XJ 4.0 running weird (crazy I know?!?)

336wheeler

Well-Known Member
Joined
Sep 9, 2006
Location
Boone
I have a ‘88 XJ with what I think to be a ‘93 4.0.

It fires up every time, but after say 10-15 minutes once it gets closer to operating temp, starts stumbling and falling off, and generally running like doo doo.

Ive replaced TPS, CPS, dizzy cap, spark plugs, plug wires, fuel pressure regulator, fuel filter, and in the rebuild of my Jeep I put in a new fuel pump and fuel tank. Running 93 non-ethanol gas in the new fuel cell.

Still runs weird. I think it’s a fueling issue because my spark is strong. I’ve read that the fuel rail needs 31-39 psi, I have not gotten a fuel pressure gauge yet but it does seem that my rail has a lot of pressure on it, when I hit the bleeder valve. It has a new fuel pressure regulator, and the “return” side of the regulator appears to be functional.

Any thoughts? Thanks - Brandon
 

336wheeler

Well-Known Member
Joined
Sep 9, 2006
Location
Boone
After doing more research tonight, I’m going to try a MAP sensor tomorrow and see what that does.

edit - also picking up the IAC valve as well
 
Last edited:

DLOR

Well-Known Member
Joined
May 13, 2007
Location
Chapel Hill
I had a xj that would stumble and shut down for no reason. Sometime immediately sometimes after 100 miles. Turned out the sock on the fuel pump had come loose and would float around in the tank until it got stuck in the pump intake. Easy test with a fuel pressure gauge.
 

336wheeler

Well-Known Member
Joined
Sep 9, 2006
Location
Boone
I had a xj that would stumble and shut down for no reason. Sometime immediately sometimes after 100 miles. Turned out the sock on the fuel pump had come loose and would float around in the tank until it got stuck in the pump intake. Easy test with a fuel pressure gauge.

I’m now running an Airtex 8248 external pump, mounted on the jeep floorboard so it doesn’t cavitate
 

336wheeler

Well-Known Member
Joined
Sep 9, 2006
Location
Boone
New IAC valve and MAP sensor did not make a difference.

@ghost I will be digging into this tech bible over the next several days with diagnostics - thank you
 

336wheeler

Well-Known Member
Joined
Sep 9, 2006
Location
Boone
Well after all the parts changing, I tested 30psi to my fuel rail. This is inadequate based on all my readings, so I changed from a Airtex 8248 pump to a Walbro 400 pump.

In this process, I found that my inlet strainer on the Airtex pump was full of gunk, maybe factory fuel cell gunk? Anyway. Now I have a much quieter and upgraded pump, and the XJ runs great.

The Airtex pump was actually fine once I cleaned it out, and I probably didn’t need to change to the Walbro pump but I will say the pump runs way quieter.
 
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