Vacuum leak?

[fields_mj]

Still working on my 036. I finally got the test gauge in, and got an adapter made out of an old spark plug. Initially, the adapter had an internal leak, but I was able to drill part of it out and refill it with JB weld to seal it up. Then the adapter leaked on the OD. I found the washer on my sparkplug had a big nick in it, so I took it off an replaced it with an o-ring. After that, I did a pressure test and it held 8 psi with absolutely NO leak. Next I tried to do a vacuum leak test but no joy. I can't get it over about 0.5 bar, and in about a second or two it bleads down to around .25 bar and holds. I can see where I could have a vacuum leak without having a pressure leak, but it seems odd that the leak is so bad when it holds pressure just fine. Is it normal to have a substantial vacuum leak at anything above the smallest of vacuum, but no pressure leak at all?

It just now dawned on me mid-sentence to deadhead the vacuum pump and make sure it's not the problem, so I'll try that tonight.... Still curious if what I'm seeing is common with vacuum leaks though.

Thanks,
Mark

 

[singinwoodwackr]

Still working on my 036. I finally got the test gauge in, and got an adapter made out of an old spark plug. Initially, the adapter had an internal leak, but I was able to drill part of it out and refill it with JB weld to seal it up. Then the adapter leaked on the OD. I found the washer on my sparkplug had a big nick in it, so I took it off an replaced it with an o-ring. After that, I did a pressure test and it held 8 psi with absolutely NO leak. Next I tried to do a vacuum leak test but no joy. I can't get it over about 0.5 bar, and in about a second or two it bleads down to around .25 bar and holds. I can see where I could have a vacuum leak without having a pressure leak, but it seems odd that the leak is so bad when it holds pressure just fine. Is it normal to have a substantial vacuum leak at anything above the smallest of vacuum, but no pressure leak at all?

It just now dawned on me mid-sentence to deadhead the vacuum pump and make sure it's not the problem, so I'll try that tonight.... Still curious if what I'm seeing is common with vacuum leaks though.

Thanks,
Mark

check your hose connections as well. I often screw that up

 

[fields_mj]

Been there, done that already on the pressure test Another though was that the thin sheet rubber I'm using to block the intake and exhaust are somehow leaking under vacuum but not under pressure. Same could be said for the intake boot. Problem is that if it were any of those, I'd expect the vacuum leak to be relatively slow since it's all holding pressure. Since I can't even pump it fast enough to generate more than .5 bar (about 7psi) of vacuum, and it drops to .3 almost instantly, and then slowly drops to .25 and holds. That that doesn't sound like one of my home made seals, or a hose is leaking. It sounds like either the check valve in my new tester isn't working right, or the vacuum is pulling one on one of the gaskets in case and causing it to leak until the vacuum drops enough that the gasket pulls itself back into place.

I've been doing automated manufacturing and testing equipment for over 25 years now, and vacuum based leak testers have always been the bane of my existence. With a pressure test, its easy to find the bubbles. With a vacuum leak, it's anybody's guess as to the source.

 

[SteveSr]

Been there, done that already on the pressure test Another though was that the thin sheet rubber I'm using to block the intake and exhaust are somehow leaking under vacuum but not under pressure. Same could be said for the intake boot. Problem is that if it were any of those, I'd expect the vacuum leak to be relatively slow since it's all holding pressure. Since I can't even pump it fast enough to generate more than .5 bar (about 7psi) of vacuum, and it drops to .3 almost instantly, and then slowly drops to .25 and holds. That that doesn't sound like one of my home made seals, or a hose is leaking. It sounds like either the check valve in my new tester isn't working right, or the vacuum is pulling one on one of the gaskets in case and causing it to leak until the vacuum drops enough that the gasket pulls itself back into place.

I've been doing automated manufacturing and testing equipment for over 25 years now, and vacuum based leak testers have always been the bane of my existence. With a pressure test, its easy to find the bubbles. With a vacuum leak, it's anybody's guess as to the source.

Did you change the crank seals? Did you use OEM? If you answered "No" to either of these that would be a good place to start looking. Another possibility is that a spring popped off a seal during installation. You can put some mix around the shaft/seal and see if it gets sucked into the crankcase.

 

[buzz sawyer]

If you are applying vacuum through the spark plug opening, it will be drawing the piston up in the cylinder at first so you won't get a reading at first. Make sure piston is at TDC when you start pumping. btw, I use a rubber gasket at the plug adaptor but the O ring is a good idea, I will try it next time I do testing.

 

[L34]

+1 on seals.
will hold pressure but lift off crank under vacuum.

 

[J D]

The way crank seals work pressure will help them seal. It is a good idea to do your vacuum test first to avoid getting a "better than actual" result

 

[fields_mj]

I don't have a way to split the case. I don't mind getting them replaced if it's needed, but the saw has to leave my hands for that to happen, and that's what I'm trying to avoid. It's less of a cost issue and more of an issue of being at the mercy of someone else. With such a gross vacuum leak, I just want to make sure it's the saw and not the test setup.

 

[lone wolf]

+1 on seals.
will hold pressure but lift off crank under vacuum.

Plus 1 on that! Grease the crank seals heavily at the inner lips that the crank rides on and re test this will prove where the leak is.

 

[SteveSr]

I don't have a way to split the case. I don't mind getting them replaced if it's needed, but the saw has to leave my hands for that to happen, and that's what I'm trying to avoid. It's less of a cost issue and more of an issue of being at the mercy of someone else. With such a gross vacuum leak, I just want to make sure it's the saw and not the test setup.

Congratulations! You just found the root cause of your scored piston.

Seals can be changed without splitting the case. Stihl makes a special tool to do this to make it easier but it is not required if you are clever and careful. There are plenty of threads on here about seal replacement.

The main things to watch is to not scratch the crank which will ruin a new seal in a heartbeat or booger the bore in the crankcase. You need to "loosen" the seal before trying to pull it by driving them in slightly.

When installing the replacements either use the Stihl seal drivers or use a suitable socket but be sure to drive them in to the same depth as the originals. Measure first! It is also recommended to put a light coat of Dirko on the outer circumference of the new seal to guard against any leaks between the crankcase and the new seal.

 

[fossil]

If the saw has a decomp, you may want to check that as well.

 

[stihltech]

Did you change the crank seals? Did you use OEM? If you answered "No" to either of these that would be a good place to start looking. Another possibility is that a spring popped off a seal during installation. You can put some mix around the shaft/seal and see if it gets sucked into the crankcase.

My bet is the seal under the flywheel. I replace those 6 to 1 of any other seal. I am not counting 051 and 041. Different story on those.
Like said above always vacuum first.

 

[Gord404]

when I install seals and it has to go over a rize I use a wrap pop can about 1/16 smaller than the crank and about an inch and a half long- sand all the edges with red scotch bright so there is no sharpness get the seal about half way down the wrap and put and lube the wrap put your sealant on the outside of the seal put the whole affair on the crank over the rize and tap the seal into place with a piece of tubing a bit smaller tha the od of the seal and tap it into place and pull the wrap off- no need if if there is no rize- in the crank- they do make a set of small thin plastic tubing that does the same thing but yet to find them.

 

[Cantdog]

My bet is the seal under the flywheel. I replace those 6 to 1 of any other seal. I am not counting 051 and 041. Different story on those.
Like said above always vacuum first.

Yep I second that!! That is a pretty small seal and hardens up and leaks like a sieve. I believe your 6 to 1 replacement ratio is totally correct. O36 it a darn nice saw over all.......not sure why Stihl thought that seal was the ticket.

 

[fields_mj]

Yep I second that!! That is a pretty small seal and hardens up and leaks like a sieve. I believe your 6 to 1 replacement ratio is totally correct. O36 it a darn nice saw over all.......not sure why Stihl thought that seal was the ticket.

It's 25 years old so I'm not going to complain about the how long the seal did or didn't last.

If the lower end of the saw needs work, I'm leaning towards replacing the bearings and all of the seals/gaskets. I haven't priced all of the parts yet, but I know the bearings are about $20 per side, so I'm guessing all of the parts together cost noticeably less than what I gave for the new OEM piston.

I'm also prone to pay someone who knows what they are doing to perform the work. If I'm getting this deep into the saw, I want it to work when I put it back together. I need to go down this path on my 064 as well, so might as well get them both done at the same time.

 

[SteveSr]

when I install seals and it has to go over a rize I use a wrap pop can about 1/16 smaller than the crank and about an inch and a half long- sand all the edges with red scotch bright so there is no sharpness get the seal about half way down the wrap and put and lube the wrap put your sealant on the outside of the seal put the whole affair on the crank over the rize and tap the seal into place with a piece of tubing a bit smaller tha the od of the seal and tap it into place and pull the wrap off- no need if if there is no rize- in the crank- they do make a set of small thin plastic tubing that does the same thing but yet to find them.

The couple of times that I have had to do this I just use Scotch tape or packing tape. I don't like the idea of sharp metal around soft rubber.

 

[Gord404]

The couple of times that I have had to do this I just use Scotch tape or packing tape. I don't like the idea of sharp metal around soft rubber.

each to there own but I do take the time to dull all surfaces and use lube on the metal but like I said everyone ha their own way and have done several hunred saws this way- imo

 

[fields_mj]

Well, I played with my tester last night and I'm pretty sure it has some issues. I used a different piece of hose and just put my thumb over the end. I could only get the tester to pull 0.7 bar. I hooked it back up to the saw and was able to pull and hold 0.5 bar. With the larger volume of air (in the saw compared to just the hose), I couldn't get it to go above 0.5 bar, but it held just fine.

So, then next question is what kind of vacuum do I NEED to pull to make sure I'm sufficiently challenging/testing everything? If I remember correctly, the videos I've watch are all trying to pull around 14 psi which is frustrating because vacuum doesn't get measured in PSI. With pressure, 14psi is equal to about 1 bar so I'm assuming that I need to draw the saw down to 1 bar to test it, but I'd like some confirmation on that.

 

[SteveSr]

Well, I played with my tester last night and I'm pretty sure it has some issues. I used a different piece of hose and just put my thumb over the end. I could only get the tester to pull 0.7 bar. I hooked it back up to the saw and was able to pull and hold 0.5 bar. With the larger volume of air (in the saw compared to just the hose), I couldn't get it to go above 0.5 bar, but it held just fine.

What kind of tester is it?

So, then next question is what kind of vacuum do I NEED to pull to make sure I'm sufficiently challenging/testing everything? If I remember correctly, the videos I've watch are all trying to pull around 14 psi which is frustrating because vacuum doesn't get measured in PSI. With pressure, 14psi is equal to about 1 bar so I'm assuming that I need to draw the saw down to 1 bar to test it, but I'd like some confirmation on that.

Stihl says 0.5bar or 7psi vacuum. If it continues to loose vacuum over time it is also no good.

 

[fossil]

From the 036 repair manual.

Oil seals tend to fail when subjected
to a vacuum, i.e. the sealing lip lifts
away from the crankshaft during the
piston’s induction stroke because
there is no internal counterpressure.
An additional test can be carried out
with vacuum pump 0000 850 3501
to detect this kind of fault.

Operate lever (2) until pressure
gauge (3) indicates a vacuum of
0.5 bar.

Note:
If the vacuum reading remains
constant, or rises to no more than
0.3 bar within 20 seconds, it can be
assumed that the oil seals are in
good condition. However, if the
pressure continues to rise (reduced
vacuum in the crankcase), the oil
seals must be replaced, even if no
leaks were detected in the pressure
test.
– After finishing the test,
reassemble all parts in the
reverse sequence.