VAF Forums

VAF Forums (https://vansairforce.net/community/index.php)
-   Traditional Aircraft Engines (https://vansairforce.net/community/forumdisplay.php?f=21)
-   -   Engine Stumble Help Requested (https://vansairforce.net/community/showthread.php?t=143125)

JonJay 10-27-2016 10:17 AM

Quote:

Originally Posted by lr172 (Post 1122136)
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

Less than a few seconds. The valve was free immediately upon enrichment. This was a partial stuck valve. Not disputing the data, just offering my observation. If you search through the posts, the last several instances describing exactly the same symptoms all proved out as stuck exhaust valves.
Keep in mind, EGT probes don't measure EGT directly. The thermal couple measures the probe, a mass of metal stuck in the exhaust stream. If it truly meausured EGT and had the dwell to do it, or lack thereof, it would swing wildly through the exhaust cycle. This was written up very well in a AVweb article a few years a ago.

YellowJacket RV9 10-27-2016 10:44 AM

Checking the exhaust valve would be a pretty easy job for Jesse to do, I would imagine, if only to rule it out. I recently checked mine after similar symptoms, and it turned out fine, although I, and many others were convinced it was the culprit. Turned out to be a fuel delivery issue (suggested by Savvy Analysis) solved with a carb rebuild. Do you have the Savvy Pro subscription, and if so, have they weighed in?

Chris

Aggie78 10-27-2016 11:05 AM

No Savvy Analysis Subscription...
 
Anymore!

I had it the 1st year after I bought the aircraft, but discontinued it and only use the website to parse/display my engine monitor data.

I may have sounded a little harsh about "throwing parts at it" (sorry Jesse! :o) but I'm really trying to be diligent w/r/t troubleshooting to narrow down-as completely as I can as owner/operator-what and where the problem is to allow the very best shot at getting a fix right on the first attempt.

The suggestions that my troubleshooting was incomplete (GAMI sweep, inflight LOP mag check) pointed to the fact that I perhaps haven't done all my data gathering yet...however, I am interested in the "sticking exhaust valve" scenario and what it might take to see if that's a possibility. I will search the website and see what pops up.

As noted, the bird is at Jesse's, and he and I are bouncing ideas around on what a root cause may be...

One other note...as to an induction leak, there is no fuel staining on the engine, and most of the time this engine is run >7500' MSL and WOT...I believe (could be wrong) that induction leak issues kind of fade away at WOT?

Thanks again, all...and I learn something every time I come to this website too!

Rob

lr172 10-27-2016 11:17 AM

Quote:

Originally Posted by JonJay (Post 1122243)
Less than a few seconds. The valve was free immediately upon enrichment. This was a partial stuck valve. Not disputing the data, just offering my observation. If you search through the posts, the last several instances describing exactly the same symptoms all proved out as stuck exhaust valves.
Keep in mind, EGT probes don't measure EGT directly. The thermal couple measures the probe, a mass of metal stuck in the exhaust stream. If it truly meausured EGT and had the dwell to do it, or lack thereof, it would swing wildly through the exhaust cycle. This was written up very well in a AVweb article a few years a ago.

Very interesting. I would not have expected a stuck valve to respond to a mixture change. I suppose it makes sense. Enrichment when LOP increases ICP, which would help in providing additional force to close the valve. However, that would only be the case if the valve was sticking in a closed enough position to still maintain enough compression to ignite the charge. I wasn't really thinking about the valve sticking in anything but a mostly open state, which would not support combustion. Clearly a flaw in my thinking.

Larry

JonJay 10-27-2016 11:46 AM

Quote:

Originally Posted by lr172 (Post 1122259)
Very interesting. I would not have expected a stuck valve to respond to a mixture change. I suppose it makes sense. Enrichment when LOP increases ICP, which would help in providing additional force to close the valve. However, that would only be the case if the valve was sticking in a closed enough position to still maintain enough compression to ignite the charge. I wasn't really thinking about the valve sticking in anything but a mostly open state, which would not support combustion. Clearly a flaw in my thinking.

Larry

Very counter intuitive I agree. I would have never imagined. I had two events over two years. It took that long to finally, almost by accident, isolate it to the valve. So, not a flaw in your thinking at all, just another scenario.

vic syracuse 10-27-2016 01:47 PM

I think if a valve was sticking in the closed postion there would be some other problems, like bent pushrods. I think this is more of a spark issue or a fuel issue.

Easy to move the spark plugs to another cylinder, and try another injector nozzle, one at a time.

Vic

lr172 10-27-2016 02:01 PM

Quote:

Originally Posted by vic syracuse (Post 1122294)
I think if a valve was sticking in the closed postion there would be some other problems, like bent pushrods. I think this is more of a spark issue or a fuel issue.

Easy to move the spark plugs to another cylinder, and try another injector nozzle, one at a time.

Vic

I didn't mean it was stuck closed. As the pressure comes off the rocker, the valve spring closes the valve. A sticking force can theoretically overcome the spring force at any point from fully opened to fully closed. I was suggesting that a valve that becomes stuck closer to the closed point would support combustion, where sticking near the open point would not.

Larry

rv8ch 10-27-2016 02:39 PM

Induction leak
 
Quote:

Originally Posted by Kyle Boatright (Post 1121943)
Your comment about it responding to increased fuel makes me think induction leak.

+1 on Kyle's idea - seems easy enough to check as described by msbobbi.

JonJay 10-27-2016 03:00 PM

Just to be clear, my similar issue was a "slightly sticky" valve, not a stuck valve, if that makes sense. That is why it was so hard to diagnose. Very few events over lots of flight hours.
I wouldn't think an induction leak would be so intermittent. I guess if this event is occurring consistently, that would make sense. I would find it odd to have an induction leak that comes and goes, or only shows its head during very specific operation conditions, but I am far from an expert.
Anyway, both induction leak and valve tests are easy to do.

Aggie78 10-27-2016 03:47 PM

Just to add a little more info...this stumble happens pretty frequently...if I'm going XC and I've got it set up for LOP cruise at altitude, it happens almost every leg...usually, well into the flight-not at/after top of climb. Like I said, usually by then I'm WOT, where I thought induction issues kinda faded away-they were a lower altitude problem.

Flying down low or quick up/downs where I'm not messing too much with the mixture ($100 hamburger/burrito/omelet runs) it runs without a hiccup.

JonJay, how did you solve your partially sticking exhaust valve problem? Is there a thread on VAF that you discussed it?

Thanks,

Rob


All times are GMT -6. The time now is 01:34 AM.