TBI flooding when on a full tank

[highrpmsbc]

I have a cj7 with a sbc swapped from a 80-90's gm truck with the tbi unit. My issue is that it runs fine as long as it is bellow 3/4 of a tank. When I go to fill it up it will shoot black smoke and wont run unless WOT. Also when it dies gas poors out onto the ground from what looks to be an overflow hose/tube in the engine bay. Sorry if its a simple fix I haven't had any experience with tbi's before only carbs.

 

[someotherguy]

Did you do the swap? It sounds like there are lines not hooked up correctly.

There are three lines on the sender* - fuel supply, fuel return, and EVAP (vent.) The TBI system has a bypass style regulator built into the TBI unit and unused fuel is returned to the tank. The EVAP line is connected to the charcoal canister under the hood and is for smog purposes as the system is designed to not vent fuel vapor to the atmosphere. There should never be liquid fuel in the EVAP line, only vapor.

I'd get this figured out quickly if I were you, as fuel dumping in the engine bay sounds like an immediate fire hazard.

*(four actually, but one is much larger and connects to the vent on the filler neck tube)

Richard

 

[highrpmsbc]

Hello Richard.
I did not do the swap it was done in the late 90's/early 2000's. The only thing that I have done is replace the fuel pump because the old one was not functioning. As far as I am aware this issue was not happening before I swapped the pump, but the jeep has also been sitting for around a year. The pump is externally mounted on the tank and I do not think it is stock to the jeep or the drivetrain. I did not see a return line while I was working on the pump, would it be coming off of the tbi back to the tank? Also why would this issue only be happening at a full tank? I could understand if this issue happened all of the time but it runs perfectly fine when under 3/4 of a tank. Tomorrow I will have a look to see what I find.

 

[GoToGuy]

Where does the return line from TBI go to.
The fuel system is a designed closed system. The only fuel access to atmosphere is opening fuel filler or TBI injectors. Excess pressure or flow is returned to fuel tank. Throttle body has internal pressure regulator.
Fuel is provided by an external electric fuel pump?

 

[highrpmsbc]

I did not get a chance to look at the system more today. Yes the fuel is provided by an electric pump mounted halfway up the tank. Is that an issue? It is my understanding that it ran fine with this set up for years before I got my hands on it. I am really just stumped why it only happens on a full tank. Also it looks like the fuel is coming out of what is called the charcoal canister?

 

[Drunkcanuk]

Ya, should definitely be a fuel return line back to the tank. That's probably what's going on is it's under pressure and has nowhere to go when it's not needed anymore at that pressure.
Definitely a scary situation you need to address before driving anymore, or you will eventually make your local evening news.
And how are you filling the tank?
Right to the top of the fill neck?
Or when the pump clicks off that's it?
Because if you are filling it right up, it will make its way to the canister.....and that's not good!

 

[Russ B]

Presuming you have a return line, need to check fuel pressure. 9 to 13 psi (1989 5.7 TBI). Pressure regulator inside TBI.

 

[GoToGuy]

Before anymore diagnostics confirm your fuel system has the proper design for TBI system. Pressure line from tank, pump to tbi fuel inlet. And a fuel return from tbi to tank. Pump out pressure no more than 13 psi on inlet side. Not less than 9 psi. Where is the charcoal cannisters vapor lines plumbed to? If it's in the wrong position at the fuel tank in could be getting fuel sloshed in when tank is at high fill level, need to check vapor line placements.

 

[95burban]

Chances are he doesn’t have a charcoal canister.