Rough engine and low EGT on #2 just after t/o. Any ideas?

[ao.frog]

Today, my plan was to do a start-up and check the engine instruments right after startup. I was thinking that if a cylinder had gone cold on me, I thought I wouldn't see any temp increase on either CHT or EGT.

Before I climbed in, I took another close look at the engine, shaking alittle on intake-tubes and exhaust tubes.
When grabbing hold of the exhaust tube for cyl #2, the tube felt loose.
I took a closer look and sure enough: the tube had broken off just below the flange.

Here's a pic:






Here's a closer look:








Here's a closer look from the other (aft) side:



There can be seen vertical crack too.




On the spark plug booth, the sparkplug and on the nearby area, there's residue from an exhaust leak.
My guess is that there was a small leak for a while, and then during takeoff yesterday, the tube broke off and resulted in a larger leak.




PS: on the above pics, I've pulled the tube downwards slightly to expose the broken area. Without downwards pressure, the break was almost not visible.



Now the big question is this: can this exhaust leak result in vibrations and a 650*F EGT on full power?

Obvioussly the EGT probably would be somewhat lower than the usual 1400 on takeoff power, but all the way down to 650??

In either case; I've contacted Vans since I bought the FF-kit from them but they told me to contact Larry Wetterman directly, which I just have done.

I'll report back how it goes.

[Neal@F14]

Originally Posted by Ralph Inkster

Also check for broken exhaust flange, would cause low EGT & near immediate fouling of plugs.

We have a winner!!!

[Ironflight]

Originally Posted by ao.frog

Now the big question is this: can this exhaust leak result in vibrations and a 650*F EGT on full power?

Definitely!! Clint (at Vetterman's) should take care of you....

[BillL]

Originally Posted by Ralph Inkster

Also check for broken exhaust flange, would cause low EGT & near immediate fouling of plugs.

OK, so Ralph is the winner and the normal CHT with low EGT makes perfect sense here. I can see that exhaust escapes on blowdown then the pulse pulls in fresh air to cool the EGT.

What I don't understand is why the plug will foul. Please help me understand why this would occur.

[RVDan]

Might consider replacing the spark spark plug and wire that have been exposed to 1200 degree F plus exhaust gasses. They are cheap and might be a problem later.

Lucky the engine was clean and there was no fire as sometimes happens if a pipe cracks fully off like that.

[miyu1975]

By the way how does a crack like that even happen in the first place? This is fairly new engine right, and I assume new exhaust? Doesn't appear to have cracked at the welds.

[Lote]

Alf--Just confirming, are your exhaust pipes supported flexibly at the other end to allow for engine movement? This is a puzzling failure if so.
LeRoy Johnston RV-6A Esperanza

[DavidBurton]

Originally Posted by miyu1975

By the way how does a crack like that even happen in the first place? This is fairly new engine right, and I assume new exhaust? Doesn't appear to have cracked at the welds.

I suspect that the crack did start at the weld. The area adjacent to where the joint was welded is called the "heat affected zone". This area can be problematic for strength issues. If the welding process, material selection or heat treatment are incorrect this area is notorious for cracking.

Given Larry's years of experience manufacturing exhaust systems I suspect that the culprit is more likely the massive structure of the flange and weld verses the relatively delicate tube. The tube shakes and acts like a long lever applying a massive force (on a PSI level) to the inflexible area next to the weld. Cracking is going to be the result. The exhaust system must move with the engine and also be prevented from oscillation.

Diagnosing the reason for this failure is likely to be an interesting adventure. I'm glad you found the crack before it caused other problems!

David

[Mark Albery]

I've had that happen twice on my no.2 cylinder. Both times repaired by Clint at Vetterman's.

The longer term fix is to add gussets fore and aft on the weld joint. Some Vetterman exhausts include them anyway.

I agree that it's a low cycle fatigue starting in the heat affected zone of the weld.

[Lote]

We brush spark plug anti-seize solution on the exhaust ball joints every time the cowling is off. Seems to help keep the joint flexible.

LeRoy Johnston RV-6A Esperanza