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Old 07-16-2001, 04:55 AM
EricSilver's Avatar
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Location: Fairfax, VA
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(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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Old 07-16-2001, 01:56 PM
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Location: New Bedford, MA USA
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Eric, you will get a ton of replies on this thread I am sure. But the bottom line is wider gap, more fuel/air exposed to spark->better combustion...up to a point, which you will reach relatively quickly. As rpms increase and mean cylinder pressure increaases, the ingition system will have to push harder to get the spark across that wide gap. At idle, no problem. Also, you will find that the percieved (operant word) life of the plugs will drop. Also, another aside, the rest of your ignition system will ahve to be in top shape to keep up with that size gap. If you must, go back to 55 thousanths, but I don't really think who will be happy with 80 thousanths for very long.
Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.
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Old 07-17-2001, 12:32 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
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The only limiting factor is the wire insulation & coil!! Also with this much gap you are pushing the coil saturation point beyond it's limits at higher RPM.
This WIDER gap has been used by most car companies for 15-20 years, so that there is a more complete combustion. THAT is why many cars now have direct coil ignition systems. However the OE Benz coil won't last long at that .080" gap. Neither will the cap, rotor, & wires!
.045"-.050" is a good gap & wont hurt your ignition system!
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Old 07-18-2001, 08:27 AM
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Jeff & MBDOC,

Since I am due for servicing tomorrow, I will certainly take your advice. I'll be replacing a thermostat and then regapping the plugs.

It really was an extreme stop-gap measure to force my engine termerature higher, and although my ignition coil is new, my wires are pretty old and something would probably fail eventually if I keep it as it is.

Still, I like the car's smoother behavior, particularly at idle, and hope I do not lose it after I make the changes.


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