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-   -   Engine Stumble Help Requested (https://vansairforce.net/community/showthread.php?t=143125)

lr172 10-26-2016 09:23 PM

Quote:

Originally Posted by JonJay (Post 1121983)
I second that request. Same symptoms I had with an intermittent exhaust valve sticking on #2.

Are you saying that you can free the stuck exhaust valve in a few seconds by enriching the mixture? The OP's logs show that the condition immediately reverses itself after mixture enrichment, which points to a lean mixture induced mis-fire. If it were a stuck valve that free'ed itself, the OP's #2 EGT would have returned to it's pre-condition state and this did not happen according to the logs.

Larry

lr172 10-26-2016 09:33 PM

Quote:

Originally Posted by Ben Meyer (Post 1121978)
Educate me, please. Is this not a stuck exhaust valve? Thanks in advance!

Ben

While I have never experienced it on a Lycoming, I would expect a stuck exhaust valve to drop EGT by around a 1000* With a stuck valve, you get no compression and therefore no combustion; EGT will quickly pummet. The OP's logs show only an approximate 200* drop and it is very short (more typical of a miss). It was my understanding that stuck Ex valves typically first present themselves when cold (morning sickness) and persist for longer than a few seconds.

Because the OP immediately added mixture and the problem went away, we can't prove cause and effect due to the enrichment. However, I would not expect stuck valves to appear for only a few seconds in warm cruise configurations.

Larry

Jesse 10-26-2016 09:41 PM

Maybe, short of a GAMI test, you could just bump up the size of the restrictor in #2 and see if the problem goes away. I'd go up 0.0005 or 0.001 for a quick test. Btw, I have those in stock and should be fairly easy when you come to pick it up and test things out.

Aggie78 10-27-2016 03:48 AM

Thanks to all...
 
For chiming in, I appreciate the thoughts and suggestions.

I do run this engine pretty LOP at cruise, which is where it was at when this happened on all three events. (There are more examples, but I just did 3 to make the point.)

Add'l info: This never happens very early in the flight...it always seems to happen after being at cruise for some time...30-40 minutes plus. Heat related? IDK...

The charts I've uploaded are "zooms" of the specific events to gather/present more detail for troubleshooting. So the time period shown is shorter and may not show an eventual return to prior LOP settings. Also, while I use Savvy's website to upload/study my data, I don't pay for their premium analysis service.

It does appear that in a couple of instances, a slight EGT spike just before a rapid drop would imply the cylinder going even leaner before going so lean it reaches a point that combustion cannot be supported and the misfire occurs. That's what's leading me to believe it's an fuel flow/injector problem...plus as I've said, in the last 10 hours I've put in a brand new P-mag/plugs/harness as well as pulled/cleaned/inspected the Tempest's running off the remaining mag.

The suggestions to do an inflight LOP mag check and a GAMI sweep/spread are good ones...but a (small) part of the issue with doing so is I don't have an RPM signal yet from the P mag side to the Skyview, I'm just feeding the RPM signal from the mag side. When running only on the PMAG side, RPM indication=0. Too much buried wiring to dig through and too close to a panel upgrade project at Jesse's to mess with it at the time. The plane is at his place, the panel is apart and that's part of the bill of work to accomplish before closing things up again.

Jesse suggested inspecting/cleaning out the restrictor and line, or now I see maybe resizing it...which I'm not opposed to, but my initial nature is to not throw parts at something without a clear diagnosis first...

First thing, the panel project needs to be wrapped up and the airplane flyable before I can inflight trouble shoot...but running the situation past the brain trust here to get some troubleshooting ideas lined up for when it's ready to go was a first step.

Thanks again,

Rob

RV8iator 10-27-2016 06:05 AM

My engine would do similar things before I balanced my injectors to peak at the same fuel flow. By getting all cylinders to peak EGT within 1/10 of a gph I now have an extremely smooth engine all the way back to where it just almost quits when leaning. Before balancing I had only a 3/10th spread and got a lot of roughness running too lean.

msbobbi2qt 10-27-2016 06:24 AM

hi all so i did not read any thing on pressure test the induction system, does the number two intake have any blue stain on it.... vacuum on blow down the intake use duct tape and soapy water spay around every thing on induction pipes. lycoming engines the induction system is not balance so #2 is leaned to much, LOP is not good on lycomings try not leaning so much and u will save your engine. put the richs nozzle in 2, swap out
gami will help...
msbobbi 540-220-8138 A&P IA 35years now

Jesse 10-27-2016 06:34 AM

LOP is not good on Lycomings? I beg to differ on that one. I run LOP every chance I get and the engine loves it. 1,300+ hours on a 540 in an RV-10 and the engine was in great shape when torn down for oil leak between the case halves.

Rob, I don't see how it can be a heat issue. The engine doesn't keep getting hotter as it runs. The hottest point is climb and then getting to peak on your way to LOP, where the CHT's should be staying low.

If you are already LOP, then a cylinder goi lean from there would not cause a spike. This would only happen if the cylinder was ROP and leans out.

We wouldn't be throwing parts at it, because it won't cost you anything but the fuel to test. We put in a bigger restrictor (<5 minutes to install) to see if the problem goes away and then we can put yours back in.

We can teubleshoot when you get here.

I don't see how an ignition problem could cause this. Both plugs would have to stop firing for this to happen. Replacing one set of plugs and going to a Pmag eliminates that possibility in my mind.

RogerG 10-27-2016 06:37 AM

On a friends recent PMAG installation running "B" curve we found starting and ground run(low RPM) was better than ever, however leveling out at altitude reducing throttle would sometimes produce engine stumble.

Timing was rechecked, and jumper installed "A" curve on PMAG, and no more engine stumble. Ground Ops, and fuel efficiency better now than ever before PMAG install.

Considering you recently installed Pmags you may want to re-check your setup.

Ben Meyer 10-27-2016 06:56 AM

Quote:

Originally Posted by lr172 (Post 1122138)

Because the OP immediately added mixture and the problem went away, we can't prove cause and effect due to the enrichment. However, I would not expect stuck valves to appear for only a few seconds in warm cruise configurations.

Larry

Thanks, Larry! I learn more every time I come here!

Ben

Adam 10-27-2016 09:22 AM

I had the same issue, I never happend on the ground only in the air, almost always when reducing power on my decent. I check everything and found that it was a stuck (sticky) valve, we had to ream it out and has be fine since.


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