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High CHTs

kaa

Well Known Member
Hi all,

I finally finished my panel upgrade to G3XT and started flying (yay!). Unfortunately, I'm seeing very high CHTs after takeoff, especially on cylinder 4 - by the pattern altitude it reaches 450F. It has never done this before. I used to have a 4 cylinder engine monitor from EI (UBG-16) before, and I don't think I've ever seen anything more than 420 or so and even that on very hot days only. I changed all of the sensors during the upgrade, so maybe the old system just didn't show this behavior since it had a significantly slower response.

During the upgrade I didn't change much about the engine, but I removed the primer from cylinder 3 and relocated manifold pressure pickup on cylinder 4. I can't really see how this can change things though.

The engine is O-360 with dual PMAGs. Fuel flow on takeoff hovers around 15-16 gph, which seems ok, but maybe it's a bit low? I'm have the 10-3878 carb, which I don't think was modified.

I checked pmag timing - it seems correct (green lights at the TDC mark). Pmags are running on the A curve.

I found a few gaps in the baffles that I fixed today (haven't flown yet), but I doubt that they can explain ~40 degrees F difference.

Here is the data from the most recent flight - I did a few laps around the pattern: https://apps.savvyaviation.com/flights/4116858/f632a500-a810-4c88-8f84-91810355a125

One weird thing there is that all EGTs go up when I throttle down at the pattern altitude - why would that be? I understand if one cylinder did that due to an induction leak, but all of them? Does it have something to do with the economizer perhaps?

At this point I'm wondering what my options are. The only plan I currently have is to try the lean test (at altitude) to check if my carb is too lean. Other than that I don't really have good ideas.
 
Hi all,

During the upgrade I didn't change much about the engine, but I removed the primer from cylinder 3 and relocated manifold pressure pickup on cylinder 4. I can't really see how this can change things though.

First step is always to check recent work. Are the holes from the former MP pickup, and the primer, plugged? Is the new MP pickup leaking (this is my bet) or missing a restrictor?
 
All holes in cylinders are plugged. MP doesn't seem to be leaking (the indications are normal). What would be a good way to test it?

I also see a bit of oil potentially coming out from between the carb and the oil pan. Would an induction leak there cause these symptoms?
 
Seeing egt?s go up when reducing throttle (full rich, or at least rich of peak) is a classic symptom of an intake leak (at full throttle there?s little or no pressure difference between intake manifold and outside world, but at partial throttle there?s a significant difference, so more air leaks in, mixture goes leaner).
 
If all EGT?s rise when you pull power, it pretty much means you have an intake leak somewhere, according to a Vic Syracuse article (I read all of his articles). Common leak locations are somewhere on induction tubes, induction tube to head gasket, an open primer port, etc. Since all four do it, I would expect an area common to all four cylinders, like the carb gasket, like someone has already said.
 
450 at pattern altitude is pretty high. Did you confirm that all CHT's now read the same ambient before start up? Do they rise in a similar manner during warm up? Do you have a CS prop? If so, 15 GPH seems a bit low for T/O fuel flow. I only have a 320 and 540, so no experience. I would have guessed that 17 is closer to normal. What are the EGTs when you reach pattern altitude?

Larry
 
Thanks everyone for suggestions, appreciate it.

I'm pretty sure the thermocouple type is set up correctly. Before the engine start all temperature sensors read ambient, as expected.

Here is some data from the first pattern at the time of initial throttle back (first number is the absolute value before throttling down second is the difference between partial throttle and full throttle):

Code:
Time: 00:12:44-00:13:01
EGT1: 1445?, 35?
EGT2: 1340?, 110?
EGT3: 1335?, 70?
EGT4: 1310?, 80?
MAP: 28.4, -6
CHT1: 421?, -5?
CHT2: 422?, 0?
CHT3: 429?, -4?
CHT4: 451?, 2?

There are more instances like this in the graphs (the link is in the first message).

It does look like an intake leak as several people noted here. I'll take a look at the carb attachment and report back.

(heh, always wanted a data logger)
 
Thanks everyone for suggestions, appreciate it.

I'm pretty sure the thermocouple type is set up correctly. Before the engine start all temperature sensors read ambient, as expected.

Here is some data from the first pattern at the time of initial throttle back (first number is the absolute value before throttling down second is the difference between partial throttle and full throttle):

Code:
Time: 00:12:44-00:13:01
EGT1: 1445°, 35°
EGT2: 1340°, 110°
EGT3: 1335°, 70°
EGT4: 1310°, 80°
MAP: 28.4, -6
CHT1: 421°, -5°
CHT2: 422°, 0°
CHT3: 429°, -4°
CHT4: 451°, 2°

There are more instances like this in the graphs (the link is in the first message).

It does look like an intake leak as several people noted here. I'll take a look at the carb attachment and report back.

(heh, always wanted a data logger)

CHTs do not seem all that unbalanced. My #6 is 30* hotter than others on my 540 and my #4 is 20* hotter than others on my 320. I would expect EGTs to drop in those scenarios (with a carb that is leaned via the red knob, air fuel ratio typically increases as throttle is pulled back, lowering EGTs), so it is likely that you have intake leaks. Reducing MAP will increase the amount of leanness due to a leak. While a carb gasket seems likely here, it is not uncommon for ALL of the intake gaskets to fail, especially if they were not re-torqued early in there life. You can spray carb cleaner near the potential leak points and listen for changes.

Getting 420 CHTs in the first minute after T/O is a problem. Could be either poor baffling or a lean mixture, due to too small of a main jet. What are the EGTs when reaching pattern altitude after take off?
 
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Garmin from EI Instruments

I recently upgraded from UBG-16 to Garmin G3X, and saw a 50 degree increase in CHT's. I was hovering around 390 in climb, after the upgrade I was seeing 440. My mechanic was suspect of the sensors.

We closed up the intake ramps, redid the baffle seals, and cleaned up the seal design around the spinner. Temps came down 40 degrees or so. Now even on a 110 degree day, I can climb at 120kts with the hottest cylinder at 395 degrees. In cruise the temps come down about 20-30 degrees.

I would take a hard look at your cooling setup, since in my case the temperature delta seen between the EI and the Garmin proved to be accurate.
 
I repaired a few small problems in the baffles, and it took ~10 degrees off the max CHT in climb to pattern. Still the maximum is at 440, which is too high for comfort. Today's flight: https://apps.savvyaviation.com/flights/4140631/33d741f0-7a87-4a57-9f73-c0bead8909e3. I flew with MP disconnected from the p-mags just to make sure that variable timing doesn't do weird things to EGTs.

The EGTs rise when throttle is pulled is still there (not surprising since I haven't changed anything). I tried to find an induction leak using the shop vac method, and the only thing I found is a leak around the carb throttle valve shaft. It looks like this: https://photos.google.com/share/AF1...?key=d0NyTUIwcUpUcG9WWjhkZzFMd3huXzRncGpVQ3RB.

Everything else seems fine, at least no obvious big leaks anywhere.

The valve shaft wobbles a little bit, enough to see, but I could not record a good video to show it, so it's not too much. I know some play is acceptable, but are leaks there expected, or is it time to overhaul the carb?
 
I installed an overhauled 10-4164-1 carb, and nothing much changed. The fuel flow went from 15 to 17.5, so that's nice, but the CHTs didn't seem to drop much (just consistent with a cooler day). I rechecked the baffles, and most of them look ok, but there are big holes (pretty much the absence of a baffle) around the bases of cylinders 3 & 4 (where they are smooth with no fins). I'm not sure how it is normally set up though.

I'm starting to question the intake leak theory though. Perhaps the EGTs rise when the throttle is closed enough for the idle circuit to kick in, and it seems to be adjusted too lean?

The data is here: https://apps.savvyaviation.com/flights/4190354/2599b8e3-86ab-47c9-8b9c-0cc9ad3cfca8

Would anyone have pattern flight data from a carbureted O-360, just to compare?
 
I just went through chasing a similar problem in a IO540. I was certain I had intake leak issues as I had rapidly rising EGT’s on throttle reduction in the pattern. It turned out the induction system was fine. What I did have was a loose wire cap on one mag and a bad plug on the other mag. Neither showed up on a traditional mag check. In addition I am actually running a bit rich at takeoff fuel flows but am hesitant to address that with the current high temps. I did not however have CHT issues. I would run a high altitude mag check with the mixture as lean as you can get it.
G
 
The older EI cht probes are known to read on the low side, up to 30-40deg, so seeing an increase after changing to the bayonet style probes is normal. I just did a swap from the old to the new style on my own engine and saw a 40 deg increase.
 
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Thanks!

Yeah, I'm starting to think that the whole induction leak theory isn't that great. I'll do proper cruise checks.

Walt, are you saying that EI is 40 degrees lower steady state or that it's slower to react?
 
You may have 2 separate problems. First, you do have leaking intake gaskets, as evidenced by the increase in EGT when pulling the throttle back. I noticed some of you even mentioned my article on that topic! So glad to hear it!

Second, do make sure the config is right on the probes. I have seen that mistake more than once, and it almost caused a panic.

Once the config is checked, then check for all of the typical cooling suspects, as mentioned in some of the other posts.

But do change the intake gaskets. They may not make a difference in cooling on climbout, because you are at full throttle, but will make a difference at idle and other power settings.

Vic
 
I'm starting to question the intake leak theory though. Perhaps the EGTs rise when the throttle is closed enough for the idle circuit to kick in, and it seems to be adjusted too lean?

Your prerogative to discard all of our advice. The idle circuits flows constantly. It is not turned on or off, though it progressively flows more as vacuum increases (i.e. MAP drops), unlike the main jet that flows linearly with air flow volume.
 
I note that you have P-Mags, and that they're correctly timed at TDC.

This won't be the ultimate fix, but may give you some relief till you find it: since the timing on the P-Mags is so easy to adjust, may I suggest retarding by 5 degrees or so? The power loss will be minimal, but the effect on CHTs is significant.

My own experience in this area (RV7, IO-375) was a long series of tweaks to control CHTs, particularly at low speed and high power. The single biggest change, with about a 30F reduction, was a friend's recommendation to time the P-Mags 5 degrees after TDC.
 
Thanks!

Yeah, I'm starting to think that the whole induction leak theory isn't that great. I'll do proper cruise checks.

Walt, are you saying that EI is 40 degrees lower steady state or that it's slower to react?

Yes steady state, the older ones are a different probe style, not the spring loaded bayonet commonly used now.
 
Thanks everyone for the help, much appreciated. I tried to plug some holes behind cyl #3 and #4 (around the dipstick and symmetrically on the other side) and it may have helped a bit, but not much. Still running 425F on #4 during the initial climbout. I guess I'll put more RTV around.

I did an induction leak test at 5000'. Here is a picture: https://photos.google.com/share/AF1...?key=amNFS09ZODd0LVpIMUZ0b0FNZXlwVWJiOUNHVTNB

Full data here: https://apps.savvyaviation.com/flights/4194603/86ecafa5-7e10-4cbd-9d20-9ee0a3c4d357

Interesting that the first two times #1 and #2 went up, and the third time all of them did. Is this typical for induction leaks?

I tried to spot the leak by pressurizing the intake system through a cylinder's intake valve, but no luck - cannot see any bubbles anywhere. I'll replace the gaskets and rubber tubes (the tubes haven't arrived yet), but I'm a little uncomfortable that the leak doesn't show up with soapy water. I also replaced intake gaskets on #2 and #4 recently, when I was installing CHT probes, so they should only have a few hours on them.

I also verified that the probes are configured correctly.
 
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pmags

Hi Kosio, I can't add more on the induction leaks, other than to fix them. You may not have seen the product that Ross sells - if you are looking for something else cool to add to your engine, he's got some very nice induction tubes and gaskets. (http://www.sdsefi.com/aircraft.html)

Also, on the pmags, you probably know that you can get serial data from them and can program them. You can use a laptop, or pick up Bill's EIcommander (https://eicommander.com) or EngineBridge (https://www.enginebridge.com/product/engine-bridge-ei/) Good luck!
 
Thanks, I just ordered a kit from Ross. Hopefully it fixes things.

I have an EICommander, and it shows that advance increases as MP is reduced, as expected.
 
Thanks everyone for the help, much appreciated. I tried to plug some holes behind cyl #3 and #4 (around the dipstick and symmetrically on the other side) and it may have helped a bit, but not much. Still running 425F on #4 during the initial climbout. I guess I'll put more RTV around.

I did an induction leak test at 5000'. Here is a picture: https://photos.google.com/share/AF1...?key=amNFS09ZODd0LVpIMUZ0b0FNZXlwVWJiOUNHVTNB

Full data here: https://apps.savvyaviation.com/flights/4194603/86ecafa5-7e10-4cbd-9d20-9ee0a3c4d357

Interesting that the first two times #1 and #2 went up, and the third time all of them did. Is this typical for induction leaks?

I tried to spot the leak by pressurizing the intake system through a cylinder's intake valve, but no luck - cannot see any bubbles anywhere. I'll replace the gaskets and rubber tubes (the tubes haven't arrived yet), but I'm a little uncomfortable that the leak doesn't show up with soapy water. I also replaced intake gaskets on #2 and #4 recently, when I was installing CHT probes, so they should only have a few hours on them.

I also verified that the probes are configured correctly.

#2 is definitely showing symptoms of an induction leak. Maybe #1 to a much lesser degree.

Larry
 
Could you explain why? Don't #1 and #2 show the same amount of EGT increase?

#2 rises much more than #1, when viewed across the average. Also, #1 EGT is not stable like the other 3 under high MAP, which indicates that something else is going on with that cylinder. The incosistent behavior makes it difficult to see what part of the rise is related to induction leak. Notice how crisp the changes are on #2 compared to #1. You can see the clear correlation between MAP and EGT and how much larger the rise is on #2 relative to 3 & 4.

Larry
 
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Well, I installed the no-leak kit from Ross, and nothing changed. The graphs looks exactly the same with #2 showing a big rise on throttle reduction. I guess it's something else then?

Could it be the primer port plugs? What's the correct sealant to use there? Although even with no sealant the potential leak path seems very small.

On the plus side, I think I might be able to bring the temps down by using copious amounts of RTV and speed tape around the baffles.
 
Am currently upgrading the 20 year old 10-3878 carb on my O-360-A1A with a rebuilt 10-4164-1 carb.

Spoke with the tech support engineer at Tempest/Marvel-Schebler today. They advise the 10-4164-1 carb should flow about 4.4 gph more at full throttle, full rich mixture, at sea level versus the 10-3878 carb.

Just chiming in here to provide for future reference some quantitative Sea Level flow data for the 10-4164-1 carb.
 
FWIW, I only saw 1-2 GPH increase on takeoff. Not sure if my old carb was modified or what.

Haven't looked at my intake leak situation more yet (I was away in the last to weeks, and now it's too smokey to fly here), but I really don't have any idea on where to look next. I guess I can try spraying starter fluid around the #2 cylinder now (I'd rather not do it around a spinning prop, but then again, I have no other ideas).
 
Finally some good news, I think. I sealed the baffles as much as I could using high speed tape and RTV, but it didn't do much, I was still seeing 425F in a climb on a 90 degree day. What helped was adding some foil dams for #1 and #2. This evened out temperatures significantly, and the climb is just barely 400F on with 90-95F OAT. Now to find that pesky induction leak.
 

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