High CHT - Engine Stumper of the Week

[Chewka]

Hi folks,

I have been seeing high CHT temperature on cylinder #1 over the last year and am having trouble nailing down the culprit.  CHT often exceeds 400 degF in cruise when running 100 degF ROP when it used to operate well below that.  We are having to run super rich (150-200 degF ROP) and often fly with the cowl flaps open.  We have recently changed this cylinder due to a small crack at the spark plug hole discovered during the annual inspection, however, this high temperature issue was apparent before this replacement.  I also notice that the EGT on this cylinder is the lowest of all.  

 

Here is what has been checked so far:

 

1) CHT probe #1 swapped with #3.  No change in readings.

2) Air cooling baffles checked - ok

3) Fuel flow divider recently checked - clean

4) New injector installed - no change

5) Fuel lines tested last year for even flow from divider to injectors - ok

6) Intake system checked - no leaks

7) Exhaust system checked - no visible issues

8) Mag timing checks out ok

9) Spark plug is black showing we are running rich (matches with what we are doing to keep CHT below 400)

 

Anyone engine gurus out there have any advice?

 

Brenden

81 M20J

Calgary

[carusoam]

Is that a new cylinder experiencing break in procedures? Or an old cylinder with new rings?

Can you post a JPI output showing what you are seeing?

GAMI spread?

Best regards,

-a-

[PTK]

How much time on the engine? Is it only on cyl #1? How about 2? Is it making good power?

Have you explored a possible intake lobe on the camshaft going bad? As in flattening out?

This shows up as lowered EGT as in excessively rich mixture and elevated CHT. Also reduced power that you may not notice initially. Compressions can still be good.

Also look at some possible abnormal combustion in that cylinder such as pre-ignition.

[Andy95W]

Very descriptive write up.

My suggestion is to double and triple check the exhaust for leaks and replace the exhaust gasket while you're at it.

I worked on an engine that the exhaust flange was not perfectly flat and we had to use a blo-proof gasket and a copper gasket to get it to seal. Difference with that was that we could see the exhaust evidence. Yours might be hiding on the back side of the exhaust pipe. Hot exhaust gases can raise the temperature in the vicinity of the CHT probe to give an indication like you're seeing. Not very common, but possible.

Good luck, please keep us posted.

[butchgilbert]

Swap coolest CHT fuel injector with the hottest fuel injector (cheap).

Try new spark plugs (not too expensive).

Double check baffling including in between cylinders.

GAMI's? Download JPI data and analyze to see if this is the only issue.

Butch

[pinerunner]

Is it possible you have a bad spark plug? In one of Busch's vids he mentions that small cracks in the ceramics can make hot spots that can lead to preignition. He mentioned on old practice of mechanics to throw away plugs that had been dropped and destroy them so noone could ever use them. I can imagine a plug might look fine but be causing a problem and drive you nuts trying to find it. 

 

PTK's post made me think of that.

 

If the cam lobe is wearing down on that cylinder's intake it would get less air and be be richer than the others right off. GAMI spread would be terrible.  

[Andy95W]

Is it possible you have a bad spark plug? In one of Busch's vids he mentions that small cracks in the ceramics can make hot spots that can lead to preignition. He mentioned on old practice of mechanics to throw away plugs that had been dropped and destroy them so noone could ever use them. I can imagine a plug might look fine but be causing a problem and drive you nuts trying to find it.

PTK's post made me think of that.

If the cam lobe is wearing down on that cylinder's intake it would get less air and be be richer than the others right off. GAMI spread would be terrible.

I would think the worn cam lobe would generate less heat, but I'm not an expert.

I really like the idea of swapping spark plugs as the cheapest and easiest thing to do if you haven't already. I seem to remember something about preignition/detonation causing high CHTs and because of the earlier than normal ignition event, less heat going out the exhaust so lower EGT.

[KSMooniac]

A worn cam lobe would affect both #1 and #2. I think that spark plug theory is a good candidate. Also check/test the harness.

Sent from my VS985 4G using Tapatalk

[Bob - S50]

Have you tried running LOP to see if that makes a difference?

[Chewka]

A recap of what's been checked:

1) CHT probe #1 swapped with #3.  No change in readings.

2) Air cooling baffles checked - ok

3) Fuel flow divider recently checked - clean

4) New injector installed - no change

5) Fuel lines tested last year for even flow from divider to injectors - ok

6) Intake system checked - no leaks

7) Exhaust system checked - no visible issues

8) Mag timing checks out ok

9) Spark plug is black showing we are running rich (matches with what we are doing to keep CHT below 400)

 

In addition to the things listed, we have also recently replaced the spark plugs.  Our AME says that there is no evidence of cam shaft lobe wear.  Compressions are good on all cylinders and other cylinders are operating in a more "normal" range.  Cylinder #2, the other one up front is operating about 40-50 degF cooler.  The AME used a handheld thermometer during ground runs to verify that the EI engine gauge is displaying the accurate temperature.  Our AME also had a conversation with a Lycoming technical rep resulting in the following response:

 

Lycoming’s published red line for the CHT on that engine is 500 deg. They recommend not running the engine at CHT’s above 435 for extended periods of time but that is just a recommendation. Operating the CHT above 400 degrees is perfectly ok and normal according to Lycoming. Lycoming also says that a difference of up to 50 deg between cylinders is normal and not to be of concern. His suggestion is that based on all the inspections and troubleshooting that we have done that there is no problem with that cylinder. He recommended that you fly the airplane based on the fuel flow/power settings in the POH and as long as you don’t exceed the redline for any parameter there is nothing to be concerned with. He also asked “If you didn’t have the EI gauge to look at would you think there was anything wrong with the way the engine was running or performing”?

 

I have two concerns with this response.  One is that everything I have ever read suggests that you shouldn't run over 400 degF for long periods.  Second, this is a definite change from what we usually see.  Ignoring it for the long term doesn't sound like a good idea.  

[Chewka]

I will graph out some CHT's and EGT's next time I fly and post.  

[Chewka]

Also, no evidence of pre-ignition during a borescope inspection.  Engine has about 1000 hrs on it since factory overhauled (1845 hours since new at time of overhaul).

[Marauder]

I guess the bigger question to ask is what is considered the acceptable range in quality that leads to the temperature ranges they feel are normal?

None of my cylinders hit 400, even on a hot summer day. I do climb at Vy+10 but run full throttle at altitude and with the flaps closed. If the rest of your cylinders run at 360 to 380, why is this one cylinder different?

[KSMooniac]

Most importantly, something changed, which makes me furious at the Lycoming rep's answer.  Sounds like Saddam's old PR guy... "nothing to see here...move along."

 

Keep digging.  How were the intake and exhaust parts checked for leaks?

[Earl]

The bigger issue is a change from your usual observations and absent some change/maintenance, etc. I think you need to keep looking.  Issues like this drive me crazy and I have had one ever since I put in an EDM-730.  I only show 33.5" of MP at full takeoff power on my gauge and have had everything possible checked with no joy.  The plane runs fine, compressions are fine, turbo is fine, no leaks identified, performance seems unchanged and yet this issue is there and really bugging me.  If I had a cylinder that showed a jump in CHT I would keep looking until I figured it out.

[N201MKTurbo]

The bigger issue is a change from your usual observations and absent some change/maintenance, etc. I think you need to keep looking.  Issues like this drive me crazy and I have had one ever since I put in an EDM-730.  I only show 33.5" of MP at full takeoff power on my gauge and have had everything possible checked with no joy.  The plane runs fine, compressions are fine, turbo is fine, no leaks identified, performance seems unchanged and yet this issue is there and really bugging me.  If I had a cylinder that showed a jump in CHT I would keep looking until I figured it out.

 

I would check it with an independent MP gauge. If it is truly only making 33.5 In I would check your pressure relief valve.

[Chewka]

 The exhaust was checked for leaks by visual inspection around the exhaust port and also by pressuring up the system and using soapy water to check for leaks. The intake was also visually inspected and then they ran the airplane and sprayed brake cleaner around the intake port to check for leaks.  An intake leak would be identified by a sudden rise in engine RPM as the flammable substance is pulled through the intake into the combustion chamber and ignited. Neither of these tests indicated any leaks on either the intake or exhaust systems.  I will do some temperature graphing this weekend.

[StevenL757]

Agree with Bob...I would try LOP ops and see if you note a CHT decrease over ROP.  Didn't see that in the list of things tried so far, unless it was done but not called out specifically here.  Scott's point is well-taken too...keep digging.  Sometimes it isn't the "what" but the "how".

[Guest]

If you are drowning the engine in fuel to keep the temps down, and the plugs are black as proof I would say its an indication error.

In reading all of the posts no where do I see mention of the CHT probe type? Are you using bayonet probes for all cylinders or gasket probes or as is common a combination of them? If you have a combination change out the odd one for testing and see what happens.

Clarence

[orionflt]

sorry, i'm a little late to the game here, just out of curiosity what are your oil temps doing? high oil temps will directly affect CHT temps

 

Brian

[Chewka]

Hi folks,  

I graphed some CHT's and EGT's last weekend.  When I compare to a test I did a few years ago it looks like all CHT's are UP about 30 degF and we are just noticing #1 because it is the highest and now gets over 400 degF.  It is also interesting to note that the EGT's are all DOWN about 35 degF.  Some of this difference can be attributed to a slight difference in test conditions:

 

2011 Test:

22", 2500 RPM, -4 degC OAT, a29.92,  8500 ASL

 

2014 Test

20", 2500 RPM, 10 degC OAT, a30.14, 9500 ASL

 

The OAT was about 14 degC cooler in the old test so I am not sure if that accounts for 14 degC of the 30 degC (also understanding air density between conditions is slightly different).  If I am interpreting the data correctly (attached) it looks like #4 is not the oddball out like I originally thought.  #1 and #4 were always the hottest running.  In response to the last post, we haven't noticed an increase in oil temps. Also, yes, I have tried LOP operations and it seems to work quite well.  

 

I was talking to a friend over the weekend about how he got his cowl flaps adjusted 1/8" on his M20K and it would affect temps significantly.  In fact, they would change the settings on the cowl flaps seasonally for the cold winters up here.  I wonder if a slight adjustment was done that just bumped all the temps up enough for us to start noticing?

 

I am attaching charts of the old (2011) and recent (2014) test.

 

Thanks again for everyone's input.

 

Brenden

March_2011_ROP-LOP_TEST.pdf

Aug 2014 - CHT-EGT Test.pdf

[Super Dave]

When I compare to a test I did a few years ago it looks like all CHT's are UP about 30 degF and we are just noticing #1 because it is the highest and now gets over 400 degF. It is also interesting to note that the EGT's are all DOWN about 35 degF.

This is exactly what you would see with an advance in the ignition timing; higher CHTs, lower EGTs.

[KSMooniac]

This is exactly what you would see with an advance in the ignition timing; higher CHTs, lower EGTs.

 

I agree with this, and wonder if something is wrong with the magneto to advance the timing in a fashion that affects #1 more than the rest.  How many hours on the mag since it's last 500 hour inspection or replacement?

[Chewka]

500 hr mag inspection occurred about 100 hrs ago and timing has recently been checked.  Still digging though.

[KSMooniac]

It might be worth checking the timing at each cylinder... I'm not sure if a mag failure can only manifest itself in one cylinder but worth asking.