Erratic Fuel Flow at LOP

[petehowell]

Hi John,

I see you are in FL - where it might actually be warm...... Any chance you are running mogas? I have seen where bubbles in the fuel will cause some issues will FF indications.

This happened to me when I had some winter mogas on a warm spring day. Mixing with some avgas made it better.

If you are running 100LL - please ignore me. I get that a lot from women anyway.........

[John Collier]

Originally Posted by RV10inOz

Check for intake leaks first.

This was the issue. Much appreciate the advice! I replaced all the gaskets and used tite seal.

On takeoff, my fuel flow was the highest I've seen it without the boast pump. The EGTs were solid and moving in parallel. However, after about 2 minutes at LOP the EGTs started to dance around.

Upon landing and de-cowling, the tite seal was out of the gasket joint and on the intake tubes...it was obvious that the intake air was getting past the new gaskets.

So...what have other done to fix this issue? RTV? I have the order invoice for the flourosilicone gaskets that were ordered a few months ago...anyone have any luck with them?

[RV10inOz]

My strong suggestion, (us Aussies would be a bit harsher on our mates, but that gets me into trouble on this site) is get all that gooey stuff out of your intakes!!!

Do not damage the surface of the intake port, as it will never seal.

Buy the Red coloured gaskets from Superior Air Parts. Check that the retaining flange is flat, either with a belt sander or wet and dry on a flat surface, and dress them back if not. Reinstall.

No goo please. I know some will say otherwise.

Glad you found the problem!

[AlexPeterson]

What is the mechanism by which intake leaks cause fuel flow fluctuations of 2 gph? I can see rough running or mixture balance problems, but I'd think they'd be more or less steady state.

[chazking]

If your meter is in the tunnel before the fuel servo, you may have fluctuations. Moved mine between servo & distributor to get metered fuel flow .. Fluctuations stopped.

[BillL]

Reviving the thread and wondering if the OP ever got to the root cause. It is hot here lately and 70F at 8000 ft pressure altitude. I was running along and trying to get peak EGT then lean for some data points, and got the same issue as the OP. wandering FF with no change in the knob. And it then ran rough. Richened up turned on the boost pump and then it settled out. I was able to reacquire peak EGT and then a lean point.

I am suspecting some fuel flashing to vapor after the servo (lowest pressure), maybe in the cube. This could cause the situation, but the root cause might be delta-T across the mechanical FP.

I am about to instrument some fuel location FWF for temperature and get a little data. I am running 100LL, and the cowl exit temperatures are in the 155F range.

I have the red cube mounted directly under the spider (IO360) at the sump split line. It is NOT insulated.

Fuel pressure dropped under a cruise condition at 17K too, after the climb at full gross weight, the boost pump-on fixed that issue.

So - if anyone has had the same condition and solved it, please post.

EDIT: Of if anyone has temperature and pressure data, that would be helpful as well. Thanks

[lr172]

Originally Posted by BillL

got the same issue as the OP. wandering FF with no change in the knob. And it then ran rough. Richened up turned on the boost pump and then it settled out.

The servo will adjust the fuel flow based upon air flow through the servo, so be sure to watch the MAP when you see the fuel flow moving about to rule out a change commanded by the servo.

Larry

[BillL]

Originally Posted by lr172

The servo will adjust the fuel flow based upon air flow through the servo, so be sure to watch the MAP when you see the fuel flow moving about to rule out a change commanded by the servo.

Larry

Good thought, Larry, the MAP is dead stable and no changes. There is likely two issues/combination: i. higher temps or suction at the inlet side of the mech pump. and 2. heat absorbed on the downstream side of the servo with the non-insulated red cube. While thermocouples will help, pressure is needed too. A bit more time investment to get that.

I have the stock firesleeve hose from the FW to the fuel pump. I firesleeved the throttle cable and stuck a thermocouple inside. At 1500F exhaust temps (a few inches away and running parallel) it read 200F, well below the max allowed by McFarland. The aluminum fuel supply tube could be picking up heat on that run through the tunnel and radiant heating from the exhaust to the center skin, though. Data will help. More to come.

[lr172]

Originally Posted by BillL

Good thought, Larry, the MAP is dead stable and no changes. There is likely two issues/combination: i. higher temps or suction at the inlet side of the mech pump. and 2. heat absorbed on the downstream side of the servo with the non-insulated red cube. While thermocouples will help, pressure is needed too. A bit more time investment to get that.

I have the stock firesleeve hose from the FW to the fuel pump. I firesleeved the throttle cable and stuck a thermocouple inside. At 1500F exhaust temps (a few inches away and running parallel) it read 200F, well below the max allowed by McFarland. The aluminum fuel supply tube could be picking up heat on that run through the tunnel and radiant heating from the exhaust to the center skin, though. Data will help. More to come.

I have struggled with getting a decent idle, due to fuel vaprorizing on the injector side of the servo. I also have my red cube next to the spider. I never have issues with heat in that circuit once the plane is > 100 MPH. Nor have I ever had erratic FF readings, other than when the red cube failed, that wasn't expected. I have had some issues on final when very hot, but that is 80 MPH.

I think that once at cruise speeds, heat in the upper cowl is a not a problem. Also, once stabilized in cruise, the fuel is moving through the lines fast enough that it can't pick up enough heat to cause issues. The real problem is taxi, when the cooling air flow is low and the fuel flow is much lower, increasing the fuel's exposure to the hot cowl environment. This extends to the T/O run once the whole system is heat soaked and hasn't stabilized to cooler temps yet.

I would not attribute erratic FF readings at 100 KT+ speeds to fuel vaporization in the red cube.

Larry