Spark Plug Blew Out Of Engine Years Ago, Now...

Stokestack

Member
Apr 5, 2010
38
4
18
Hi all.

Quite a while ago, a spark plug blew out of my '96 Cobra. The dealer said Ford had a problem with this on pickups, but it wasn't a known problem on Cobras. Nevertheless, it happened.

The incident wrecked the threads in the block, but the dealer repaired it with a Helicoil and the car seems to have been fine since.

Today it was time to replace the spark-plug wires, and of course the plugs. I found that one of the rubber connector assemblies didn't want to slide over the spark plug and sit flush against the engine the way the others did. I tried another connector, but had the same problem. I also tried reinstalling the plug. I suspect that this is the hole that was repaired years ago (it's on the correct side).

I replaced the wires before, but don't remember this happening. I may have simply dismissed it, but now I'm concerned that the plug is actually not screwing into the block as far as the others, because of the repair. I've attached a picture showing the connector plunger in question.

FbXTrPK.jpg


Wouldn't it cause a performance problem to have a spark plug not extending into the cylinder properly? Any suggestions on what to check or do about this? The car does seem to run fine, but of course subtly reduced performance may not be noticeable.

Thanks.

Gavin
 
  • Sponsors (?)


Hi Gavin. Thinking out loud here...

--Would that difference in spark plug seat height matter? Seems to me it would. The spark plugs do not usually take too many turns of the wrench to get in place. Each turn is a couple millimeters, I would imagine. Could we equate the difference in the spark plug seat height you show to not tightening any other working spark plug fully? Or tightening it fully, but backing it out a half turn? Not sure, but seems logical to think so. Would that difference be enough to affect the ability to properly ignite the air/fuel mixture in the cylinder? That seems to be the question.
--You know, issues tend to be simpler for more complex. Could it be debris or residue built up in the spark plug cavity? Perhaps with a mirror and flashlight you could get a glimpse down there? Or with a small hose-camera?
--I was not familiar with the heli-coil solution, so I did a quick search on the interwebs... given this video:
View: https://www.youtube.com/watch?v=-_azEsNkA9Q
, it seems plausible that the thread in the affected cylinder would have flexibility properties different than that of the other threads. Perhaps expansion/contraction over time has affected the integrity of the original heli-coil. Not sure. Just thinking out loud here.
--Perhaps there is some way for you to look back through your paperwork to identify the specific spark plug cavity that had the heli-coil work done to it?
 
  • Like
Reactions: 1 user
Thanks for the reply. As it happens, my car is now misfiring badly, and sure enough it's this same cylinder. My mechanic concluded that it was bad spark-plug wires, which I've encountered before and yes, they produced similar symptoms.

So I replaced the wires, and the problem came back quite rapidly. This time my mechanic concluded that the wire to this cylinder was defective. I've replaced it, and still the same result: misfiring. I tested the newest wire against the other (almost-as-new) one and found the same 3.3K-ohm resistance.

Even more alarming: The gap in the fit of the wire assembly is now larger than it was in the above picture. This indicates to me that the plug may be coming out of the engine. Further research reveals that it's not uncommon for helicoils to fail.

I found a video that discusses this problem (and a repair with Cal-Van inserts) on Ford Triton engines. Unfortunately, Cal-Van doesn't make inserts for the 4.6L Cobra engine:
 
Last edited:
Just to clarify, have you already checked/replaced all of the sparkplugs too or are you avoiding that due to the possibility of further damaging the threads in the head?

What other recent repairs/maintenance were performed by your mechanic? Start with the simple stuff and doublecheck things like air filter fitment/sealing, vacuum hose leak, etc.

Also, have you looked into replacing the helicoil with a timesert? They are (were?) a better choice when repairing damaged sparkplug hole threads...can google/search for a little more info on that, g'luck !
 
Thanks for the reply. The plugs are new and carefully gapped. When investigating the misfiring, my mechanic pointed out evidence of arcing on the plug wires. I've had bad wires go bad before, and agreed that the symptoms were consistent. So I replaced the wires and the plugs (I've done this before).

People have recommended Timesert. I will apparently need an oversized insert because of the previous helicoil repair.
 
I finally used my borescope to take a look at this hole. Here's what I can see from the top:

kjjeV93.jpg


I don't know what that fragmentary shape is at the 3:00-4:00 position. Although the scope has a 90-degree mirror attachment, its focusing distance is about an inch and a half; so I can't get a clear side view of the thing.

I pulled a plug from a neighboring cylinder and was about to look at it when the camera battery died. I'll try again tomorrow, but meanwhile any feedback is welcome.
 
  • Like
Reactions: 1 user
Hi Gavin (Stokestack),

I also have a 96 Cobra and I am LITERALLY in the same exact situation! My car had a cylinder (or cylinders) repaired by a Ford dealership with heli-coil and the second time I changed plugs I couldn't get the boot all the way down on one of them.

I also did the run around with new wires, plugs, etc. but that one boot on the cylinder closest to the firewall on the passenger side of the car, just wouldn't seat properly. I was able to drive the car for a short time with no trouble until one day it started misfiring bad and I thought maybe I should put some fresh gas in it. Well I go to start the car after filling it up with gas and BOOM! It shot the plug right out of the head, still connected to the plug wire and all! I was able to "thread" the plug in enough to drive the car home but the next time I attempted to drive it, the plug shot out within less than 5 minutes of driving.

So now the plug will not tigheten in the head at all and I cannot drive the car. I am worried there are more spark plug threads that may have been "repaired" this way so I am trying to find someone who has the knowledge and experience to remove the heads and repair all 8 of the holes in the heads.

I too have researched the Time Sert options and yes, I believe it would require the "over-sized" inserts for our cars. But with the tool kit and a bunch of inserts, that option starts getting very expensive. That is why I am now trying to gather more information about removing the heads and having them reworked by a machine shop. Do you happen to know if the cams, valves and such need to come apart every time the heads come off these engines?

Thanks,

Chuck
 
Hi Chuck!

Nice to hear from you, but sorry for your troubles of course! So... the plug blew out a second time after the Helicoil was installed? I figured that this was imminent on mine too, so I stopped driving it.

I am still dismantling my engine. I have the driver's side valve cover, brake booster & cylinder, and a bunch of pulleys and stuff off the front of the engine. Unfortunately I found I also have to pull off the right valve cover to get the engine front cover off and then deal with the timing chains.

I bought the TimeSert kit. It's sitting next to my box of removed parts awaiting use when I finally get the head off. I'm only planning to fix the one hole.

I have the same question you do: Do the cams and valves have to be removed from the head in order to bore and tap the spark-plug hole? I don't see why they would, but I don't have the head off yet.

What part of the country are you in?
 
I went through all of this fun myself. I recommend replacing the heads completely, but with 9-thread versions--especially if you're going to remove the heads anyway.

For those wanting to fix, my mechanics did not even remove the valve covers. And, their fixes held up fine.
 
I went through all of this fun myself. I recommend replacing the heads completely, but with 9-thread versions--especially if you're going to remove the heads anyway..

Thanks. Where are these versions available? The only alternative heads I remember hearing about were from a different car and required the construction of an adapter plate to mount the manifold.
 
May have jumped the gun. The 9-thread heads are from the 04 Cobra - a C-head. Did a little poking around and it looks like you'd need to upgrade your manifold while you were at it.

My 4-thread heads had repaired holes on at least 3 cylinders. I got tired of blowing plugs out, so I bought some used 9-thread heads on e-bay and had them cleaned up.
 
That's why I said I jumped the gun. I assumed the 96-98 Cobras used more-or-less the same heads. I've got 01 and 04 Cobras and a 98, but it's a GT.

For me, the cost for the machine shop to fix my 4-thread heads was around 500, and used 9-thread heads were 1,200 on E-bay. So, for 700 (was going to refresh my existing heads anyway), I didn't have to worry about blowing plugs out in the future. I said the repairs (inserts, helicoils, etc) held up fine because they lasted for awhile; probably 20,000 miles. However, at the Arkansas Mile (drove from Austin TX), I only got 2 runs in because I damaged a spark plug on the first run and when I went to remove it, the insert came out with it. That was enough for me to be over it.

For the 96-98 crowd, it sounds like its more difficult unless you can score a set of those adapter plates (which do sound pretty neat). It also looks like the C-heads provide a nice power boost.