FI Engine Surge at Idle

[Taildrgr]

I believe I have found the RPM drift problem.

I removed the sniffle valve and found it to be stuck open.

I put a plug in the sniffle valve port and made two test flights.
The open sniffle valve was acting like an air leak.

The engine still surges at idle when hot which is likely vapor in the spider, but when cold it idles smooth again. And in flight RPM is back to steady.

After soaking the valve in acetone for a week, it is still quite sticky so I will just leave the plug in the intake sump.

[AX-O]

Originally Posted by Taildrgr

I removed the sniffle valve and found it to be stuck open.

Same thing happen to me after my third flight. I was able to get another one for free by calling Superior directly. The new one has a few flights and it is doing ok but I carry a "plug" just in case it fails again away from home.

[DanH]

Originally Posted by AX-O

...but I carry a "plug" just in case it fails again away from home.

So do I. It's installed in the sump, about 4 years now

Actually checked the inside of the intake plenum at 500+ hours. There was no significant evidence of oil or blue dye. I'm not saying everyone should just ditch the sniffle, but it seems possible to get along without one.

[madhun]

Originally Posted by Don at Airflow

From the aircraft that customers bring to our facility for rough idle performance, 90% of the time we find the installation is the culprit. That?s not to say that there is an issue with part of the fuel injection system, ignition, or engine itself. We typically find excess hose routing, flow meters mounted on the firewall, gascolators on the firewall, loops in the hoses that can trap air bubbles etc. Sometimes we find issues in the airframe, poor flairs on tubing, flow meters on the suction side of the pumps, cut O-rings and bad pipe thread connections. All these things contribute to poor hot idle operation and hot restart problems.
Even if the installation is correct, poor idle operation can be an issue in tightly cowled or pressure cowled installations. There?s no magic here, we all know avgas boils at around 130 degrees F at sea level. So if the nozzle line temperature gets to 130, the fuel boils in the nozzle lines at idle where the pressure at that point at idle is around 0.04 PSI for a 4-cylinder engine. Raising the idle nozzle line fuel pressure to 5 PSI would be great but then you would need 900 PSI to make the system work at 16 GPH. Certainly you can install smaller nozzles up to a point and that would help. And NO, it will not reduce the take-off fuel flow if the size is calculated to work with the existing inlet fuel pressure and system pressure drops.
One thing to check is to run the engine on the ground with the cowl off. If proper running is achieved, then you know you have a heat issue due to the cowling, so then concentrate in trying to reduce under cowl temperature, reduce the heat transfer to the fuel or increase the nozzle line pressure. If poor operation at idle remains with the cowl off, then look for air entering the fuel supply (typically this is on the suction side of the pumps).
Running a re-circulation system may improve hot idle operation, but then there are fuel management and plumbing issues to deal with.

Hi Don,

Not attempting to be a Smart A here, but where else but the firewall would you mount a gascolator; the only other place I've seen one mounted was in beneath the floor of a footwell, and it was only that way due it's design?

Regards,

Quentin

[DanH]

Originally Posted by madhun

Hi Don,

Not attempting to be a Smart A here, but where else but the firewall would you mount a gascolator; the only other place I've seen one mounted was in beneath the floor of a footwell, and it was only that way due it's design?

Regards, Quentin

Don is currently banned from the forums because he decided not to renew his advertising. You'll have to contact him directly at Airflow Performance to learn why he finds fault with a gascolator on the firewall.

[JonJay]

Originally Posted by DanH

Don is currently banned from the forums because he decided not to renew his advertising. You'll have to contact him directly at Airflow Performance to learn why he finds fault with a gascolator on the firewall.

Banned? Kicking someone off the board because they didn't continue to advertise doesn't sound like Doug's style. You don't even have to donate and he still allows you to play.
Don's contributions to the Forum have been huge. He has helped many of us offline as well and has always been approachable and willing to help, even when it wasn't his stuff.
I hope this isn't true.