Gas Tanks Build pressure...

Trying to get rig ready for a run tonight! Going for a test drive, it shuts off... like there is no fuel. Pull the gas cap, and lots of pressure is released. Jeep still won't run. Just changed the gas tank (prior to problem)

Anyone have any ideas what could cause this?

88XJ Aw4 Np231

Gas tanks build pressure on purpose, that is to detect a leak. Drive with your gas cap off for any amount of time and you will get a code saying there is a large leak in the system. Also, if it is warmer out now than when you last filled up the tank, normal expansion will create pressure.

ebishop said:

Gas tanks build pressure on purpose, that is to detect a leak. Drive with your gas cap off for any amount of time and you will get a code saying there is a large leak in the system. Also, if it is warmer out now than when you last filled up the tank, normal expansion will create pressure.

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This didn't SEEM to be the quick little "swoosh" that normally happens. Also, this is something that just started today.

Did you have this before the fuel tank change?

Can you hear the pump run when you turn on the key?

Saudade said:

Did you have this before the fuel tank change?

Can you hear the pump run when you turn on the key?

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Did not do it before.

Now, the pump does not come on with the key, and NO pressure on the rail!

Check your fuse, relay and pump connections.

Saudade said:

Check your fuse, relay and pump connections.

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No power to the Pumo or fuel pump ballast resistor. Where are the fuses?

Under the dash, driver side near the kick panel.

Not getting any power to the pump or fuel pump ballast resistor. What would cause no power?

All fuses by the kick panel look good! Are there any relay under the hood that would crap out and cause no power to the pump?

Got it running...
Now when I give it over half throttle, it dies. Any suggestions?

BTTT for help from the Weekday readers!

Right now the Jeep run fine until you hit about half throttle. Then it just shut down. No stutter, just dies. I'm not sure if this is related to the Gas Tank or not.

I could use any recommendation!

check your relay as well...


Do you have enough fuel in it? Warning light on? Sometimes when my '89 Heep is down to the last little bit, I can hear the pressure in the tank and then she spits and dies at half throttle.

Another thought would be the pressure regulator on the fuel rail... there's a test proceedure for it as well, you will need a fuel pressure gauge...

1. Remove fuel tank cap
2. Remove cap from pressure test port on fuel rail. place rags under this spot so you don't leak fuel onto your manifold.
3. Connect fuel pressure test gauge to test valve on fuel rail
4. Remove vacumn line from fuel pressure regulator
5. Start engine, let idle
6. with vacumn line to regulator disconnected, pressure should be around 39 psi
7. connect vacumn to regulator, pressure should be around 31 psi
8. if pressure does not read 8-10 psi higher with vacumn to regulator, check vacumn line for leaks or blockage.
9. if pressure is less than stated in #6 or #7, pinch off the return line and recheck pressure (vice grips work great for this) if it remains lower, check fuel supply hose, fuel filter, and fuel rail inlet for blockage. (8 out of 10 times it's the filter)
10. if pressure is high when return line is pinched off, check fuel return line for kinks or blocks, if none are found, replace the regulator

WITH RETURN LINE PINCHED OFF FUEL PRESSURE CAN BUILD QUICKLY! TURN OFF ENGINE IF YOU NOTICE EXCESSIVE AMOUNTS OF PRESSURE BUILDING! (I like to position the guage so I can see it from the drivers seat, just in case!)

Might also want to do a leak down test if it's not the regulator... could be bleeding past outlet check valve on the fuel pump... let us know if you need the procedure or pressure specs for the leak down test.

11 posts and no one asked what year or which engine. If it is a Renix then the EGR may have become un hooked or lost a connection. Another place to look would be a stuffed up CAT. O2 sensor. Try those and let us know what kind of engine/years we are working on here.

rig year is in his 1st post at the bottom... Renix EGR may be a place to look, but I've got 2 renix heeps and the EGR is disconnected on both, they run fine, little rich, but fine.

Thank you guys for your help!

Yes it is a Renix 88, sorry for the confusion.

We have swapped the relays, they seem to be working ok.

We're now working on the other suggestions! It seems to be starving for fuel, but it shuts down so abruptly I'm not sure. No sputtering or anything!

jrowell said:

Not getting any power to the pump or fuel pump ballast resistor. What would cause no power?

All fuses by the kick panel look good! Are there any relay under the hood that would crap out and cause no power to the pump?

Click to expand...

could be a faulty cps or bad cps wiring harness connection.

Round Two!

It seems like power is being cut off to the Fuel pump. Sometimes when the key is turned on, it does NOT prime. If you run a jumper to it, it will pump fuel. Is there a pressure switch or sensor that could affect the Pump getting power?

jrowell said:

Round Two!

It seems like power is being cut off to the Fuel pump. Sometimes when the key is turned on, it does NOT prime. If you run a jumper to it, it will pump fuel. Is there a pressure switch or sensor that could affect the Pump getting power?

Click to expand...

according to a couple manuals, there is a ballast resistor to help reduce pump noise, but I can't seem to locate it on any wire diags... might be on the pump itself.

Since you said you can jumper the pump, you may want to test the wiring going to the pump with an multimeter, might have a short somewhere preventing the pump from getting juice all the time. I'd start at the relay and work back. I know the line goes through a few connectors and the ECU, one of the connectors may be bad...or a wire is rubbed through. If you can make the pump run, I wouldn't waste money on a new one, they're around $120, and if the issue is electrical, the new one won't work either...

Maybe someone has a wire diag for the renix that shows exactly what you need to trace back... If I find one, I'll post it up...