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Unexplained extremely high #4 CHT event

scottmillhouse

Well Known Member
First stock RV-7A with new Lycoming IO-360, Hartzell CS prop, 60 hours time since new. For first few hours I had high CHT across the board. Eventually traced to one of the new mags timing drifting over first few hours. Stopped at about 10 hours and has not drifted at least at last check about 10 hours ago.

Yesterday a typical flight only difference at landing with power pulled the engine appeared to be miss firing. Slight noise but could feel the exhaust misfiring pulses under my feet. Upon taxi from Savvy analysis of Dynon data showed 1000-1400rpm with #4 climbed over 2 minutes from 325-480. No evidence of any blockage. Initially assumed a malfunctioning CHT. Upon startup and taxi temperature was about 400 for #4, run up normal, back taxi about 20 knots did not cool but stayed about 400. Did another runup and #4 immediately climbed in 15 seconds to over 500 before I shut down. Audible misfire noise at shutdown and high temperature smell. For all events EGT stayed in line with other cylinders with no other parameters out of line. Shutdown runway for a few minutes to cool. Challenging IO hot start but then taxi to FBO temperatures stayed in safe zone. Let cool a half hour with another challenging hot start but they otherwise normal to circle and climb high just in case before heading home. Being late I just hangered it.

I plan to pull cowl and check timing, compression, remove and clean all fuel filters and flush fuel injection. Any ideas from the brain bank on what happened and what to look for?
 
After you do your checks, do the ?shot glass? fuel flow test. This is a fast and accurate way to determine if you have a fuel starvation problem upstream of the injectors.

Carl
 
Check the baffles, especially around the oil cooler, if it is hung on the baffles by #4. That's about how many hours I had on mine when it cracked and #4's CHT went up.

Check the probe, switch it with another and see if the problem follows the probe.

And finally, recheck your mag timing. It is unlikely that bad timing will only impact one cylinder but stranger things have happened.
 
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Perplexing to say the least! Does seem to be some kind of fuel delivery problem. The only thing that popped into my head that might affect that is a sucking fuel from a tank with a clogged vent line. I have no idea if this thought could lead to your symptoms but hey... easy enough to rule out.
 
Following is assuming you confirmed that the CHT source is not hot air blowing on it from exh or plug:

480* CHT at 1400 RPM is not likely from any typical engine anamole, like rich, lean, traditional timing drifts, baffling, etc. That is really hot for that RPM. Given the noticeable misfiring symptom from exh pulses, I would be looking at detonation or pre-ignition. It seems to be the only thing that can get CHTs to rise that much in a short amount of time, beyond a complete air block, like a nest or something else heating up th probe. Detonation can be caused by extreme advance, as well as other issues. Pre-ignition is usually from deposits. 400*->500* in 15 seconds at 1800 is also extreme and classic detonation/pre-ignition symptom. That is a typical runaway scenario.

Easiest first step is to put a timing gun on each plug of that cyl and confirm their timing (timing advanced too far is a typical cause of detonation). Then a borescope to look for a deposit that could be causing pre-ignition.

It is not a fuel flow issue, as anything rich or lean of best power should give lower temps, not higher. A loose exh flange can also cause noticeable exhaust popping that could be mistaken for misfires. These tend to loosen up on a new engine. I had to tighten mine twice in the first 50 hours.
 
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lr172.... those are good thoughts. Other that verifying the mag timing is correct, (and not running too hot of spark plugs), I was just wondering what can be done if pre-ignation/detonation is found to be the problem?
 
lr172.... those are good thoughts. Other that verifying the mag timing is correct, (and not running too hot of spark plugs), I was just wondering what can be done if pre-ignation/detonation is found to be the problem?

You don't want to just do mag timing. You want a gun on each lead and confirm the timing, based upon that actual spark event. There are various issues that could cause a spark at the wrong time, even if the mag timing is correct.

The fix can't be determined until the cause if found. It could be something simple. Pre-ignition can usually be solved by removing the deposit that is causing it. Detonation is far more likely here, given that it is not occuring at all times. Problem seems intermittant. It should have been worse at cruise power. That said, all issues have been seen in situations where cooling air flow is low relative to cruise.

Larry
 
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Detonation or pre-ignition would usually leave the top of the piston clean of deposits with a slight melted looking surface. I think this would be very unlikely with the very low manifold pressure at 800 to 1800 RPM.

This event was on only one cylinder: a more likely cause was a partially plugged injector that cleared the foreign matter,on it's own, before you could pull the injector and find a "smoking gun".
 
Detonation or pre-ignition would usually leave the top of the piston clean of deposits with a slight melted looking surface. I think this would be very unlikely with the very low manifold pressure at 800 to 1800 RPM.

This event was on only one cylinder: a more likely cause was a partially plugged injector that cleared the foreign matter,on it's own, before you could pull the injector and find a "smoking gun".

How does an overly lean condition create 500* CHTs? A plugged injector usually results in lower CHTs; Often a lot lower. I have seen that firsthand. I agree that detonation at these power levels is atypical, but how else do you explain those crazy high CHTs combined with perceptible misfiring, assuming it is not a loose exh flange?
 
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I've dealt with engines that had excessive lead deposits which were causing preignition, which in turn caused runaway CHTs on application of full power.
 
Keep the ideas coming so I have a better idea for trouble shooting. With pandemic, there being no place to go right now and with my focus on another project, I have not rushed back to start checking this out. In a few days I will dig in and start going through systems as part of my first annual.

I'm thinking fuel injection, especially since the hot starts were so challenging. CHT was running about 340 and EGT about 1250 on all on return flight but I also did not lean much since I was concerned about the fuel injection working properly. Trying to figure out how to check individual leads with a gun for timing. Lycoming had a service letter about the mags drifting. They approved a warranty replacement for my drifting mag but the local distributor never processed it. Quit drifting at 10 hours so I quit bothering them about it since the bureaucracy was so daunting. Not sure how a mag malfunction could only affect one cylinder either.
 
High CHT #4

You might check the intake runner seals for cylinder #4 and/or the connection of the runner to the cylinder.


Don
 
Not sure how a mag malfunction could only affect one cylinder either.

spark can jump to the wrong terminal in the cap and several other issues inside the mag. Spark energy can also jump from one wire to anther, etc. The issue is not mag timing, as all other cylinders are behaving properly. You want to be sure that energy isn't getting to one of the plugs at a point well advanced of 25 BTDC on the compression stroke.

I'll reiterate that a fuel mixture variations alone cannot cause 500* CHTs in one cyl with all others at <400*. "Best power" fuel flows produce the most heat. All others, richer or leaner, produce LESS heat. Even if the others were very lean or rich and the offending cyl was at best power, it still wouldn't produce a delta of 100+ degrees. It would be closer to 10-20* at those RPMs and would take much longer than 15 seconds to register.

Detonation and pre ignition are about the only things that can raise true CHT (assuming accurate sensor without external heating effects) 100* from normal operating temp in 15 seconds. Mixture variances simply can't do that. Nor any other problem that impacts air fuel ratio, such as leaking intake gaskets or plugged injectors. I do suspect that a significant exhaust leak could do that, but only in low cooling air flow conditions, like your runup. This would explain why you only saw the issue on taxi and runup and not in flight.
 
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Trying to figure out how to check individual leads with a gun for timing.

Timing guns have a clamp on tool that goes over the spark plug wire. Like a hall effect sensor. You put the clamp on each wire, individual, and point the gun at your reference point on the flywheel to get the timing of that plugs firing. Not really a simple test.
 
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Timing guns have a clamp on tool that goes over the spark plug wire. Like a hall effect sensor. You put the clamp on each wire, individual, and point the gun at your reference point on the flywheel to get the timing of that plugs firing. Not really a simple test.

Agreed, own one for my car, just not sure how to do that with the whirling meat slicer in view area. What you said does make sense if a mag had an internal fault to fire a plug way, way before TDC. That might also explain the hard start if it was the impulse mag since I believe the other does not fire when cranking.

Talked to Lycoming today. They said not likely for mag but most likely a hot spot, lead fouling on plug or in combustion chamber. Challenge is that it most likely corrected itself since I had an uneventful flight home. Started annual and so far found nothing out of line. No deposits on plugs or evidence of any leaks or anything loose. Also no evidence of heat damage, change to plugs or painted strip.
 
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Update

Did bore scope on #4 cylinder and adjacent #2 and they looked great and exactly the same. Both mags at 25 BTDC and all compression 74-76/80. Fuel system appears fine and all intake and exhaust is tight.

Thinking back now, it did appear the bottom #4 plug could have been inadequately torqued. It was stiff coming out and the deposits on threads could have been a mix of carbon and anti-seize. If not seated then I suppose it could have gotten hot enough for pre-ignition. I used a new copper gaskets but without my glasses, I could have installed backwards and that could have caused it to not adequately seal.
 
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