airguy

airguy

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Yesterday coming home from Tulsa to Midland I had my #1 EGT suddenly drop from its "normal" cruise temp (I don't have the temps, only the temp bars on the display) by about 125 degrees. I was in cruise at 8500 at 18 degrees C, WOTLOP at 10,900 density altitude burning 7.8gph of 100LL on an IO360 with GAMIjectors and FP prop at 2500rpm. This of course got my full undivided attention and I played with the engine for quite a while to make sure I was not about to make a forced landing, but I convinced myself that I may have an EGT probe going bad or a wire connection going bad. I'm asking for other opinions here.

As soon as I noticed the drop, I started paying VERY close attention to all engine parameters. It was not running rough at all, all CHT's were completely normal, only the #1 EGT was off-nominal. I was about 50 degrees WOTLOP at 2500 rpm, and did an in-flight mag check in that condition. All cylinders reacted as they should, EGT and CHT rising together, smooth running and normal RPM drop, so that ruled out ignition. I went back on both mags and riched up to peak, all EGT/CHT came up the appropriate amount (#1 EGT still lagging by about 125 degrees, but tracking normally otherwise), RPM rose to about 2625 which would be relatively normal for that prop at that density altitude, then went back LOP and kept leaning slowly until the fire went out at about 2200 rpm while watching temps, #3 was the first to extinguish as it always has been, with the others very close in trail just like normal - so this leads me to believe I don't have a fuel flow problem like a blocked injector. I also pulled the fuel cutoff to completely starve the injectors and then restored fuel, thinking that the fuel divider might be at fault and this would dislodge debris stuck in it somewhere, no effect. The CHT on #1 never varied from its normal position - so that leads me to believe that it was still producing normal power, which agrees with the fact the engine was very smooth and never indicated a cylinder misfiring - even at 650rpm idle on the ground when I landed. The very smooth running even at idle with the normal CHT makes me think the cylinder valves and piston/cylinder itself are fine and at this point I'm believing my next most likely culprit is an EGT probe starting to die. The probes have 650 hours on them at this point, so it's entirely possible.

In the next day or two I'll swap the #1 and #3 EGT probes and see if the problem follows the probe. Any other suggestions?
 
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Possible

An EGT drop of this value may indicate a crack in the associated exhaust pipe. I had this happen to me. Check closely around the EGT probe - the clamp may hide the crack.
 
With the engine still running smooth and the CHT behaving normal... it's gotta be a problem with the EGT probe or its wiring.

EGT probes wear out, and it's a miracle they last as long as they do under their harsh operating environment.
 
sjhurlbut said:
An EGT drop of this value may indicate a crack in the associated exhaust pipe. I had this happen to me. Check closely around the EGT probe - the clamp may hide the crack.
Click to expand...

Good point - I hadn't thought of that. I didn't smell any exhaust but that doesn't mean anything. I'll check that carefully.
 
engine power must be good

One advantage of a fixed pitch prop is in diagnostics like this. You are getting normal RPM at typical manifold pressure setting. That means the power's got to be normal.

With a C/S prop, you would not know if the engine power was less than normal, and the prop was just compensating by reducing pitch. Only exception I guess is when the prop is on the flat pitch stops - if it won't turn up to normal rpm then, I guess that means the engine power is reduced.
 
Not necessarily, Steve.

I was cruising home on a long ferry in an Agwagon with an IO-520 Conti , with a CS prop, many years ago, well trimmed and hands-off, in smooth air.

For no apparent reason, the airplane started descending because it was a bit slower than minutes ago...hmmm??

I decided to check the mags with their individual toggle switches...first click...engine dies!!:eek:

Yep, the prop had compensated.

Best,
 
May not be the probe...

I had several of the cheaper EGT probes fail at around the 300 hour point, and my experience was that they started fluxuating, not necessarily dropping and staying there. I was seeing about 100-200 degree fluxuations, up to a couple a second, and sure enough, probes were dying. I was also able to see a probe failure coming when looking at data, the first indication was high frequency, low amplitude fluxuations in the data, not normally seen by the eye on the gauge.
 
I swapped the probes between #1 and #3, and the problem followed the probe. The airplane is going in for annual next week anyway so that goes on the squawk list. It's a relief knowing it's not an engine problem though...
 
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