Brain trust help - #3 CHT HIGH

[Tankerpilot75]

My engine was torn down and rebuilt with dual PMags replacing Slick mags following a prop strike at last year’s AirVenture by Popular Grove with Myers Aviation handling the removal and reinstall plus some other work. Had cylinder heads IRAN during rebuild with new guides installed. Picked aircraft up in late March and flew it home. Ran great with all CHT temperatures below 400 degrees.

CHT #3 is now having a hard time staying below 400 degrees in cruise. The engine has 20 hours since rebuild. Wish it was more but other issues got in the way of flying.

I’ve basically been flying at 24”, 2400 rpm and 12 gph at cruise since leaving OSH last March (break-in recommendation). Did my initial oil change at 12 hours post rebuild. Had a PMag problem with rpm dropping 200 rpm when doing mag checks at around 12 hours. EMag replaced the bearing on one PMag which corrected the issue. Infant mortality - it happens!

I also had to rewire my CHT/EGT sensor wires to the EIS because, for some reason, they didn’t align with their correct cylinders (1=2, 2=1, 3=4, 4=3). I think it had been wired this way since before I bought it eight years ago by the builder. It took the CHT issue to highlight this discovery with identifying and correcting the problem taking lots of time and patience. Anyway, CHT and EGT readings now align correctly but CHT #3 is running high and it appears to not be going away after 20 hours of break-in run time.

#3 CHT is at least 25-30 degrees higher than #4 CHT and 35-40 degrees higher than #1 and #2. I’ve looked at baffling and even put a washer between #3 cylinder and the rear baffle wall. That helped a little but not enough.

At 2400 rpm, 24” and 12 gph fuel flow (break-in settings) CHT#3 will run about 410-420 degrees CHT in these hot summer temperatures. Takeoff and climb are not a problem because of my higher climb out speed (110 kts) and full rich fuel flow (~14.4 gph).

So I’ve been generally at 24”, 2400 rpm and 12 gph for these last 20 hours. If I throttle back to 2300 rpm, 23” it’ll drop back to around 390-400 degrees. If I try to lean to -100 ROP (around 9.8 to 10.2 gph) the #3 CHT will climb to 420 and higher. Other cylinders generally stay between 360 - 390 at all fuel flow settings during cruise. It was suggested I not run LOP until at least 25 hours. With CHT#3 still wanting a very rich fuel flow for cooling I dare not run LOP.

I would think all cylinders would be broken in by now. Oil consumption is stable. What do you think is going on? Any ideas?

 

[Bruce]

Jim

I would isolate some things to identify the source.

Move both plugs #3 to #2

Test fly
See if anything changes.

Then move CHT sensor #3 to #2

Test fly

See if you can pinpoint movement with changes.

This should send you down the right track.

Boomer

 

[Walt]

I’d turn the timing back 10 deg on those pmags. That will cool it off then you can work on other stuff.

 

[Bavafa]

Do you have the jumper in your PMAGs, if not then you will benefit by putting the jumper which will limit the advance shift. You may also want to retard the timing by a few degrees or program the PMAG for even less advance which will help your CHTs in general. But the difference of the CHT between your cylinder is something that you will need to work on and tweak to get a better airflow through all cylinders. One area to look at is the lower section of the baffle and make sure the air is staying within the fins and not escaping easily.
Just for a reference, the delta between my highest and lowest CHT is often around 8-10F and three of them are within 2-3F

 

[blaplante]

Engine was torn down & rebuilt...

Thus all the baffles were pulled and reinstalled. By a shop, apparently? Ever check baffles on the typical 172 or Cherokee? You can stick pencils thru.

So, that raises the question - did the shop RTV up all the little gaps in the baffles? Or did they just 'clean all that **** off' and do the job like every 172?

 

[Taltruda]

How is #1 CHT? If number 1 isn’t too hot also, you can put a little aluminum tape across the baffle at #1, and it will cool down # 3.
Not my picture, and the tape is much too high for what I’m suggesting, but it shows the concept. I would start with about 1/2 inch high. I believe I ended up with about 3/4 high on #1, and 1/4 high on #2.

 

[jakej]

#3 cyl, hot

‘Washer between the baffle’ - I’m really surprised no one picked this up.
Dan Horton’ s baffle mod is the new way to go

 

[lr172]

While reducing timing will drop temps, it will do it universally across all the cyl's and doesn't solve the #3 problem. I would start by doing a mixture sweep to find out where #3 EGT peaks relative to the other 3 cyl's, as there is a chance there is debris in the injector/restrictor or possibly you had balanced restrictors and the tech mixed them up. You can drop the RPM while doing this if you are concerned with the CHTs, but the data is more valuable if done at the 2400 you are seeing the issue at. With the fuel flows you are running, this is a real possibility. At 20 hours, there is NO issue running peak or LOP, especially just for a few minutes. At 20 hours, the VAST MAJORITY of ring seating is complete. Even if not, there is no issue running at other phases, just as you do for taxi and warm up.

If you don't identify a lean condition on #3, I would start looking for something that is reducing air flow over the #3 cyl head fins.

Larry

 

[lr172]

‘Washer between the baffle’ - I’m really surprised no one picked this up.
Dan Horton’ s baffle mod is the new way to go

He said he put a washer behind it. Also, we can presume the same baffling that was there before the O/H went back on and that this issue did not occur before the O/H. That points to something other than the classic air flow obstruction behind #3.

 

[D-Dubya]

All good advice so far. I would absolutely want to do everything possible to lower the CHTs on all the cylinders before doing the "GAMI Lean Test" to see if there's a plugged or under-sized injector. Those CHTs across the board seem a bit high to me, even with the unusually hot weather most people are seeing this year. Mike Busch has written several articles and recorded a webinar or two on this subject, so that's a good place to start if you need some background.

Close up every single gap in the baffling that you can. Little gaps can make a huge difference. Many people told me that when my engine was new but I didn't fully understand or believe it. But it's true--close up every single gap, leak, hole, crack (or whatever) that's allowing air to bypass and not pick up heat by blowing air past a cooling fin. Get a flashlight and look for gaps where it's not obvious. Pay special attention to the fit of the inter-cylinder baffles, as those are often overlooked and neglected. Does your plane have a plenum or regular baffle seals? If it has regular seals, are they all in place and folding the correct way? Was RTV that was sealing up against the case or metal baffles removed during the rebuild and not put in place afterward? If you're not sure what to look for, get someone with some experience to help you.

Get all of that in order (along with the timing that others have referenced) so it's the best it can be, then perform the tests to see if the injectors could be a problem. It's not that unusual for a rebuilt engine to pass a tiny piece of trash that can adversely affect an injector.

 

[Bavafa]

Get all of that in order (along with the timing that others have referenced) so it's the best it can be, then perform the tests to see if the injectors could be a problem. It's not that unusual for a rebuilt engine to pass a tiny piece of trash that can adversely affect an injector.

A clogged or more restricted nuzzle in the injector will affect your EGT a lot more. But if you see one of your EGT abnormally different than the other, you can test the flow but disconnecting the injector lines from your nuzzle, put a small cup under each and run your pump. Then measure the fuel in each cup which should be about the same amount.

Good luck.

 

[Tankerpilot75]

I’d turn the timing back 10 deg on those pmags. That will cool it off then you can work on other stuff.

Walt,

As you know PMags are supposed to be set at 0* TDC. When you say turn back ten degrees are you saying 10 ATDC or 10 BTDC? 10* ATDC would have it firing much later - right? That should produce less advance but how much is too much retardation? Just asking.

Bavafa,

The jumper is in. Timing is currently set at 2* ATDC per manual conservative alternative setting. I did this when I reinstalled the repaired PMag. Thinking about your’s and Walt’s comments, it seems to me setting timing after TDC should cause a later spark, less aggressive advance - shouldn’t it? Or am I backwards in my thinking? This is what I want to cool things down isn’t it?

I think I’ll go back to 0* TDC (where it was on my trip home from OSH) and see if this cools things down. That’s where it was on my flight home in March and CHT temperatures never got above 400*. But outside air temperatures were also fifty degrees cooler in March then they are now in late July.

I’m sure a GAMI test update is called for. Three of my Airfow Performance restrictors are 0.0280 while one is a 0.0285. With my previously described CHT/EGT sensor wiring mix up, it’s likely the 0.0285 restrictor was reinstalled on the wrong cylinder or possibly one of them is partially clogged. I’ll try cleaning all of them and installing the 0.285 restrictor in #3 and see if that helps the balance and possibly cool #3 down.

I’ve got two things to try. Thanks guys, you’ve refreshed my memory and given me some positive things to check out. The VAF brain trust is great!

 

[Carl Froehlich]

All good advice so far. I would absolutely want to do everything possible to lower the CHTs on all the cylinders before doing the "GAMI Lean Test" to see if there's a plugged or under-sized injector. SNIP

No need for this concern.

As your engine temps were good but now one cylinder is hot when ROP, something changed. That cylinder is getting to much air or not enough fuel.

I assume this is an injected, 8.5 compression parallel valve engine. I suggest:
- Verify the pMag jumpers in. This provides a max limit of 9 degrees advance when MP is low. Any more than that does no provide gains, but does increase CHTs.
- Time the engine per the lable plate (I assume 25 degrees BTDC). This is done with the crank at TDC then set timing per the pMag instuctions.
- Perform the “shot glass test” to rule out a possible spider issue. The attached photo is a shot glass test to figure out why an engine had a rough idle. We found a piece of crud in the spider.
- Look for an air leak on #3.
- Perform the GAMI test at 6000’ plus and wide open throttle. If #3 peaks early you have verified a fuel problem. Do the same test at something at perhaps 3000’ and less than wide open throttle verify or rule out an air leak on that cylinder.

Carl

 

[cubdriver]

#3 high CHt

I had a similar problem and changing timing helped alot. To retard timing start at TDC and then move the prop past TDC in the normal direction of the prop about 6 degrees which is 2 to 3 teeth on the flywheel, Leave the jumper in.

You should drop 20 degrees in temps. I ended up drastically cuttimg down the front baffles to get more air to the front cylinders as well

 

[Tankerpilot75]

Update - CHTs now staying below 400*

Just wanted to thank the brain trust. Did as several of you suggested and moved my timing further ATDC (approximately 6 degrees - 3 teeth after the TC mark) and cleaned the injector restrictors insuring the one larger 0.0285 restrictor was installed in #3 cylinder. Results: all CHTs are now cooler with none of them getting above 400 degrees without extreme effort and all stay within approximately 15 degrees of each other. Number 3 still runs hotter than the other three but not by much.

To confirm my work I went the the local air work area this morning and did some power on and power off stalls, steep turns and a little light acro. There was a brief moment where #3 reached 405 degrees but that was during a power on stall where the aircraft was 25 - 30 degrees nose high with airspeed decreasing below 54 knots. The aircraft doesn’t really want to stall with power and the MT9 three blade prop. It seems to try to hang on the prop resulting in climbing CHTs before it breaks into the stall. Anyway it’s now flying well and engine temperatures are definitely cooler.