What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Erratic Fuel Flow at LOP

John Collier

Active Member
Seeking advice/experience with fluctuating fuel flow when operating LOP. I balanced my nozzles a couple of years ago and had good success operating LOP. Recently, my fuel flow is indicating anywhere from 8 gph to 6 gph after I set the mixture. The EGT is unstable and I can tell the engine is not running smoothly...my guess is because the fuel flow is indeed inconsistent.

I'm planning on cleaning the nozzles and looking at the divider.

My typical cruise altitude is 8500 to 12500.

The OAT has cooled off considerably in the Southeast...I understand a higher fuel flow would be required based on a lower density altitude...but would that contribute to this problem?

What am I missing?
 
System Info....

Mattituck IO-360
Dual PMags (timed correctly & monitored with EICommander)
Precision FI (balanced nozzles from Airflow)
Dynon Skyview
 
John,

If you have individual EGT readouts, are you currently seeing one or more cylinders going LOP early? I've had two cylinders that leaned before the rest, and once those cylinders get too lean, I could hear and feel the roughness, and noted the FF drop more rapidly per twist (or partial twist) of the mixture knob. Not sure if that is what you are seeing, but injector cleaning seems like a good troubleshooting step. Good luck!

Cheers,
Bob
 
4 EGTs...

I do have EGTs for each cylinder. When I first balanced the nozzles the EGTs peaked within 20 degrees across the cylinders. Now, I can't get consistent peaks. One attempt will have a 30 degree spread...another a 10 degree...another a 50.

My real question is why I don't have consistency on fuel flow. The EGTs don't stay at a solid reading...cylinder EGTs are +/- 20 degrees as the fuel flow fluctuates. My fuel flow has varied up to 2 gph after setting the mixture for LOP.

I believe if I could get the fuel flow to stabilize after I set the mixture...my LOP operation would be fine.

Also, the fuel flow is stable ROP and the engine runs great.

Maybe a higher being is trying to tell me to stop running LOP :D
 
John, perhaps you have a fuel flow indication problem that shows up at those flow rates. Others (Pete - what was your experience?) have found that the little window inside the transducer gets clouded or otherwise obscured, giving erratic readings. The roughness may just be that you are running more lean than you believe. If the 2 gph variation were real, that is a lot, and enough to take you from LOP to ROP.

Just a thought.
 
"If the 2 gph variation were real, that is a lot, and enough to take you from LOP to ROP."

It does take me from LOP to ROP....or I have to lean so much that the engine gets very rough. The 2 gph was the extreme i've seen since the OAT cooled off...but 1 gph fluctuation is common.

I'd thought about the transducer being inaccurate...but I'm using the Dynon's LOP feature and watching the EGTs peak. At peak to LOP...that is when the fuel flow gets weird.
 
Thanks Dave.

The manifold will be my first check. Then I'm thinking I need to look at the size of my nozzles as I might be bringing the fuel divider into the equation at low fuel flows. That makes sense in my mind...as the problem only shows itself at approx 7gph fuel flow.

thanks!
 
erratic reading at LOP

John,

I posted a week or so ago about an erratic flow rate at LOP. I had never an issue with my D180 install but I upgraded to Skyview and just completed a long cross country and had fuel flow swings +/-.3 without any noticable engine performance changes.

My fuel usage was off as well. I still have not done investigation to do but it only occurred at LOP @9.5k but did settle down. My fuel totalizer was accurate with the D180 system but apparently not was the Skyview.

250+ hours on my bird and loving it. I am glad you kept your RV8. I have pictures of your bird off my wing hanging in my office. That camo paint scheme attracts a lot of attention.

Shotgun moved to CO flying 16's with the COANG. I am helping another C17/T6 pilot with his 7. Hope to get it flying at the end of January. He just accepted an AZANG KC135 job in Phx. I must be scaring all the Arizona AF RV guys away.
 
Mogas?

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.........
 
Yep...how to fix?

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?
 
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!
 
Curiosity question

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.
 
Fuel Flow meter location

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.
 
Fuel Flow fluctuation - - me too

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
 
Last edited:
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
 
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.
 
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
 
Last edited:
Back
Top