Rough engine, high CHT. Clogged injector and Detonation or Preignition?
Any A&Ps or other engine gurus out there? I had a rough running engine develop at 4000 feet, mixture leaned to 100 rich of peak. It started out similar to the roughness that develops when leaning the engine to the point of roughness, but progressively worsened over 20 seconds, to the point where I called Mayday and turned to the nearest airport. I noted cylinder #3 had CHT of 450 degrees. I enriched the mixture, the roughness resolved gradually over 20 seconds, and the CHT came back to normal, all normal to the nearest airport, so normal I gave brief consideration to continuing home. See the attached JPI engine analyzer data.

On the ground, phone conversation with two mechanics immediately felt it was highly likely a partially plugged injector nozzle in my IO-360. I questioned how a plugged injector, even if it resulted in peak EGT, could cause the CHT to climb at 1 degree per second to that level. Surely it could cause a high EGT and a somewhat higher CHT, and loss of power, but 450 degrees at 1 degree per second??

I drove home and asked a mechanic to check the engine over with borescope, plug check, injector examination and cleaning. He pronounced it all good except for a massive electrode style top plug on that cylinder with a cracked insulator.

Being a Mike Busch reader, and liking his philosophies on engines, I know a fair amount about engines. I am thinking that something precipitated detonation or pre-ignition, and I'm lucky it didn't destroy the engine. I flew home, zero problems.

So I'm thinking that there are two types of clogged injector scenarios, I'd be curious what you all think of it. First type would be a rough running engine from power loss, and that the EGT might go up if it happens to be closer to peak EGT mixture level which may cause a moderate rise in CHT over time. Second type would be the scenario here, with CHT temp rising fast to dangerously high levels.

If this was detonation or pre-ignition, it is said that a cracked plug insulator and being overly lean can in fact precipitate these events.

I don't know what else I can do except ASSume that it was in fact a clogged injector and soldier on, but you can bet your life I'll be cross-checking that JPI more often than my usual scan of about every ten minutes.

Big question is, can a partially obstructed injector nozzle all by itself, i.e. without detonation/preignition, in fact cause such a precipitous rise in CHT? I am skeptical. I bet essentially all of these cause engine roughness until the obstruction clears itself or the injector is cleaned, and the CHTs rise some but not this much.

Read on if you'd like a painstaking tour of this tracing, with numbers on the tracing.

1: Normal 100 ROP leaning, all normal. My cylinder #3 always runs cooler.
2: Maybe this is the spot where the injector clog took place with an initial drop in EGT, then the rise as the mixture went to nearly peak EGT. CHT starts rising at this point.
3: Mixture pushed to full rich, EGT drops, engine smooths out, and CHT cooling starts.
4: I wonder if the clog got worse here, and even at full rich mixture the mixture leaned, EGT started up again,
5: ... And possibly here the obstruction corrected itself, and all normal. Could have flown her home, but I didn't know this. 

Incidentally, I miss the AOPA Forums format better. This tiny box to enter the post is not good, the overall look and feel is not good either. I like the "Answered by AOPA" feature though. If an AOPA person is reading this, I'd be glad to help in improving this.


5 Replies
Since the experts haven't given you an answer, I (not an engine expert) will try.  I think the cracked plug may have been your problem.  If the plug stops firing, you get nonuniform ignition in that cylinder.  The flame front will propagate nonuniformly and combustion will be slower. EGT will go way up.  (For this reason, when you do the mag check, EGTs rise running on only one mag.) I can't explain how nonuniform combustion affects CHT, but anything that interferes with the normal combustion process seems like a candidate for unusual CHT.

Another possibility is a sticky exhaust valve. That would certainly interfere with normal combustion.  You might want to do diagnostics like a compression test and boroscope that cylinder's valves to make sure the valves are healthy.  If you have an exhaust valve failure in flight, it can lead to engine failure.  BTW, If you have roughness when you start a cold engine that persists long after the start but clears up when the engine is warm, you may have a sticky valve.  Sometimes that is referred to as "morning sickness," and it indicates a valve problem.

The clogged fuel injector will cause roughness, but it seems less likely to cause high temps, because the highest temps occur right at peak EGT, and if the reduction in fuel flow moved the mixture to the peak EGT mixture after you have already leaned to 100F ROP for cruise, that should not redline the CHT.
Exactly as I understand it, as a non-expert i.e. A&P, but as someone who's made a study of airplane engines in depth including Advanced Pilot Seminars, Mike Busch's articles, and other sources. It doesn't seem that a clogged injector would cause a runaway CHT, but it is true that a leaned cylinder due to the clogged injector nozzle can trigger detonation because of the lean mixture itself. And yes a cracked plug insulator can cause detonation or pre-ignition. So with the replaced plug I'm hoping that's the end of this, and the long flight home after being checked out with borescope too was totally uneventful. My amateur reading of the JPI engine monitor data tells me that it was a clogged fuel injector, where the elevated EGT started, triggering detonation, perhaps the cracked insulator promoted this too. I've got Carlo, an A&P and Pilot Information Center person on the case, speaking to him later. Might contact Lycoming too.

I emailed Mike Busch too.
Carlo, an A&P Pilot Information Center person, concurred with my thoughts that this had to be a detonation or preignition incident. A partially clogged fuel injector nozzle could cause the mixture to be at or near peak EGT which could cause a modest increase in CHT that might not be healthy in the long term, but would not cause the rapid CHT rise seen here. In fact, I would think that the odds of the mixture being leaned just right to be at peak EGT can't be terribly high, that it would more likely be well lean of peak where it would result in lower EGT and lower pressures in the combustion chamber, and lower EGT as anyone who operates LOP knows well. Any leaner would result in even lower EGT and roughness which I did have eventually.

So the fact is it would take detonation or pre-ignition to cause this. What was the trigger? It's a check vs. egg situation. I think a partially clogged injector started the situation and the detonation itself caused the ceramic damage on that top plug. But it's also possible that the ceramic damage and/or the ultra lean mixture triggered detonation or pre-ignition.

Probably will never know. The borescope was satisfactory, so I think I'll have the timing checked and just monitor, nothing else to do really.
I would like to provide what I hope is a good explanation of excess leaning.  (I am hoping to be helpful, not argumentative.)  

My opinion is that Carlo is right that excess leaning can cause detonation, but only at high power settings.  Above 75% power, fuel is being used as a coolant, and if we lean out the mixture, we stop cooling the cylinder with fuel.  When that happens temps can run past redlines, and detonation can follow.  However, at normal cruise power settings of 75% and less when flying level with good air cooling, most engines like the IO-360 are designed to run leaned out to best economy, using air flow as the coolant.  Except for conditions with high OAT or high angles of attack that limit air cooling (conditions that may call for running extra rich for cooling), our engines don't need excess fuel for cooling, and that means the engine can be leaned beyond best economy as much as desired without overheating.  

So, if you were in cruise at 75% power or less and you leaned out the mixture, and then a fuel injector got partially clogged, that one clogged cylinder became more lean.  It produced less power than the other cylinders which made the engine run rough, but it can't make it run hotter, it actually should run cooler.  This is the case in my TSIO-520 which at cruise power can run as lean as I want, the only consequence being roughness from power imbalance.  My opinion, for whatever it's worth, is that if you were flying level at 75% power or less, a clogged injector did not cause your temperature spike.

I hope this is helpful, let us all know what Mike Busch says.

I agree completely with what you say. The point I am trying to make is this:

A rough running engine could very well be due to one cylinder being too rich or too lean with the resulting loss of power. This could be due to a lot of things, among them too little gas (injector nozzle plug) or too much air (intake manifold leak). If you leaned out at 75% power to say 100 ROP the worst that can happen as far as EGT is for the EGT to go to peak EGT, something we do all the time, either on purpose or accidentally, without short term serious consequences. This would indeed, I would think, cause that cylinder to run somewhat warmer if at peak EGT (not 450 degrees warm), and if somewhere too lean or too rich it would generate less power, less combustion chamber pressure, lower EGT, and cooler cylinder, as anyone who runs their engine LOP. Anything that results in loss of power will cause the rough engine.

SO: rough engine with higher CHT, or lower CHT, could very well be a clogged injector on a cylinder, but would not cause a runaway CHT elevation. At least not all by itself.

That missing factor that could be compounded by the acutely more lean cylinder would be things that can trigger detonation, pre-igntion (or both) like: Cracked plug insulator, timing too far advanced, carbon deposits which I imagine would be visible on borescope, too much power, too low of an RPM, perhaps more.

So runaway high CHT may be triggered by a lean mixture under some circumstances, so one has to investigate both the cause of the temporarily lean cylinder (most likely plug injector) AND the precipitating factor. Perhaps mine was that cracked insulator. What bothers me is, detonation can crack an insulator, so it's a chicken or egg situation.

I did in fact copy Mike Busch, maybe he'll respond and maybe he won't, he has in the past from me, and I certainly understand that he's a busy man in a profit-generating business where random inquiries and tracings are coming in by the boatload. For that reason I sent an inquiry to Lycoming, I'll report the response.