Surging Fuel Flow

[Jim Worley]

All,
Looking to see if anyone has seen this problem or has any suggestions.

During cruise, fuel flow will ocillate at much as +/- .5 GPH (maybe even a little more). It seems to be be cyclical on about a 1 min interval. It came to light as I was doing some experimenting with cruising lean of peak. When I am running 25 deg LOP, the FF ocillation is enough to make my engine run rough and feel the loss of thrust at the bottom of the ocillation. Running the boost pump doesn't seem to affect the oscillation which, to me, rules out the engine fuel pump.

I had noticed the fluctuation on my digital FF before, but when running ROP it didn't affect engine ops and I thought it was just a "gremlin" in the FF gauge. I am really interested in getting finding the cause now since I want to explore LOP ops.

There is an old thread from '99 on this board which discusses a similar problem, but no terminating action is discussed. Some of the things that are discussed are: Fuel Servo (I just replaced my fuel servo for a different issue, problem was there before I replaced it and is still there), intake leak, (I replaced all intake pipes and gaskets 2 years ago), leak in the fuel supply lines (no visual indication of a leak).

The place I am going to start looking is the alternate air door. I understand that if that door is allowed to move it can cause FF fluctuations.

Background: '61 24-250, IO-540 (engine upgraded to IO in the 80's)

Anyone seen anything like this before?

Jim

[Michael Rath]

Here is some info from Sacramento Sky Ranch:


Fluctuating Fuel Pressure

1. A broken O-ring on the inlet side fuel fitting can cause an inlet suction leak. An inlet air leak will result in low fuel pump output pressure.
2. Worn fuel pressure gauge

3. Clogged fuel vent or fuel cap creating suction in fuel tank.

4. Fuel line blockage.

5. Incorrect fuel pump

6. Defective fuel pump

7. Foreign material in the pump such as fuel tank sealant.

8. Sucking air. Check calculator and fuel selector

 

[N3322G]

I'd add fuel door gaskets, transducers and injectors

Also for digital, ground or other electrical anomalies such as fixin' to fail alternator.

[larry0]

I have a similar problem on my IO720 with a JPI 730. It seems that the McFarlane mixture control cables have slop in them if there are any bends in the installation. I had my mechanic hold the engine end still while I moved the mixture knob. There was slop equal to more than +- .5 gph. My gph varies at about 6 cycles per minute, per the JPI fuel flow and EGT.

[Kevin Ebsen]

Jim, I have a similar problem when running LOP which is all the time. Mine usually will happen about 30-40 minutes into the flight and usually only once will it make the engine stumble. I posted the problem on another forum and the explanation that makes most sense to me is Bold print cut and pasted from other forum guy has a RV10.

"Kevin, the IO 540 is a familiar beast, and a great engine, but it does suffer from fuel pump heat soak.

We have a cooling tube and shroud on ours and anther tube pointing directly at the top of the pump that made the biggest improvement. on the -10 of course I can do these things easily without paperwork more than ....OK forget that!

So unless there is an STC for your aircraft you might want to talk to your A&P about this.

What we found is running the boost pump during the climb, and for a while after levelling out helped, but when you do the BMP the fuel flow drops by a significant amount and spends more time getting heat soaked, and not cooling the pump either. So a vapour bubble goes through and gives you some big shudders, and the fuel pressure drops from 23-27 down to 8-15 PSI and bounces around a bit.

I have watched it slowly manifest itself and can predict the first stumble!

Word of caution, while this may well be the issue you are experiencing, make sure you check everything else, just in case you do have some other serious matter creeping up on you."


I don't have a fix for it yet, but am thinking about running a little less on the lean side and adding shroud & cooling tubes. I just got this answer yesterday so have not done anything yet. See link for engine monitor data. set the data to FF and fuel pressure and you can see about 40 minutes into the flight the ff goes up and fuel pressure goes down for about 10-15 seconds this sometimes will cause the engine to stumble and then go back to normal. I don't know if this is right or not does it make sense to folks on this forum?

https://www.savvyanalysis.com/my-flight ... 98426b8703

Kevin

[md11flyer]

I find it a bit strange that the small IO 320 would have a shroud and cooling tube to the fuel pump and not the big bore 540's.
I would think that you could easily get an approval to run a cooling tube from the baffles down to the fuel pump. Installing a shroud around the pump to accept the scat hose might not be as easy, but at least you can direct some cooling air over the pump.

Gary

[Kevin Ebsen]

[Wayne Haristy]

Yes. My '62 250 with 0-540 definately has a cooling hose running to a shroud surrounding the fuel pump from the factory.

[Kevin Ebsen]

Looks like I have the shroud and cooling tube, but the tube has a kink in it, need to replace and see if it helps. Is that how everybody's looks just the shroud and one cooling tube coming off the airfilter box?

[md11flyer]

On the Twin comanche the hose is connected to a hole in the rear baffle. This position would give a positive
flow of air over the fuel pump. Connected to the airbox I guess the engineers would consider that position as also a good airflow position from where to tap airflow to cool the pump. But if the scat tube is crimped it would restrict the airflow.
I would guess you have found your problem. Let us know after you replace that scat hose if the symptom goes away.

Good luck,

Gary

[Ray B]

Both my '62 250 and my current '60 250 had and have cooling shrouds over the fuel pump. My '60 250 also has two cooling tubes directed at the mags. Don't remember if the '62 model had the mag tube or not. Ray B

[Jim Worley]

All,
That is great info. I have the Eagle XP cowling and do not have a shroud or scat tube going to the fuel pump. I guess I need to find out if that is something that was removed as part of the cowling install or if it has been removed and forgotten at some point after. I also think that the control cable slop can be a factor too. I can put pressure on the fuel flow knob without actually moving it (it's vernier) and make the FF change significantly. Definitely going to take a look in the next couple days when I make it back to the hangar.

Jim

[Kevin Ebsen]

Jim, and thread followers,

I am having second thoughts on my first response to surging fuel flow of "heat soaked fuel pump LOP". Yesterday I flew like normal after reaching altitude went LOP (about 18 minutes on attached data link), but then about 10 minutes later realized we were running late for son's B-ball practice so went ROP to speed up the trip a little. About 27 minutes into the data link if you put FF in one of the drop down menu's you can see where I went to ROP. There is a more sporadic FF ROP than LOP. In my situation I think I notice it LOP, because every so often it is significant enough to make the engine stumble. So for me while I plan on replacing the duct tubing because of the kink, am doubtful that will solve my problem.

Kevin

See flight data on 1/27/2013 flight.

https://www.savvyanalysis.com/my-flight ... 98426b8703

[Ray B]

Why would the amount of "heat soak" at the rear of the engine change from LOP to ROP? If no cooling air is getting to the pump as originally engineered fixing that problem may still be the answer! Look on the bright side! Ray B