FI Engine Surge at Idle

Taildrgr · #1 03-27-2016, 01:29 PM

I have searched and found threads dealing with rpm surging at full power settings, but my surging issue is at idle speeds.

I have a new Lyc IO-360-M1B, Hartzell CS C2YR-1BFP/F74972 prop, and MT P-860-4 Rev G governor from Van's, all with 32 hours time.

I set my prop pitch screw for max static rpm of 2690 rpm. I have not touched the governor.

The rpm is rock solid while flying.

At about 5 hours into phase I testing, my idle/taxi rpm started surging about 200 rpm after I stopped at the fuel pump after a flight. When I restarted up and taxied to my hangar, the engine was surging. I thought it was vapor lock (I only use 100LL). 90 degr day in Texas.

Days later, on the next flight, the engine was still surging on initial startup (cold) and did not stop when the engine warmed up or taxiing to the runup area. The prop cycled normally at 1700 rpm, so I took off and stayed over the airport, landed, and the surging was gone. It did not surge again for 27 hours so I thought it was just an initial break in issue.

Yesterday, at 32 hours total time, after a 2 hour flight, I landed, got fuel, and again, on startup it was surging. It was a cool day at 54 degr OAT but the engine was hot. Still 100LL. I have not restarted the engine since yesterday so I don't know if it will continue to surge when cold.

On the ground, changing rpm from 650 to 1200, changing the mixture from full rich to lean, and turning on the boost pump, all have no effect. I have not tried to move the prop control because I thought the governor has no effect at idle speeds. I also checked my prop governor control linkage and it is solid. The lever is on the stop screw with the prop control out 1/8 inch from the panel.

So, why would a fuel injected engine surge at idle? Hot or cold. Is it the prop or a fuel flow issue?

Any suggestions on what to investigate would be appreciated.

lr172 · #2 03-27-2016, 01:47 PM

Have you checked your idle mixture. A lean mixture can cause surging. Mixture knob cannot enrichen idle mixture beyond its setting, regardless of its position.

flyinga · #3 03-27-2016, 01:53 PM

Almost certainly not a prop or governor problem. Look elsewhere.

Taildrgr · #4 03-27-2016, 08:23 PM

Quote:

Originally Posted by lr172

Have you checked your idle mixture. A lean mixture can cause surging. Mixture knob cannot enrichen idle mixture beyond its setting, regardless of its position.

I have adjusted the idle mixture to give a 50 rpm rise on shutdown.
I lean while taxiing, but the surging exists at full rich and my leaned taxi setting, too.

Thanks for the reply.

nigelspeedy · #5 03-27-2016, 08:41 PM

Just out of interest what sort of Mags do you have? My engine has dual Mags and before I installed a restrictor in the MAP line it idled quite rough, hunting up and down 50 to 100 RPM, when I tried to idle below 800 RPM. The restrictor improved this quite a bit. Even so when I restart the engine after a brief shutdown my fuel pressure sometimes fluctuates and unless I turn the boost pump on the idle is rough.
Cheers
Nigel

AX-O · #6 03-27-2016, 10:31 PM

what kind of fuel injection system do you have? I am having the same issue and can't figure it out.

Kevin Horton · #7 03-28-2016, 05:30 AM

Many Lycoming engines with mechanical fuel injection (i.e. Bendix, Airflow Performance) do this when the engine is hot, as the fuel boils in the fuel injection lines above the engine when the fuel flow is low. This causes uneven fuel flow and rpm variations. The surging stops when you increase power, because the higher fuel flow requires higher pressure in the lines, and the boiling point of the fuel is now higher than the actual temperature.

Switching to slightly smaller injection nozzles (sold by Airflow Performance) increases the pressure in the fuel injection lines, which increases the boiling point of the fuel. But, the smaller nozzles pass less fuel at full rich, so there is a risk of being too lean at high power. I've chosen to live with the issue, as I don't want to risk being too lean at full power.

mike newall · #8 03-28-2016, 06:06 AM

Normal behaviour.

Ours does it all the time after fuelling - the area around the engine gets hot very quickly after the taxi in and once you restart, there are vapour bubbles in the lines which take a while to work there way through.

If you have a full return fuel system, you can run the electric pump with the mixture in ICO - we used to do this on big Cessnas after fuelling when we were skydiving.

IO-360 180hp (ECI)
Silverhawk Injection
Dual P-Mags
MT prop and governor.

David-aviator · #9 03-28-2016, 06:24 AM

Quote:

Originally Posted by mike newall

Normal behaviour.

Ours does it all the time after fuelling - the area around the engine gets hot very quickly after the taxi in and once you restart, there are vapour bubbles in the lines which take a while to work there way through.

If you have a full return fuel system, you can run the electric pump with the mixture in ICO - we used to do this on big Cessnas after fuelling when we were skydiving.

IO-360 180hp (ECI)
Silverhawk Injection
Dual P-Mags
MT prop and governor.

It may be normal behavior or the controller may need recalibration by a service center. I had an intermittent problem with AFP at idle, sent it back to have it checked, they found nothing wrong but it has worked normal ever since.

Only explanation was it may have had some contamination within the unit that got cleared out during service. It was a new unit with very few hours.

lr172 · #10 03-28-2016, 08:12 AM

Given that this is a new installation, I would try idling with the boost pump on to see if anything changes (rule out problem with mech pump). I would also be looking for a loose connection on the suction side (before mech. pump). Loose connections can introduce air bubbles, which would be more noticeable at idle. You may find slight leaks (blue staining) at the loose connections.

Larry