Number 4 CHT

[texdog]

O-360, CS, RV6A, 1400 hours. Last two flights the CHT on number 4 cylinder started running 20 degrees hotter than ever before. EGT is a little hotter. I switched the probes from 2 to 4 and I don’t think it’s the probe. Replaced intake gasket and connection pipe. Compression check cold 75/80, bore scope looks good. 1 mag and one E-Mag, mag check is good. What have I overlooked?
Thanks

 

[sansoneservices]

Baffles secure?

 

[BillL]

If it was internal (timing-exhaust valve), the EGT would go up while the CHT went down. Agree - look for leaning causes or broken baffles or some blockage of the fins. Missing the lower intersection baffle? ETC.

My #4 CHT is warmer by 20F than other cylinders, but have tracked it to a smaller baffle gap exiting under the head/cylinder. Just to lazy to remove all the stuff to get to it.

 

[texdog]

Bill,
Are you living with the CHT temperature increase or increasing fuel flow. I’ve tried to keep my CRT temps below 380 in cruise,
Thanks

 

[lr172]

O-360, CS, RV6A, 1400 hours. Last two flights the CHT on number 4 cylinder started running 20 degrees hotter than ever before. EGT is a little hotter. I switched the probes from 2 to 4 and I don’t think it’s the probe. Replaced intake gasket and connection pipe. Compression check cold 75/80, bore scope looks good. 1 mag and one E-Mag, mag check is good. What have I overlooked?
Thanks

Were was your mixture when this event occurred? Lop , rop, etc.

 

[texdog]

I tried LOP,ROP and everything in between. If I increased fuel flow 2GPH it would come back down. I have the intake hose off now and I’m going to try putting it on a little differently. I noticed it wasn’t over the band toward the engine sump.

 

[mahlon_r]

Maybe one plug isn't firing during high power operation. try mag check in flight at power. Good Luck, Mahlon

 

[Raymo]

Maybe one plug isn't firing during high power operation. try mag check in flight at power. Good Luck, Mahlon

Wouldn't that result in less power/heat from the cylinder?

Checking the plugs is a good idea to see if one is badly fouled and igniting the mixture early, which would drive CHT up. If they are good, baffling is the most likely source of the higher temp.

 

[lr172]

I tried LOP,ROP and everything in between. If I increased fuel flow 2GPH it would come back down. I have the intake hose off now and I’m going to try putting it on a little differently. I noticed it wasn’t over the band toward the engine sump.

If the EGT changed only a small amount from normal, but CHT changed a moderate amount, an intake leak is not likely. Heat due to a leaner mixture should be seeing a significant change in EGT. Also, once at peak or anywhere LOP, leaner mixtures will reduce CHT, not increase it. Have you looked for FOD in the #4 cyl fins? Maybe a bird got started with nest building. They get the straw way down in there. I would be looking for what may have changed that would reduce airflow past that cyl.

 

[lr172]

Wouldn't that result in less power/heat from the cylinder?

Checking the plugs is a good idea to see if one is badly fouled and igniting the mixture early, which would drive CHT up. If they are good, baffling is the most likely source of the higher temp.

In my experience, dropping one plug in a cyl will increase EGT ~150* and create a small CHT drop at higher power settings. If you pay attention to EGT during runup, you will see this, though the rise is smaller due to the lower RPM.

 

[ve0kog]

In my experience, dropping one plug in a cyl will increase EGT

this is what i see during runup - all EGsT raise during mag check. Interesting the #1 raises a lot more with the right mag off.
However on the takeoff roll #4 has the hottest EGT and CHT raise which is not detected during the 1800rpm runup (very similar to the OPs problem)

runup:



takeoff:



I wonder if #4 is getting less oil flow compared to the other cyls for some reason .

 

[lr172]

this is what i see during runup - all EGsT raise during mag check. Interesting the #1 raises a lot more with the right mag off.
However on the takeoff roll #4 has the hottest EGT and CHT raise which is not detected during the 1800rpm runup (very similar to the OPs problem)

runup:

View attachment 90809

takeoff:
View attachment 90810


I wonder if #4 is getting less oil flow compared to the other cyls for some reason .

The higher EGT rise on 4 on TO, is not that uncommon with carbs. #3 and/or #4 tend to run leaner due to the very poor distribution in the lyc plenum. Oil flow has nothing to do with EGT.

 

[bjdecker]

@texdog

Fred,

Along the lines of what Larry suggested -- did you catch some bees in the fins of #4? Maybe a mud-dawber got busy back there...

 

[texdog]

Thanks gentlemen, I been over the baffles, fins and I flew twice this morning. The number 4 is still 20+ degrees higher in cruise, ROP AND LOP. In climb full throttle, 2500 RPM, full rich the temperature is the same. Only in cruise is it higher. My next move is to check the wiring on the thermocouple, and the EIS. 7500 ft. cruise, 2400RPM, 21 MP, RPM drop on mag check is 10 RPM each mag. More to come.

 

[lr172]

Test your cruise with the carb heat on and see what happens to the egt spread

 

[Raymo]

Thanks gentlemen, I been over the baffles, fins and I flew twice this morning. The number 4 is still 20+ degrees higher in cruise, ROP AND LOP. In climb full throttle, 2500 RPM, full rich the temperature is the same. Only in cruise is it higher. My next move is to check the wiring on the thermocouple, and the EIS. 7500 ft. cruise, 2400RPM, 21 MP, RPM drop on mag check is 10 RPM each mag. More to come.

Does #4 have a primer line going to it?

 

[texdog]

It does have a primer line and I checked the tightness on all connections.

 

[Raymo]

It does have a primer line and I checked the tightness on all connections.

Assuming this higher CHT is a new symptom, I would disconnect the primer line and cap it off to see if it makes a difference. You can also try opening the primer while running on the ground and confirm that makes a difference.