EGT Drop

[RV7AJeremy]

Hello all,
I have had an issue now on three separate flights where EGT4 had dropped about 300*F when I have gone to idle while slowing to get my flaps down. I have noticed no roughness, nothing else abnormal, and I would never have noticed anything was unusual if I had not had this engine monitor. As soon as I add any power everything returns to normal.

Here are the links to the three flights:

https://www.savvyanalysis.com/flight...e-f78778d72571
Event at 1:10:56

https://www.savvyanalysis.com/flight...f-d85163eaee2e
At 0:25:45

https://www.savvyanalysis.com/flight...d-d2c35cac0c52
at 1:08:52

Any comments appreciated.

[Bill.Peyton]

I would check for a vacuum leak, or the possibility of two dead plugs

[Jesse]

When you do a mag check, watch all 4 EGT's. They should all go up when on only one mag. If not, then you may have a bad plug. Otherwise, it could be an idle mixture thing, somehow getting too much fuel to that cylinder. See if you can get it to repeat on the ground. Another thought is to switch both plugs from 2 to 4 and see if the event stays with the cylinder or follows the plugs.

[RV7AJeremy]

Originally Posted by Bill.Peyton

I would check for a vacuum leak, or the possibility of two dead plugs

Bill, can you expand on the "vacuum leak"? I do not have a vacuum pump.

Originally Posted by Jesse

When you do a mag check, watch all 4 EGT's. They should all go up when on only one mag. If not, then you may have a bad plug. Otherwise, it could be an idle mixture thing, somehow getting too much fuel to that cylinder. See if you can get it to repeat on the ground. Another thought is to switch both plugs from 2 to 4 and see if the event stays with the cylinder or follows the plugs.

All the plugs check ok during the mag check. (I have had fouled plugs before)

Could this be an injector nozzle issue?

[Jesse]

It could be an injector nozzle that is partially clogged, but I would think it would show up all the time. You could also swap restrictors from 2 to 4 and see if it follows the restrictors or stays with the Cyl. Make sure to do that separately than other tests.

[BillL]

When you close the throttle, the engine becomes a vacuum pump - thus lowering the manifold pressure. So - with a low MAP, if there is a leak if the intake tube gasket/ o-ring etc, then it would lean out that cylinder. Thus, a vacuum leak.

Good luck!

[Bill.Peyton]

The vacuum leak I speak of would occur in the intake manifold. When you reduce the throttle to idle, you close the throat of the fuel servo, this causes the pressure in the intake manifold to decrease as compared to the static pressure surrounding it. If you have a leak in the intake system on one particular cylinder, it will cause that cylinder to run leaner than the rest of the cylinders . At idle this will become most noticeable. Check the intake manifold gasket at the cylinder head.

[RV7AJeremy]

Originally Posted by BillL

When you close the throttle, the engine becomes a vacuum pump - thus lowering the manifold pressure. So - with a low MAP, if there is a leak if the intake tube gasket/ o-ring etc, then it would lean out that cylinder. Thus, a vacuum leak.

Good luck!

Got it! Thanks

[Bill.Peyton]

I see Bill and I posted at the same time. Have an A&P take a look

[BobTurner]

Except that if you started rich of peak, an intake leak should show a rise in EGT. If you're 300 deg lean of peak I'd think you'd feel the power imbalance.