p-lead check

[fangzz]

Fairly new 170 owner here. Getting to know her still. Did a p-lead check. Switched mags to off. master was still on. Engine stumbled to 'almost' off. I would describe as 'gasping'. But it did not quit. I did not turn the mags back to on for fear of a huge backfire. I shut down with mixture. During normal mag checks, preflight, I get an rpm drop on both of them. So, my thought is, the engine was firing on hot carbon particles. That's the best I can do. I am a religious leaner. During ground ops, taxi, everywhere except takeoff, landing, climbouts under high power settings.

Weigh in! Drew

[minton]

I think your thoughts are correct. A good high power run at way--- lean might straighten it out.

[GAHorn]

Perform the mag-switch/p-lead check again. It should shut down. If it continues to run, you should perform a continuity check of your mag switch and P-leads. This condition is very hazardous.

[blueldr]

In my opinion, it's very unlikely that the stock C-170 engine would fire on hot carbon particles if the engine is running reasonably the rest of the time.
I think you have a dangerous elictrical electrical problem. It well could be in the mag switch if it is of aircraft age.

[Brad Brady]

I think I agree with BL, while you are doing a continuity check, as prescribed by George, make sure you wiggle the key around. So as to make sure the switch isn't the culprit.

[fangzz]

You know, this is exactly why owners of this great bird really need to belong to our assn; the way you guys come to the aid of each other this way. Various views all based on millenia of experience (I'm not calling us old - just experienced. Well....old too. But that's good).

Thanks for the input. I think I have a problem that could have been disastrous. We're gonna fix it. Drew

[Jimmy M.]

Try it again with a cold engine............run it only a few seconds and then turn the ignition off,
that should eliminate " run-on " or some people call it " dieseling ". If it still acts up then, for sure you have an
electrical problem..............
good luck...............Jim

[wingnut]

Since it checks fine during a normal rpm for mag drop check (engine running smooth), I would think the most likely cause would be the P lead terminals at the mags. When the engine is at very low rpm it shakes around alot more, and poorly crimped or corroded terminals could cause intermittent continuity when grounding the mags.
That's just the easiest, and one of the most common problems I've found with the problem your describing

[fangzz]

Don't know if this will get seen down here at the bottom of this issue, but, an update......

It is very apparently hot carbon. Did a start and immediate shut-down, and she quit immediately. No dieseling whatever. Repeated. Same. Last flight, shut down with the mags, and it dieseled until I shut down with mixture. Only barely running. I believe this resolves this issue in this particular bird. New plugs on the way. Gonna keep the current ones, and have one squeeky clean set to stick in at every oil change (50 hrs w/ a spin-on). Drew

[canav8]

Don't know if this will get seen down here at the bottom of this issue, but, an update......

It is very apparently hot carbon. Did a start and immediate shut-down, and she quit immediately. No dieseling whatever. Repeated. Same. Last flight, shut down with the mags, and it dieseled until I shut down with mixture. Only barely running. I believe this resolves this issue in this particular bird. New plugs on the way. Gonna keep the current ones, and have one squeeky clean set to stick in at every oil change (50 hrs w/ a spin-on). Drew

Fangzz if it is carbon that is causing your deiseling then new plugs will not fix it unless you clean them and that problem goes away. I would get a borescope and look at carbon in your cylinder tops and on tops of pistons. New plugs will not fix this problem with the mags off. You could have leaking intake or exhaust valves that have allowed oil to build up and cause the coking or running excessively rich with the mixture. I would try an experiment and run the engine mixture control leaner while taxing around then going to full rich just before takeoff for a few weeks. I have never had a problem with the deiseling on any of my aircraft in 30 years. I have always got TBO out of my motors, and I have always disregarded TCM recommendations of full rich below 5K. Except for takeoff. FWIW. Doug

[fangzz]

Doug, thanks for the input. This is a newly TBO'd 0300. 70 hrs since new. The plugs were not replaced, however, which surprises me. When I timed the mags, which were well off, the plug I inspected already had some lead fouling on it. I believe the plugs were simply meticulously cleaned. I lean like crazy, and the more so the more I read about the benefits of it and the hazards of NOT. I lean even on warmup. And taxi. And aggressively (just like it advises in our great 170 book - so that I will never be able to takeoff with the mixture out). Always rich of peak however. I want to prolong that beautiful engine. In our case, it's about longevity, not economy. We accept the economy as a side bennie.

The plug idea is one I've wanted to begin (change with each oil change) for a while now. Just seems right. Might change my mind when I begin to fully appreciate the getting under that cowling on a regular basis. We'll see. But thanks for the input, and look forward to posting you when I get some results here. Drew

[blueldr]

I can't immagine that a seventy hour engine could possibly have enough carbon build up to cause your problem.

[GAHorn]

Drew.... just exactly what plugs are you using? (Make, model, series)

[fangzz]

UPDATE...

George, darn good question. We've had her such a short time, haven't had the plugs out to see, but the new one's I've ordered are Champion REM40E's, a hotter plug than mid range. Your thinking is right on. AS WELL, another factor in this is that we've been using almost exclusively autofuel, ethanol free of course. My IA and I ran some live tests (various methods of run-ups and shut-downs) and the p-leads are fine. Dieseling is only occurring after reaching full operating temperature. Since this is a newly TBO'd engine (with old plugs), I''m hoping that the change-out will solve it. During all phases of operation and all ranges of power, the engine is smoothe and strong. I'm not aware of pre-ignition or detonation. But, I'm not sure I would recognize it, audibly, if it were slight, and with the other cockpit noises. Gonna try and get a boroscope in her sometime very soon.

I am thankful the p-leads weren't the issue. Hate to think of the tragedy that could have caused.

And Dick, sorry I missed you before you went galavantin' off. Wish I coulda bought yours. Those little rascals are EXPENSIVE. I keep looking for gold electrodes, but they say they are only tungsten!

Thanks for continuing to help me think about this. Glad you're interested. I'll keep you 'posted.'

[GAHorn]

... another factor in this is that we've been using almost exclusively autofuel, ...Since this is a newly TBO'd engine ...

Of course, everyone knows I'm not a fan of autofuel in aircraft....so...with that in mind.... Drew, you ARE aware that virtually ALL engine and cylinder mfr's insist that new engines be run-in on AVGas, ...?