Stihl ms181 pressure in tank and vent

[pozidriv]

Hi all!

So while fixing another problem on this chainsaw I came across another one. The chainsaw runs fine for a while and then very suddenly stops.
I found out that there's pressure in the fuel tank, but when I release it by unscrewing the fuel cap, the pressure remains in the fuel line.

When I pull the carb away from the fuel line, fuel squirts out, even though the fuel cap has just been released??
Also, when I remove the tank vent line, the same happens, fuel gushes out.

So I'm a bit puzzled where this pressure is hiding itself How can the tank be pressurized when the cap is off?
Also how did the pressure get there in the first place?
When I release the pressure from the fuel line, the saw starts and runs fine, until... the same thing happens, very sudden stop.

Thanks for thinking along!

 

[hotshot]

Gasoline has vapor pressure that increases when you shake the fuel tank or add heat. Pressure builds in the tank & remains there due to the duckbill vent valve (#0747 313 6810) being a one way valve in.

The tank vent allows air in, but not out, so it is working correctly. The tank cannot be pressurized when the fill cap is off, you might be seeing the metering needle valve trap some pressure on the fuel pump side.

You’ve got another issue causing the saw to stall out, unless the fuel filter or fuel tank vent is fully plugged up. Use your Mityvac tester to check that.

Check for a vacuum being pulled in the fuel tank & also check for loss of spark when it “suddenly stops”.

 

[pozidriv]

Thanks for the reply!
I had already checked the vent valve with the vacuum pump, it's not a duckbill, but this type:

It lets air in one way but not the other, so seems fully functional.
However I did have a good amount of pressure in the tube leading up to this valve, even after opening the fuel cap. That's what's so confusing to me.
I also thought about heat killing the spark, but it still sparked, at least after taking the spark plug out (which might not be the best indicator? e.g. spark working in atmospheric pressure but might not work in cylinder?)

Will spend some more time with it tonight. (I'm 8 hours away from you

 

[hotshot]

OK, I was looking at a MS180 IPL.

Stihl’s changed to the 0000 350 5802 vent on the MS181 & it now has an integral fuel & oil tanks.

 

[lone wolf]

Thanks for the reply!
I had already checked the vent valve with the vacuum pump, it's not a duckbill, but this type:

It lets air in one way but not the other, so seems fully functional.
However I did have a good amount of pressure in the tube leading up to this valve, even after opening the fuel cap. That's what's so confusing to me.
I also thought about heat killing the spark, but it still sparked, at least after taking the spark plug out (which might not be the best indicator? e.g. spark working in atmospheric pressure but might not work in cylinder?)

Will spend some more time with it tonight. (I'm 8 hours away from you

There may also be a tiny brass valve in the fuel line believe it or not im not sure on that model but likely.

 

[pozidriv]

hmm, in the actual fuel line, or in the vent line going up to that valve?
What's it's function, to provide a restriction? In any case the vent system works, I can pull air through the line.

If there's another one way check valve between the fuel tank and the actual fuel outlet at the carb side, that could explain why I'm still seeing pressure at the carb, with the fuel cap removed..
I might just take it apart a bit further until I can get to the fuel line.

 

[lone wolf]

hmm, in the actual fuel line, or in the vent line going up to that valve?
What's it's function, to provide a restriction? In any case the vent system works, I can pull air through the line.

If there's another one way check valve between the fuel tank and the actual fuel outlet at the carb side, that could explain why I'm still seeing pressure at the carb, with the fuel cap removed..
I might just take it apart a bit further until I can get to the fuel line.

Actual fuel line might have one take it out of the saw and look.

 

[lone wolf]

OP is the temp there when operataing 85 F or so? Because it could be winter gas acting up. Google winter gas.

 

[hotshot]

There may also be a tiny brass valve in the fuel line believe it or not im not sure on that model but likely.

 

[lone wolf]

There is in the 200T's for sure.

 

[rd35]

2 questions:
1) when you say you are removing the fuel cap....are you putting it back on before you start pulling the carb/lines off?
2) If you are removing the cap completely is there still fuel in the tank?

I ask because the last time someone had this problem and asked me about it they were loosening the cap to relieve the pressure and then tightening it back up so fuel wouldn't spill. The fuel builds pressure in the tank immediately when the cap is tightened. The only way to avoid a spill is to dump the fuel from the tank. It will also run out if the cap is off and the saw is on its side if the fuel line is lower than the fuel level in the tank. Just asking to make sure it;s not one of these situations.

 

[pozidriv]

Hey Rd!
You're right, I am screwing it back on, before pulling off the lines, to avoid a massive spill. I had no idea that the pressure could build so quickly and greatly!
There is still fuel in the tank.. That really surprises me, can't be more than 20 seconds between tightening the cap and pulling off the fuel or vent line.

But that implies that the problem of stalling has nothing to do with a pressurized tank? (like hotshot and lone wolf suggested) Strange, because already two times it started working after releasing the carb from the fuel line.
I hope I get some time tonight or tomorrow, I will keep you guys updated.
Thanks for thinking along!!

 

[lone wolf]

Hey Rd!
You're right, I am screwing it back on, before pulling off the lines, to avoid a massive spill. I had no idea that the pressure could build so quickly and greatly!
There is still fuel in the tank.. That really surprises me, can't be more than 20 seconds between tightening the cap and pulling off the fuel or vent line.

But that implies that the problem of stalling has nothing to do with a pressurized tank? (like hotshot and lone wolf suggested) Strange, because already two times it started working after releasing the carb from the fuel line.
I hope I get some time tonight or tomorrow, I will keep you guys updated.
Thanks for thinking along!!

Does your country sell winter gas? Google winter gas.

 

[pozidriv]

OP is the temp there when operataing 85 F or so? Because it could be winter gas acting up. Google winter gas.

Not that hot, more around 23°C (74 F)
I don't really think we get winter gas in western europe, it's just aspen two stroke mix

 

[pozidriv]

Hey!

I did some more poking. Removed the entire fuel line, but didn't find any brass insert. Everything can be squished (except the filter ofcourse):

rd35 was completely right about the fuel pressure building up very quickly: I made a small video about it, you see me tightening the cap and within seconds the fuel rises in the line:
https://www.dropbox.com/s/woitp9on0icl3qo/20200610_155628.mp4?dl=0
So I'm probably barking up the wrong tree here that this pressure is causing the problem.

 

[pozidriv]

I'm afraid we can put this one to bed...
Couldn't get it to start with brake fluid in the carb, nor with a known good spark module, so I removed the spark arrestor and to my horror:

It bubbles:
https://www.dropbox.com/s/rqvdfa9ox8zkmex/20200610_175314.mp4?dl=0
Contacted the owner, asked him what fuel he ran it with:
"same as my lawnmower"... I'm surprised it held out as long as it did.

He seems to want to repair it. Is replacing cylinder and piston sufficient or might there be damage to big end crank bearings etc?