Hello Fellow Crown Vic Owners,
I am the very proud owner of your sister automobile, a 1999 GM, and have recently developed the dreaded Cylinder #5 misfire code. I brought the car to my mechanic to have a ball joint/ tie rod replaced and asked him what could be causing this problem to occur when the engine warms up. When it is cold (thermostat closed) car runs fine, when it warms up the car misfires/bucks when under a light load (30 - 50 mph). My mechanic said to check the sparkplug boot first thing in the AM and if it is wet, the intake is cracked. OK fine. The problem is the cost and/or length of time to replace it. My question to everyone or anyone is, what actually causes the misfire. Is it the moisture getting into the spark plug well? This would make sense, since it exhibits symptoms of wet wires. Can't the well be sealed by silicoing the spark plug boot at the top, thus preventing the coolant from entering? I also pulled the coil and inspected the boot and it was cracked on the inside where it mates the plug. I intend to buy all new boots, since the car is 12 years old now and could be another source of the arcing. I will also use dielectric grease to further weatherproof the plug/coil connection. Does this sound like a feasible temporary fix? My coolant leak is very negligible at this time (doesn't make it to the block)and don't want to replace the intake until it starts leaking onto my driveway.

that intake will only get worse and worse. i had the same problem for awhile. i used alot of bars leak stop leak for a good six months. it worked, but every time i went wide open throtle it would blow the seal. as the months dragged on the crack in the intake manifold only got worse to the point that the stop leak did not work anymore because the crack just kept enlarging.

my advice, save your self all the head aches i endured trying to beat the leak and keep the coolant level. just go get yourself a Dorman intake manifold and get it replaced. the sooner you do it the better, no more head aches once its done.
good luck!!

Using silicone should get you by for a little bit without misfires, dependent on the severity of the leak. It may migrate to another cylinder. If you do go that route, make sure to use a hi-temp product, as it'll be right on the cylinder head.

Something to keep in mind: loss of coolant is never a good thing. Keep an eagle eye on both the level and the temp gauge. You absolutely do not want your engine overheating. Cracked heads and worse could be the result.

And by the time your coolant hits the driveway, it may already be too late. So, having said that, I sure hope you make it through the winter, and plan on replacing it when the weather warms up a little.

I marked the coolant reservoir 6 weeks ago and I cannot notice it going down at all. I don't see the seepage either. Just a damp spark plug boot when the coil is removed. No coolant in the well either. When I see it going down I will replace. Right now I just want to ensure that the intake is the problem. If this temporary fix doesn't solve the problem, then I'll need to focus on the fuel delivery (injectors). Either way, i need to get the car to run right, now. I've read too many posts about people replacing the intake and the problem still exists (although the leak stopped).

Replace the intake manifold and coil pack boots.If this does not help than you may have to go as far as replacing the sparkpugs and coil packs them selfs.

Good luck to ya.

Sparkplugs were changed at 100K about a year ago with Motorcraft double platinums. Coil packs all tested good. Boots all suck and are cracked at the inside bottom of the boot (where it mates to the plug). Intake will eventually get done when the weather warms. I'll post my results this weekend. I'm doing the boot replacement, silicone sealing the entire driver side with high temp silicone. I just want to get this thing to run good again. If this clears the check engine light and the misfiring go away, I'll order the Dorman replacement manifold next week. I just can't confirm the coolant leak. Nothing green around the thermostat, or the driver side of the engine and the level is constant.
Has anyone ever seen the primary wiring to the coils open up ?

Fix the boots and dry out the wells, then keep an eye on them for moisture. I wouldn't go sealing it up/replacing the manifold until you are sure it wasn't just water from driving thru a puddle or something.

If you do end up needing a manifold, they aren't too difficult to do yourself. I did mine in ~4 hours w/ only basic hand tools, and I had never done one before.

Follow Up:
Cylinder #5 boot split apart at bottom, well dry, spark plug removed for visual. Put back with new spark plug boot, sealed with high temp silicone.
Cylinder #6 boot had a few splits at the bottom and wet. Wiped it off with white rag and was clear. Looks like condensation. Removed spark for visual and put back with new boot and sealed with silicone.
Cylinder #7. Spark plug well soaked with oil. Removed spark plug and found that the insulator (ceramic) is cracked in several places (pics to follow). Cleaned well, replaced spark plug and boot with new and sealed with silicone.
Cylinder #8. Almost the exact same as Cylinder #7 except the ceramic wasn't cracked as bad. Replaced plug, boot, and sealed.

So far the car is running fine, but I don't want to declare success until after a few days. As far the #7, I first thought I cracked it taking it out. But after inspection, you can see the dirty oil imbedded in cracked insulator. #8 was just starting to crack. What do you think caused it crack? Do you think the oil was boiling from the temp in the well and that stressed the plug?
Remember, this all started with a CEL and code stating Cylinder $5 misfire. So much for that diagnosis.
Also, no coolant detected on the block, spark plug boots. Only a trace around the bolts that hold the thermostat down. I probably should have used teflon tape when I changed my thermostat over a year ago.

How about 1-4. Replace those plugs/boots and you should be good to go after you fix your leak.

The NAPA store near me only had 4 boots. I ordered the other 4 and should have them sometime next week. As far as the leak, I can measure any noticeable change in the reservoir level. And that is after 3 weeks when I marked it. We'll see. Time will tell. Warmer weather is just around the corner and I may just replace the intake anyway. For fun !!

Let me know what you find. I'm having a similar problem. 1999 P-71 owner for 4 months and I've noticed since the day I brought the car home it seemed to misfire. What's really odd is it started out just when the accelerator was at cruising speed. Then, as time progressed it became worse and worse. I had to replace the intake manifold last weekend and no more coolant leak....YAY! blah....

Now, she's running like crap consistantly...

I'm curious about the boot and spring thing...

Also, does anyone else know if double platinums are ok or should I go with OEM plugs from the dealer???

Thanks Boys!!


1999 Crown Victoria P-71
120K...Former Baraboo, WI Police Department unmarked
No mods as of yet...just trying to bring her back to life...

Misfire Update:
Car is running perfectly for five(5) straight days and about 150 miles. If you suspect a leaky valve cover gasket, especially on the top, you may end up having a situation like me. Seems like the oil would slowly fill the spark plug well, boil from the cylinder head heat, and eventually breakdown the insulator of the spark plug. I know that by installing new spark plug boots and sealing them with silicone would temporarily stop the leakage of oil into the wells. I will consider my job complete after I install new valve cover gaskets. The silicone will only last so long.
Can't hurt to replace the spark plug boots if you've never have. Twelve years of exposure to heat breaks them down and lends them to arcing. They are about $4 a boot. Also, remove each spark plug and inspect the plug, and the well. If you notice any condensation on the plug, boots, or in the well, make sure you seal it out with silicone. Get the high temperature type sold in automotive stores. Good Luck and keep me informed.

BTW, I picked up Motorcraft double platinum spark plugs from EBay Motors for $25 (free shipping) for a set of ten(10). Don't know why the guy sold them in sets of ten, but at $2.50 a plug, I wasn't questioning him. Got them in 3 business days too !!

I will have to try the boot swap. When I had them all out while replacing the manifold, the boots looked to be in good shape. A friend of mine told me that I should use OEM plugs, but I'm running Bosch Double or Triple Platinums and they aren't cheap. I notice no moisture on the boots or even on the top of the motor at all since replacing the manifold.

Also, just a dumb question...is it possible that a coil could be bad? I was just thinking that before the new manifold I found the passenger side rear plug well was completely full of coolant. Is it possible that the coil could have moisture build up that only causes issues once the motor is warmed up?

I'm just running out of ideas...

Take those junk bosch plugs out and install double plat. autolite APP104.