Resistance testing spark plugs

[Kevin Ebsen]

Ran across the attached article while trying to trouble shoot my in flight engine roughness while LOP. I know there are several thoughts on LOP operation, and not trying to start a pro or con thread on that, but found the resistance testing information interesting. I'll say upfront I'm from the pro LOP school of thought. Being pro LOP I spent the money for good engine monitor, turbo gami's, and took the APS online course. My engine will run half-way smoothly at about 20 maybe 30 degrees on the lean side of peak past that it gets pretty rough. Downloading engine monitor data and looking at the gami spread I have all cylinders reaching peak at the same fuel flow or within the acceptable "Gami" limits .5-1 gal difference.

In this process I talked to the Gami folks about my problem and they told me to resistance check my spark plugs and throw away any that were over 3-5,000 omhs. I pulled one and it tested 8800 ohms, so I ordered new ones mine were 10 years old and installed I think new by previous owner with 240 hours on them. Before I installed the new Tempest UREB37E spark plugs I tested them they ranged from 907-1189. To see where the old ones were I tested them and the following is where they came in at. 3800,4400,7600,8100,8800,11000,21000,34100,49600,96700, and 4.6 million (missed writing down one plug) they were Champion REB37E. Haven't flown it yet do to snow, but interested to see if any improvement. Mag check on ground prior to spark plug change was fine, so was in flight mag check LOP.

I guess the purpose of this post is just FYI. The attached piece IMO reads a little like an ad for Tempest I'm not for or against champion plugs just passing on my situation.

http://www.flyplatinum.com/blog/?p=690

[N3322G]

Always interesting what other folks have found - thanks for sharing.

[Michael Rath]

I ran across this old post last week and and I found it interesting. Ever since I bought my Comanche I had a slight engine roughness at altitude when the prop was dialed back below 2500 RPM, above 2500 it was smooth as silk. Long story short I replaced both mags which were due and installed new lord engine mounts. No change. The Champion plugs I had looked good and only had 200 hours on them.

After reading this post last week I decided to pull my plugs and check their resistance. I pulled the plugs and they all looked good. I then checked their resistance. You have to clean them first or you won't get a good test. Two of the plugs tested at infinity, two more were suspect and the other eight tested good.

I ordered six new Tempest massive electrode plugs and installed all of them running off the left mag. I reinstalled the six good champion plugs on the right mag.

The results amazed me. All the vibration below 2500 RPM is gone! I dialed back to 20/2200 at altitude and no vibration at all. I would have never been able to do that before.

My advice is to check the resistance of your plugs no matter how good they look. It is cheap and simple and might save a lot of money in the long run.

MR