Engine analyzer and fouled plug

MAKG1

MAKG1

Touchdown! Greaser!
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MAKG
As a renter, I've run across fouled plugs a LOT. This time, I had an engine analyzer handy. Cessna 182T with G1000, CAP transition instruction.

During taxi, I commented that the engine felt a little rough, and we'd check it out at run-up. The initial run-up showed a nice 50 RPM drop on the right mag, and 250 RPM on the left. Obviously no good.

We turned on the "lean" page, and it showed rather clearly what was happening. On left, the #6 cylinder had no EGT reading. Consistent misfire. On both, the #6 EGT was extremely high.

That high reading told us when to stop "clearing." It needed quite a bit; more than two minutes aggressively leaned at 2000 RPM. But when #5 had the highest EGT on both, we retried, and it was all good.

The no-reading EGT was really obvious to explain, but I'm not sure I understand the high EGT when running "both." Is that telling us that the mixture is still burning at BDC on the power stroke?
 
I think it's telling you that there's more than the usual amount of unburnt fuel in the exhaust so it continues to burn as it leaves the exhaust port and passes by the EGT probe... or something like that.

I just had my first run in with a seriously fouled plug waiting in line for departure at Oshkosh. I had the same indication, i.e. no EGT temp on #1. I was unable to clear it and had to replace the plug. It looked like a lead cauliflower. When I got home I pulled and cleaned all the plugs. All the lower ones were a bit fouled. The #1 looked like it had actually shorted out.

We were idling for about an hour due to a temporary airport closure. I was careful to lean aggressively and keep the RPMs at 1,000 or above. In retrospect, I think 1,200 might have been a better way to keep things hot enough to prevent the fouling. Don't know.
 
Typically, a high EGT on one cylinder with both mags on indicates one plug isn't giving you as much spark as it should. You can determine this by doing a mag check at run-up power. When you go to the mag powering the clean plug, the bad plug cylinder EGT stays high while the EGT's on the other cylinders catch up. When you go to the mag powering the bad plug, the engine runs very rough, RPM drops a bunch, the other cylinder EGTs rise, and the bad cylinder EGT goes off the bottom of the scale. Now you know it's a plug, and further, which plug it is. Often, burning the plug clean by running to 2000 RPM or so and leaning to peak RPM for 20-60 seconds solves the problem, and you're back in business. If that doesn't work, it may be a bad plug, mag, or ignition lead, and those aren't things you can solve in the run-up area.

OTOH, if one cylinder is running higher EGT than the others in all mag switch positions, it's usually because that cylinder is running lean, perhaps due to an intake leak, or a valve issue, or a bad injector, or a number of other possibilities. That's a bit harder to troubleshoot, and you probably won't fix it outside the shop.

BTW, checking for EGT rise on mag checks is a good idea if you have all-cylinder EGT. This will also identify a P-lead failure -- the EGT will not change at all when you select that one mag.
 
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MAKG1 said:
The no-reading EGT was really obvious to explain, but I'm not sure I understand the high EGT when running "both." Is that telling us that the mixture is still burning at BDC on the power stroke?
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Yes.

After a while you'll get used to the analyzer and what you should be seeing, and you won't need to go through as much to diagnose a problem.

One my first long cross county with my analyzer in my PA32 I was doing a run-up at the mid-point airport before departing... mag drop seemed fine, no issues. But I noticed #5 had NO EGT reading... left or right. Yeap... you guessed it.... both plugs were fouled which I never would have caught with a mag check and the engine also wasn't running rough.

The problem is I had had to do a very quick descent from 12,500 due to weather and I had fouled both of those plugs with the very rich low power mixture (I dialed out the turbo to come down). Never would have caught the issue without the EGT.
 
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Ron Levy said:
BTW, checking for EGT rise on mag checks is a good idea if you have all-cylinder EGT. This will also identify a P-lead failure -- the EGT will not change at all when you select that one mag.
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My mag check is now just a check of the EGTs to make sure they all rise on each mag.

I'm not even looking at the RPMs any longer - maybe I'm skipping a key check but the EGT seems definitive.
 
A leaning forum I attended at OSH talked about the two times you should run full rich, on start up & take off only. All other times lean the engine.
 
Geico266 said:
A leaning forum I attended at OSH talked about the two times you should run full rich, on start up & take off only. All other times lean the engine.
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What about a go around?
 
Jim Logajan said:
At high altitude airports (anything over about 3k MSL) I wouldn't even run rich on take off or go around.
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Concur. At least in my airplane full rich is never needed. Not even for startup. I usually do go full rich for startup but I've done it leaned also and some other high altitude guys do this always.
 
Bravo said:
What about a go around?
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A "go around" is considered a take off. Don't go full rich on final, keep it leaned. If you need to do a go around push the mixture in with the throttle. Landing & taxiing full rich is where a lot of fouled plugs occur.
 
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