Rough Idle - (mine) finally solved

Dean Pichon · #1 09-13-2009, 05:24 PM

I've been plagued with a rough idle on my -4 all summer long. I have a 180HP lycoming with AP fuel injection and dual P-Mags. Last week, I elected to abort a flight because the idle was too rough to inspire the necessary confidence to initiate a take-off. (The engine has, however, always performed flawlessly in the air.) After searching the achives for other's idle issues, and slowly eliminating all the "classic" causes, I traced my idle problem to air in the fuel system (Thanks to the troubleshooting guide supplied with the AP injection system).

Early in the summer, I had replaced and relocated the fuel filter to be before the boost pump to protect the pump from any debris in the gas or tanks. I pressure checked all the individual lines, but did not pressure check the filter itself. I wasn't too worried about it as it was on the suction side of the pump. Yesterday, I removed the filter with its AN hardware and pressure tested it. I found one of the pipe threaded AN fittings to be leaking at the joint. I had used fuel lube rather than Teflon tape or paste. I have little experience with Fuel Lube, but this was either not an appropriate application or a poor execution. I replaced the Lube with Teflon tape and re-assembled the system. Idle tests earlier today were beautiful.

Apparently, I had been sucking enough air into the fuel stream to produce rough idle conditions. For reasons not yet apparent to me, under high flow conditions, everything seemed to work fine.

I would expect this is an unusual cause to a common problem, but hope this experience may be of use to others.

Dean Pichon
RV-4
Bolton, MA

vlittle · #2 09-13-2009, 06:21 PM

Hi Dean.
I would second-guess the use of teflon tape anywhere in your fuel system. In fact, an inspector would not approve it as airworthy.

The problem is that bits of tape can become dislodged and be trapped in the fuel system, plugging an orifice or injector. Not a pleasant experience thereafter.

If you just retorqued your coupling with fuel lube, it should be fine. If not, replace the couplings and try again.

Just some friendly advice that could be important some day.

Thanks,
Vern

mannanj · #3 09-13-2009, 06:40 PM

Quote:

Originally Posted by vlittle

Hi Dean.
I would second-guess the use of teflon tape anywhere in your fuel system. In fact, an inspector would not approve it as airworthy.

The problem is that bits of tape can become dislodged and be trapped in the fuel system, plugging an orifice or injector. Not a pleasant experience thereafter.

If you just retorqued your coupling with fuel lube, it should be fine. If not, replace the couplings and try again.

Just some friendly advice that could be important some day.

Thanks,
Vern

DITTO!!!!!!!!!!!!!!

scard · #4 09-13-2009, 06:45 PM

This thread will go long, but it is worth many voices against Teflon tape. Open it back up and do something different.

IowaRV9Dreamer · #5 09-13-2009, 07:21 PM

I think the best thing you could use on a tapered thread in the fuel system is Loctite 565. It is a teflon paste, specified for use on tapered threads in the presence of petroleum. It is also recommended for tapered threads in the hydrualic system.

Hope this helps.

Dean Pichon · #6 09-13-2009, 07:48 PM

I re-torqued with Fuel Lube, then again with Teflon paste and both leaked with 80psi test pressure. It was only the Teflon tape that held. I, too, have stayed away from tape, but found neither of the other options worked. I inspected all threads with a magnifying glass and all seemed fine. Again, I have limited experience with Fuel Lube.

Regards,

rocketbob · #7 09-13-2009, 07:57 PM

Fuel lube isn't a sealant. Its a lubricant. It doesn't work well for sealing pipe threads.

I use Titeseal, and for years have yet to have anything leak using it.

N941WR · #8 09-13-2009, 08:23 PM

The "seal" is made when the aluminum tubing is pressed against the angle portion of the fitting. The aluminum deforms just a little to make that seal.

If you are getting a leak, there is a chance that the flair has a small crack. I would take that fitting off and take a close look at both sections with a magnifying glass.

L.Adamson · #9 09-13-2009, 10:24 PM

Quote:

Originally Posted by N941WR

The "seal" is made when the aluminum tubing is pressed against the angle portion of the fitting. The aluminum deforms just a little to make that seal.

If you are getting a leak, there is a chance that the flair has a small crack. I would take that fitting off and take a close look at both sections with a magnifying glass.

I'm assuming it's a pipe joint, instead of a flare fitting. For all my pipe joints, I use Rectorseal 5. Been using it for gas lines for years (HVAC business); and it's even available at Home Depot. This is recommended for many petroleums, oxygen, and more.

In regards to teflon tape, the chance for damage will usually occur if the joint is taken apart, and some of the pieces rip apart and remain on the female thread, only to spread thoughout the system. That's why it's not generally an acceptable practice. In reality, a combination of teflon tape & pipe dope, has the best chance of sealing, over pipe dope alone. But for our airplanes, the sealant is good enough.

Some aircraft instruments call for using teflon tape on their nylon fittings. But......you have to make sure that the tape doesn't get to the end of the threads, when wrapping.

L.Adamson --- RV6A