Engine Overheating Help (High CHTs)

[Jarerh]

Hey everyone,

Looking for some help troubleshooting overheating issues on my 1984 M20J IO-360-A3B6. I’ve owned the airplane for about 60 hours now. No major problems for the first 50 hours other than slightly higher CHTs, topping out at around 400 in climb and typically cruising with cowl flaps in trail and the hottest cylinders (#1 or #3) running around 380°F.

However, over the last 10 hours, both the #1 and especially the #3 cylinders have been running significantly hotter. By about 1,000 ft AGL, both are over 400°F, and even climbing out at 120 KIAS, #3 stabilizes around 420°F before slowly dropping. A few weeks ago, I had an engine stumble on initial climb and #3’s CHT spiked rapidly to 435°F. After I landed we did a compression check and found Low compression on #3 (50psi). After pulling the cylinder, we discovered a broken piston ring and a bent exhaust pushrod. We don’t know the exact order of events, but they were likely related. The cylinder, rings, and pushrod were replaced, and I’m now in the break-in phase of the new cylinder.

The problem is, I’m having a really hard time keeping temps under control. I know new cylinders tend to run hotter during break-in, but even climbing at 120–130 KIAS, #3 is hitting 440°F (the Lycoming-recommended max for break-in) by 800 ft at 120-130kts, and it keeps climbing if I don’t level off and reduce power significantly. #1 is also very hot, around 425–430°F by 800 ft.

What we’ve checked so far:

• Baffling is in good condition with solid contact between gasket material and cowling.
• Inter-cylinder baffles are properly installed.
• During the cylinder replacement we noticed that all lower baffle springs had been removed at some point before me owning the plane; they’ve since been reinstalled (before the most recent flight with the highest of CHTs).
• Takeoff fuel flow is 17.5 GPH.

At this point, I’m looking for ideas on what else to check to get these CHTs under control. I got a Savvy analysis trend report that puts my plane in the 94th percentile for climb and cruise CHTs, so I know I have a cooling issue. Any insight or experience with similar issues would be hugely appreciated.

Thanks, Jared

[PT20J]

Where is the timing set? It should be 20 deg BTDC for an -A3B6.

[Bartman]

Assuming all baffling is good here are a couple of suggestions  

Remove and clean each injector. Do the baby food jar to confirm equal fuel flow, with the injector inserted installed. 

Check ignition timing. 25deg BTDC is allowed on my A3B6D but it runs hotter than 20deg. 

[hazek]

Can you share a flight from your Savvy uploads so we can see the whole data set?

Also, did you fly this?: https://www.savvyaviation.com/flight-test-profiles/

[Shadrach]

30 minutes ago, PT20J said:

Where is the timing set? It should be 20 deg BTDC for an -A3B6.

Has the TCDS been changed? The copy thatI have does not authorize 20° timing. In fact, it specifically excludes setting an A3B6D to 20°.

[Shadrach]

I seriously doubt that this is a timing issue. I run maximum advance in my engine for the TCDS and it runs much cooler than yours. I would start by thoroughly inspecting all baffle material. Do not neglect to check the internal baffle seals in between the cylinders.

With a relatively new installation, there can be multiple factors that are causing problems. Indeed, you could have a baffle seal issue and also if you flow issue. I’m of the opinion that 17.5 GPH is not adequate full rich fuel flow for an IO360 departing a  sea level airport. Your fuel servo may need to be serviced.

[mike_elliott]

Check rear baffle seal about half way across the engine. They have a tendency to droop there and let air escape vs being forced down thru the cylinder cooling fins

Look at the baffle seal for evidence of contact with the cowl and maybe use a light at night to check for fenstration 

[hazek]

An induction leak would present itself as overly lean condition. I just had all my induction gaskets and hoses replaced on a 18 year old engine and they were all brittle and destroyed. There was evidence of AVGAS on the induction pipes (with a turbo the leak happens both ways at different phases of flight). With your NA engine it would happen only to the lean direction, so you wouldn't see any traces of AVGAS.

[PT20J]

40 minutes ago, Shadrach said:

Has the TCDS been changed? The copy thatI have does not authorize 20° timing. In fact, it specifically excludes setting an A3B6D to 20°.

OP said he has a A3B6. 

[Jarerh]

Thanks so far,

I will double check the rear baffling seal. The magneto is times to 20 degrees, though I also have a SureFly ignition on the left side that is set to the variable timing option. 

This is the flight in which I had some engine roughness and CHT #3 tended to have a mind of its own, pulled the cylinder after this flight: https://apps.savvyaviation.com/flights/shared/flight/9962458/5b23d3f3-71e9-4feb-ac34-01b25341f1dd

This is a break-in flight where CHT 3 rapidly reaches 440: https://apps.savvyaviation.com/flights/shared/flight/10132193/0238ab27-4e50-41be-a418-cc720035a6eb

I have not run a test profile yet, as I'm not able to climb about 1000' without the CHT continuing to rise. 

-Jared

[Shadrach]

5 minutes ago, PT20J said:

OP said he has a A3B6. 

Whoops…misfire on my part. My apologies!

I know that you’ve received a lot of Lycoming specific information from the factory over the years. I have never timed a dual mag. Do you have any insights as to why the optional 20° timing is prohibited with D suffix engines? 

Having flow behind my engine when times at both 20 and 25°, I prefer 25°.

 

 

[Shadrach]

36 minutes ago, hazek said:

An induction leak would present itself as overly lean condition. I just had all my induction gaskets and hoses replaced on a 18 year old engine and they were all brittle and destroyed. There was evidence of AVGAS on the induction pipes (with a turbo the leak happens both ways at different phases of flight). With your NA engine it would happen only to the lean direction, so you wouldn't see any traces of AVGAS.

At closed throttle, high vacuum scenarios, yes. In climb at wide open throttle, an intake leak isn’t going to have much of an effect on mixture unless the leak is huge.

[PT20J]

When you check the baffle seals, make sure that after flight they are curved inward. If not installed correctly, they can look fine when the cowling is installed but then blow back from air pressure causing a leak. Afterwards, they can appear to seal the cowling when the engine isn’t running. Another trick to check for air leaks is to put the airplane in a dark hangar and insert a light through the oil filler door and peer through the inlets by the spinner.

A significant induction leak should cause a rough idle.

For comparison, my IO-360-A3B6 in a 1994 J generally runs about 360 deg F CHT or less with cowl flaps open during a sea level climb at 105 KIAS with full rich mixture and about 18 gph fuel flow. It will get hotter on a warm day at lower IAS but never over 400. In cruise I usually see about 340 with the mixture leaned peak or LOP and the cowl flaps closed.

If your left cowl flap is flat rather than curved like the right one, you might try rigging it to be  open about half an inch when the cowl flaps are closed.

[Shadrach]

10 minutes ago, Jarerh said:

Thanks so far,

I will double check the rear baffling seal. The magneto is times to 20 degrees, though I also have a SureFly ignition on the left side that is set to the variable timing option. 

This is the flight in which I had some engine roughness and CHT #3 tended to have a mind of its own, pulled the cylinder after this flight: https://apps.savvyaviation.com/flights/shared/flight/9962458/5b23d3f3-71e9-4feb-ac34-01b25341f1dd

This is a break-in flight where CHT 3 rapidly reaches 440: https://apps.savvyaviation.com/flights/shared/flight/10132193/0238ab27-4e50-41be-a418-cc720035a6eb

I have not run a test profile yet, as I'm not able to climb about 1000' without the CHT continuing to rise. 

-Jared

OK, well the Surefly installation muddies the water a bit. 

[PT20J]

12 minutes ago, Jarerh said:

Thanks so far,

I will double check the rear baffling seal. The magneto is times to 20 degrees, though I also have a SureFly ignition on the left side that is set to the variable timing option. 

This is the flight in which I had some engine roughness and CHT #3 tended to have a mind of its own, pulled the cylinder after this flight: https://apps.savvyaviation.com/flights/shared/flight/9962458/5b23d3f3-71e9-4feb-ac34-01b25341f1dd

This is a break-in flight where CHT 3 rapidly reaches 440: https://apps.savvyaviation.com/flights/shared/flight/10132193/0238ab27-4e50-41be-a418-cc720035a6eb

I have not run a test profile yet, as I'm not able to climb about 1000' without the CHT continuing to rise. 

-Jared

You might check the Surefly timing. Sureflys are timed to 0 TDC on the engine and the advance is set with dip switches. There have been instances of mechanics timing them to 20. You might also try setting the Surefly to fixed timing as a test. 

[PT20J]

11 minutes ago, Shadrach said:

Do you have any insights as to why the optional 20° timing is prohibited with D suffix engines? 

Bendix never made a dual mag with a lower lag angle for the impulse coupling, so if you set the timing to 20, it will fire 5 deg late during start. 

[hazek]

4 minutes ago, Shadrach said:

At closed throttle, high vacuum scenarios, yes. In climb at wide open throttle, an intake leak isn’t going to have much of an effect on mixture unless the leak is huge.

MAP would still be less than ambient pressure at sea level, no? Granted not by much, but 4" of pressure difference is enough perhaps for what he is seeing. No?

This is curious:

FF decreases while EGTs rise twice when I presume power was reduced. Classic induction leak? Also the small rises and drops in EGT could be going so lean that it's over the top and to the lean of peak side.

[Pinecone]

Fuel flow looks a bit low.  Rule of thumb is WOT, full right, 1 GPH per 10 HP.  So 200 HP engine should be 20 GPH or higher.

[Jarerh]

I certainly wont rule out an induction leak, however I always thought that an induction leak would lead to a rough running idle? This engine runs as smoothly at idle as any IO-360 I've ever flow, for what its worth.

My left cowl flap is flat, I will double check the rigging. I know the right side is about 1/2" open when close, but I don't remember if the left one is as well.  

[PT20J]

Fuel flow isn't field adjustable on an RSA injector. I've never been able to find a spec for full power fuel flow on a Lycoming IO-360-A.

I have test cell data from Lycoming for my rebuilt, but they only measured airflow at full power, not fuel flow. However fuel flow was measured at three lower airflows (500, 900 and 1000 pph) and extrapolating shows that fuel flow would come out to about 17 gph at 1118.5 pph measured full throttle airflow. 

I got the bench test specs from Precision Airmotive for the fuel servo. Pertinent test points are at 600 and 1400 pph airflows and that works out to about 16.2 gph 1118.5 pph airflow.

The Lycoming Operator's Manual shows a best power fuel flow of 15.7 gph at full power. Full rich would be greater.

If I recall correctly from past posts over time, it seems most people are getting around 18 gph at full power at sea level. But I am doubtful that 1/2 a gph would account for the high CHTs.

[Ragsf15e]

Can you tell from your past engine data that it was colder at some point?  Especially before some specific maintenance??  Like maybe before the Surefly was installed?  I had one on my F and I really liked it.  Never had heat issues, but as @PT20J said, it times a little different than a mag, so could be installed wrong.  Or the dip switches set wrong, or maybe even the MP tube not rigged right allowing it to advance earlier than it should? I’m just spitballing here, but I might look back at the data and see if you can tell when it started getting hot.  And how long has the SF been on there?

Just for clarity, installed properly, the Surefly has a pretty good history of working fine and didn’t cause any heat issues in my F.  It should not be advancing at all below about 5,000’ anyway.

[hazek]

2 hours ago, Jarerh said:

I certainly wont rule out an induction leak, however I always thought that an induction leak would lead to a rough running idle?

The leak may only appear when the metal is hot.

[Shadrach]

The operating manual for my F calls for 18.6gph at full power and full rich.

@PT20J is absolutely correct in that fuel flow is not field adjustable. It is adjustable by the overhauler. Years ago, I had my servo overhauled locally by a Bendix fuel system specialist. I asked that it be set as rich as possible within spec. When I reinstalled the servo all of my takeoff EGT‘s had dropped into the high 1100s or low 1200s. I’m not sure what the fuel flow was it that time because I did not have a transducer installed. I can say that my takeoff EGTs have gradually become leaner since I first installed the servo. My takeoff fuel flow currently runs between 18.5 and 19gph. It will be interesting to see what the numbers look like this winter.
 

Managing EGT‘s was never really a problem, but the engine has run very cool since the servo overhaul about 12 years ago.  

 

 

[GeeBee]

To check the cowling, do a light test. Place a bright light in the lower backside of the engine compartment, place the airplane in a dark hangar and look through the front for "light leaks". If you can see light, air is leaking. Seal it up. Sometimes seats will push back and leak under pressure. Lycoming says in climb you should have a minimum of 4 inches of water pressure as measured on a water manometer on the high pressure side. If you don't have a manometer use an old airspeed indicator, 4" is about 90 mph. The formula is airspeed (MPH) squared divided by 1980.

[blaine beaven]

On your next flight, perhaps try turning off one of the magnetos to see if that makes any significant difference over a few minutes? This could be a quick way to determine if you have a timing issue with a rogue ignition source.