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Cylinder #3 high EGT

Michaelpk3

Well Known Member
I have O-360 that runs fine, compressions in the 70's, and just replaced intake rubbers and gaskets. Cylinder #3 runs about 150 degrees hotter than the coolest cylinder. The CHT's all seem to be the same. I could probably save an extra gallon per hour if I could get #3 down. All the other cylinders run about the same EGT (with 15-20 degrees of each other). In cruise I run 70 degrees rich of peak and cylinder #3 peaks first. Also to note is on decent, if I pull throttle back and do not touch mixture, then cylinder #3 climbs into the 1500 degree range, CHT is normal and the other cylinders EGT drop slightly. I swapped EGT's on #3 and #4 to rule out a sensor issue.

Is this just the nature of a carb and the fuel distribution charictoristics or should I investigate a little more?

https://photos.app.goo.gl/1zHCcd4w5iDUIHky1
 
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Michael,

The absolute EGT number is not all that meaningful. The placement of the probes can shift the reading a lot. What is meaningful is the fuel flow when each cylinder peaks. This is what will tell you if you have an issue or not.

The photo you provided shows 11.2gph while you have the engine set at modest cruise (and a much lower power than your displayed 163hp). For this power setting, I'd expect something around 8.5gph if ROP, 7.5gph if LOP. In other words you are running very rich. Is there a reason why?

Recommend you do a GAMI spread of the cylinders and establish a good base line before anything else.

One other note - it appears your alternator voltage is low. Typically this should be 13.8vdc to 14.2vdc.

Carl
 
Carl is dead on about absolute temps not being very useful. Measure the distance between the cylinder exhaust flamingo and the EGT probe for each cylinder. Get the measurement to the nearest sixteenth of an inch - a little bit gives a big difference in EGT. That will tell you if the temps should be close to each other or not. Report back here - it will be interesting to see what you find!
 
1/16

WOW
I knew the distance was suposed to be the same for accurate readings but, I never dreamed 1/16th would make a measurable difference.
Good info Paul....so should we have a EGT correction card for measurment or placement variances?
 
Carl,

Thanks for the info. The electrical system had an issue and had be repaired since. As far at measurements, I haven't checked that closely. I'm out of town for a few days but I'll measure when I get back. I will also get some trends.

That picture was taken at 3000' and cruise. I ran it the other day, at 8000' and ROP 60 on #3 (#3 is always quickest to peak) then I was getting 9.5 GPh. If I used the next fastest cylinder to peak (typically #4) then I can get 8.5 GPh at 8000' but cylinder #3 is running LOp and all other cylinders are ROP.
 
I will be following this one close as I have chased my tail for years with #1 and #3 both 100-150 hotter than others while all CHTs are within 15-20 degrees of each other.

I've accepted it's the nature of carburation and a difference of air flow to each cylinder. I will re-visit probe distances thanks to paul.

I have steam gauges so no hi-tek print outs.

doug
 
Air leak?

Is it possible you have an air leak on that cylinder? Possibilities: 1. Intake hoses; 2. Intake flange; 3. Manifold pressure tap; 4. Primer line. Those would make your cylinder #3 run leaner (and peak first).

I spent a lot of time fiddling with my O-290-D2 to get the EGTs to be in a straight line at cruise. What others have said regarding EGT numbers and probe location is all true. That said, the geometry of the throttle plate tends to throw more fuel toward the front or back cylinders depending on position. Also, if your carb has a one-piece venturi, I read where that can mess up your fuel distribution. Marvel-Schebler's solution to that was to offer a "peppermill" nozzle which had some aeration holes drilled in the shank to aim more fuel toward the back cylinders. I think those are available for your MA-4SPA carb, but I had to make my own.

These days when I level out in cruise, I lean out where I want, at which time my cylinder #1 EGT is usually higher than the others. I retard the throttle just a tad and that evens everything out.

Good luck!
 
I had up to a 250 degree spread between my EGT's and then the CHT's would reflect the difference. Ether my rear two cylinders would be cool and the front hot at WOT, bring the power back to 25 square in the climb and the rear cylinders would get hot. My leaning was way off. One cylinder would peak and the others were way off. If I brought the throttle back about 1/4" the EGT's would come with in 50 degrees of each other.

I cleaned out the intake tubes, they had a little varnish in them and I replaced the intake gaskets and hoses. That made a small difference.

Lycoming says a 150 degree spread is normal on a carbureted engine. I even sent the car in and had it rebuilt.

The butterfly position has a lot to do with the turbulence in the intake system.
I found this on here. Great info. It should answer all of your questions.

http://www.vansairforce.com/community/showthread.php?t=97025
 
Update

So I double checked the rubber boot tightness and intake torque and they are all good. (replaced all last week). I also measured the location of EGT and they are all spot on and equal length.

Yesterday, on a relative cool Florida morning (75 degrees), I took plane up to 5000' and the EGT's were all within 30 degrees F of each other. I flew from Sarasota FL to Melbourne FL which was a 1 hr flight, and they stayed 30 degrees F apart the whole trip (1 HR flight, CHT's were all 15 degrees apart). I landed, picked up a passenger, turned and burned. Now with the engine hot, I took off (cowl flaps open), 210 degree oil temp, 375 CHT, and now cylinder #3 EGT is 175 degrees F hotter than coldest EGT. It was like that the entire trip back to Sarasota at 6000'.
 
throttle position?

One thing I have proven lately on my plane is, after landing, the carb will get heat soaked from the hot oil in the pan. I just installed a carb temp gauge and in that situation, it will rise up to 65C after shutdown. I don't see how that would effect anything after you got airborne again, but I thought I would mention it.

To me, it sounds like a fuel distribution "problem". I mentioned earlier that I drilled a couple "areation" holes in my carb nozzle to direct more fuel towards the rear two cylinders. That worked, but in flight, my four EGTs are very sensitive to throttle butterfly position. A tiny change in the throttle will make a big difference in my EGT distribution. I can give you numbers if you want.
 
Had a similar situation on my RV-9A (O-320), except that #3 had high EGT in high ambient temperatures and on climb out. At cruise, the EGTs and CHTs didn't have nearly as much spread. It went away at the last annual, but it had been there through several annuals.

At least it's gone now.

???

Ed
 
numbers

Here are selected numbers from my flight last Sunday - similar to yours in that I stopped for lunch for an hour and the plane sat in the 90F ambient and heat soaked the carburetor. On the return flight, reaching cruise and before leaning, the EGT numbers were (highest in boldface):

1360, 1338, 1403, 1399 (65 deg spread), and CHTs were 367, 371, 385, 387. Fuel flow was 11.1 GPH at 4,500'

During leaning, the peak EGT numbers with the throttle backed off slightly were (my EGTs run hot):

1516, 1501, 1512, 1530 (29 deg spread), and CHTs were 344, 364, 387, 383. Fuel flow was 7.9 GPH at 4,500'

After enrichening 60 degrees and backing off the throttle slightly for even EGTs, I got:

1448, 1420, 1423, 1448 (28 deg spread), and CHTs were 331, 341, 353, 353. Fuel flow was 5.6 GPH (O-290-D2, remember).​
 
Poor fuel distribution is pretty typical on the carbureted Lycoming's. When I had a carb I had a similar situation. I was burning well over .5 GPH more than necessary due to the early peaking cylinders and that was after installing the better, pepperbox jet. I believe there just isn't enough plenum volume to smooth out the transition to the 4 intake pipes. The only real fix is to move to fuel injection. I purchased a used setup and now save around $3 per hour in fuel. At 300 hours since installation, it has already paid for over half of the investment and I haven't even sold the old carb yet.

There is a guy selling a new FI setup right now in the classifieds for $1600, which is a great deal.

Larry
 
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