What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Cooling - High CHTs on ground

bryanhorgan

bryanhorgan

Member
I made some changes to my setup and now I have CHT problems (new carb, pmags and baffling). The CHT problems really only exist on the ground if I taxi or sit for an extended period. At top of climb (4000 ft, 110 knots, 85 degree day) I am seeing around 380 degrees CHT. Then after landing, I've got about 7-8 minutes before my CHT's start to hit 400. I let them go to 410 before shutting down. All cylinders are within 10 degrees or so. There is a slow and constant rise in CHT on the ground with seemingly no end in sight. That is at idle and FULL rich.

0-360 with Marvel Carb (10-4164-1) and Pmags. Mag timing was done TDC plus 5 degrees measured from case split, with inclinometer (which should give me 20 degrees BTDC). Jumper wire is installed on the mags. I also recently redid the baffling. I did a backlight test and can't find any leaks in the baffling (I backlit the engine bay with the cowling on and used a borescope to check for light).

I'm all out of ideas and was hoping one of you had some.
 
wirejock said:
Dumb question but did you time 5 degrees "after" TDC?
5 before would give you some higher CHTs
Click to expand...
When you set PMAG timing, you set it at TDC, not the 25 degree timing marks.
The mag wants to know where TDC is and then sets timing to 25 degrees Before TDC.
My target timing was 20 degrees Before TDC, so if I set it 5 degrees after TDC the mags should fire at 20 degrees Before TDC.
And just reading that is sort of confusing...
 
bryanhorgan said:
When you set PMAG timing, you set it at TDC, not the 25 degree timing marks.
The mag wants to know where TDC is and then sets timing to 25 degrees Before TDC.
My target timing was 20 degrees Before TDC, so if I set it 5 degrees after TDC the mags should fire at 20 degrees Before TDC.
And just reading that is sort of confusing...
Click to expand...
Yes. I have two P-mags. Like I said. Dumb question
 
Another way to determine 5 deg on the starter gear cogs.
There is 149 (older engines 122) cogs on the starter gear.
If we divide that with 360 deg, we get:
360/ 149 = 2,42 deg / cog.
So 2 cogs would be 4,84 deg or about 1/2" after TDC. (looking from pilot side, to the Right of TDC).
Does this mag timing position sound right on your engine ?

Good luck
 
Last edited:
I'm not sure why we are all jumping on timing (ok, the PMAGs are new...). The climb CHTs seem OK. Ground idling and taxi isn't.
If the PMAG timing is all wrong, then wouldn't it have high CHT on climb out?

What is the air temperature on the ground?
 
blaplante said:
I'm not sure why we are all jumping on timing (ok, the PMAGs are new...). The climb CHTs seem OK. Ground idling and taxi isn't.
If the PMAG timing is all wrong, then wouldn't it have high CHT on climb out?

What is the air temperature on the ground?
Click to expand...
it was 85 degrees OAT on the ground when I had the issue.
 
What is your fuel flow at WOT/departure @full rich. FP or CS prop? You getting full throw on carb/wide open? You leaning heavy on ground eg: lead fouling prevention?
 
I'm leaning toward something amiss with the carb idle. If it were the P-mag timing or the baffling, then you would likely have higher CHTs throughout all regimes of flight. You state that you're at full rich when idling, but the new carb's idle may not be as rich as the old carb was. There should be a procedure in your carb manual to set the idle mixture.
 
BH1166 said:
What is your fuel flow at WOT/departure @full rich. FP or CS prop? You getting full throw on carb/wide open? You leaning heavy on ground eg: lead fouling prevention?
Click to expand...
My issue is not in the air, its on the ground. At idle I'm somewhere between 2 and 2.5 gph, 800 rpm, full rich.
 
BH1166 said:
What is your fuel flow at WOT/departure @full rich. FP or CS prop? You getting full throw on carb/wide open? You leaning heavy on ground eg: lead fouling prevention?
Click to expand...
Full flow is something to looks at. I hadn't checked if I'm at the full rich stop on the carb when I'm full rich in the cockpit...
 
bryanhorgan said:
My issue is not in the air, its on the ground. At idle I'm somewhere between 2 and 2.5 gph, 800 rpm, full rich.
Click to expand...
Fuel flow looks normal to me.
I would have expected to se the cylinder temps to drop 20-30 F as power is reduced to idle, mine does.
What happens if you run it at idle for 15 min ? Does the temp skyrocket?
From the manufacturer:
"For maximum service life, cylinder head temperatures should be maintained below 430°F during high performance
cruise operation and below 400°F for economy cruise powers".
I would consider it a cause for concern continue flying but I don´t think it will hurt the engine as it´s on low power.
If temp goes above 430 F on GND I would investigate before I do anything else.

Good luck
 
Avanza said:
Fuel flow looks normal to me.
I would have expected to se the cylinder temps to drop 20-30 F as power is reduced to idle, mine does.
What happens if you run it at idle for 15 min ? Does the temp skyrocket?
From the manufacturer:
"For maximum service life, cylinder head temperatures should be maintained below 430°F during high performance
cruise operation and below 400°F for economy cruise powers".
I would consider it a cause for concern continue flying but I don´t think it will hurt the engine as it´s on low power.
If temp goes above 430 F on GND I would investigate before I do anything else.

Good luck
Click to expand...
This is what's happening. After landing, once the engine is heat soaked, when I idle it takes between 7-10 minutes for the CHTs to slowly rise to 410. That's as hot as I've let them get before shutting down. I have no idea how high they *would* go. The bigger concern I have, is if I get a long taxi my CHTs will be super high right before takeoff. That could put takeoff CHTs into the red.
 
bryanhorgan said:
This is what's happening. After landing, once the engine is heat soaked, when I idle it takes between 7-10 minutes for the CHTs to slowly rise to 410. That's as hot as I've let them get before shutting down. I have no idea how high they *would* go. The bigger concern I have, is if I get a long taxi my CHTs will be super high right before takeoff. That could put takeoff CHTs into the red.
Click to expand...
What’s your idle speed? I idle at 680 and lean my engine into the 50RPM rise on the ground (730-750 rpm) and I can taxi at 700rpm. I’m talking about maximum leaning, such that if I move the throttle in more than a little bit, the engine starts to die. (Safer because you can’t accidentally begin a takeoff with it leaned that far) also I can go 200 hours or more without needed to clean plugs.. just gap and rotate them.

If you have dragging brakes or low tire pressure, perhaps you are using 1000 rpm or more and the additional power is making the engine work harder than it needs to? Whenever you stop, are you able to swing the nose into the prevailing wind?
 
Canards have almost zero cooling on the ground... we lean (very) aggressively to get cooler CHTs on the ground so we are able to taxi and hold short without getting too high CHTs before takeoff. Works even for OSH departures, but i will admit I taxi so that the guy in front of me is pushing air across the fuselage. Don't forget to use the one had, two lever method to ensure you are full rich when you hit the throttle for take off.
 
Taltruda said:
What’s your idle speed? I idle at 680 and lean my engine into the 50RPM rise on the ground (730-750 rpm) and I can taxi at 700rpm. I’m talking about maximum leaning, such that if I move the throttle in more than a little bit, the engine starts to die. (Safer because you can’t accidentally begin a takeoff with it leaned that far) also I can go 200 hours or more without needed to clean plugs.. just gap and rotate them.

If you have dragging brakes or low tire pressure, perhaps you are using 1000 rpm or more and the additional power is making the engine work harder than it needs to? Whenever you stop, are you able to swing the nose into the prevailing wind?
Click to expand...
It's not a moving issue. While I'm parked it's happening. Doesn't matter if I lean or go full rich. Slow rise in CHTs at 800 rpm to 410 degrees.
 
I’m almost always wrong about CHT issues but there is a chance it’s related to the thermocouple instrumentation. When you are on the ground the temperature of the thermocouple wiring and instrumentation typically rise substantially . Things like disimaler metals in the wiring to the instrument or errors in the cold junction compensation could cause this. Getting it hot and shut it down and quickly measuring the temperature of the cylinders with an inexpensive IR thermometer might be useful.

Prop outflow has a big effect on cooling at low speeds and I have a prop with 8 inch wide blades but my CHT at shutdown is typically 100 degrees lower than inflight.
 
Last edited:
bryanhorgan said:
It's not a moving issue. While I'm parked it's happening. Doesn't matter if I lean or go full rich. Slow rise in CHTs at 800 rpm to 410 degrees.
Click to expand...
Why are you idling at 800rpm? Sure when the engine first starts, I do just under 1000 for splash lubrication on the cam with cold, thick oil, but after a flight, I idle it lower with an extremely leaned mixture.
 
Bryan, my CHTs are cooling off down wind. I land, taxi and same, continues to cool. What engine info system do you have? I just looked at 10-15 random flights engine data, same mid 300’s at cruise, mid 200’s while taxiing in. Like another mentioned, sure seems like you are having INSTRUMENTATION issues. Id borrow or buy ( harbor freight cheap) a point and shoot thermometer. On next flight land, taxi to safe spot, get out and measure #1 #2 through cowl openings to compare what the panel EIS says. Odd for all to be off, and not a probe or two, but not impossible. Ive read if you are using the cht probes under plug vs a bayonet style, readings will not be as accurate, but not a huge difference.
 
I am curious why you are running full rich while taxiing. I run as lean as I can just before it wants to stall and my CHT temps drop
 
Bryan

i might have missed it in the posts. But have you checked the idling mixture? I know for fuel injected engines that you should aim for a 50 rpm increase if you very slowly pull the mixture back. A carb should give similar.
If you’re getting a higher increase it shows you’re too rich- no or little increase too lean.

regards Peter
 
Idle mixture won't cause it to overheat at idle power on the ground.
Whether or not the pmag jumper is in or out won't cause it to overheat at idle power on the ground. Besides, it isn't overheating at takeoff power, so we can rule that out.
Full-rich fuel flow won't cause it to overheat at idle power on the ground. Besides, it isn't overheating at takeoff power, so we can rule that out.
Whether or not it's heat-soaked after landing won't cause it to overheat on the ground. Besides, it shouldn't be heat-soaked after landing because you entered the pattern at 100-120 knots and near idle power so it's had plenty of time to cool down, and it's probably somewhere near CHT=300F at touchdown.
It's probably not instrumentation because it seems sensible in flight.

There's very little actual data in this thread.

"I recently redid the baffling." And, "I made some changes to my setup and now I have CHT problems."

What were the changes? Show us the baffling. If you have engine monitor data, show us that too.

Lot of theories floating around here with very little that's actually helpful. Start with data and photographs, please. You seem to be overheating when you don't have ram-air through the engine bay. Let's find out why.

- mark
 
BH1166 said:
Bryan, my CHTs are cooling off down wind. I land, taxi and same, continues to cool. What engine info system do you have? I just looked at 10-15 random flights engine data, same mid 300’s at cruise, mid 200’s while taxiing in. Like another mentioned, sure seems like you are having INSTRUMENTATION issues. Id borrow or buy ( harbor freight cheap) a point and shoot thermometer. On next flight land, taxi to safe spot, get out and measure #1 #2 through cowl openings to compare what the panel EIS says. Odd for all to be off, and not a probe or two, but not impossible. Ive read if you are using the cht probes under plug vs a bayonet style, readings will not be as accurate, but not a huge difference.
Click to expand...
I have a Skyview system. While possible, I doubt all 4 channels of my CHT probes have failed in the same way that gives me high readings. They are bayonet style.
 
What make & model aircraft?
We are all assuming this is an RV, but I don't see any statement to that effect. Your member signature doesn't say.

Help us help you! [Still waiting for photos...]
 
bryanhorgan said:
I have a Skyview system. While possible, I doubt all 4 channels of my CHT probes have failed in the same way that gives me high readings. They are bayonet style.
Click to expand...

This is the first time you've mentioned that you have an engine monitor that logs data.

Which is great, because all the data that should have appeared 30 posts ago is sitting right there on a flash card, ready for publication in the thread.

Brilliant!

- mark
 
newt said:
This is the first time you've mentioned that you have an engine monitor that logs data.

Which is great, because all the data that should have appeared 30 posts ago is sitting right there on a flash card, ready for publication in the thread.

Brilliant!

- mark
Click to expand...
Roger newt. We will hold off until you solve this. You have told us what isn’t the problem. What will you be looking for in the flight data to determine what the problem is? VAF is social media and not a review board. Anyone looking for answers gets what they pay for. Ideas and inspiration. Often conflicting or redundant and rarely with consensus.
 
Last edited:
You must log in or register to reply here.
Back
Top