Cj5 Fuel Tank Drama

ECJ5 Team, I’m a first time poster. I have found much valuable advice here on this site. I hope y’all can help me as my current challenge has me and two very good mechanic buddies stumped.

The patient: 1975 CJ5, 258 4.2 I6, T15 transmission, 5,000 miles, brand new Carter 2 bbl carb, vacuum choke, brand new MSD distributor, coil and ignition control module (got tired of old points system and upgraded), new plugs, wires. Owned one year, PO not well maintained, lives in hot sandy beach town.

Symptoms: after a new carb, and entirely new ignition system, and all the right adjustments to the mixture and timing, she was running Great. Starts right up. Idles perfect. Revs and drives under load great. Then... I fill up the gas tank. On a full tank, she barely starts. Won’t idle at all. Sputters out, blows black smoke, and acts like she’s misfiring under load. This exact same scenario has happened 3 times in the past two months.

Checks: Carb bowl getting a solid squirt of gas when I hit the throttle. Timing, verified with light still at 8 BTDC. Tried to start/run with gas cap off, same symptoms.

As a side note, the fuel and temp gauges went out a little while ago but it has been running perfect without those, that’s the next project when this mystery is solved.

Thoughts: venting issue with tank (but still won’t run with cap off), choke issue (but seems to work normally and is opening at temp), vacuum issue with carb hoses routed wrong somehow (but ran perfect with hoses where they are before filling up), sending unit in tank not working (but that’s just for the gauge and shouldn’t affect fuel flow).

If anyone out there has any ideas on what may be causing this, Please let me know. Thank you!!!

 

Saved round... it’s a rear tank set up.

 

Factory setup is the rear tank. The TSM has a nice drawing of the plumbing in the vicinity of the tank. Do you own the TSM? I'd check all that. Do you have vapor recovery? If not, why not?



I'd also measure the fuel pressure, empty and full. A '75 with a 258 came with the T-14, not the T-15. The 2V BBD only appeared in 1977 in wagons and trucks, not in CJs till '78 or '79.

Welcome from Boston.

 

Well, I had issues with my 75/304 when I first got it. It had low original [15,750]miles and had set a lot.. While mine would start and run, it would also puke and die at odd times. I finally decided to replace the filter sock in the tank. I siphoned out what fuel I could by removing the filler hose at the tank. I used a clear hose. Near the end of the siphoning a whole bunch of brown nasty stuff came through the hose. When I finally got the tank out and open it was a nasty mess. Cleaned er up, blew out the fuel lines and it never missed another beat,

 

It has T-15 written on the side of the transmission, but it may be a T-15, who knows. I've learned it's quite the Frankenstein. Old distributer was off a '74, power steering is off an '83, and the carb wasn't the original Carter single bbl, but a double so I reckon the manifold is a different year as well. None of those are my issue though, all those components are fully functioning.

There is no vapor recover connected anywhere. Only hoses coming from the tank are the fuel to the pump and the return from the carb. It runs perfect on 3/4 tank or less. Checked the fuel too coming from the pump before the filter, good pressure and all clean and clear coming from the tank.

 

Sorry, "has T15 written on the side... may be a T-14"

 

Thanks Zila, I'm hoping to not have to drop the tank and replace it or parts. It was running perfect until I filled it up, and that same thing has happened 3x. Something about a full tank makes it stop running, yet there is no problem with fuel delivery to the carb. Carb is new and tuned correctly. Entire ignition system is new and correctly timed. Still a mystery.

 

Sounds like it's running rich. It happens after you fill it so it sounds like you're getting gas into the engine through the fuel return line.

How is the fuel return plumbed? That's not normal on a carb.

 

Sounds like water in the gas to me. I've had that problem several times in the 42 years I've owned the Jeep.

PIA, siphon tank, add dry gas and known fresh gas.
Jay

 

It's definitely running rich, fouling my plugs. That's an interesting thought AKjeff. I may disconnect that fuel return and see how it behaves. It only happens after I fill the tank.

Jay, I thought this too, but can't logically understand how it would only happen immediately after filling the tank and not at any other time. Draining it and adding known fresh gas may still be something I need to do to diagnose it to completely rule out water in the gas.

 

Akjeff, YOU NAILED IT! THANK YOU! For whatever reason, will still investigate it, either gas was getting back into the tank through the return or the return was blocked or something. Either way, I removed the return line, capped it off and it started up and ran, idled smooth, no more rich black smoke. There are many posts and opinions out there on whether a return line is even needed or what it may or may not help with. If anyone wants to chime in with a recommendation on using one or not I'm open to suggestions, please do. But it was definitely the problem because once removed, it's back to normal running conditions.

 

Great and welcome from Ga. Without the return connected make sure you have a vented gas cap.