One rear EGT "takes off" but CHT, OT and OP behave normally

[pschwenn]

Aerovee 2.1 / Aerojector / Xenos built 2010 / Sensenich climb prop

Main symptoms: as throttle is increased and engine warms up, anywhere above about 2300RPM at about 1100 F the rear EGT takes off toward 1450 and aloft engine rough. CHTs always look good and oil pressure varies appropriately. Won't idle smoothly, and not below 950 - (which is too fast for landing a glider.)

Valve clearances correct and equal. All plugs look good, light brown - far from too rich.
No Gascolator, exhaust wrapped, oil not over filled, #2 needle, large K&H filter

Passes Rich/Lean test - tho I can't do it fully because I can't quite reach WOT.

Odd(?): Oil temp increases very slowly; same behavior flying and tied down, same behavior cowl on or off; pulling mixture to lean - best lean occurs very far on pull and engine wont stop until mixture knob is held full lean for 2-3 seconds.


What I'll check next (from advice read on this forum, esp "Crazy EGT"):

Try take off with warmed up but not hot engine - so that WOT for take off doesn't run EGT over limit.
Try richer needle and recheck plugs.
Test (candle) / Replace EGT sensor.
Deflector cowl flap for rear cylinders.
Reduce entry to oil cooler.
Compression test.
Increase cowl exit flow by changing exit aluminum flaps angle and maybe increase size of exit
Replace rubber tubes in intake track and check other possibilities for air leaks.

Thank you,

[lutorm]

You have 4 EGTs? What do the other 3 EGTs do, specifically the other rear one? And what do they all do if you start pulling the mixture while at 2300?

There is no real EGT "limit". The absolute reading is far too dependent on where the sensor is mounted. What really matters is how they behave with mixture changes. If the EGT starts shooting up and the engine goes rough, that would seem to indicate that it's leaning out. If you slowly pull the mixture at that point, does the EGT go up or down?

With our intake system there's no way to just lean out one cylinder. Even an intake leak at the head should affect the other cylinder on that side (and intake leaks are most noticeable at idle when the manifold pressure is lowest.)

Do you have a fuel flow meter? What's your full throttle fuel flow?

[sonex1374]

Peter,

A few things stood out in your initial post.

1) Oil temp is slow to come up. What temp does it reach at maximum (sustained climb), and what is a typical cruise oil temp? I think you may be over-cooling your oil. If you have a bottom mount oil cooler, you may need to reduce the size of your air inlet to the cooler. If you have a top mount, try blocking off some of the cooler surface.

2) If your plugs are light brown, you may be running too lean at full throttle. Using an aerocarb/aeroinjector and tuned for a good, rich-enough mixture at WOT, it is very common to need to lean aggressive at low throttle to keep the mixture lean (and the plugs tan-to-brown in color). If you don't lean at idle, your plugs will almost certainly be black after even a few minutes. So here's the question: are you leaning aggressive at idle and getting tan plugs, or are you only leaning a little and getting tan plugs? On your new engine I'd expect to hear that your plugs are black but WOT runs well, rather than your plugs are tan, and the engine runs hot.

3) Idle at 950 is typical of a new engine. It will be possible to idle lower once broken in, but probably not below 800. If you set it too low early on, the engine may die on short final as you pull the power all the way back.

4) Needing to pull the mixture knob almost all the way out at idle is typical. This doesn't really tell you much about the tuning of the carb. A well-tuned carb will still allow you to lean aggressively at idle, just as a very-lean tuned carb will.

Regarding your testing plan, I have a few thoughts as well.

1) You can check the EGT sensors with a candle, but it's hard to really calibrate them. You can learn more by swapping probes in your exhaust pipes. It's a pain, but if you really want to determine if it's a probe problem or a EGT problem, swapping probes around and comparing the temps registered by different probes will tell the tale.

2) You can check for leaks by spraying down the joints on your intake system with carb cleaner or starting fluid. If there's a leak, the intake will ingest the fluid and change the way it runs. You'll know it.

3) Making the exhaust deflector lips larger won't help as much as simply increasing the size of the exits. Really small lips are a problem, but the stock ones are fine.

4) Switch to a bigger needle. This is my suggestion as well. Install a 2.5 and tune for a good, rich WOT mixture. It's OK to even need to lean a touch at WOT to run smoothly. Don't go crazy, but do ensure you're getting enough fuel at WOT. Don't be afraid to lean it a bit on climb out - if you are running a bit rough and it smooths out with a little leaning, then you were a bit on the rich side and have now confirmed that. An added bonus of the 2.5 needle is that once you get the WOT mixture tuned, the mid-range and idle will both be leaner, and you'll need less leaning at low power than with the 2.0 needle.

Jeff

[gammaxy]

Passes Rich/Lean test - tho I can't do it fully because I can't quite reach WOT.

I'd focus on figuring out why it won't reach WOT. If no mixture setting will get there then it sounds like it is too lean at WOT. I'd adjust the current needle richer and see if it improves before switching to the 2.5.

My mixture isn't as tricky as others describe. The engine generally runs smoothly at idle, takeoff, and in cruise full rich. As the engine warms up after startup, I do lean it aggressively at idle, but as a precaution against fouling spark plugs and not because it won't run smoothly. I've never had black spark plugs. I get extra power in climbout if I lean a bit, so I always do. In level flight at low altitudes while WOT I go full rich to accommodate the extra air the engine is breathing due to higher RPMs. I have noticed with a full fuel tank (higher fuel pressure) and warm engine, the engine seems too rich during takeoff, so a little extra leaning is occasionally required.

[pschwenn]

Responder,

Thanks very much

I only have two EGT sensors. I will swap them, then move them fore'n'aft. I've ordered one in case one is defective.
I just tried occluding oil radiator cowl entry by half - seemed to help - previously oil temp might get to 175f after forever.

Next I'll richen a bit more. And then try a 2.5 needle.

Pushing mixture rich always lowers EGT and and RPM,. And vice versa. Closest I've come to a full WOT rich/lean test is (after richening by a third needle turn clockwise since my post) at 2800 rpm, pushing from opt lean all the way in lowers EGT by 70f.

I'm going to judge in future more by plugs, which I'm very familiar with, ability to reach smooth WOT, and idle. (But making sure initially that my EGTs are neither defective nor way out of line.)

I'm still digesting these very useful replies.

Thanks again

[gammaxy]

(after richening by a third needle turn clockwise since my post)

If you really did turn clockwise, you actually made it leaner by pushing the needle into the fuel port. Also, if you don't already know, make sure the needle doesn't move when you tighten the set screw, sometimes you might get an extra turn of the needle without realizing it. I draw a mark on the needle so I can see if it moves while tightening.

Since your EGT reduces when you go rich, it doesn't sound like you're too lean. Does it still not reach WOT? Does anything improve when you switch ignitions?

[Onex107]

Responder,

Thanks very much

I only have two EGT sensors. I will swap them, then move them fore'n'aft. I've ordered one in case one is defective.
I just tried occluding oil radiator cowl entry by half - seemed to help - previously oil temp might get to 175f after forever.

Next I'll richen a bit more. And then try a 2.5 needle.

Pushing mixture rich always lowers EGT and and RPM,. And vice versa. Closest I've come to a full WOT rich/lean test is (after richening by a third needle turn clockwise since my post) at 2800 rpm, pushing from opt lean all the way in lowers EGT by 70f.

I'm going to judge in future more by plugs, which I'm very familiar with, ability to reach smooth WOT, and idle. (But making sure initially that my EGTs are neither defective nor way out of line.)

I'm still digesting these very useful replies.

Thanks again

Peter,

A few things stood out in your initial post.

1) Oil temp is slow to come up. What temp does it reach at maximum (sustained climb), and what is a typical cruise oil temp? I think you may be over-cooling your oil. If you have a bottom mount oil cooler, you may need to reduce the size of your air inlet to the cooler. If you have a top mount, try blocking off some of the cooler surface.

2) If your plugs are light brown, you may be running too lean at full throttle. Using an aerocarb/aeroinjector and tuned for a good, rich-enough mixture at WOT, it is very common to need to lean aggressive at low throttle to keep the mixture lean (and the plugs tan-to-brown in color). If you don't lean at idle, your plugs will almost certainly be black after even a few minutes. So here's the question: are you leaning aggressive at idle and getting tan plugs, or are you only leaning a little and getting tan plugs? On your new engine I'd expect to hear that your plugs are black but WOT runs well, rather than your plugs are tan, and the engine runs hot.

3) Idle at 950 is typical of a new engine. It will be possible to idle lower once broken in, but probably not below 800. If you set it too low early on, the engine may die on short final as you pull the power all the way back.

4) Needing to pull the mixture knob almost all the way out at idle is typical. This doesn't really tell you much about the tuning of the carb. A well-tuned carb will still allow you to lean aggressively at idle, just as a very-lean tuned carb will.

Regarding your testing plan, I have a few thoughts as well.

1) You can check the EGT sensors with a candle, but it's hard to really calibrate them. You can learn more by swapping probes in your exhaust pipes. It's a pain, but if you really want to determine if it's a probe problem or a EGT problem, swapping probes around and comparing the temps registered by different probes will tell the tale.

2) You can check for leaks by spraying down the joints on your intake system with carb cleaner or starting fluid. If there's a leak, the intake will ingest the fluid and change the way it runs. You'll know it.

3) Making the exhaust deflector lips larger won't help as much as simply increasing the size of the exits. Really small lips are a problem, but the stock ones are fine.

4) Switch to a bigger needle. This is my suggestion as well. Install a 2.5 and tune for a good, rich WOT mixture. It's OK to even need to lean a touch at WOT to run smoothly. Don't go crazy, but do ensure you're getting enough fuel at WOT. Don't be afraid to lean it a bit on climb out - if you are running a bit rough and it smooths out with a little leaning, then you were a bit on the rich side and have now confirmed that. An added bonus of the 2.5 needle is that once you get the WOT mixture tuned, the mid-range and idle will both be leaner, and you'll need less leaning at low power than with the 2.0 needle.

Jeff

Don't hesetate to try the #3 needle. I don't have to touch my mixture from one flight to the nest. From idle to wot. I lean 1/4 inch to cool an EGT on climbout and put it back on curise. Idles fine.
Onex 107

[bschell]

I had a similar issue and found that the gap on the rear plug was way off and also that I had a Helicoil for that plug that had worked it self out a bit and the plug would not sit deep enough in the hole. Made these two repairs and the EGT is just fine. Bela

[Mrcaseymyers]

I’m having the same issue. My cylinder 3 cht is about 325 during climb out but cylinder 1 and 3 egt’s go up to 1400 pretty quickly. I’ve tried many different things. I’ve finally narrowed it down to an issue with my primary ignition. I did a inflight timing check today and during cruise my egt on cylinder 3 was about 1350, within 10 seconds of switching off my secondary ignition my egts were at almost 1400. Thinking my bottom ignition probably loosened up on me. So that will be the next thing I check

[lutorm]

Switching off one of the ignitions will always raise EGT somewhat, because the fuel will burn later. What EGT did you see when you turned off the primary?

I wouldn't worry about the absolute value of the EGT, it's mostly a relative measurement. Too many things affect the exact temperature sensed, its use is relative to the temperature at peak EGT mixture.