Engine question - high fuel flow & low fuel pressure at idle

[MChadwick]

Hi all - first time posting here, hoping someone may have seen something like this before or have some pointers.  I got a strange situation when I fired up my M20J this morning to head to Oshkosh from Virginia.  Couldn't find any clear instances of this on a search so figured I'd try a new post.

When I engaged the aux fuel pump to prime, I noted that fuel pressure didn't climb as I normally expected.  I suspected it was possibly instrumentation (a tech ran my battery near flat earlier this week) and fired up.  The engine started normally on the third blade and ran smoothly, but on initial instrument scan I noted that my fuel pressure was still showing as near zero.  At the same time, my fuel flow was 8.6gph, far higher than the normal ~2gph at idle at the same mixture setting.  I shut down shortly after - the fuel pressure had climbed to 5psi and the fuel flow down to 6gph by the time I did so.  Upon inspection, there was what I originally thought was oil along the right side nose gear door.  Now I think it may be some mix of fuel, oil or something as it isn't the color or consistency of the fairly fresh oil I'm running.  Photo of that attached.

After poking around for a few hours with the cowling off I did not finding any obvious leaks, drips or stains.  I fired up again to see if the issue persisted and everything ran perfectly fine - normal pressures indicated, normal fuel flow of around 2gph, same smooth engine as always, and no fluids.

Fuel Pressure is factory gauge, fuel flow is through a JPI EDM700 with fuel flow sensor.  M20J with IO-360-A3B6D

Not an A&P - I've contacted some A&P's but am not anticipating getting a mechanic's eye until Monday.  Any ideas or things to check in the meantime would be massively appreciated.

 

Mike

 

Saavy logs:

Odd FP/FF: https://apps.savvyaviation.com/flights/6035132/6aeae3b3-8459-4141-956b-eb8301b51992

Previous Flight:  https://apps.savvyaviation.com/flights/6035128/34d80b1a-b045-4b18-a125-4092d2766c3d

Edited July 24, 2022 by MChadwick

[N201MKTurbo]

I would have to guess it is an instrumentation issue. That engine will not run with that low of a fuel pressure and it wouldn’t idle with that much fuel flowing into it. 
 

You didn’t say which year your plane was. The early Js had wet gauges and the later ones had electronic gauges with remote sensors.

[Ragsf15e]

I agree with @N201MKTurbo, but recently I did have a (self inflicted) leak at my fuel pressure sensor and it did look like oil at first - streaked down the nose gear doors just like that too.  Looked just like your picture.  Do you normally get oil there?  Did you smell it?  Rub it on your fingers?  I was able to see a faint blue tinge in the dirty streaks and smelled fuel on it.

[ArtVandelay]

A poor ground connection may cause bad sensor readings, I would check the engine ground strap. And anything else the tech may have touched.

[PT20J]

Different M20Js had different fuel pressure senders. The later ones have Kulite transducers incorporating a Wheatstone bridge/strain gauge circuit that gets power and ground from the gauge and doesn't depend on the engine ground. The EDM 700 uses a FloScan flow transducer that has it's own ground return to the EDM and also does not depend on the engine ground. However, there may be some common electrical problem because the readings you saw don't make any sense and the engine runs normally. You said that the shop ran the battery down so perhaps they caused some other problem. Shops usually run an airplane on ground power to avoid using the ship's battery. I would run up the engine with special attention the the voltmeter and ammeter.

The oil on the nose gear door may or may not be related. But a fuel leak is pretty easy to detect. Run the boost pump and look at all the fittings for leaks. If you touch each and then smell your fingers even a small seep will leave a very noticeable odor.

Skip

[Ragsf15e]

Also, kind of adding on to @PT20J s good advice, you can run your electric boost pump with mixture in cutoff and pressurize much of the system to ~25psi.  Turn off the boost pump, it should hold pressure for a good long time which allows you to get out and look for active leaks.

[PT20J]

I thought about this some more. Low pressure and high flow is indicative of a significant fuel leak. In fact, if this happens in flight, it should be treated as an emergency with a landing asap because a fire can start at any time. What's strange in this case is that the problem seemed to go away and the next engine run was normal. I would pressurize the system with the boost pump and wiggle things to see if there is a fitting or hose that leaks intermittently. I would also talk to the shop that last worked on it about what they might have done to affect the fuel system.

Skip

[Ragsf15e]

Also, the “oily streaks” are they new or normal for your airplane?  They seem significant, especially if they smell like fuel.

[MChadwick]

Thank you all for the thoughts.  Heading to the hangar shortly to try pressurizing the system and looking for leaks.  To answer a few questions

• This is a 1980 J - it has an electrical fuel pressure transducer
• I didn't smell the oily streaks on the gear door.  I will do that when I get to the airplane today
• All of these symptoms are brand new to the airplane - the engine hasn't leaked fluids in my two years of ownership.  Makes me think something broke, something is in the process or breaking, or something became loose on my last flight
• As for the possibility of a significant fuel leak - this was the only way I could make sense of the oily streaks + low fuel pressure + high fuel flow.  The engine normally takes ~2gph at a leaned out ground idle, which to me means to get to 8gph measured requires upwards of an additional 6gph going overboard - the equivalent of a can of beer worth of fuel every minute.  I didn't see any physical evidence of a leak of this magnitude (aside from the oily streaks).  Also assumed the measurements are accurate, but I am inclined to believe the measurements since it is from two different sensors hooked to two separate systems.
• The battery ran down earlier this week during a pitot-static check due to a faulty ground power unit.  We didn't notice the ground power unit wasn't doing its job until volts were low.  Fixed the ground power unit and topped up the battery.

Last thought - fuel flow measurements would imply the leak is after the fuel flow sensor.  I confirmed that the fuel flow sensor is located between the mechanical fuel pump and the fuel servo.  Implies the leak would be downstream of the fuel flow sensor.

More updates to come...  Thanks all for the very helpful thoughts and advice

[MChadwick]

Conducted the following checks:

1. Inspected the oily streaks - didn't smell like fuel but its been >24 hours so not sure that is a valid conclusion
2. Mixture idle cutoff, ran fuel pump to pressurize the fuel system upstream of the fuel servo.  Ran for 5-10 seconds and got ~16psi (i.e. not exhibiting 0psi symptoms of yesterday).  I also ran the JPI while I was doing this and fuel flow was 0gph, as it should be.  Couldn't find any indications of a leak upon visual and manual inspection of all fittings.
3. Mixture rich, ran fuel pump to pressurize the entire fuel system.  Ran for 5-10 seconds and got ~16psi.  JPI showed ~2gph while this was happening and decreasing as pressure rose.  Inspected for leaks visually and manually and found none.
4. Fired up engine - once again ran normally from start.  Inspected for fuel or oil leaks after and found none.

Only other observation is that the aux fuel pump puts some small amount of fuel overboard out the drain when in operation, maybe a tablespoon.  This does not seem to be normal, though I have never looked at it while running it.  This behavior exhibited at both full rich and idle cutoff mixture settings.  Makes me think the aux pump may need some work, though not sure that explains the other behaviors.

I'll try to get it into a shop tomorrow and see what they have to say.

[EricJ]

13 minutes ago, MChadwick said:

Only other observation is that the aux fuel pump puts some small amount of fuel overboard out the drain when in operation, maybe a tablespoon.  This does not seem to be normal, though I have never looked at it while running it.  This behavior exhibited at both full rich and idle cutoff mixture settings.  Makes me think the aux pump may need some work, though not sure that explains the other behaviors.

That's definitely a smoking gun, and generally indicative of impending failure.   It's a good catch to get that early.

 

[PT20J]

1 hour ago, MChadwick said:

Only other observation is that the aux fuel pump puts some small amount of fuel overboard out the drain when in operation, maybe a tablespoon.  This does not seem to be normal, though I have never looked at it while running it.  This behavior exhibited at both full rich and idle cutoff mixture settings.  Makes me think the aux pump may need some work, though not sure that explains the other behaviors.

That would indicate a leak from the internal pump seal. But it does not sound like enough to cause a problem in operation yet. Perhaps the seal leaks more if the pump hasn't been used for a few days and leaks less if it has been used recently. However, I would tend to agree with your earlier observation that the high fuel flow would imply that the leak was after the flow transducer. There is only one fuel line between the transducer and the servo. I suppose it could be something internal to the servo, but the extra fuel would have to go somewhere. Since the RSA servo doesn't have a fuel return to the tank, the fuel either has to get burned or leak out somewhere.

Have you tried a full power runup?

Skip

[Ragsf15e]

@MChadwick You’ll eventually need Aeromoters in Wisconsin to rebuild the electric pump.  You’ll also want to find the source of that leak. Did you just have your oil changed and the mechanic spilled some?  If not, I’d find the source of that leak.

[MChadwick]

Just wanted to throw an update here.  We found the source of the leak!  The hard part was getting the system to reproduce the intermittent issue - it seems to only happen when the airplane is at ambient temperatures and/or hasn't run in a few days.

I reproduced by going mixture rich, fuel pump on, note 0psi fuel pressure, continue to run until fuel pressure needle moves, turn off fuel pump, mixture idle cutoff, exit aircraft to look for leaks.  It took around 30 seconds of aux pump running at 0 fuel pressure for the leak to show up, leak started just as the fuel pressure built up to 1-2psi.

Significant leak on the fuel line just after the fuel servo.  As predicted by @PT20J, the leak was after the fuel flow sensor, after the servo and before the fuel pressure sender.  Leaked down the servo housing and down onto the exhaust system.  Will be fixed soon, will report back on if it was the servo, fitting, or line.  Glad I grounded the airplane until this was sorted, even though it only happened very intermittently.

Thank you all for the assistance troubleshooting!

Mike

[carusoam]

Nice follow-up!

Amazing that it didn’t carry blue color and fuel aroma….

Great catch!

Best regards,

-a-