Why did #5 die?

[Desert Stallion]

Tried posting this in the SVT forum because it has to do with a 4V motor, but not getting any response over there. Hopefully someone over here can help.

Okay, what started out as a simple misfire has turned into a friggin' mechanical pergatory. Here's the scoop...
-Drivin' along minding my own business, all of a sudden I have a harsh vibration start. I figure I tossed a wheel weight or something, pull over only to discover it's the motor that's developed a hard miss. I limp home and do some basic trouble shooting, decide I've got bad plug wires because one I had an intermittent break in and the others all had high resistance.

-I rebuild the broken wire, but no change.

-I measure the coil packs and discover they're all out of spec, so I replace them as well as finishing rebuilding the rest of the plug wires. No change.

-Now the car has all new ignition stuffs, plugs, wires, coil packs.

-Just to shotgun parts, I had a set of 24lbs injuctors laying around, so I swapped them in. No change.

-Now the car has new ignition and injectors, plus good fuel pressure.

-I run a compression test, and all cylinders are between 175 and 190psi, the lowest being #3, and the miss is on #5, opposite sides of the motor, go figure.

All this, and the computer is throwing NO CODES. Not even a check engine light for the motor going lean/rich.

So, new ignition, new injectors, good fuel pressure, good compression throughout the motor, no vacuum leaks that I could find and a strong 19 inches at idle, bearing in mind that the car is running on 6-7 cylinders right now since #5 being gone is somehow affecting #6.

So WTF is going on?



Then today...


-Put the scope on the injector harness, and it's fine. Got a good, steady pulse on all 8 injectors, so they're firing.

-Pulled the valve cover (in 30 minutes might I add, I'm getting waaayyyy too good at taking this engine apart) and I didn't find anything amiss. No broken valve springs, no damaged/missing followers, nothing out of the ordinary.

-Ran another compression test for *****s and giggles, and still have good compression, showing that the valves aren't damaged.

Okay, so there's nothing physically wrong short of an imploded piston with that cylinder, and even there it would've shown up in the compression test. It's not fuel, or air, so it's gotta be spark still, but I don't see where the fault is.

Can the EDIS module fail in a way that would only take out #5 without affecting #3? Because #3 still works from what I can tell. If I pull the plug wire on it the motor responds by dropping RPM's and dogging down like it should. If I pull the wire for #5 nothing happens. The coil is still firing though, and the wire is new, as is the spark plug(s) that I've tried.

I've still got the valve cover off, anything else I should be looking for?

Thanks all.

 

[crew_dawg16]

is that #5 wire getting spark? I couldn't tell from your post if you actually tested that. You may just have weak voltage to that wire in particular and it's not a hot enough spark to ignite the fuel. That is the only thing I can think of right now.

 

[Desert Stallion]

Yeah, the wire is carrying the current from the coil to the plug and causing a good spark.

Kinda stumped here.

What about an exhaust blockage?

 

[jstreet0204]

Try checking the crank position sensor. I know mine was throwing a consistent #3 misfire when my crank pulley wan't pulled up all they way. It seemed odd that it would only misfire on that on cyl, but when I got it torqued down all the way the misfire went away.

 

[Desert Stallion]

Very interesting. How was the pulley not 'pulled up' all the way?

 

[jstreet0204]

Very interesting. How was the pulley not 'pulled up' all the way?

The bolt was a little too long when I switched the stock pulley back on before the supercharger. I got an oem bolt from ford wich allowed me to torque it down right. With it not pulled in all the way it allowed the sensor wheel to vibrate slightly.

 

[UVBNHAD]

It's interesting that you posted this. I am having a very similar problem with my 04 GT. I may have missed this but what year is your car?

This is the third time that I have had problems out of this car. So far it's been the #3 cylinder crapping out on me. The other day it happened when the woman was driving. She had the cruise control set at 70ish and all of a sudden, *thud* and the car jerked real hard. It happened almost exactly like you said. Pulled over and discovered I had a dead cylinder. The dealer did a leakdown test and found that it was either the #3 or #2 cylinder that wasn't holding compression. As I said this is the third time that it has happened and the second engine so I don't know what's going on.

 

[Desert Stallion]

The problem is that I still have good compression in all 8 cylinders. There isn't an obvious reason for this cylinder to be dead.

Since I've got it out now I think I'll spend the $15 on a new CKP and see if'n that helps any.

 

[hotmustang331]

Um I didnt hear you mention the change of your spark plugs, could have fouled.Thats probley whats wrong, unless I missed the part of you changing the spark plugs. If you did, then there may be A. excess fuel going to that cylinder fouling plugs, B. an intake restriction (you dont have one of those tornado things right?) If its not either of those I dont know what the craps wrong.BTW pull that spark plug and see what it looks like.

 

[slow87GT]

I agree, what does the plug look like? I'd still be looking in that direction, theres a million things you can still do to figure this out, maybe swap wires w/another cylinder see if you can move it, or test a new plug by resting it on strut tower for ground, than you can see the quality of spark. I'd also use a noid light on just that cylinder, if you get depserate, do running compression check, that will check for weak or broken valve springs

what does plug look like? wet(fuel soaked, oil?), if you change the plug, then check it immediately after running, it should be wet w/gas or look like it never ran, in that case it would be fuel concern

sounds like you got bases covered w/coil packs if you swapped em out already