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Old 07-16-2001, 04:55 AM
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EricSilver EricSilver is offline
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Join Date: Mar 2000
Location: Fairfax, VA
Posts: 1,274
(Apparent) Myth: Spark Plug Gap should be set at .032.
(Observed) Fact: Spark Plug Gap set at .080 and car is running fine -- and smoother than before.

So, what's the deal here? Common sense dictates that a bigger spark gap should offer better combustion, so why specify a small gap?

I made this realization when trying to troubleshoot a cold-running engine on my 89 260E (152,000 miles).

After discovering my thermostat is apparently faulty -- not closing/opening as far as it should, I checked my plugs for any sign of fouling. They were OK but work down -- with gaps of about .055 so I adjusted them down to .032.

After doing that, the car ran even colder than before, with a pretty rough idle, so I opened that gaps back up to where they were before, and the car ran warmer, but not warm enough.

After some experimentation, my gaps are now at .080 and the engine temperature is normal, except when it is about 75 degrees or less outside and I am driving above 65 mph; then the temp dips just a hair below 80.

I was tempted to keep increasing the gap to see what the limits were, but since the car was behaving as I wanted it to, I left it alone.

These gaps may seem large, but when I pulled each ignition wire looking for a failed plug/cylinder, I noticed *multiple* 1/2 to 3/4 inch sparks between the wire and the engine block, so a .080 inch gap seems trivial in comparison.

The performance difference I have observed is a somewhat smoother idle, and an apparently flatter power utilization curve from standing starts. Instead of a surge of power and abrupt shifting on acceleration, there is now a more gradual acceleration with less perceptible shifting -- but I get from 0-60 in the same time as before.

Fuel economy is also apparently improved, which I discovered after being caught in Washington DC stop and go traffic for a long while without the usual excessive consumption of fuel.

So what's the deal? This may sound like a lot of non-tech voodoo, but it is what I am experiencing -- and I like it.

Eric Silver
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