Fuel tank evap lines, rough idle, etc

[sth5591]

After completing a complete drivetrain swap from a 95 XJ into my 87 MJ I'm experiencing a rough idle that almost seems like low fuel pressure. I can hear a hissing noise coming from near the gas tank which I assume is the vacuum lines on the fuel tank, they are not connected to the evap canister right now. Anyone have some pics of how these should connect, and if this would cause a fuel pressure problem? 

[sth5591]

Anybody? Really would just like to see how the two vacuum lines come out of the tank and go into the evap canister

[buckwheat]

Just replaced my tank. The two lines on the vent valves come together at a T junction right next to the tank. Then a single line went forward under the truck. I didn't trace it all the way, but the only place for it to go was the charcoal canister. Is that what you wanted to know?

[Strokermjcomanche]

I have done the same swap . What pump / sending unit are you using ? They are run as Buckwheat described .

[sth5591]

Original MJ pump and sending unit, I believe part of my problem may be the TPS again.

[Strokermjcomanche]

I would use the fuel pump from the xj in the mj sending unit . Your gas gauge will be backwards though . If you had a 2.5 I don't believe the fuel pump has the pressure to keep up . Maybe someone on here can confirm?

[buckwheat]

If it was me I would get a Fuel Pressure Gauge and see if my pressure was actually low before replacing things. Someone can correct me, but 31 with the vacuum, 39 without is what comes to memory.

 

Strokermjcomanche: Why would changing the pumps cause the gauge to read backwards? Wouldn't that only change if he used a different sending unit?

[Strokermjcomanche]

Using a renix sender with a HO harness makes the guage read backwards and I'm also told will burn up the pentiometer . The pump has no effect on the guage , sorry if I implied that . Checking the pressure is a good idea .

[ParadiseMJ]

Just replaced my tank. The two lines on the vent valves come together at a T junction right next to the tank. Then a single line went forward under the truck. I didn't trace it all the way, but the only place for it to go was the charcoal canister. Is that what you wanted to know?

 

From the single line going towards the engine...the rubber line attaches to a short metal line, secured to the lower part of the firewall.  Then the metal line (about 10-12") curves up and attaches to a single rubber line again and into the canister.  On the canister, there's a big line and a small line.  The big line is the source vacuum to the manifold, it sucks the fumes coming out of your tank through the canister (via the little line).  If the little line is disconnected anywhere along the line...you should be able to hear it...AND...you may smell fumes.

[sth5591]

I swapped in a complete 95 4.0 drivetrain/interior and all wiring and am using the original MJ renix fuel pump and sending unit. I think the issue may have been old gas in the tank and also my TPS. Thanks for the help with the tank vacuum lines.