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willysmjeeps.com :: View topic - M38 Fuel Tank Vent
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M38 Fuel Tank Vent

 
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Mike_B
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Joined: Dec 10, 2017
Posts: 259
Location: Southern Maryland

PostPosted: Tue Nov 10, 2020 7:52 pm    Post subject: M38 Fuel Tank Vent Reply with quote

I'm working on my M38 as a driver at this time and want to put a filter or something on the gas tank vent (I'm not plumbing it back to the air filter). What's the best way to keep it simple and keep the bugs out?

Thanks,

Mike B Smile
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Mike B
1953 M38A1 Brush Truck
1952 M38
1951 M100 Trailer
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RonD2
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Joined: Oct 02, 2014
Posts: 1112
Location: South Carolina, Dorchester County

PostPosted: Tue Nov 10, 2020 8:29 pm    Post subject: Reply with quote

Hey Mike,
As I understand the M38 vent system, the gas tank vent line that runs into the air cleaner is for underwater fording purposes when the fording valves are closed and the system is sealed.

If you have no intention of fording anytime soon, I think you can safely not install the line to the air filter and just tape off the connection on top of the tank (or screw a pipe cap over the fitting) to keep crud and bugs out.

This assumes that you have the proper vented gas tank cap, and that the valve on the inside of the cap is in the open position so the tank can breath. The tank needs air to breath so the pump can pull fuel. Non-vented gas cap will stop the flow.

At least that's how I understand things. Wes will straighten me out if I'm mistaken....good luck!
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Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

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wesk
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Joined: Apr 04, 2005
Posts: 15472
Location: Wisconsin

PostPosted: Tue Nov 10, 2020 8:41 pm    Post subject: Reply with quote

Mike,

It is smarter to retain that vent in the event the cap is left in the wrong position you won't have any annoying issues. The best course of action if you do not wish to run the vent line to the air filter is to do exactly what Willys did when they decided not to run the vent line to the air filter on the 53 & up M38A1's.


This is the later M38A1 gas tank.

Just download the M38A1 ORD 9 and look up the part number.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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RonD2
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PostPosted: Tue Nov 10, 2020 8:51 pm    Post subject: Reply with quote

Hi Wes,
Awesome.....that's what the crowd came to see! Thanks! A little bit more towards my education....always a good thing.

Do you know if that filtered vent plug is a common item that can be bought today, or is it one of those rare items that can't be had at any price? I never saw one before.

Possible to fabricate one I suppose?
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Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

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wesk
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Joined: Apr 04, 2005
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Location: Wisconsin

PostPosted: Tue Nov 10, 2020 11:07 pm    Post subject: Reply with quote

Quote:
Do you know if that filtered vent plug is a common item that can be bought today, or is it one of those rare items that can't be had at any price? I never saw one before.


They are not easy to find. I will assume you did not follow my advice and research the availability of the part number like I advised Mike to! No matter which model jeep you own it is always a good idea to get familiar with similar models and their manuals.

I would bet there are dozens of similar purposed vent caps on the market for other industrial equipment that can be easily substituted for the original.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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wesk
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Joined: Apr 04, 2005
Posts: 15472
Location: Wisconsin

PostPosted: Tue Nov 10, 2020 11:11 pm    Post subject: Reply with quote

Quote:
Do you know if that filtered vent plug is a common item that can be bought today, or is it one of those rare items that can't be had at any price? I never saw one before.


They are not easy to find. I will assume you did not follow my advice and research the availability of the part number like I advised Mike to! No matter which model jeep you own it is always a good idea to get familiar with similar models and their manuals.

I would bet there are dozens of similar purposed vent caps on the market for other industrial equipment that can be easily substituted for the original.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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Mike_B
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Joined: Dec 10, 2017
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Location: Southern Maryland

PostPosted: Wed Nov 11, 2020 7:18 pm    Post subject: Reply with quote

Thanks for all of the reply's...the vent Wes has shown is what I'm looking for.

I do have a follow up question...it seems to me that the vent assembly riveted to the top of the tank fuel pickup assembly works as a one-way check valve. You can suck air out but can't blow it in, so how does that work to "vent" the tank??? I'm confused...seems like it will only pull air out of the tank and suck it down the carb.

Mike B Smile
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1953 M38A1 Brush Truck
1952 M38
1951 M100 Trailer
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RonD2
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Joined: Oct 02, 2014
Posts: 1112
Location: South Carolina, Dorchester County

PostPosted: Wed Nov 11, 2020 8:41 pm    Post subject: Reply with quote

Maybe something like this?

https://www.tanksinc.com/index.cfm?ptype=product&product_id=98&category_id=-1&home_id=-1&mode=prod
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Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

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Mike_B
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Joined: Dec 10, 2017
Posts: 259
Location: Southern Maryland

PostPosted: Wed Nov 11, 2020 9:17 pm    Post subject: Reply with quote

Thanks Ron,

I know about those as I have one one my '56 Chevy Pickup .

Might be simpler and cheaper to just run a piece of copper tubing back to the air cleaner...lol

My biggest problem is I didn't take the original tubing out, so I really don't know exactly how to route it. I need to find someone with a M38 that's close by that I can go look at.

Mike B Smile
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1953 M38A1 Brush Truck
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1951 M100 Trailer
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RonD2
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Posts: 1112
Location: South Carolina, Dorchester County

PostPosted: Wed Nov 11, 2020 9:38 pm    Post subject: Reply with quote

Hey Mike,
The route for that vent line from the tank to the air filter is well shown in several posts in this forum, the manuals, and Wes' album --- plenty of photos. It's tricky getting it through a couple of the correct holes going up to and through the correct hole in the firewall, but patience will do it. If you need more photos let me know.

Here's a good one to start: http://willysmjeeps.com/v2/modules.php?name=Forums&file=viewtopic&t=12593&highlight=fuel+vent
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Ron D.
1951 M38 Unknown Serial Number
1951 M100 Dunbar Kapple 01169903 dod 5-51

“The only good sports car that America ever made was the Jeep."
--- Enzo Ferrari

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wesk
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Joined: Apr 04, 2005
Posts: 15472
Location: Wisconsin

PostPosted: Wed Nov 11, 2020 11:03 pm    Post subject: Reply with quote

Quote:
I do have a follow up question...it seems to me that the vent assembly riveted to the top of the tank fuel pickup assembly works as a one-way check valve. You can suck air out but can't blow it in, so how does that work to "vent" the tank??? I'm confused...seems like it will only pull air out of the tank and suck it down the carb.


You must have mis-read the system description. It allows air IN As fuel is removed from the tank to prevent a disruption of the fuel supply to the carb. The only purpose of routing it to the air cleaner was to prevent water from entering the tank during fording operations since the air cleaner uses a snorkel extension during fording ops. In early M38's the fuel cap was unvented so this was the only way to avoid fuel starvation. Later vented caps with built in open/close valve assisted this tank vent but if the cap was inadvertently left closed after a fording op the tank's integral vent would continue to prevent fuel starvation to the carb.

Intake air flow to the carb from the air cleaner does not have the ability to actually suck anything from the tank's vent line. The maximum suction of the carb occurs at the venturi inside the carb.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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Mike_B
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Location: Southern Maryland

PostPosted: Thu Nov 12, 2020 1:39 pm    Post subject: Reply with quote

Wes,

I understand the tank venting concept completely...fuel flow out must have air flow in...I'll revisit everything again.

OK, I just went out and looked at everything again and found that the Nichol sized metal wafer valve seat that rides against the rubber diaphragm was stuck to it and not allowing it to open to the tank. That's why I thought it was a one-way valve when it is in fact a two way valve. Mystery solved!

Thanks for your help!

Mike B Smile
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1951 M100 Trailer
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