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Engine miss-fire LOP diagnosis

Wunderon

Well Known Member
New IO-360M1B, 13 hours. Starting to investigate LOP operations. As I lean past peak (8000', around 60% power) I get continuous 'miss-fires' or 'burbling', which continue as I lean further until the engine starts to quit. Enriching back to ROP clears any symptoms.

I'm assuming injector nozzle or perhaps spark plugs. All other engine operations seem normal as far as I can tell. Mag check at altitude seemed ok, generally increased EGT but smooth running.

Avstar servo, spider, and injectors. Slick Impulse and PMAG. Checked timing, cleaned and gapped plugs before first start.

Can we tell which one or more cylinders might be causing the issue from this info?

You can see that number 2 stays pretty rich, hardly peaking, while the misfiring appears in the RPM. #2 CHT also runs higher, which I suppose makes sense. I did check the #1 and #2 probes for accuracy.

The gami spread is around 0.5 I think.

4hKaxuC.jpg


Thanks!
dave
 
I have an M1B but two Slick mags. Are you WOT? CS prop? If so you are really low in RPM by my experience. But . . . I have found the following in no particular order:

A - as I lean, one plug will begin to misfire first, that causes the EGT to go back up then crashes as the second plug will begin to misfire. (a 2- mag issue)
B - Lower RPM (with my mags) allows greater EGT drop
C - I have to twiddle the mixture very very slowly as it takes a couple of minutes to really stabilize FF an temps with each change.
D - maybe it is my bird, but the fuel temps rise quite a bit from the tank to the fuel pump, and after high EGTs the FF can wander and if really close to the lean limit, drop/dip fast enough to yield rough running.
F - I find the best indicator of what is happening is to plot airspeed against FF after the EGT peak to see where the linearity of the leaning breaks over. It is seen happening in the data well before any cockpit indication.

You will be eliminating one plug (the mag) from firing the mixture completely if you have advance on the pmag at that altitude. Therefore, any dip in fuel, or rapid change in mix can quickly drop out multiple cylinders. If it gets rough, it will take a .8 GPH ( or more) increase to reestablish thermal stability and then the FF can be dropped again.

I would not try all this for extended periods at 13 hrs on a new engine.

It takes me a lot of study of the Savvy data, much more than posted along with other data to see what is really happening. It is very subtle in the data. Honestly my double Slick does not run very well at super lean settings, and a lot of effort has been invested in testing and setting of the timing.

I found learning about leaning was a quite the journey with lots of hours in cruise and more in data study.
 
At only 13 hours, perhaps you are seeing some oil or carbon fouled plugs.

At 8000' you can run wide-open-throttle and lean to any setting without harming the engine. (because it can only make 75% power). I would start from WOT and lean to peak power and run that way for a few minutes to stabilize the plugs. Then lean gradually and see how the engine performs. Try switching off one mag at a time and see if there is a difference. Yeah, they talk about avoiding the "red box" while leaning but for testing the engine, it doesn't hurt to lean gradually and see what the engine is doing. The RPM will climb, peak and begin to drop off and eventually begin to run rough.

5.2 gph FF seems pretty low for an IO-360 so maybe you are reaching a practical limit for non-matched injectors. My experience is only with carb'ed engines but I that's how I'd approach it.
 
Slow your leaning process if you want good data. The plot shows 15 seconds or less between peak and least EGT. Tweak a little, wait, let things stabilize, repeat.

Tighten up the GAMI spread if you can. A lot of us have gotten it down to 0.2 spread or less. Checking it might take 10 minutes of tweaking and waiting for the EGT probes to settle. No need to start at full rich. Start at some EGT higher than your EGT at takeoff to shorten the process.

Lower RPM will exacerbate the roughness.

Fuel flow does look low...the cursor line says 5.2 GPH at peak? I'm thinking more like 7.5 is normal, and the indications are misleading here because of probe lag. Put another way, you're leaner than you think.
 
5 GPH is very low for cruise power. Wondering how lean you really are. #2 EGT looks like it is unbalanced from the others. Agree with others - change FF by .2 GPH and wait 30 seconds, observe and repeat. Make a chart of results. You simply can't see the true peak behavior at the speed you are leaning at.

From the limited test data, #2 looks to be quite a bit richer than the others. You can send your chart to AFP and they will recommend a set of new restrictors to help balance.

Larry
 
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Same setup.

We have the sames setup. We just did the test runs and talked to DON at AFP.
We are ordering restrictors and see how it works then. Although my spread was pretty great (0.3g/h) for a stock engine. Really only #1 needs to be adjusted she is not happy on the Lean side. I hope that changes with the restrictors.

but at 8500ft and WOT at 2300 we were at 6,9-7 g/h. So nowhere near 5,5g/h
 
Thanks so much for the input. I will slow down my process for further testing. Remark: The fuel flow should be taken with a grain or cart of salt as I’m still calibrating the sender. It is certainly still reading low at this point.

dave
 
LOP

For what it's worth, my IO-360, with dual P-Mags will cruise at around 2250 rpm, 50 degrees LOP at 5.2 gph. About 145 TAS.
You HAVE broken in the cylinders, haven't you?
 
Send the data to Savvy Analysis!

+1. They can look at ALL the data and give you an informed opinion. They can also look at different flights.

However your rough running aside....I'm a bit surprised that you are running at such low power settings at 13 hours on a new engine. You say you're running LOP at 60% power. However if your fuel flow is even remotely accurate at 5.2 gal/hr that would be less than 50% power.

I'd be worried that you could be glazing your cylinders at those low power settings at 13 hours. That could be very expensive. I would be running at much higher power, only ROP, and at lower altitudes until the engine oil burn has stabilised. In some engines that can take a lot longer than 13 hours. I'd be running at 75% power until the oil consumption had stabilised. As they say...it's not good to baby the engine on run-in.
 
The gami spread is around 0.5 I think.

Thanks!
dave

Shouldn't have to guess at the spread, the Savvy site will tell you this exactly.

I'm tuning my new AFP system right now. Here's a chart from a test run this past weekend. Click the "G" in a circle icon at the upper right to enter GAMI mode. Then click the cursor at a point before the first cylinder peaks, and again after they've all peaked. Savvy will immediately show you the peak for each cylinder and the spread.

GAMI+example.jpg


I also agree with what others have said, the lean sweeps need to be done as slow as you can stand to do them for best results. This is not the best example since it was only a little over 30 seconds from first peak to last peak.

This 0.3 spread was not bad with all restrictors at 0.0240, but I think I can do better. I have a 0.0235 & a 0.0245 arriving today from AFP.

Note that I'm also still re-calibrating my FF sensor after the change from carb to injection, so these FF readings should be taken with a grain of salt.
 
Thanks for the input. My oil consumption stabilized at around 4-5 hrs Tach and CHTs dropped around 30 or so around there. It was my understanding that low power, with low CHTs, could only delay break-in, rather than cause glazing by themselves.
 
Thanks for the input. My oil consumption stabilized at around 4-5 hrs Tach and CHTs dropped around 30 or so around there. It was my understanding that low power, with low CHTs, could only delay break-in, rather than cause glazing by themselves.

I think it's widely accepted that low power operations at low hours can cause glazing...or partial glazing. At 13 hours I would personally be pushing it a bit harder. Better to burn a bit of fuel than risk the possibility of having to re-hone the cylinders...or have high oil consumption forever.
 
Thanks for the input. My oil consumption stabilized at around 4-5 hrs Tach and CHTs dropped around 30 or so around there. It was my understanding that low power, with low CHTs, could only delay break-in, rather than cause glazing by themselves.

Glazing only occurs with high heat and no risk of that at low power settings. However, proper break in require high pressure ( created only at high power levels) behind the rings to properly wear the rings and cylinder walls to the proper finish (incorrect finish will burn oil just like glazed cylinders). Two camps on this. Some say it will happen eventually at low power. Other camp says you only have a certain amount of time for proper wear and it will eventually stop wearing even if enough wear hasn’t occurred. I am in the latter camp but have no evidence to support it. Best to follow general guidelines which say to run it hard (75%+). This is the recommended approach from all aircraft engine manufacturers. Doubt that they all got it wrong. It was also the recommendation in the auto world prior to the inception of plateau honing.

Larry
 
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In addition to the post above, it is all about Mean Effective Pressure.

a rule of thumb is 75-100% power, and to get the best of it, do it LOP. But some engines need injectors tuned to do it so they are best off full rich the whole time.

A good zone LOP is 2500-3000' and 2500 RPM with about 40dF LOP. That will be around 75% power. Cleaner and cooler.
 
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