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Engine Stumble Help Requested

Aggie78

Well Known Member
Howdy Gang,

Having a problem diagnosing this engine stumble...Cylinder #2 appears to lose power:

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I've kind of ruled out ignition system as a source due to cleaned plugs and a newly installed P-mag (10 hours ago). A little richening of the mixture and she smoothes right out and runs good, even leaned back out to the same degree.

Had the FF nozzles cleaned out a year or so ago, but did have the servo re-flowed earlier this year and wondering if gunk from that could be in a restrictor?

Thinking about swapping restrictors around as a first step unless someone has a better idea of something else to do first?

Thanks,

Rob
 
In flight mag check?
EGTshould go up when you do. The spike tells me one plug is miss firing.
When I first swapped to pmags, I had the same issue. I got religious about leaning while taxing. I also had to adjust my full rich mixture. It was very rich. You should see 50-100 rpm increase when you pull the mixture on shutdown.

Replace both plugs, clean the injector nozzle, restrictor.
 
It does not seem to be ignition (at least lower down the probability list), as it responds to increased fuel. You will notice that it does not fully recover with more fuel, though I don't know your engine. Notice that #2 is the second highest EGT in the beginning. After recovery, it is the lowest.

I would be looking at fuel flow and blockage, especially with a history of this problem. I would pull the #2 restrictor for cleaning and would also flush the SS line. If it persists, I would flush the whole system.

Given the FF, I assume you are running LOP at the time. My guess is that #2 gets restricted (a piece of junk floating around that blocks flow to different degrees at different times) and net #2 flow drops. This causes the miss or lack of firing. The increased gross flow gets it firing again, but is still more lean (net flow is still lower than normal) than it was before the event, as evidenced by your logs.

Larry
 
When you had the servo serviced did they also do the flow divider? I have trouble imagining that nozzles could plug and then unplug repeatedly. It seems more like junk in the flow divider to me.

You can see a little egt spike on #2 before it drops off when the fuel flow is increased from what appears to be the LOP condition. So it appears that cylinder is peaking well before the others.
Have you done a GAMI spread test on it?

Another thought is an intake leak.

Good luck
Tim Andres
 
When you had the servo serviced did they also do the flow divider? I have trouble imagining that nozzles could plug and then unplug repeatedly. It seems more like junk in the flow divider to me.

You can see a little egt spike on #2 before it drops off when the fuel flow is increased from what appears to be the LOP condition. So it appears that cylinder is peaking well before the others.
Have you done a GAMI spread test on it?

Another thought is an intake leak.

Good luck
Tim Andres

According to the logs, the problem occurred on three different occasions and always occurred on #2. I don't see how random blockage in the divider could always occur on the same cylinder. If the debris were stuck in the V slot of the divider's #2 outlet, it would not be intermittent at the same flow. If it is floating around in the divider chamber, the odds of it blocking the same port on three separate occasions are pretty low. The cylinder/piston in the divider creates a wiping motion over the v slots, opening and closing them based upon fuel pressure. The only blockage that can occur there over a period of time would be that which becomes stuck in the slot and does not protrude into the interior. I don't see how that type of blockage could be intermittent at constant fuel flows.

Just my thoughts. Clearly cleaning out the divider in this case is good advice.

Larry
 
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Just another opinion, but . . .

1. Intake leak -maybe possible, but since it is injected the fuel/air would be richer if there was a leak AND at WOT a leak has little effect anyway. = Not likely the issue, move on.

2. Lower fuel flow - could be. A GAMI test, as mentioned, would verify this definitively (irrespective of ignition effects). = possible - do GAMI check first. If bad fix that, then consider other options.

3. As mentioned - inflight mag check would help. Also, a weak spark will push the lean misfire towards the richer mixtures. Plugs, gaps, wires, etc all have an effect, but it sounds like you are appropriately checking those. = Possible, but do a GAMI first to eliminate the fuel issue as you could chase ignition for a while.

Just one cent.
 
It looks like you have your data on the Savey server - what do they say - That is what you are paying the for.
 
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
 
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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
 
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.
 
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
 
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.
 
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
 
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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.
 
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.
 
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
 
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.
 
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.
 
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
 
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! :eek:) 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
 
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
 
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.
 
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 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
 
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.
 
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
 
My experience is similar to Jon Jay. My #2 exhaust would stick open randomly/periodically. But it only ever lasted a few seconds, so anything I did seemed to cure it. Cut power, cured. Richen mixture, cured. But always accompanied by loss of EGT. This went on for a long time because it was so intermittent. When I removed the valve cover, it was clearly a valve tight in its valve guide. Reamed it out and no more symptoms. In retrospect, I could hear the valve occasionally not closing fully when at high rpms. Just an almost imperceptible miss.
 
The End of The Story!

So, to circle back to the beginning of this thread and wind things up...while the aircraft was at Saint Aviation for an avionics upgrade, I asked Jesse if he would mind adding popping the #2 cylinder head cover off and checking the exhaust valve for freedom of movement after the way it had been running and the info learned on this thread.

He agreed, and we added it to the bill of work.

I arrived today (as the project is reaching its end) and Jesse informed me that the mechanic he'd brought over to check this out for him opened things up, and they discovered a verrrry difficult to move exhaust valve-so much so, that it took a socket extension and a bit of persuasion with a hammer to move it at all.

The mechanic disassembled the associated valve train and then cleaned out the guide with a hone (removing a fair amount of gunk) and the valve moves silky smooth now. Test flight tomorrow, but I gotta believe this is going to resolve the engine stumble.

Wouldn't have guessed an exhaust valve issue without this website! Thanks to all who contributed, and I'll PIREP how she runs (especially lean of peak) after the flight home.

Thanks again, all!

Rob
 
As noted above, it's not injection nozzle or similar since I have a carburetor.

I would like to persue the intake tube theory. Is there a test for an individual intake tube?

As for the sticking valve, I'll ask my mechanic. I do occasionally run 100LL but as noted, I burn very little of it and a recent inspection of the plugs were very clean. That wouldn't completely discount a valve.
 
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If you search through the posts, the last several instances describing exactly the same symptoms all proved out as stuck exhaust valves. .
So, we can add one more to our list. Our engine monitoring tools are great, but as Bart LaBlonde previously stated to me, things happen very fast during the combustion cycle.

Very glad you found the issue and it was an easy fix. Yay!
 
So, we can add one more to our list. Our engine monitoring tools are great, but as Bart LaBlonde previously stated to me, things happen very fast during the combustion cycle.
Hmm... Wonder if there's a way to detect a sticking valve somehow with instrumentation that we don't already have in our planes... I see an untapped market... :)
 
Easier is better . . .

Hmm... Wonder if there's a way to detect a sticking valve somehow with instrumentation that we don't already have in our planes... I see an untapped market... :)

There is a way. The acceleration/decel of the crankshaft creates an rpm fluctuation during a revolution. This is easily detected with instantaneous speed sensor (with adequate number of teeth) and then algorithms for detection can directly identify an event and offending cylinder. This is used on large 12-16-18-20 cylinder engines to detect combustion anomalies. It takes a pretty high speed processor, but those are widely available these days. I don't know if these are commercially available for smaller engines. Maybe Ross will be interested, I know the people who have done this.

Example Publication $$: HERE

Taking time to remove the VC and check each valve annually would not seem like much work. It seems a lot easier to notice abnormal engine operation and then do the check.
 
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As noted above, it's not injection nozzle or similar since I have a carburetor.

I would like to persue the intake tube theory. Is there a test for an individual intake tube?

As for the sticking valve, I'll ask my mechanic. I do occasionally run 100LL but as noted, I burn very little of it and a recent inspection of the plugs were very clean. That wouldn't completely discount a valve.

I could be wrong, but I doubt the build up in the exh valve guide is lead. It is likely carbon and produced by the oil carbonizing (i.e. burning) due to the excessive heat that Lyc's see on the exh valve stem due to very limited oil cooling in the rocker box. The exh valve stem has very little exposure to lead in it's pre-heated state. I would expect to see the same problems in a lead free environment.
 
Flight Home PIREP...

3 legs, no problems!

Pushed it too, running it very LOP and looking for any issues...

Ran like a top! :D

Now I need to research the suggestion about running Marvel Mystery Oil as a preventative.

Also chasing down a lead that we might be able to get Swift Fuels 94UL at my home base, too, the use of which I understand would help as well.

Thanks again to all for the help!!

Rob
 
I could be wrong, but I doubt the build up in the exh valve guide is lead. It is likely carbon and produced by the oil carbonizing (i.e. burning) due to the excessive heat that Lyc's see on the exh valve stem due to very limited oil cooling in the rocker box. The exh valve stem has very little exposure to lead in it's pre-heated state. I would expect to see the same problems in a lead free environment.

Yes, this was explained to me today as we checked various possibilities. We did the "wobble test" on the exhaust valve and it was "nearly perfect" with zero sign of sticking or buildup.
 
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