The International Cessna^® 170 Association

Looking for your collected wisdom – have just installed two new millennium cylinders on 0–300 C, on break-in flight everything good, temps, pressure, power – but once on the ground an odd whining or squealing could be heard. Like you might hear if a belt on a pulley were running badly – but there are no belts and no pulleys. We tightened up a few bolts that we thought might have led to some induction system leakage, but no change in the sound. It has a pulse to it, keeps time with rpm, so it either relates to the rotation of the engine, or if it is a bearing in mag or generator it is only on part of the rotation, not constant.

Anyone encountered this before? We have ruled out any squeaks in baffles, and are baffled.

Stumped at Stanhope.

Doug

Does the engine turn over smoothly by hand? You may might to remove a plug from each cylinder to get a better feel.

Do you get the noise while turning it by hand? If you do can hear it can you localize it to the new cylinders or something else? If the noise is in the new cylinders can you inspect them internally to see what you might see?

Is there any metal in the screens or oil filter?

Engine turns over smooth by hand, no grind. Have not checked oil screens - just one flight of just under an hour.

Do you have venturis? Cover them up and see if the sound goes away.

I'd check the screen or cut the filter and check for metal. This is probably nothing and you will feel better about running the engine again.

Thanks - I'm going to check the oil for peace of mind. My AME is puzzled, wants to pull generator to check bearings. I guess these generators are prone to bad seal, looks like about a pound of some kind of goop around the gen base. I'm not eager to get into fishing expeditions like pulling peripherals off and creating more troubles. I also replaced the engine mount rubbers (costly little brutes!) and he wonders about these.

Second on the venturis. When I still had venturis they would whistle and change tone with rpm at low idle and taxi speed. When I first heard the sound I was a little concerned like a dog hold it's head side ways when it hears something strange that it does not understand. Not to make light of your dilemma however I have never heard anything like you describe after installing new cylinders.

A generator would be turning consistently and the sound, I'd think would not pulse.

I would check the oil for metal. I would then tape over the front of the venturi(s) and run the engine. I'm thinking your venturi always made noise but you where not hypersensitive to the noise and did not take note before.

I personally had a mag bearing fail. My scary story is this. I was about ready to depart on on a Young Eagles Flight. We were operating at a controlled field and so loaded kids and started engine. Sitting at an idle I heard something different. After listening for about two minutes waiting for taxi clearance. I decided to shut the engine down and abort to investigate. At the least I was wigged out an had lost my nerve to fly at the moment specially with the kids. As I pulled the mixture and the prop came to a stop I heard a snap and the prop came to a quick sudden stop. That was the mag gear jamming the accessory case gears when the armature of the mag jammed. The prop could not be turned.

If proven not to be venturi noise, I'd spend some time VERY VERY carefully listening to the engine running with a stethoscope to isolate the location of the noise.

If the noise is from the back, I'd check the mag bearings before the generator bearings if the noise is at the back of the engine because of the failure I had and the mages are easier to pull than the generator.

I agree it is Venturis. Due to your heightened awareness of the senses after engine maintenance, I am thinking you finally realized what the venturis have been doing all along. It is the same thing when a guy has his batteries quit in his noise cancelling headset. I see airplane owners shreiking over normal. Better to be safe and educated then in jeopardy and tested! D

When I was a kid I looked at light planes and figured the venturi was a horn to alert people. Now maybe it is honking after all.

While it is hard to locate the sound, it doesn't seem to be coming from that side, but i'm going to check everything. Thanks for the advice.

BTW, the C172 forum pretty convinced it is an intake leak, and I found one thread that was precisely the same story (Intake and exhaust off to do Real Gasket tube conversions, rhythmic whistling after) and it was an intake gasket not well settled down. So I will delve into that as well. My AME needs brakes on his truck, I may as well finance that...

onward and soon, upward.

Doug

To test for an intake leak, remove the carb heat hose from the muffler and duct tape it to a leaf blower. Turn on the carb heat and turn on the leaf blower. Spray soapy water from a pump bottle all over the engine. Any leak will blow bubbles. An intake leak will usually cause at least one cylinder to misfire due to being too lean and the engine will not idle smoothly.

Good directions. Thanks. Will a shopvac running on 'blow' accomplish the same pressure? Compressor, set to ?? psi? Should I tape off the main air inlet to carb air box?

hilltop170 wrote:To test for an intake leak, remove the carb heat hose from the muffler and duct tape it to a leaf blower. Turn on the carb heat and turn on the leaf blower. Spray soapy water from a pump bottle all over the engine. Any leak will blow bubbles. An intake leak will usually cause at least one cylinder to misfire due to being too lean and the engine will not idle smoothly.

Richard, will it also show the black sooty signs as Doug showed in his picture? Not sure this trail is hot. Doug

Bikerboy6 wrote:Good directions. Thanks. Will a shopvac running on 'blow' accomplish the same pressure? Compressor, set to ?? psi? Should I tape off the main air inlet to carb air box?

Leaf blower or shop vac pressure, makes not difference. The idea is to pressurize the intake so you can see bubbles at any leak like you would test a tire under water.

Much much more dangerious on an airplane, on a car mechanics will shot ether (starting fluid) around the intake and hoses of an idling engine and listen for a rise in RPM. Same idea reversed.

OK – here's the update: back to the airplane today with my engineer, here's what we found. One nut not installed on the cylinder stud of the number 4 intake stub. Looks like the exhaust got put back on and covered over the intake stubs before we double checked our work back there. Fixed that, checked torque on all the others, ran the shop vac and soap bubble pressure test, all good, and on run-up no more squeal! Yee haw.

Did another break-in flight, and the only bad news was that the oil leak I have been chasing for three years, and which all this work was supposed to resolve, is still there. We replaced two cylinders to deal with bad rings that were likely pressurizing the crankcase, and new real gasket pushrod tube conversion for that whole side. I am going to go back in and re-do the pushrod housing gasket on that number six cylinder. On the other hand, the engine work is all signed off, we did an annual at the same time and so at least we are back in the air after two months, just have to keep wiping some oil.

I have heard that the reason that Continental motors never got into the computer business is because they couldn't figure out how to make one leak oil.

Thanks for all the advice!

Bikerboy6 wrote:... I have heard that the reason that Continental motors never got into the computer business is because they couldn't figure out how to make one leak oil. ...

That's the funniest thing I've read all day; I'm glad I didn't have mouthful of beer!!