Fuel Vapor System

I know this issue has been discussed a lot. After reading many posts on this site, I think my situation is unique.

Keep in mind, nothing is hooked up right now. I'm putting everything back together finishing a frame off rebuild.

I have a 1972 CJ5 with the 232 I6. My TSM shows the expansion tank off and four-vent fuel tank in the rear and no charcoal canister under the hood. But my Jeep does not have the expansion tank and has a two-vent tank. 1972 was a transition year and often contains some elements of different years. I think that's what is going on here.

I'm trying to figure out what to do to hook everything up correctly so I don't smell like a rolling gas station.

If you look at the pictures: My actual Jeep fuel tank looks like the first picture. My TSM shows the second picture.

The lines under the hood were set up like the third picture (with the PCV valve going to the manifold).

But the fourth picture seems more likely to work with the liquid check valve and the rollover valve and two vents.

What I'm thinking of doing is adding a two-port charcoal canister, hooking it up like the fourth picture with the charcoal canister purge line going to the small third hole toward the back of the valve cover.

Does this make sense?

Also, I don't see any vendors selling what is called the "limit fill valve." If the point if just to stay closed until a vacuum pulls, can I just rig up a second PCV valve and set it high on the firewall so liquid doesn't flow into the valve cover?

Sorry for the long post. I'm really curious for y'all input.

 

You are on the right track. What you have is puzzling.

I assume your Jeep actually has a J2 VIN and the VIN characters and the engine build date match the presumed build date of the Jeep. The pre-73 rear-mount fuel tank for these Jeeps does indeed have 4 separate vent lines, one from each top corner. AFAIK there is no exact replacement for this tank. The later tank has two vent lines, and I believe the only difference is the plumbing for the additional two vents is moved inside the tank. If you had the expansion tank, you could simply eliminate two of the hoses and plug those taps in the expansion tank.

How much of the plumbing shown in the first picture is actually there? AFAIK the roll-over check valve was not present until 1978. Jeep started using the liquid check valve instead of the expansion tank in 1973.

PCV is mostly separate from vapor recovery. In 1972 (and perhaps in '70-71) the arrangement with the limit check valve was used. It appears this setup vented fuel vapors from the fuel tank directly into the crankcase. The PCV is only involved insofar as it purges crankcase vapors via the PCV valve and engine vacuum. The connection to the air cleaner is a supply of clean air to the crankcase, keeping the crankcase close to neutral pressure even though the PCV valve is constantly sucking any crankcase vapors.

Suggest you can't use the original configuration unless you have the expansion tank and a working limit fill valve. Finding a working limit fill valve will be difficult to impossible.

In '73, this strategy was replaced by the charcoal canister. The vapors that passed the liquid check valve were vented to the atmosphere through the charcoal bed in the canister, located in the engine compartment. This charcoal bed has a weak affinity for the fuel vapors in the vented air, so it scavenges them while allowing the majority of the vented air free passage to the atmosphere. Again, this keeps the fuel tank at neutral pressure, without venting a bunch of fuel vapor to the atmosphere.

These charcoal canisters have a purge cycle that clears the scavenged vapors from the charcoal. When the engine is running, vacuum is applied to the canister and clean air drawn through. The clean air has more of an affinity for the vapors than the saturated carbon, so the captured gasoline exits the charcoal and is drawn into (in 1973) the air cleaner to be burned. The 1973 uses a passive system, where a tube at the air cleaner snout makes a small vacuum by the Bernoulli effect. This in turn applies a small vacuum to the top side of the charcoal canister and draws out the vapors. Later systems are more active, and have internal valves and multiple connections to engine vacuum.

 

What would you like to do here? If this were my Jeep, I would skip forward a few years and fit an actively purged canister. You'd need to find a working canister, either in the wrecking yard or as a new part. I replaced the canister in my J20 with an XJ canister, but I think that's NLA. I'd look ca '70-80 GM products, with vapor recovery but before TBI/EFI. A suitable canister will have a purge connection (to manfold vacuum), a purge signal connection (to ported vacuum) and one or more connections for vapor input. You only need one vapor input for the gas tank, unless you want to capture vapors from the float bowl. These canisters have a valve inside them that controls air flow.

ALSO - looking at some GM canisters, there are some that have an external purge valve, and some with a purge valve on top. The purge valve is plastic and looks something like a pork pie hat, maybe 1.5-2" diam.

 

"I assume your Jeep actually has a J2 VIN and the VIN characters and the engine build date match the presumed build date of the Jeep."

- Yes. J2 code stamped into the frame and engine is original.

"How much of the plumbing shown in the first picture is actually there?"

- The liquid check valve is there. There was no rollover valve. I assumed it was just missing, but if they didn't use it until '78 that make sense.

"I would skip forward a few years and fit an actively purged canister."

- I like this idea and am very glad I asked. I didn't want to drop money on a two-port canister only to find out I wanted a different one. Let me absorb what you have written here when I have more time and I'll get back to everyone with my decision and -- I'm sure -- more questions.

Attached are pictures of what is left of my liquid check valve and the new one I fashioned based on the one people are selling on Ebay. No offense to anyone who works to make a buck, but I couldn't justify spending $60 to $80 on something I could cobble together with $20 in parts from the hardware store. BTW, I ultimately went with copper. PVC gets eaten by fuel and I couldn't find a funnel shaped coupler in rubber (or neoprene, nitril, etc.). After some research, it seems some people get worried about fuel causing a gumming of auto fuel, but several other have debunked that. Any opinions are welcome.

 

Very few of those original check valves are still around. Seems like the plastic crumbles to bits after 50 years of use. I was surprised that the one on my Jeep is intact.

I like your copper body for the check valve. This is the valve that goes in the plastic gas tanks? Seems that's what MTS is selling. They made it work by making the rubbery body to hold it - this is where most of the added value comes from. The rest is just parts selection.

 

This is what I did for my J20:



The original canister was round and fit in that location. At some point it had fallen against the engine and the exhaust manifold melted a big hole in it. The original canister is NLA, so this is an XJ canister. I made the bracket to support it, and plumbed it like the original canister.

 

"This is the valve that goes in the plastic gas tanks?"

I don't know that it is specifically for the plastic tanks. I'm using it with steel. I figure it just provides enough space for vapors and some splashing fuel to congregate before going out the top or funneling back into the tank. I'm going to attach it with a bracket (same one they are selling with the MTS valve unit) to the small panel under the body panel near the driver's wheel well, where the old check valve was located. That's a roll-over valve on top, so I've got that covered in case of an "Oh SH....!"

Thanks for the pic of the canister. If you have a diagram or can say which lines go where it would save me some time. I plan to look up some diagrams though.

Tim, you always point out the benefits of the original air cleaner and the light vacuum produced via the Bernoulli effect through the snout. I don't have the original air cleaner. If I hook up a line to the hole in the bottom side of a round FLAPS air cleaner, will that not produce enough of a pull (or too much inlet area possibly to produce an effective pull)? I can always source an original but I've got a Motorcraft 2100 with the wider top, so I may have to poke around a bit. Also, the after market round ones do have an aesthetic that I have to admit I prefer. Aesthetics, of course, are secondary if I smell like gas or have a faulty vapor system.

 

Also, where you say: "A suitable canister will have a purge connection (to manfold vacuum), a purge signal connection (to ported vacuum) and one or more connections for vapor input."

I see GM vapor canisters with three ports. One is listed as "purge," one as "tank," and one as "air."

Purge and tank make sense. The third, you described as "to ported vacuum." What is the difference between "ported vacuum" "manifold vacuum"? Is this the one you say should go to the snout of the air cleaner? If so, my previous question about after-market air cleaners applies.

Thanks for all your expertise on this. It is invaluable.

 

These are all from the factory Jeep CJ parts manual, 1973 and prior.. Not saying it's right or wrong. What we had to work with at the time.

1972 CJ



1973 CJ




1973 CJ 232 canister and PCV routing. Note the trans controlled spark switch in-line with distributor vac advance.

 

Very helpful jpflat2a!

Looks like all the new canisters I see are in the $100 range. Probably means a trip to the junkyard (or a good excuse to go to the junkyard). It's always funny to me how fast a $20 project can become a $200 project. Just Empty Every Pocket....

 

So if I buy the canister in the below picture, it looks the same but the markings as to what goes where are different. The second, smaller picture is from a 70s Jeep TSM

My gut tells me to set it up as marked on this unit and cap the purge inlet. Seems this canister just has a different style purge valve.

 

Looks good if it's not broken. This is an "active" canister (my description) that has a valve in the top. There is a rubber diaphragm in the valve that can fail. Some have opened the valve and repaired the diaphragm - no personal experience with that. The small port on top of the valve is definitely the purge signal (ported vacuum). I would cap the port labeled "bowl" and connect the tank line to "tank" and tee in the "purge" line to the PCV line between the carburetor base and the PCV valve.

I would also test that the purge line holds vacuum when there is no vacuum on the purge signal. I would use a MityVac for this, maybe with the purge line plumbed to the engine?

[Later Note] I'm not sure this will work. Certainly the purge signal should hold a vacuum always. However, according to the '79 TSM, this type of canister has a two-stage purge, where it purges slowly at idle and faster when the valve opens. Not sure how you'd measure that. Anyway, I believe you can test the diaphragm in the valve by pulling vacuum on the purge signal line and watching for a leak.

Looking at the drawings that Jim posted, I would say that the drawing that says S6 (83/86) is wrong for your vehicle. As confirmed by my '74-80 parts book, Jeep used a 2-port canister for '74 and early '75. There is no reason to believe that the later canister, as shown in Jim's drawing, was used in '73, much less '72. My personal experience owning a factory-new '73 says it had the 2-port canister, conforming to my '74-80 book.

According to a review of the TSMs I have, the passive system (with no valve) was in use as late as 1977. In mid-1975, a third line was added to scavenge vapor from the carburetor float bowl when not running. '78 uses the active canister, so that was the year they changed to that type of device.

I'd also point out - my book only shows one drawing for all the different canisters that were used '74-80. In this case, I would trust the TSM more than the parts book drawings. The parts book only needs to show enough so the counter man can get the right part from the tables listing the various years and models. There are plenty of cases in these books where the drawing (see below - to be added) only shows a schematic view of the systems it's covering.

Also, Jim points to the connection to the TCS valve. The canister for '73 has no ported vacuum connection, and thus will not be connected to the TCS switch.

Earlswrench also wrote something about how I advocate the original 2-port canister with the air cleaner snorkel connection. IMO that's the wrong interpretation - I am in favor of vapor recovery in general. I think it's terribly wrong-headed to remove it if you have it. If you have the passive system with a snorkel connection, it won't work without the correct air cleaner and that snorkel connection. You could go to a later active-purge system and get rid of the air cleaner, but I think that's the only option if you want to change the air cleaner and keep vapor recovery.



Two-connection canister for the "passive" system.

 

Are you still confused about this?

There are GM canisters that do not contain a purge valve. I believe these are used with an external purge valve. Possible they are compatible with the air cleaner Bernoulli system? Don't know. The external purge valve is typically listed along with the canister. Even later, there were solenoid purge valves that were under the control of the EFI computer.

The canister in the photograph you show contains a valve. If this is not clear to you, please reply.

 

I think I've got it now, Tim. A big thanks for all your help! I'm probably going to buy the canister I posted in the pic, test it, and go from there. I might even replace the charcoal in it.

Any further questions on this topic will probably come once I've got it set up if I notice problems.

 

For more confusion, there is no listing for a charcoal canister for a 72 CJ model.
As I stated in my opening post, the diagrams in the parts manuals weren't always what you found/saw on the vehicles.

 

Ah, but that's correct, and shown in the TSM. '73 was the first year for the charcoal canister. '72 and maybe previous years (at least with the CJ) dumped the vapors into the crankcase. If the drawing of the canister includes '72, it's wrong.

This was the original complaint by Earlswrench. The back-half of his Jeep has the vapor recovery plumbing for a '73 or '74, but no charcoal canister under the hood. Kinda suspect someone who does not understand the system has been mixing and matching parts here. You need the expansion tank if there's no charcoal canister. You also need the special air cleaner with the early "passive" canisters.

Sorry if I'm beating a dead horse ...

 

I've read about people doing that. However, I think it's been done when the canister has been breached somehow, and there's carbon in the float bowl? Something like that. If it holds a vacuum, I'd add a new filter (service part on the bottom) and run it. I don't think the charcoal wears out, unless it's filled with oil or something that fills the pores in the grains.

 

I did see a new filter for $8 on Amazon. Makes sense about the charcoal not needed to be new. Good point.

 

$8 is robbery. Parts store item:
1973 JEEP CJ5 5.0L 304cid V8 Vapor Canister Filter | RockAuto

 

You're right again. Local flaps has one in stock. It's at 50% more than the price of your RockAuto link ... $2.49 .... I think I can swing the extra 87 cents though