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Momentary RPM jump

jwilbur

Well Known Member
Question for the experts.

I have 115 hours on my RV10. Hartzell Governor, stock IO540, Hartzel prop, and mags, all new when I installed them. Today I had prop set at 2300, 13 Inches MAP, 3000 feet level flight (~90 knots) just burning fuel for a while practising some stuff. Pushed MAP up to 20, climbed 500 feet or so. As I pushed to level off and accellerate, RPM jumped. The Dynon voice said, "over-speed." By the time I looked it was falling back through 2400 back to the set point of 2300. It happened again while I watched. It jumped to ~2600. Then back down. It settled back with another jump or two. I noticed nothing unusual with the engine or sounds or feel, so it might be just a wire. I've got Dynon hooked to the P-leads. Early test flights had RPM jumpy so I increased the resistance in the lines back then. Never had any issues in the last 100 hours.

Back to today's flight. I pulled power back to 15" to be safe and turned for home. No more troubles. Near the airport I moved some knobs around and in and out and things seemed to behave normally.

Some time back I know there were troubles with another governor where props suddenly went fine pitch and caused troubles. What I don't know is if there were symptoms leading to those events. I'll be pulling the cowl this week to look over the wires and cables and all. Hoping to get some advice from the experts here on what not-so-obvious thing I might be interested in looking for. Or any theories on what might have happened today.

Thanks,
 
What I can say is that if it had actually jumped from 2300 to 2600 you would very definitely have felt and heard that, since you say you did not detect any difference in the engine operation then you probably have an indication error.
 
What I can say is that if it had actually jumped from 2300 to 2600 you would very definitely have felt and heard that, since you say you did not detect any difference in the engine operation then you probably have an indication error.

For sure I would if it were sustained for any length of time. Not sure I would if it were a few hundred milliseconds. Also since I'm connected to the mags, could something like this indicate a problem brewing in a mag?

I'm going to check the logs. I'm guessing if there were a real RPM change there would probably be some other things like MAP that might have blipped, too.
 
When we switched from a narrow-deck to a wide-deck on our RV-10, I asked a few people which governor to get (I still have the narrow deck 540 Hartzell governor for sale!), and it was recommended to get the PCU5000X because of similar issues that others have had with the Hartzell.
 
What I can say is that if it had actually jumped from 2300 to 2600 you would very definitely have felt and heard that, since you say you did not detect any difference in the engine operation then you probably have an indication error.

+1

You can't really miss hearing a 300-400 RPM change. I would be looking at the RPM sense wires and consider yet another increase in resistor size. Sensing the RPM from a P lead is challenging and most mfg's recommended to keep increasing resistor size until the RPM is reliable/stable.

Larry
 
For sure I would if it were sustained for any length of time. Not sure I would if it were a few hundred milliseconds. Also since I'm connected to the mags, could something like this indicate a problem brewing in a mag?

I'm going to check the logs. I'm guessing if there were a real RPM change there would probably be some other things like MAP that might have blipped, too.

I don't think the governor can change 400 RPM and then change back in a few hundred milliseconds. They don't react that fast. You get a sense for governor reaction time during your runup, if you cycle the prop. Regardless of the time, I still don't believe your ears would miss it. If your resistor is too small/large or have a bad connection on the P lead, you will not find other signs of problems in your logs. It is not likely indicative of a problem in the mag without other performance indicators. Though changes in the mag's E gap over time will change the current / energy out of the coil and that could push past the threshold of sensing the RPM in the EFIS. RPM is sensed by monitoring the spike that occurs on the coil's primary winding, as the field collapses. The nature of that spike will change as the charge energy fed to the coil changes and this can change with a change in E gap. E gap drifts as the points wear over time. A bad condensor can accelerate that wear.

Larry
 
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I don't think the governor can change 400 RPM and then change back in a few hundred milliseconds. They don't react that fast. .... The nature of that spike will change as the charge energy fed to the coil changes and this can change with a change in E gap. E gap drifts as the points wear over time. A bad condensor can accelerate that wear.

Larry

Thank you, Larry. I appreciate the explanation of what's going on in the mag. If I find that I need more resistance that would indicate a change in the mag(s) since I have not had this problem in 2 years.

Regarding the governor. The prop is controlled through oil pressure. Is there anything in the oil system that could cause a momentary change in the prop pitch outside the control of the governor? For example, I had just finished a climb as the event occurred. The airplane had been nose high, gravity pulling oil away from the prop, etc. Point of information, I measured just over 6 quarts before the flight. Anything important about this information?

Let me emphasize that I believe this is almost certainly a deteriorating connection in the wires between the mags and the Dynon or maybe a resistor or a ground connection. ... But I like to consider whatever other alternatives there may be and to take the opportunity to learn what more I can about my systems ... and avoid expectation bias if I can.
 
Clean and precisely gap your plugs, check the ignition wires and magneto condition This is an ignition problem, not a resistor problem.

A misfiring plug will generate a wonky waveform on the P-Lead (a wonky is the international unit for messed-up). This is interpreted by the tach circuit as an overspeed.

Since it's been stable for some time, something has changed. The airplane is speaking, listen.

V
 
Regarding the governor. The prop is controlled through oil pressure. Is there anything in the oil system that could cause a momentary change in the prop pitch outside the control of the governor? For example, I had just finished a climb as the event occurred. The airplane had been nose high, gravity pulling oil away from the prop, etc. Point of information, I measured just over 6 quarts before the flight. Anything important about this information?

No, assuming that your oil pressure didn't drop to a ridiculously low level.

As I and Vern have stated, something is going on in the ignition or your wiring. I agree with Vern that it is likely in the ignition system and not the P lead wiring. As he mentioned, problems on the secondary winding side can also create P lead irregularity.

Larry
 
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No, assuming that your oil pressure didn't drop to a ridiculously low level.

As I and Vern have stated, something is going on in the ignition or your wiring. I agree with Vern that it is likely in the ignition system and not the P lead wiring. As he mentioned, problems on the secondary winding side can also create P lead irregularity.

Larry

I understand. Thank you very much. I'll clean the plugs and check everything I can before next flight.
 
I have one elect mag on my IO360 with an EI digital tach. Every now and then at random when I start from cold it over reads 200 RPM. At first I thought a Gov issue but after much searching, swearing and checking by having the guage pulled and looked at etc as well as all the obvious it was just an incorrect indication on the EI display. I confirmed it by using my RC tacho which I check my RPM of my model planes, it was indeed over reading. I now just accept it happening on the odd occasion, somethings are what they are -)
 
I understand. Thank you very much. I'll clean the plugs and check everything I can before next flight.


Was having similar issues with my RV 7/IO360 with Slick mag and light speed. Continually got worse, Continual Swearing by myself, however even though the EIS and EFIS were reading wrong I was positive it was not the prop governor. Especially when it was reading 0 and 3500. Took it to an avionics shop/a&P. they thought it was the wire leading from the Lightspeed prop sensor. That would have required pulling the prop(didn't really want to do that). I checked all the wires multiple times. Cleaned all spark plug wires multiple times. replaced spark plugs. then someone here nailed it, make sure the wires from the slick mag(where the RPM info on my plane comes from) are not close to the Spark plug wires leaving the mag. Sure enough they were touching (probably bent down during an oil change)the plug wires. I lifted them up, tied them with some twine, and now have rock solid Tach again. Sometimes its the simple little thing that is the culprit.

Jom
 
Upload your engine monitor data to Savvy Analysis and see what is shows. If RPMs changed, MAP and FF likely would have too.
 
Clean and precisely gap your plugs, check the ignition wires and magneto condition This is an ignition problem, not a resistor problem.

A misfiring plug will generate a wonky waveform on the P-Lead (a wonky is the international unit for messed-up). This is interpreted by the tach circuit as an overspeed.

Since it's been stable for some time, something has changed. The airplane is speaking, listen.

V


Here's an ignition waveform for an IO540, captured in flight:

magneto-1-bmp.2916


The top waveform is the P-Lead waveform, the bottom one is from an electronic stabilizer I developed.
 
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Question

So here's a question. In my data I see an unusual jump in the #6 EGT. 2 minutes and 20 seconds after I pushed in the throttle to climb, #6 EGT smoothly goes up another 50 degrees exactly coincident with the RPM jumps. Up until this point all the EGTs react together.

At the time of the RPM event, MAP was 20", fuel flow was almost 14 gal/hour. In my experience this is plenty rich and all other EGTs were well behaved so I don't think I was too lean.

What does this tell me? I assume (as has been suggested) I should clean and gap all the plugs. Can a mis-firing plug cause this symptom of a jumpy RPM indication?
 
So here's a question. In my data I see an unusual jump in the #6 EGT. 2 minutes and 20 seconds after I pushed in the throttle to climb, #6 EGT smoothly goes up another 50 degrees exactly coincident with the RPM jumps. Up until this point all the EGTs react together.

At the time of the RPM event, MAP was 20", fuel flow was almost 14 gal/hour. In my experience this is plenty rich and all other EGTs were well behaved so I don't think I was too lean.

What does this tell me? I assume (as has been suggested) I should clean and gap all the plugs. Can a mis-firing plug cause this symptom of a jumpy RPM indication?

This tells you that you had some issue with one of the ignition feeds to #6. When one plug stops firing or produces too weak of a spark, the effective timing is retarded on that cyl and EGT goes up accordingly (50-100 is typical). The issue/failure on that plug causes funny things to happen in the coil, which drives the P lead, which drives your RPM display. Vern pretty much covered this is in his first post.

Clean and test #6 plugs. Looking for 1000 ohms; tip to center conductor. Test both ignition wires for #6 (don't recall how many ohms/ft for those). Pull cap off of the mag and check springs and bronze contacts for corrosion. If those check out, you may need to dig into the mag; Possibly too much eaten out of the #6 post.

When one plug doesn't fire, you don't see the wave form that Vern posted above on the P lead. The EIS is looking for that spike 3 times per revolution. When the spike disappears for one cylinder, the computer gets confused and cannot accurately determine the RPM.

Larry
 
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update

The #6 plugs were not fouled, but I cleaned and gapped them anyway. One of the plugs was out of spec (open too much on one side). I checked the resistance. One measured 1.4 KOhms, the other was open circuit. I thought I had a smoking gun. The local shop tested the plug for me on their test equipment and they said it was fine. Measured the resistance again and it was still open. Not sure what to make of that. I decided I would trust the expensive plug tester.

I put it all back together and went for a ride. I stayed within gliding distance and tried to reproduce the issue for an hour. Nothing. The engine ran great and the RPMs were rock solid no matter what I did. I pulled the logs and checked them when I got home. Everything looks great to me.

I really wish that plug had been bad. That would have been a satisfying answer. Can one side of one plug with a gap out of spec cause my symptom?
 
The #6 plugs were not fouled, but I cleaned and gapped them anyway. One of the plugs was out of spec (open too much on one side). I checked the resistance. One measured 1.4 KOhms, the other was open circuit. I thought I had a smoking gun. The local shop tested the plug for me on their test equipment and they said it was fine. Measured the resistance again and it was still open. Not sure what to make of that. I decided I would trust the expensive plug tester.

I put it all back together and went for a ride. I stayed within gliding distance and tried to reproduce the issue for an hour. Nothing. The engine ran great and the RPMs were rock solid no matter what I did. I pulled the logs and checked them when I got home. Everything looks great to me.

I really wish that plug had been bad. That would have been a satisfying answer. Can one side of one plug with a gap out of spec cause my symptom?

In my experience, yes. These digital tachs are an early warning system.
 
Can one side of one plug with a gap out of spec cause my symptom?

IMO no. They have two electrodes for redundancy and to increase the time before wear creates a critical gap opening (mags are very weak and large gaps will burn out the coils). One neg electrode in spec is fine to spark properly. Would be far more concerned with the open condition. That is a major red flag. It may work some or most of the time, especially on a tester that is pumping out more energy than your mag. I would replace that plug, as the failure condition that is causing the open reading is the most likely cause of your recent experience. The mag energy can jump an open circuit in the plug, but it further reduces an already low source of energy. At some point, the plug won't be getting enough energy to fire. These components most be in optimal condition to have the greatest reliability.

Larry
 
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new question

Based on the comments here I did a bunch of spark plug research and decided to change all 12 plugs in my airplane to Tempest UREM38E from the Champion plugs I had. They were from an era where apparently there was a known resistance problem. In addition, in the course of my research, I realized I had gotten some bad advice about cleaning them with a wire wheel on a bench grinder. So now I've got all new plugs in my engine.

Here's the question. The plug which formerly measured as an open circuit in the resistance test now measures 1.6 K. In fact, all the plugs I removed show a resistance in the proper range. I am 100% sure this plug measured as an open circuit when I checked it last week. I measured it multiple times and even lightly sanded the tip to be sure I had a good connection with the DMM. The only change is it went back into the engine and I went for a 45 minute flight. Have there ever been reports of the Champion plugs being intermittent with regard to the resistance test? This might explain a lot.

And is it worth saving these plugs given I've cleaned them inappropriately?
 
Based on the comments here I did a bunch of spark plug research and decided to change all 12 plugs in my airplane to Tempest UREM38E from the Champion plugs I had. They were from an era where apparently there was a known resistance problem. In addition, in the course of my research, I realized I had gotten some bad advice about cleaning them with a wire wheel on a bench grinder. So now I've got all new plugs in my engine.

Here's the question. The plug which formerly measured as an open circuit in the resistance test now measures 1.6 K. In fact, all the plugs I removed show a resistance in the proper range. I am 100% sure this plug measured as an open circuit when I checked it last week. I measured it multiple times and even lightly sanded the tip to be sure I had a good connection with the DMM. The only change is it went back into the engine and I went for a 45 minute flight. Have there ever been reports of the Champion plugs being intermittent with regard to the resistance test? This might explain a lot.

And is it worth saving these plugs given I've cleaned them inappropriately?

I am sure it is possible to have an intermittent open. However, any plug that reads open, once or every time, should be replaced. Just not worth the potential problems to save $20.

Wire wheel should not have caused major damage unless you hit the ceramic with the wheel.

Larry
 
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And is it worth saving these plugs given I've cleaned them inappropriately?

The internal, replaceable, resistor in the old Champion plugs can cause the behavior you saw. Can't hurt to keep them around for emergency spares (even for other people). You can remove and clean or replace the resistor from the top of the plug.
 
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