Hi, looking for a little troubleshooting help, our mechanic is a little stumped. We have a Cherokee 180, and have a strong vibration above 2300 rpm, with it being the worst at about 24-2500. Our mechanic is pulling his hair out, they can't seem to find the cause. The compression checks out fine, they removed 3 of the 4 cylinders and the cam seems fine. It doesn't have any intake leaks, and the ignition leads and plugs are good. He sent us to another mechanic to balance the prop, but he felt the vibration was a combustion problem(just like our guy thought), not a prop balance issue. He did some troubleshooting, and was not able to come up with anything other than the baffle in the rear muffler was broken out(gone, not rattling around) and checked with a few sources and they didn't feel like that would be the problem.

We ran up the plane for about a minute and pulled the cowl to check cylinder temps with a laser gun, and all the cylinders were about the same, with the only difference being the #4 exhaust pipe was about 100 degrees cooler than the other three (400 compared to 500)

Any thoughts?

Is it possible to have a valve stick at rpm that wouldn't show up easily?

Thanks, Lance

Sounds like an ignition problem. Check your mags, if one is firing out of time it will cause the symptoms you describe. It may be OK at a static test but let go at high RPM.

lancef53 wrote:Any thoughts?

Is it possible to have a valve stick at rpm that wouldn't show up easily?

Thanks, Lance

yes.



the most stumped i have ever been by a lycoming was a case just as you describe. it turns out that one of the exhaust valve springs was SO worn out that you could actually push the valve open with your thumb. so it would "float" or "flutter" in the open position, moreso at certain rpm's and especially at full power. it had excellent compression on a gauge, because the valve closed and seated nicely. i had the cylinders off and nothing appeared wrong. the only thing i noticed was a light gray chalky residue inside the rocker box of the offending cylinder, which turned out to be exhaust stains from a badly worn valve guide. further inspection found the bad spring and worn guide, worn stem, etc.... worst vibrating lyc i had ever seen. the owner didn't even give it much notice because it came about so gradually he just adapted to it

if it were in my shop i would pull all 4 rocker covers and complete the lycoming service bulletin on valve guide wear. it will alert you to a loose guide or a very tight guide that could also cause severe roughness.

also check valve lift at the rocker arm to check for bent pushrod, bad cam lobe, faulty hydraulic lifter etc.......

good luck. let us know what you find.

Thanks for the replies, we will check these out. The second mechanic thought about valve guides, and said that the ones he has seen leave a mess in the rockerbox like you describe. I will pass this on to the AP.

Any body else have any ideas?

I've seen an old C-85 with valve springs so weak the motor just wouldn't turn up RPM, it would shake and spit and sputter and give all of the classic weak ignition symptoms. Those springs may have been 60 years old.

So, here we are a month later, and no real progress. We sent the prop in, the prop shop said it was tracking wrong and out of balance. After putting the prop back on, the test flight didn't show any progress.

One thing I noticed was that the plane seems to work well cold, and the vibration seems to get worse with the increase of temp in the engine.

Any thoughts??

Sounds like you have a few problems there alright.

First if you exhaust core is gone how do you get it passed 100 hr inspection?

Also you state the cam seems fine, is it or does it just seem that way?

You need to run a dial indicator on all your rocker arms.

Also on that 100 hr inspection lycoming has a bulletin on how to check your guides, and that should always been done, but many times isn't.

They also have one on how to check for induction leaks.

What condition is your airbox in?

How does each cylinder feel when you rock the piston at tdc?
Does one feel different?

I suspect your problem may be you in your mags, how long has it been since you sent them off for 500hr inspection.

If they are slicks and have more than 600 hrs your asking for trouble.

Having your mechanic check the points isnt good enough, when you send them off they get new gears and impulse springs along with new points cond, and many time coils, the coils break down when warm and at higer rpms.

Its also not uncommon for the gears to be going away and your static timing seems fine but at higher rpm its not.

Try running it on one mag through the complete rpm range and see how it responds.

Also how long has it been since your switch was serviced? I seem to remember that's a 1000hr requirement on that airplane.

Like Mister Scout, I would also suggest checking LH versus RH mag at vibration rpm's, see if one or the other makes it worse or better.

Thanks for the replies, the vibration is essentially the same on either mag, the vibration is less at a lower rpm, so it is slightly less as you switch from both to either one, but behaves the same at the same rpm.

It is a private plane, no 100 hour, and the problem was just noticed by the AP who was going to balance the prop, and has not been back to our AP since. Annual is due in November, it will be addressed then.

I am not sure what it meant when they said the cam was good, they had 3 cylinders off and inspected it. I don't think the rockers have been dial indicated. I assume that that that is done hot, with the lifters pumped up? I have only done that on a diesel, Is it about the same procedure?

The airbox is not in perfect condition, I will take a closer look at it.

What do you mean by "what does the cylinder feel like when you rock the piston at tdc"??

I will check the logs for the time on mags, I don't know off hand>


By the way, the prop was reconditioned, I forgot to specify that in my previous post.

Thanks, I am still thinking a sticking valve at RPM?????