Fuel return for 4.0 conversion in CJ

widnrf1j1

New member
I have an 86 cj7 and have installed a 4.0 from a 91 cherokee.

It runs great for about 20 minutes and then won't run below 2000 rpm without surging, sputtering, and sometimes dying.

Over the last year, I have:
Complete engine rebuild
Replaced every temperature sensor
Replaced throttle position sensor, cam position sensor, crankshaft position sensor, and tried an alternate computer.

After running the full trail of electrical issues, the new diagnosis is vapor lock. This week, we wrapped the fuel lines in heat insulation and rerouted them a bit so we have actually improved it greatly and now it takes about an hour to show up at idle and it goes away at 1300 rpm.

My theory is that there may be an issue with the fuel return line so I am hoping that someone can tell me the exact modifications that should have been made to the oem gas tank to allow the fuel return process to be correct. Does it need to flow freely, should there be a regulator that holds a certain amount of pressure in this loop to facilitate the correct pressure to go through the regulator on the fuel rail?

I have an external fuel pump and filter mounted on the passenger side frame rail. Is there something special about the wrangler fuel pump that makes this work better and if so, can I buy one that fits my tank or a complete tank and pump that has a modified fill neck for the CJ?

Thank you for your help!
 

widnrf1j1

New member
Final chapter. Collins Brothers identified the issue in short order and replaced the computer which had a short along with the O2 sensor which was a generic from Autozone rather than OEM. Done, runs great.
 
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