Spark plug boots - the whole truth

Twice I've had a "stumble" at idle and in 4th when the hammer goes down. Both times it was diagnosed as a coil, and the first time the cost was more than $100.

The second time I dug into it myself and found that the boot on one cylinder had deteriorated. Under a microscope, here's what I saw:

In removing the boot, some mechanic (me?) had slightly torn it. That tear picked up dirt, oil mist, whatever - but the detritus provided a path for the high voltage, and over time it just got worse and worse. Hence, stumble.

The purpose of the boot is to seal the juncture between the nipple on the coil and the ceramic of the spark plug, preventing dirt from entering that high-voltage area. That's the entire objective! There is nothing mysterious or critical about it. If there were no dirt in the area the system would need no boots.

The original boot has three important characteristics. First, it's of high-dielectric material, and therefore does not pass high voltage. Second, it withstands high temperatures. Third, it meets the mechanical requirement: ~3/8" plug hole, ~5/8" nipple diameter, about 1 1/2" total length.

Possible solutions:

1. Buy a coil - it comes with a new boot - and Subaruparts sells it for under $80.

2. Use nothing. This is okay until things get dirty, and since some of those cylinders are difficult I don't recommend it. However, if you experience a stumble, this is a definitive diagnostic step! Examine all boots and remove any that appear damaged. Another way to check is to disconnect the injectors, one at a time, until the bad cylinder has been identified.

3. Make a boot.
A. Make a mold (trivial), and use high-temp RTV.
B. Use a piece of 3/8" dowel and a piece of 5/8" as a form, and use four layers of heat-shrink. Obviously it will get soft with heat, but seems to work. C. Find two high-temp, high-dielectric hoses (without fibre reinforcement), cut short sections, and bond them. In my experiment the smaller hose fit inside the larger one, making a good bond. The problem with this is the bonding - with heat it may deteriorate, creating a high-voltage path.

4. Adapt an existing boot. NAPA stocks two boots, the larger of which fits the coil nipple perfectly, but the wire end is only 7mm. Cut it. I have 200 miles on it so far - seems okay.

[Mr. Pockets]

I adapted existing boots from a set of Bosch plug wires that I bought at an auto parts store. They're snug, but I figure that just makes 'em better. I've have them in the car for a year now.

Somewhere on this site I specifically said which set I bought...

[ensteele]

I don't know about anyone else, but I am still interested in having some sets made. I think you ought to ask again and get a count of how many may want them. I would buy 2 sets myself, and a friend of mine wants 1 set - 3 sets so far.

If your experiment works, please let us know and tell us what boot it is that you used.

Thanks