Fuel pressure dips

[AndreiC]

Hello all,

I noticed something a bit unsettling during my one hour flight today. The fuel pressure on my IO-360 has a fairly regular “dip” of about 3-4 psi every 5-6 seconds. It would stay at 24 psi for 2-3 seconds, then suddenly dip to 20 psi, and slowly recover back to 24 psi during the next second or two. And repeat. It never goes out of the green range, but it is unsettling. The engine shows no irregularities in how it works. It has about 500 hours SMOH by Penn Yan. 
 

if I turn on the electric boost pump, the gauge stays rock solid at 26 psi. The gauge and the electric pump were overhauled (for unrelated reasons) about 5 months ago, but I had noticed this behavior before the overhaul. 
 

Any ideas, comments? I have a video of the gauge doing its thing, but don’t know if I can post video to this site. 

[N201MKTurbo]

   Most likely an air leak in Your fuel selector.

[AndreiC]

Here is a link to the video, I managed to post it to YouTube. 
 

 

[AndreiC]

3 minutes ago, N201MKTurbo said:

   Most likely an air leak in Your fuel selector.

Is this something that needs to be addressed immediately, or can wait until the annual? Is it something relatively easy to fix, like changing the O-rings in the fuel selector?

[N201MKTurbo]

7 minutes ago, AndreiC said:

Is this something that needs to be addressed immediately, or can wait until the annual? Is it something relatively easy to fix, like changing the O-rings in the fuel selector?

You are probably ok to wait, especially if the boost pump restores the pressure. It hasn’t dropped enough for the engine to lean out and stumble.
 

Have you noticed if it is better or worse depending on the tank selected?

Also does altitude affect the problem?

[AndreiC]

Not sure if it changes with the tank and/or altitude. I’ll try and report on this when I go flying next time. Sometimes the dips are subtle; some days it is obvious, as in the video I posted. I’ll try to see if this correlates to altitude or which tank I use.  On this flight I was at 8500ft. 

[AndreiC]

@N201MKTurbo: how difficult will it be for a skilled mechanic to rebuild the fuel selector? Is this a major endeavor, or something that takes a couple of hours? Just for planning purposes, as they say...

[N201MKTurbo]

23 minutes ago, AndreiC said:

@N201MKTurbo: how difficult will it be for a skilled mechanic to rebuild the fuel selector? Is this a major endeavor, or something that takes a couple of hours? Just for planning purposes, as they say...

It is pretty easy. If you have the parts. Mostly o-rings. 

If you have the plane opened up, you are halfway there. You have to unbend the pull ring to get it off, and remove the set screw to get the pointer off.

If you are fast with cap plugs, you can remove the fuel tank fittings and plug them without draining the tanks. If you are quick you can do it without spilling more than a teaspoon of fuel. 

disassembling and reassembling the selector takes about 1/2 hour.

[AndreiC]

OK, I followed up on some of the suggestions, but so far have come up empty in understanding the pressure fluctuations. I talked yesterday to Mark at Top Gun aviation (MSC in Stockton, CA), and he said that he thinks it is unlikely to be the o-rings in the fuel selector since I have no fuel smell in the cabin and no blue stains anywhere in the area of the fuel selector/strainer. (This time around I did not take off the panel that covers the fuel selector and strainer assembly, but that area had been opened up about 6 months ago when I had the boost pump refurbished, and there were no visible leaks, even though the fuel variations were around at that time as well.) Mark suggested as a first thing to do to check the fuel finger screen on the intake to the fuel servo. I had my local mechanic do that today, and the filter was clean. I also took the plane up again today after the filter was blown out with compressed air, and the variations are still there.

To answer @N201MKTurbo's questions, changing the fuel tank makes no difference. However, the altitude does seem to play a role, oddly enough. At lower altitudes the dips get much smaller, or disappear completely once I am at a much lower altitude (say, around 2500 feet). The indicated fuel pressure on the gauge is also higher at lower altitudes, closer to 26psi (up from 24 up high).

Any other ideas on what to check, before I start taking apart the fuel selector?

[PT20J]

The suggestion that such things are caused by air leaks in the fuel selector comes up here often.

Has anyone actually solved such a problem by overhauling a fuel selector or is this just a theory that gets repeated but has no confirmation?

[47U]

1 hour ago, AndreiC said:

Any other ideas on what to check, before I start taking apart the fuel selector?

Do you have fuel flow… or just the pressure gauge?  

How old is the input hose to the engine driven fuel pump?  This hose on my C was sucking air.  Fluctuations in both pressure and flow.  The hose did not leak externally.  I removed the hose, capped one end, put it in a bucket of water and blew into the other end.  The bubbles were dramatic.

I think Skip is right.  I would check the hoses (and hardline fittings) first before tearing apart the selector valve.  

[AndreiC]

3 hours ago, 47U said:

Do you have fuel flow… or just the pressure gauge?  

How old is the input hose to the engine driven fuel pump?  

 

I have both FF and the pressure gauge. But the FF is digital, and I think it averages over a certain interval of time to make the readings more smooth. It varies a bit even with this averaging, say from 9.5 gph to 9.1 and back. I did not check if it is correlated with the dips in pressure.

Not sure how old the fuel lines are. The engine was overhauled in 2012. I would have guessed they would have replaced the fuel lines at the time of overhaul, but nothing about this is in the logs (they don't say either that they reused the old ones or they put new ones). What is the standard procedure -- is it typical to replace them? This was a good overhaul, by Penn Yan. Is there a way to look at the lines and read off from them some manufacturing date? And if I need a new line, how hard is it to get one? I have a big trip planned in about 2 weeks, I don't want to be stranded here because the line is hard to find...

[PT20J]

The lines should have metal tags with date codes on them unless they were field built. 

[AndreiC]

I actually found an entry from 2006 that states "Replaced all oil and fuel hose assy's forward of firewall with new Teflon/silicone hose assys." (This was from a previous engine installation.) So at worst they are 18 years old. How long are they supposed to last? I will go and check for metal tags tomorrow.

[AndreiC]

@PT20J, @47U, @N201MKTurbo : the oil and fuel lines indeed appear to be from 2006. I am planning to replace all the fuel and oil lines under the cowling, since they are now 18 years old (and my local mechanic says the recommendation is for them to be replaced every 10 years, independent of whether they show signs of aging or not). 

Two questions, though:

1) What is the best way to get these lines relatively quickly made? Does anyone have them in stock, or should they just be ordered from Aircraft Spruce or a similar outfit (recommendations?) and wait for them to be made?

2) Is it safe to fly the plane as it is for another 3-4 weeks? I have a big trip over the mountains planned in two weeks, and need to decide whether it is safe to do it if the fuel lines are not ready in time for it.

[PT20J]

There are lots of places that make hoses. Teflon hoses are an upgrade that don't have a stated service life and should be good to TBO. You can get them with molded-on fire sleeves. I got mine from Precision Hose Technology www.aircrafthose.com. 

Who knows about safety? You are assuming that the problem is the hoses because they are old, but you haven't verified that the hoses are actually the issue.

SPD-PTFE_Hose124.pdf

[AndreiC]

Interesting. I guess that reduces the likelihood of the hose being the issue. This makes me think more and more that it is better to take the conservative approach and wait until I get back to my home base and let my mechanic there investigate things, instead of trying to repair/replace things I don’t know for sure are broken. (The only reason I was planning to replace the hoses was for age, as the mechanic here said it is a good idea to replace them anyway if they’re older than 10 years. But I had not told him they are Teflon.) 

[Ragsf15e]

4 minutes ago, AndreiC said:

Interesting. I guess that reduces the likelihood of the hose being the issue. This makes me think more and more that it is better to take the conservative approach and wait until I get back to my home base and let my mechanic there investigate things, instead of trying to repair/replace things I don’t know for sure are broken. (The only reason I was planning to replace the hoses was for age, as the mechanic here said it is a good idea to replace them anyway if they’re older than 10 years. But I had not told him they are Teflon.) 

Replacing old hoses might be separate from your issue (and might not be an issue since yours are teflon).  The theory is that the fuel psi bobbles around when air bubbles get in the lines.  I’m guessing that would normally happen at a joint.  It might be a very small air leak that isn’t actually leaking fuel yet.  Could also be from heat causing fuel to vaporize and fluctuate pressure.  There are a lot of theories.  Lots of people with electronic engine monitors see this activity.  Some find help with a snubber.  Usually you don’t see it on a mechanical gage.

The thing that scares me is deciding that it’s normal and missing something that might be important.  Unfortunately, troubleshooting it isn’t simple either. Trust me, I’ve tried…

[PT20J]

7 minutes ago, AndreiC said:

Interesting. I guess that reduces the likelihood of the hose being the issue. This makes me think more and more that it is better to take the conservative approach and wait until I get back to my home base and let my mechanic there investigate things, instead of trying to repair/replace things I don’t know for sure are broken. (The only reason I was planning to replace the hoses was for age, as the mechanic here said it is a good idea to replace them anyway if they’re older than 10 years. But I had not told him they are Teflon.) 

I probably wasn't clear: I meant only to suggest that you install Teflon hoses when you replace them. I have no way to know what kind of hoses are installed on your airplane.

[47U]

9 minutes ago, AndreiC said:

This makes me think more and more that it is better to take the conservative approach and wait until I get back to my home base and let my mechanic there investigate things

 I like the conservative approach.  

[Ragsf15e]

1 minute ago, PT20J said:

I probably wasn't clear: I meant only to suggest that you install Teflon hoses when you replace them. I have no way to know what kind of hoses are installed on your airplane.

The writeup he quoted from his engine said the hoses were teflon/silicone so he should be ok…

[N201MKTurbo]

FWIW, you can order hoses from Aero Performance in Chandler AZ, or you can order the hoses from Aircraft Spruce and they will be made by Aero performance in Chandler AZ. It is a good hose shop. I was trying to get them to make some custom hoses for a defense project a while back and spent a few hours in their hose shop. They have a good shop and do good work.

Unfortunately, they couldn't make the hoses I needed. We finally got Swagelok to make them.