FI Engine Surge at Idle

[BillL]

FYI - during testing of a 10 fuel system last week, we were pumping 12gph with a variable restrictor before the spider and saw bubbles in the clear discharge line. We added a clear section before the restrictor to see if it was suction. It was not. Ambient was 59F. 100LL. Boost pressure was 29 psi.

I can now see that a restriction dropping the fuel pressure rapidly can induce cavitation bubbles even in cold fuel.

If my (new) M1B did this . . . after reviewing VAF, and the manual, I would exercise my fingers and call Vans/Lycoming Support to discuss the possibilities with a list of the conditions and things checked/tried. I have gotten very good response from them on other questions so far. YMMV

[vlittle]

Originally Posted by lr172

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

There are a lot of fuel line connections on the suction side of an FI system. Any leak will introduce air bubbles into the system. Usually, this is not noticeable at high power/high fuel flows but is at idle power. These air bubbles expand rapidly when heated, so a hot engine makes it worse. This is in addition to fuel vapour bubbles caused at idle in a hot engine.

You won't see large blue fuel stains, but if you wipe all the fittings with a shop towel and notice any stains on the towel, you have a leak.

In Canada, we must have gascolators, even in FI systems, and I have found that they can be (are always) an air leak problem.

It can take several hours of operation to find the leaks, so good luck.

[lr172]

Originally Posted by BillL

FYI - during testing of a 10 fuel system last week, we were pumping 12gph with a variable restrictor before the spider and saw bubbles in the clear discharge line. We added a clear section before the restrictor to see if it was suction. It was not. Ambient was 59F. 100LL. Boost pressure was 29 psi.

I can now see that a restriction dropping the fuel pressure rapidly can induce cavitation bubbles even in cold fuel.

Bill,

I am curious what your resolution was to this? Or is it just an accepted flaw of the mechanical FI system.

Larry

[erich weaver]

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.

NOT normal behavior. Original post says engine was still surging on intial start up (cold) and did not stop after warmed up or on taxing to run up area. Fuel will not be vaporizing in injection lines under these conditions. Sounds more like there is a leak somewhere letting air into the fuel system.

erich

[BillL]

Originally Posted by lr172

Bill,

I am curious what your resolution was to this? Or is it just an accepted flaw of the mechanical FI system.

Larry

Larry, there is no problem here.

This was a test to measure unusable fuel in each tank, and at different attitudes. The servo was bypassed with a coupling, the spider inlet hose was removed and connected to a tube with a ball valve and then a length of clear vinyl tubing to allow it to flow into a 1 gallon jug.

The boost pump was pumping fuel through the full system, i.e. the mechanical fuel pump and the red cube (between the servo and spider).

The test was conducted at 59F. The boost pump was turned on, the valve opened until it ran clear, then the valve shut down to either cruise or TO fuel flow to check pressure at flow conditions.

We noticed that the clear tube was discharging bubbles when restricted to 12 gph. The EFIS was reading 29 psi. The entire system had already been pressure tested with air at 30 psi and being left overnight to see if there was a drop. Issues were repaired until it held w/o drop overnight. Something could still have surprised us, so the tubing upstream of the ball valve was separated and a 6" length of clear vinyl was installed. 8 more test conditions were run and each time after the bubbles cleared from upstream with full flow, the flow dropped to 12 or 24 gph, the bubbles were still being generated through the restriction of the ball valve.

So, there was not problem, just learned that if the pressure is dropped from 30 to zero via a turbulent orifice, then some cavitation bubbles are generated. I don't know what the pressure drops are though the servo, spider, and injectors, but one would have to think that gas is generated somewhere. It would be best if it were after the spider or the metering function could be compromised.

It probably has nothing to do with the issue here unless the spider is having an issue, or there is some leak in the system sucking air. In order to ensure that air is not being pulled in between the tank and the mechanical pump, a systems test (as described above) would be in order. The OP did not indicate any test (like above) was performed on the system prior to first flight. I will be running a full fuel systems test on my 7 before first flight.

Originally Posted by erich weaver

NOT normal behavior. Original post says engine was still surging on intial start up (cold) and did not stop after warmed up or on taxing to run up area. Fuel will not be vaporizing in injection lines under these conditions. Sounds more like there is a leak somewhere letting air into the fuel system.

erich

Agree.

[Taildrgr]

Thanks for some ideas guys. I will check again for leaks. I has put a wrench on all the fittings at my last inspection 7 hours ago but I will inspect the lines and connections for leaks too. Have not seen stains anywhere.

Another thing I will check is the inlet fuel strainer on the servo. I have checked the filter at the boost pump but it is always clean.

BUT one thing is interesting, when the plane sits after a flight, I get about a 3 inch diameter fuel stain on the ground under my sniffle valve drain after about 24 hours. I thought it was normal but apparently some people don't get any leaks from the sniffle valve. Could this be a related issue?

FYI, my red cube is hard mounted to the firewall at a low point between the mechanical fuel pump and the servo.

The servo is an Avstar AVX-5VA1 unit that came with the engine from Vans.
Mags are Slick 66GC25SFNN on left and 66GP-0SANN on right also came with engine.

Again, thanks for taking time to reply.

[Carl Froehlich]

Other things to check:
- Intake leak. Make sure all your intake gaskets are in place and connections tight.
- Intake leak. Take out the sniffle valve and replace with a bolt to see if the problem goes away.
- Intake leak. Watch EGTs at 1000 rpm or so as you lean. Is one cylinder rising faster than the other?
- Spider issue. Had this recently happen. Do a fuel flow test with a shot glass to collect fuel at each injector (line from the spider disconnected at the injector and fuel directed to the shot glass). The spider has very small bypass holes for fuel flow at idle. If one is blocked this can look like a lean cylinder. This will also show up during the idle lean as you watch EGTs.

Carl

[Taildrgr]

Originally Posted by Carl Froehlich

Other things to check:
- Intake leak. Make sure all your intake gaskets are in place and connections tight.
- Intake leak. Take out the sniffle valve and replace with a bolt to see if the problem goes away.
- Intake leak. Watch EGTs at 1000 rpm or so as you lean. Is one cylinder rising faster than the other?
- Spider issue. Had this recently happen. Do a fuel flow test with a shot glass to collect fuel at each injector (line from the spider disconnected at the injector and fuel directed to the shot glass). The spider has very small bypass holes for fuel flow at idle. If one is blocked this can look like a lean cylinder. This will also show up during the idle lean as you watch EGTs.

Carl

Thanks, Carl. I will check the spider too.

[Taildrgr]

Carl,
I have done 3 GAMI tests while flying at 75% and 65% power. All had a GAMI range of 0.4 gpm.

Would this test show the spider and injectors are ok?

Also, all my EGT's at cruise are within 50 degr F when leaned to 100 ROP = 1300 degr to 1350 Degr. Probes are 2 inches from exhaust flange.

Max EGT spread at idle is 100 degr. Lowest is 900 degr and highest is 1000 degr.

[mike newall]

Originally Posted by erich weaver

NOT normal behavior. Original post says engine was still surging on intial start up (cold) and did not stop after warmed up or on taxing to run up area. Fuel will not be vaporizing in injection lines under these conditions. Sounds more like there is a leak somewhere letting air into the fuel system.

erich

Erich,

Sorry, the cold bit was buried in the text

Hot it will surge, whatever you do.

If the cold surging re occurs, I would investigate. If it is only hot, I would not.

Mike