# Mower engine gas cap leaking



## Green (Dec 24, 2017)

This is the second time, after a repair mechanic swapped out for a good cap with a good seal 9 months ago. Now, it's leaking too, as of today. The O-ring is now loose, and not sealing tight, so the gas starts leaking out while the engine is under operation. Not sure if it also affects operation, but at any rate, it's annoying and hazardous. Engine is a Briggs and Stratton 675ex (Quantum) L-head. What part should I get and where can I get it? Just the seal? The whole cap assembly?

@MasterMech


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## silvercymbal (Sep 17, 2018)

I would replace the entire cap with a factory original assembly. Look closely at the threads on the tank too. It's not common but sometimes they get messed up if someone put the cap on wrong and forces it.

In the meantime a trick you can use SHORT term is take a zip lock bag and put over the filler cap, THEN put the cap over that. This will usually keep is sealed. You should remove it when done but this should keep your gas in for now.

Good luck.


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## MasterMech (Sep 24, 2017)

O-ring? Most of those engines just have a rubber gasket in the top of the cap that makes the seal. But if it really is an o-ring seal, the fix is the same as for the gasketed caps. Replace it with a new OEM part after checking the tank and filler neck areas for damage to the sealing surface, threads, or any cracks.


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## Rockinar (Jul 21, 2017)

I had the same issue on my last mower. It was a 22" Toro. It seemed to be a design issue to me. The lastest mower (same one) has a different design and does not leak.


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## Green (Dec 24, 2017)

Thanks.

https://www.amazon.com/Briggs-Stratton-799585-Replaces-799684/dp/B00CNWW4KC

I wonder if there's a replacement with a better design.


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## kaptain_zero (Jun 2, 2018)

Older engines have a breather hole in the cap... not enough for the gas to leak out, but enough to allow air into the tank. Sealing it off completely with a plastic bag or some such would result in the engine not getting fuel after a while. Thankfully, it's hard to get a really tight seal, so it may not happen.

As far as I remember, MY 30 year old Quantum engine filler cap had a flat rubber gasket and some sort of deflector to prevent gas from sloshing out the breather hole. I'm afraid I sold my old Jacobsen this spring to fund a new Toro mower, so I can't go look.


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## Green (Dec 24, 2017)

kaptain_zero said:


> Older engines have a breather hole in the cap... not enough for the gas to leak out, but enough to allow air into the tank. Sealing it off completely with a plastic bag or some such would result in the engine not getting fuel after a while. Thankfully, it's hard to get a really tight seal, so it may not happen.


Is that bad for the engine?


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## kaptain_zero (Jun 2, 2018)

Unless it is just exactly right (not quite sealed), causing the engine to run lean and overheat, then no. If it is fully sealed, gas will stop flowing to the carburator and the engine will stop.


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## kaptain_zero (Jun 2, 2018)

Steve's Small Engine Saloon on YouTube just posted a video on what happens when a gas cap vent gets plugged or you seal the cap off. I like his videos.... he's been around the block and has the knack for explaining things to people who are NOT small engine mechanics. The title of the video is: How To Fix a Lawn Mower That Quits, Dies or Stalls After 2 Minutes, Video.

He covers a lot of common problems that can be fixed by the owner for little or no money, well worth the time it takes to watch the videos. Also, if you click on the gear symbol in the video playback screen, you can bump up the speed from normal to 1.25x, 1.5x and 2.0x speeds. It does not make voices sounds like the chipmunks, it just speeds things up. I always watch at 1.5x myself.... normal speed makes presenters sound like they are spaced out on something. I do the same thing with podcasts.... I play them at 1.8x on my phone and save almost half the time, so I can listen to more stuff.


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