Fuel tank vent suggestion

Bretto · Post by **Bretto** » Tue Aug 07, 2012 2:21 pm

I dont have a stock fording system or crossover tube to hook the vent line to. I am currently set up using a 2A aircleaner and crossover tube. Would it be ok to run the tank vent line directly to the intake manifold? Or any better idea?
Thanks,
Brett

jimm · Post by **jimm** » Tue Aug 07, 2012 3:29 pm

Not to the intake manifold - that is after the throttle plate and can see full idle vacuum levels. You want to vent it upstream of the throttle plate. Original was vented into the air cleaner. You could braze a fitting onto the 2A air cleaner, I presume.

Bretto · Post by **Bretto** » Wed Aug 08, 2012 10:33 am

Prev owner had it piped to the manny, I dont know how long it had been that way or the ill effects. Was the 3A setup with a tank vent or did they just use the cap to vent? Maybe I should get a vented cap and run that way?

evanso1975 · Post by **evanso1975** » Wed Aug 08, 2012 2:16 pm

As far as I know, for the CJ3A the filter was not the in-tank type like the M38; the only hole in the top of the tank was for the sender. I believe it vented through the fuel pump, but other folks in the know can confirm if this is the case. A couple of suggestions:

1) use the later M-series vented tank cap, which allows you to switch between fording (closed) and non-fording (open). Obviously, you'd want to keep it open. Those caps show up often as NOS on Fleabay for very little $$. You can then just plug the fuel tank vent line.

2) fit a breather (similar to you have on the axle diff covers) to your filter/fuel pickup unit, and do away with the vent line. You can replace it at a later date, if you want to do the stock M38 setup.

Owen.

Post by **wesk** » Wed Aug 08, 2012 8:42 pm

Just do the same thing with your tank vent port that the M38A1 did when they discontinued the fording plumbing.

Bretto · Post by **Bretto** » Wed Aug 08, 2012 9:26 pm

Sounds promising Wes, what did they do with it? Sorry I don't know.
Thanks,,
Brett

evanso1975 · Post by **evanso1975** » Thu Aug 09, 2012 12:08 am

The picture on this post will help:
http://www.willysmjeeps.com/v2/modules. ... pic&t=6261

Post by **wesk** » Fri Aug 10, 2012 2:19 am

jimm · Post by **jimm** » Fri Aug 10, 2012 5:33 pm

Bretto wrote:Prev owner had it piped to the manny, I dont know how long it had been that way or the ill effects. Was the 3A setup with a tank vent or did they just use the cap to vent? Maybe I should get a vented cap and run that way?

At idle manifold vacuum, I'm surprised it didn't collapse the tank! Perhaps a testament to the strength of an M38 fuel tank.

On mine, a PO had cut and crimped off the vent line. I noticed it, but not knowing anything about the fording ventilation system in the pre-internet-information era 28 yrs ago, had no idea what it was for. I had driven it quite a bit, including on some long trips, without any problems until my wife and I were headed south from Moscow, ID to California and we hit the first big hill out of Lewiston on a hot summer afternoon. The engine stumbled like we were out of gas, and I could hear the electric fuel pump running full speed. I figured out the fuel was vaporizing in the fuel pump from the vacuum and heat. I rigged up a fuel pump cooling system with a funnel in front of the passenger feeding a length of vinyl tubing to a rag on top the pump. Had all that in my emergency kit. My shotgun rider's job for the rest of the trip was to dribble a little water from a gallon jug into the funnel whenever we hit a rough spot, which, traveling across the Great Basin desert and the eastern Sierra, was quite often. My wife still comments on her role on that trip. Now I know it was due to the fuel tank not being properly vented and that poor fuel pump was working like mad to overcome the vacuum it was pulling on the tank.

Once again, thanks to Wes and Ryan for this site, and to Bob Westerman for his M38 venting diagrams. I feel like I'm living in the Age of Enlightenment.

Bretto · Post by **Bretto** » Sat Aug 11, 2012 6:33 am

Months ago when I got the tank back with still lots to do under the hood, I ran the vent line pigtail unterminated to the engine bay. I just figured I'd leave it there till I got back to it. Perhaps I could just nail it up high in the firewall like you would an axle vent?
Is the vent itself on the tank a one way vent? Is it intended to let air in to fill the void from the fuel going out or is it to let the build up of pressure out? I have read of fuel being pushed back up thru the vent line and getting into the air cleaner so it obviously allows pressure out. On the other hand though, air needs to come in somehow.

Post by **wesk** » Sat Aug 11, 2012 7:23 am

Yes, it is a two way valve.

vuldub · Post by **vuldub** » Wed Apr 12, 2017 12:38 pm

wesk wrote:

Sorry to dredge up an old post, but I would like to know more about this "vent valve". There's no part number. How does it work in conjunction with the valve built into the fuel pickup assembly, downstream of the vent outlet?

I would like to vent my m38 but don't have the original powerplant and my V6 has no carbon cannister. Can I simply plug the vent and use a vented gas cap?

4x4M38 · Post by **4x4M38** » Wed Apr 12, 2017 1:26 pm

Yes.

Something wrong with your mouse? I deleted the two other duplicate posts!

Post by **wesk** » Wed Apr 12, 2017 10:18 pm

Sorry to dredge up an old post, but I would like to know more about this "vent valve". There's no part number. How does it work in conjunction with the valve built into the fuel pickup assembly, downstream of the vent outlet?I would like to vent my m38 but don't have the original powerplant and my V6 has no carbon cannister. Can I simply plug the vent and use a vented gas cap?

The adapter PN 808998 shown used on the later M38A1 tank is a filter not a valve. The valve just below it functions independently of the little filter the same as it would on the earlier A1's with the vent tube connected there going up to the main oil bath air filter.

I would not cap it off. If something went bad on your fuel cap you would force the carb to run lean and eventually quit on you out on the road. If it were mine I would just run a vent line from there to the air cleaner. The tubing is 44-T-5319-55 (3/16 steel .028 wall) and the fitting nuts are 137396, Nut, tubing, inverted flare 3/16" tube.

https://www.wbparts.com/rfq/4730-00-288-9385.html

4x4M38 · Post by **4x4M38** » Thu Apr 13, 2017 5:15 am

Long night, too much coffee Wes!