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SDS EGT question

agent4573

Well Known Member
Full EM5 system on an IO375. During low power or idle descents 3 of the 4 egts will drop below 700ish degrees. Cylinder 4 stays up around 1150-1200. All 4 egts read within 50 degrees at cruise and climb and respond correctly during normal ops. There's always a some "lean popping" from the exhaust during extremely low power descents, but if this popping was coming only from cylinder 4, would it be enough to keep that egt higher than the rest?

Because installation varies absolute egt values, peak egt is normally 1550ish. Normal ROP/LOP range is 1350-1400, and very LOP is 1250-1300 range.
 
Full EM5 system on an IO375. During low power or idle descents 3 of the 4 egts will drop below 700ish degrees. Cylinder 4 stays up around 1150-1200. All 4 egts read within 50 degrees at cruise and climb and respond correctly during normal ops. There's always a some "lean popping" from the exhaust during extremely low power descents, but if this popping was coming only from cylinder 4, would it be enough to keep that egt higher than the rest?

Because installation varies absolute egt values, peak egt is normally 1550ish. Normal ROP/LOP range is 1350-1400, and very LOP is 1250-1300 range.

I see idle EGTs around 900-1000 on my bendix FI, though I have honestly never looked at EGTs during an idle decent. Yours are much lower and with lean popping, have to assume you are too lean at that vacuum level. May want to try playing with the fuel tables to get a bit more fuel. During decent with the lean popping, observe the MAP or vacuum (don't know how SDS tables are arranged) and then add a few milliseconds to the fuel at that MAP and see what happens. I am guessing the higher EGT on #4 is unrelated to this but not enough info to venture a guess.

Definitely worth giving Ross a call. I thought he had pretty well sorted baseline maps, but maybe your 375 is a bit different or there is some other reason for the lean condition. With a speed density FI, I would expect a vacuum leak to make things rich, depending how much bias the throttle position indication gets. It also could be Alpha-n, with no MAP input and the advice above would not necessarily apply. Again, no experience with SDS. If it is Alpha-n, there could be something wrong with the throttle position sensor, giving an erroneous reading when throttle plate is fully closed, giving the lean condition only at idle.

It is also possible that 1-3 are normal and #4 is filthy rich, due to a blockage or other issue with the injector. What you hear as lean pops could be from an excessively rich mixture and you are getting the after fires when that raw gas gets exposed to the hot gas on the next cycle.

Larry
 
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We normally do most of the descent in a lean of peak setting with the mixture knob, so I'll try it again with the knob back to vertical on the SDS and see if the popping goes away. While slightly annoying, I've never heard that lean popping at idle can cause damage, so I'm not overly worried about fixing that. I'll try it though and see if the #4 cylinder behavior changes at all. Hopefully it's lean popping and not #4 after firing.
 
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I have the same thing on my 540 with #2&3 cylinders sometimes. Not all the time, but usually on my base to final. I can’t find a correlation between power or rpm settings, it’s always around the same area but not exact. Ross said to look for intake leaks, as it’s getting too lean but I haven’t been able to find any leaks. I’ve never had it stumble or lag on adding power, so I just monitor it. I am curious why it happens though.
 
We have heard of this from a number of people. It's typically caused by the MAP values being a bit too small just below the idle range which is around 10 inches on most aircraft. Values will vary a bit between CS and FP props as well.

In most cases, this can be resolved by increasing the MAP values about 20% between 4.59 and 8.51 inches. When the values are too low, the mixture is too lean for the plugs to ignite. The mixture is carried out the exhaust port and explodes in the hot pipes, causing popping.

By enriching slightly, the plugs ignite the mixture normally.

This generally happens mostly on final where MAP is lowest, though you could encounter any time the RPM is relatively high and the throttle is closed or almost closed completely. Some cylinders mixture may be on the edge of ignitability others slightly too lean, hence the reason some are firing and some not as shown by EGT.

The TPS is only used for transient acceleration enrichment as the throttle is rapidly opened so doesn't enter into this issue at all.
 
As a quick check, you could turn the mixture knob slightly richer as you encounter the popping. If it cleans up, you'll know you can fix it with programming.

The knob % in Gauge 2 will show you how much to increase the MAP values by to stop it. If the value was 30 and it took +20% on the knob to stop it, you'd enter the new value using 30 X 1.2= 36 in that MAP slot.
 
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