Magneto or Fuel Injector or Both?

dfw11411

dfw11411

Pre-takeoff checklist
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dfw11411
I know only the basics about the TIO-540 in my new-to-me Cherokee 6XTC. I'd like opinions of what might be wrong. Six cylinders, constant speed prop, fuel injected, turbo charger. AvGas was clean.

Last week, while in the pattern with my CFI, the engine began running rough on the third climb out. We looked at the engine monitor and saw TIT at 1400 (we were running it rich for landing/takeoff) and all cylinders at about 400. We assumed carbon fouling of a spark plug and leaned the mixture to increase temps to burn it off. The roughness became much worse and we saw cylinder 3 temp dropping. A mag check confirmed it was the right mag. Both mags are due for service in December when it undergoes an annual.

We immediately terminated our pattern work and headed home - about 7 miles away. We made sure we were within gliding distance of a runway. The roughness continued to get worse and cylinder 3 temp dropped to almost zero. I kept her high and dove for the runway with the engine idling. The almost-power-off landing was uneventful, smooth even, and we exited the runway. The engine was back to running normally. We stopped and did a run up with mag check. No problems. Hmmm. Intermittent mag issue? But cylinder 3 wasn't firing at all and was cold. Could it be an intermittent issue with both mags on only one cylinder simultaneously? That's just too strange for reality.

Next morning was my final training flight for my High Performance endorsement. She started and taxied out just fine, but began running rough again when I increased power for the run up. Again, cylinder 3 temp began dropping. My CFI played with the settings for a few minutes and then we scrubbed the flight. The A&P played with it a while as well, then pulled both spark plugs for cylinder 3. The plane is new to him as well. He explained to me that they were "fine wire" plugs and are particularly immune to carbon fouling. Both plugs were, indeed, not fouled.

My CFI initially dismissed my thought of a weak mag or mags combined with a clogged fuel injector, but later warmed to the idea. The A&P kept his opinion to himself. I suspect he's eaten crow too many times in the past!

Since my plane was going to sit for a week before anyone could work on it, I decided to go ahead and have the annual done now rather than December. And this would include servicing both magnetos.

I bet my CFI a nice lunch on the cause. I think it's a fuel injector and tired mags. He's thinking some stars aligned to cause both mags to fail on cylinder 3 simultaneously. Chicken bones may be involved as well. :)

So, do you think I'll get my steak lunch?
 
I think you were a little, uh, optimistic to attempt a flight the next day.

I'd check the harness or the injector.
 
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Occam's razor. It's probably the injector... The chances of both mags failing on one cylinder simultaneously are effectively zero.
 
Is 400f CHT normal for that engine?
 
It took a while to solve this cold cylinder problem as I had the annual done plus ADS-B out/in installed plus other upgrades.

"Lycoming engine morning sickness". Yes, it has a name. What surprised me is that three A&Ps plus a tech at Lycoming were baffled after the fuel injectors, harness and mags were determined not to be the cause. The valve guide sleeve builds up a coating causing the exhaust valve to stick slightly open. Lycoming has a service bulletin, detailed service guide and a maintenance document addressing this issue since the 1980s. She's running great now. Gav1n wins the steak dinner.
 
Lycoming:
Service Letter: 197A Recommendations to Avoid Valve Sticking.
Service Instruction: 1425A Suggested Maintenance Procedures to Reduce the Possibility of Valve Sticking.
 
So what does it say? And as for the wild guess of "injector clog", I think it is highly unliklty. Avgas if not lilkley to clog unless you are using car gas and its sitting for years. I don't recall ever seeing or hearing of an injector being clogged over my decades. As for a mag problem, no not if the whole cylinder is cold, ie not firing. A mag would only be wired to one plug, its failure would run rougher, but still fire the other plug from the other mag.
You could have asked a certain group of experts down south and they could have sold you complete new set of Tempest spark plugs, which cure everything from a golf slice to flat feet, especially if your feet have no problems to begin with.
 
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I had a similar issue in my Mooney (io-360) I came back from a trip and noticed that the CHT was cold on number 1. Mag check was fine. So I assumed a bad plug. I changed the plug and it was worse. Hmm. Cleaned the injector and it was fine. Did a run up and it was fine. Took off and #1 started coing down. I tried leaning a little and got violent shaking. I went full rich and landed. On the ground it was fine. Took the injector out again and there was a very small piece of debris. I suspect that cylinder was running a little lean at idle but acceptable and then when power was really needed it could not flow enough.

After that cleaning I haven't had any issues.
 
Bill Greenwood said:
So what does it say? And as for the wild guess of "injector clog", I think it is highly unliklty. Avgas if not lilkley to clog unless you are using car gas and its sitting for years. I don't recall ever seeing or hearing of an injector being clogged over my decades.
Click to expand...

Injectors can and do periodically clog. I’ve cleaned a few over the years but I have seen far fewer problems with plugged injectors than the internet seems to suggest.
 
mondtster said:
Injectors can and do periodically clog. I’ve cleaned a few over the years but I have seen far fewer problems with plugged injectors than the internet seems to suggest.
Click to expand...

It's the mechanical equivalent of the ubiquitous bad ground that causes all electrical problems.

Owner:

"It (fill in the blank) when I (fill in the blank)"

Internet:

"You have a bad ground somewhere".
 
For Greg, if the "mag check was fine" why did you "assume a bad plug"? As for as I know, with about half a century of history, if you do a mag check, ie a runup and it is rough or you have too much rpm drop, then you can assume an ignition problem. However it can be a plug, a mag, or ignition wires, etc. Now if if runs fine, within specs why would you assume the plug or any of the other ignition items is at fault?

As for injectors clogging, where does your fuel mixture get a small piece of debris? Do you use clean fuel out of the fbo fuel truck which is filtered at the pump and truck or something out of an old beer can? Do you have an air filter on your engine? Do you have a fuel filter? As for injectors periodically clogging , is that worse or better in times of a full moon?
 
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The mag check was fine, the cht were fine on the ground. But the #1went cold initially in flight. It never crossed my mind that it was debris. I just incorrectly assumed that it must have been the plug (fouled). I believe the debris was a small piece of fuel tank sealant.

When looking through the injector you could just see the debris.
 
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