To All
Recently I had my PA24-180 manifold pressure gage overhauled. About twenty flight hours after it was installed, it exhibited the same sticking problem but even worse. I returned it to the same company; they found that the manifold pressure gage was “contaminated with fuel”. They suggested checking to see if there was a drain hole in the aluminum manifold pressure line at the low point near the fitting at the cylinder. There is no drain hole. Should there be one, and if so where can I find the reference to find the correct diameter of the hole?
Russ

I did a 1/32" hole in mine, as after overhaul there wasn't one. Everything seems to work great.

This seems to be a common problem, and I wonder whether some A&Ps fail to understand that the hole is SUPPOSED to be there. Without the hole, the residual vacuum in the guage after engine shut-down draws fuel back into the lines and eventually into the manifold guage itself.

After some maintenance on the right engine a couple of years ago, we had sticking of the MP guage, which was sent out for repair and was found to be contaminated by fuel. Within a few flight hours, the same thing happened again, with fuel again found in the guage at the re-do overhaul. This time we discovered "no hole in the line", which was apparently replaced at the time of that maintenance, perhaps because someone discovered a hole in the line! For some reason, it seems I am frequently in the position of having maintenance done twice: first time to learn how to do it, second time to fix it properly!

My A&P drilled a 1/32" (I think) hole, and we've been fine ever since. As I understand it, the size of the pressure relief hole doesn't matter much, so long as it's some small fraction of the diameter of the line itself.

Bryan McIver
1969 PA-30
N8807Y

Bryan,

While I agree there needs to be a hole, to state that it is for "pressure relief" reasons is not accurate. If that were the case, why would Lycoming direct that it be placed at the bottom of the line in the lowest position on the line. If it were simply for pressure (vacuum) relief, it would not matter where it is located. Lycoming also referes to the hole as a "weep-hole".

As the engine runs, there is constant pulsing of pressure (vacuum), in the manifold line since it is drawing off of one cylinder. That is why there is a dampening orifice in the gauge at the inlet. Without the dampening orifice, the needle would fluctuate wildly and be un-readable. The constant pulsing of higher vacuum and lower vacuum creates a migration of air-fuel mixture within the line during engine operation. It does not happen at engine shut down. The idea of the hole is to capture that migration of fuel at that point.

Trust me when I say it happens during engine operation. I watched the fuel level rise in my MP gauge during a 5 hour flight from Michigan to Florida once. I did not have a weep hole. I called Lycoming and they told me where to place it and how big to make it. (I thought they wanted 3/64 but I do not remember for sure). The explaination above is from them, not me. Lastly, the call to Lycoming took place after a reputable shop told me that the intake valve on that cylinder must be leaking and pressurizing the MP line to the gauge!

Chris

Chris:

Interesting..... and informative... thanks!

What I find fascinating, though, is that - no matter how it works - the hole is clearly a required element of the system...... how come so many A&Ps don't seem to be "in the loop" on that??

Bryan

Bryan,

I am not sure that it is "required". Since I experienced the issue on my 180 several years ago, I have paid attention to other aircraft including my current 260 which does not have it. At 2300 TT, & 1 engine overhaul, I believe that the line is original. Rough observation indicates about 50% of the airplanes that I see have it. I question whether the hole was there coming out of the factory. My dad's Piper Dakota did not have the hole and there is no indication that the line was ever changed. We added it after he had MP guage sticking issues and residual fuel was discovered in the guage at the overhaul shop.

I believe that the hole is often being added in the field once a MP gauge issue is found. Also, on my 260, the line is routed up over the top of the engine mount then back down to the firewall fitting. I imagine that it helps to keep the migrating fuel running down hill. I have not had an issue in the 350 hours that I have owned it so I have not added the hole.

Chris

I put this question out to a few other places, and will post replies as they come in.

Here's the comment of Clyde Smith Jr ("The Cub Doctor"), who used to teach for Piper:

"Not only did the PA-30 have a bleed hole, but so did the Arrows and many others. I had to put one in my Comanche, because I got tired of taking my gage over to Keystone Instruments once a month and draining the 100LL out of it. When the bellows goes back to the ambient position after shutdown, it sucks any fuel that may be laying in the line due to overpriming, etc., back up into the gage. The bleed hole prevents this suction from being formed. I think there might be a Piper service letter or something about adding this hole to any of the normally aspirated aircraft manifold gage lines. Make sure it points down so dirt or engine cleaning solvent won't enter the line. Regards...Cub Dr"

I also have a call in to Lycoming and Piper.... will post their comments if/when I get them.

Bryan McIver
N8807Y
1969 PA30C