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Discussion Starter · #1 ·
After the swap: fuel Injection problems and ?\'s

Hey guys...
Just when I thought I had ALL the bugs worked out of my motor swap.....

Here are the details:
- 1988 YJ (used to have the F.I. 2.5L)
- 1998 4.3MFI motor swapped in w/Howell Harness
- TH350 tranny
- 1991 YJ 4.0L in tank fuel pump (to match the Chevy's pressure)
- Factory fuel lines (3/8" and 5/16")
- Gas tank vent to charcoal cannister is hooked to manifold vacuum (no cannister)
- Factory non-vented gas cap
- new high cap fuel filter
- no driveability problems

Here is the problem:
When driving for long distances 25mi(+) or if I am on bouncy terrain like bad gravel roads, fuel drips out from the passenger rear inner fender. There is a three-way fitting that just sticks out from under the inner fender and is wet with gas. It isn't leaking from there, but that is the lowest point and the gas drips onto the frame there. If I open the tank when this occurs, air rushes out?? If I drive with a rag stuck in the filler neck - no leak.

I dropped the tank and the vent seems OK - I can blow through it from the tank side.

Here are my ?'s:
Why does it leak if the vent is open?
Does anyone have a diagram of the fuel plumbing in a YJ? (I did not alter anything at the tank, or on the passenger side around the "rollover valve")
 

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Discussion Starter · #2 ·
Re: More fuel delivery problems and ?\'s

I edited my first post...I mistakenly listed the motor as TBI, it is a multi port injected setup requiring a much higher fuel pressure than the older GM TBI.

After making some phone calls, I may have gotten one step closer to the cause of the problem. The rollover valve apparently opens at 3psi to prevent tank over pressuring. On the original YJ setup, the vent went to the charcoal canister and was evacuated to the intake by a purge valve on the canister. The chevy setup (I am going on sketchy info here) is similar except there is no rollover valve - the tank vent is plumbed to the charcoal canister and is evacuated by a solenoid valve on the intake.

My motor has the purge valve, but it is unplugged electrically and from the canister (which is gone). I have the harness connection taped up. Would connecting the tank vent to this be better? It is hooked up to manifold vacuum now.

Any ideas?
 
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Re: More fuel delivery problems and ?\'s

Are you sure you have the vent going to the charcoal canister going to the right port? There should not be that much pressure inside the tank....

I think your looking in the right direction....

BTW...did you try blowing through the vent while the engine was running?
 

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Re: More fuel delivery problems and ?\'s

Hank- I just had a similar problem with my 88 YJ. I have a built 383 stroker that I just installed MPI on. I had the same problem with the fuel leaking out, and when I took the gas cap off the air and gas would just pour out. I took my tank down and did everything you have done so far. I finally turned to the boating industry. On large boats they have a fuel air seperator that lets air out, but it will not let fuel out. So I installed a thru hull vent tube (like on the side of a boat) with the fuel air seperator valve in line to the tank. This has worked pretty good. A little fuel still seeps out through the valve but the tank can breathe now. The valve is made by Racor. Check out this link to their web site. I mounted my vent on the outside of the jeep body just inside of the rear left tail light. Also, go with the smaller seperator if you do this. I got the bigger one, and it is way too big for where it went. One day I'll switch it out. anyway, let me know if you have any questions.

http://www.parker.com/racor/fas.html
 

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Discussion Starter · #6 ·
Re: More fuel delivery problems and ?\'s

Thanx for the responses guys...

Nick...Doesn't this serve the same purpose as the rollover valve? I'm not sure I understand where you hooked up the air/fuel seperator.

I am not sure I understand the factory plumbing. I don't have a good schematic.
 

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Re: More fuel delivery problems and ?\'s

It is supposed to keep fuel from passing through it, so it will work like a rollover valve. Just to test it, I tried to run some water through it before I installed it, while holding it upside down and nothing went through. Leave all the factory hoses and vents alone. Don't do anything to them. I tapped in a new vent hose to the top of the tank with a plastic fitting with an elbow at the top. I ran the hose from that up inside the left rear panel up behind the taillight. That is where the fuel air seperator is located. Then from the fuel air seperator I ran more hose to a thru hull vent that comes out beside the tail light to the outside of the Jeep body.
I hope all that made sense.
 

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Discussion Starter · #8 ·
Re: More fuel delivery problems and ?\'s

Are you still running a charcoal canister? If not, where did you hook the vent line from the gas tank?
 

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Re: More fuel delivery problems and ?\'s

Tha charcoal canister is still in the Jeep but not doing anything. I just never took it out. Just leave the old vent lines in place. Nothing will come out of them because the MPI is creating so much pressure they will not let any air out. I just left mine connected on top of the tank.
 

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Discussion Starter · #10 ·
Re: More fuel delivery problems and ?\'s

Nick...
Did you actually tap the tank?

Do you have a plumbing diagram for the original Jeep setup? I have a suspicion that it may be screwed up - I let a buddy help me troubleshoot it ;-)
 
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