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Engine Run On/Backwards (!) problem

D

davro14

Member
Greetings

I wonder if anyone has come across the following and/or could help please?

I have a 2017 RV7 which I have had from new and has flown for five years and done approximately 250 hrs. All is good with her except the following. She has twin 2017 E Mags, a Precision Airmotive Silver Hawk servo, and a Red Cube flowmeter. When very hot (e.g. with OAT mid 20s, or higher, and after a long taxi) she does not like shutting down (she runs on). This is due to the well known, heat soak, fuel boiling/vapourising problem. When it is simply this, I am not over concerned, however, very occasionally (a couple of times a year, perhaps but including yesterday! … hence this posting) she will then run backwards, not just one or two blades but two or three revolutions. The mystery to me is where she is getting the ignition/spark from as avgas 100 can not self ignite as far as I am aware. Given this occurred near the beginning of the engine’s life I do not believe there is red hot carbon to ignite it. I use NGK BR8ES auto plugs and I am told it won’t be because the plugs are hot enough to cause ignition or that a colder plug would help. Ignition timing is spot on and Ts and Ps in the air are all very good.

Any work around advice (if not a cure!) would be much appreciated.

Best regards

David
 
Run-On

I too have seen this on my high time engine. My solution was to run full rich in the approach to landing phase of flight, and landing. This helps cool the engine down. When parked, run the engine at full idle for .5 -1 min, then pull full lean to shut it down.... (NOTE: I am using 93 oct E10 auto gas too)...

davro14 said:
Greetings

I wonder if anyone has come across the following and/or could help please?

I have a 2017 RV7 which I have had from new and has flown for five years and done approximately 250 hrs. All is good with her except the following. She has twin 2017 E Mags, a Precision Airmotive Silver Hawk servo, and a Red Cube flowmeter. When very hot (e.g. with OAT mid 20s, or higher, and after a long taxi) she does not like shutting down (she runs on). This is due to the well known, heat soak, fuel boiling/vapourising problem. When it is simply this, I am not over concerned, however, very occasionally (a couple of times a year, perhaps but including yesterday! … hence this posting) she will then run backwards, not just one or two blades but two or three revolutions. The mystery to me is where she is getting the ignition/spark from as avgas 100 can not self ignite as far as I am aware. Given this occurred near the beginning of the engine’s life I do not believe there is red hot carbon to ignite it. I use NGK BR8ES auto plugs and I am told it won’t be because the plugs are hot enough to cause ignition or that a colder plug would help. Ignition timing is spot on and Ts and Ps in the air are all very good.

Any work around advice (if not a cure!) would be much appreciated.

Best regards

David
Click to expand...
 
Usually it means the engine is still getting fuel even at ICO, might be time for a servo overhaul.
 
My fuel injection guru (not me) suggested a simple test to make sure the servo mixture circuit wasn’t leaking at idle cut-off. Disconnect the fuel line from the servo to the “spider” at the servo, and put a little cup under the fitting. Put the throttle to idle, mixture ICO, then turn on the boost pump. It shouldn’t flow any fuel - if it does, its time to look into the servo.

Paul
 
David, how do you shut your engine down? I'm assuming mixture cutoff but maybe not? Do you lean aggressively on taxi in?

My engine will occasionally miss a start and run backward for a rev or two. Also electronic ignition (EFII)
 
Shutdown method

Thanks for replies so far. I shut down using LCO and also turn the mags off as last rev should be happening - still runs on and then backwards if very hot. Yes I go rich before LCO to help, or at least not hinder, engine cooling. The servo was checked and even another one tried, servo good ... same result. I know fuel gets past the spider springs because too hot but the mystery is how does it then run backwards. I can't easily eliminate the heat soak/long taxi fuel vaporisation but is there not a way of eliminating running backwards?! ... how does the fuel get ignited when doing this?
 
David. You mention that "Yes I go rich before LCO to help, or at least not hinder, engine cooling" ...

I don't follow your logic, and a guess is that running rich is loading up the manifold with fuel that then is pulled into the cylinders and continues the combustion process for a few seconds. As far as running backwards goes, the kick back on the final blade may be enough to reverse the direction and put you in a "diesel" mode, for just a few turns. No ignition required.

If you're willing to do more troubleshooting, pull a plug on each jug and do a borescope. If you're seeing a lot of carbon inside the heads or on the plugs, you may have a spot hot enough to cause the diesel effect.

Or just on taxi in, lean the engine until it just barely idles at 750 rpm or so (mine IO-360 will idle there at about 1.4 gph) for a few minutes (maybe 5) and try going straight to ICO from there. No richening up before shutdown. Observe the result.

Hopefully your issue is one of technique that you can change and send this issue to the bin!

Good luck
 
Your throttle blade may be open a bit too far as well. You did not mention your normal idle speed, but you might consider backing the screw out a bit more and on shutdown make sure you are hard against the idle stop.

Back in the old days, cars had an "anti diesel solenoid" which held the throttle at the "normal" idle speed when running, but as soon as the ignition was switched off, the throttle closed fully, which solved the run on problem as you describe.
 
davro14 said:
Thanks for replies so far. I shut down using LCO and also turn the mags off as last rev should be happening - still runs on and then backwards if very hot. Yes I go rich before LCO to help, or at least not hinder, engine cooling. The servo was checked and even another one tried, servo good ... same result. I know fuel gets past the spider springs because too hot but the mystery is how does it then run backwards. I can't easily eliminate the heat soak/long taxi fuel vaporisation but is there not a way of eliminating running backwards?! ... how does the fuel get ignited when doing this?
Click to expand...



Update? Have you found the problem?
 
FWIW, Lycoming recommends setting the throttle at 1000 RPM (and keeping it there) prior to moving the mixture to ICO. This will result in a higher demand for fuel, and would create a larger delta between fuel demand vs. fuel supplied. So, if the fuel system is allowing a slight flow at ICO, this method might be effective. Many of these fuel systems have some minimal flow at ICO, carbs typically worse than FI.
 
Run on

1000 rpm, lean slowly till it dies then full idle cut off.
Engine sucks in a little cool air to cylinders (not enough
to hurt anything) but enough to cool hot spots.
And running backwards is making your oil pump suck
oil from some expensive stuff.
 
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