparked, we called it good. If it didn't spark... well, you get the idea.

I don't see why pulling a coil and putting the spark plug end close to the motor would hurt the coil, but you never know.

Of course, if you're not careful, you might be in for a... shocking (no pun intended) experience.
 
On these high voltage coils, it can be bad for them to randomly "short" to the engine block as you hold them, and also, just because they are arcing it doesn't mean you're getting the full potential. You may get enough spark to look like they are doing their job, but when under load, the spark isn't enough. Ya know what I mean?
 
I was with Mark when he looked at them. The boots looked excellent, although we did not vigorously bend and twist the boots looking for cracks. They were obviously changed at least once. The hairline cracks he refrences were on the plastic coil casing. They were very small, but obviously it is a point for moisture to get in the coil and cause rusting. The cracks were no where near as bad as the ones on Trixie's car when she had a missing problem.

There was oil in some of the plug pockets. No signs of moisture, which was good. Each goil boot and srping was very clean (no rusting).
 
We tried pulling codes with our chip programmer, but I can never get that thng to pull codes well. I usually borrow the premium $400 code reader from the auto parts store.
 
I'm still all for replacing the cracked coils, even if the cracks are just hairlines. Again, if it's not the problem, it'll turn into one eventually. I can pretty much guarantee you that.

And still, the best and most accurate way to test a coil is to do a resistance check. Yeah, it'll take a bit, figuring that there's 8 coils... But this thread has been going on for days, and it shouldn't take days to do the check. Remember: A coil can still work and appear to be functioning properly while having excessive resistance. A digital multimeter isn't that much money if you don't have one and it's really worth it just to check.

As far as the drivability issue is concerned, yeah there's other possibilities for the misfire as well. I agree with J in the starting over part. What you really need to do is a visual inspection on the small stuff. Even the air filter or intake can be an issue. Then determine if you have proper fuel and spark, THEN start looking at sensors and codes.

Do you have the means to do a fuel pump pressure and volume test? The kits aren't that expensive and they'll pay themselves off. They're just a gauge, some hoses, a release valve, and an attachment to connect to your shrader valve on your fuel rail. A good rule of thumb is that you should be able to fill a water bottle 1/4 within 15 seconds. I'm not certain what the output specs are for the Mark VIII, but most vehicles are within that range. This'll be much easier than replacing the fuel filter and hoping for the best... Although a fuel filter is something to be replaced regularly anyway, so you might want to consider it in the back of your mind while you tackle this job.

Do you have a spark tester that allows you to adjust the tester gap? They cost almost nothing and are a great way to measure the output of your coil for COP setups as long as you have a coil that you KNOW is good. Seeing as how 3 were already replaced, I'd use one of those as a "control"... This isn't as nice or as accurate as a resistance check, but it's faster and a bit more convenient. I prefer the other way, but I thought I'd throw this out there.

Anyway, good luck. Let us know what happens.
 
I agree with Lucas...except the order of checking for codes. (which is irrelavent in your case since you already tried) Anytime you have issues, you should go straight to the computer and scan it for codes. Then go onto the mechanical side of diagnosing. :)
 
Yeah, well... You're ugly! :p

Actually, they teach us to do a visual inspection and check for sufficient spark and fuel at school, so... Yeah. I disagree. The reason is because if you use any type of software to aid you in your service (we're slaves to AllData and ShopKey5), all of the procedures for specific codes assume that you have sufficient spark and fuel unless it's requesting a specific procedure with exact values determining the course of action you're supposed to take. Whatever works for you, though. As long as it works.

EDIT: I still have no clue how to turn that little :p into a smiley. I can fix a car but I can't do forums! GAH!

EDIT 2: So I gotta hit "post reply" instead of quick reply and click the individual smileys... Meh, it's too late now. That's my almost-smiley and I'm stickin' to it!
 
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Thanks everyone for the replies. The car's been running OK for the last few days (only an occasional miss) so I'm holding off until this weekend to try your suggestions...

...which means my car will catch fire by Friday afternoon, with my luck.
 
I'll stock up on beer. ;) And good luck getting a fuel filter changed around here - it's a job no one ever wants to do for me. In fact, I probably have a new fuel filter sitting around here somewhere....
 
The fuel filter was replaced a year or so ago, but I've been known to drive it on fumes so I'll give that a shot too. I have the filter and the little tool to disconnect the lines from it, but what's the simplest way to release the pressure from the lines, short of buying yet another special tool? I've heard of leaving the fuel cap off, then turning off the pump (switch at the rear of the trunk behind the trim right?), and running the car until it runs out of gas. Will that work?
 
Find the Schrader valve at the front of the driver's side fuel rail. Take the cap off, and put a rag on top of it to absorb any fuel that may spill.

Realistically though, there shouldn't be much pressure there with the fuel pump not running.
 
AssFoe said:
Yeah, well... You're ugly! :p
Click to expand...

Lol! :D

AssFoe said:
EDIT: I still have no clue how to turn that little :p into a smiley. I can fix a car but I can't do forums! GAH!

EDIT 2: So I gotta hit "post reply" instead of quick reply and click the individual smileys... Meh, it's too late now. That's my almost-smiley and I'm stickin' to it!
Click to expand...

Negative....look at the very bottom left of the page. It will say "smiles". Click on it and it will open a new page showing you how to put in the "code". I usually just copy and paste.



M Darrah said:
the simplest way to release the pressure from the lines
Click to expand...

Remove the plastic line off the fuel filter. After the car sits long enough for you to pull the wheel off(or just turn it all the way to the right like I do) and remove the back side of the fender liner the pressure will be a quick spert and thats about it. Ofcourse once you "break the seal" the fuel will start draining out of the fuel rails and fuel filter so make sure you have a pan down.

Wear safety glasses...always. Ask anyone here....I know, lol. Unfortionally, so does Dukey. :(
 
I just replaced the plugs with 764s gapped at .054 and so far the car is running smooth as silk. The old plugs had a lot of white/light gray deposits on the side electrode and the center electrode was practically worn down to a nub on every one. Of course it doesn't match any pictures I can find for inspecting them, but the closest match seems to indicate the plugs were too hot or lean fuel, so I'll be sure to replace the fuel filter. I just didn't have time for the filter today. Maybe I'll try to post pics of the plugs when I get the chance.

In any case, the misfire seems to be gone for now, which is a good thing.
 
DANG! White is bad....gray is O.K....more dark gray/black is normal. Worn down to a nub is BAD, lmao.
 
What was the gap of the worn plugs? Likely way over 0.060 which could've caused firing issues.

I usually gap new plugs at 0.048".
 
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