Having some fuel issues....

So I've been having a fuel delivery issue ever since I started driving my Cherokee. It's a 96 4.0 5spd. It runs fine untill about half throttle, then it just flattens out and doesn't have more power. Sometimes it bogs. I put on a pressure gauge and it was right where it should be at 50psi. Thinking fuel wasn't the issue I changed the rear o2, no change, checked the TPS and MAP and they were fine. I pulled the hood off and hooked the guage up again and drove it around and at more than half throttle the pressure would drop, sometimes down to zero at WOT. So I changed out the pump and regulator and nothing really changed. I bought a second pump and just installed it today and still nothing has changed. Pressure still drops. I checked for kinked fuel lines and they were fine. I also swapped in another set of injectors just to rule those out. I also unplugged the pump and ran it straight to a battery and still same issue. I'm stumped and done know what else to do. Any ideas?

Oops, I meant to say I changed the front o2, not the rear

does a 96 have a fuel filter somewhere?

Yeah that was the first thing I changed

hrmm could there be something in the fuel tank? or buy a new TPS
I know it doesn't completely pertain to XJs but my dads samurai had fueling issues. It would fire right up, you could drive it about a mile and it would lose power and then die out. give it 5 mins and itd fire right up and idle fine. we found out that the tank had started to rust and the sediment was clogging up the fuel filter sock. so that might be something to look for also

Would it be possible to create this problem if the fuel tank vent line was pinched, or somehow plugged? I am just thinking out loud here...wondering if a vacuum situation in the tank could create a problem.

I suppose an easy check would be to loosen the gas cap and take it for a spin. It may throw a code for a loose cap, but it should tell you if a vacuum is the issue.

It is an interesting problem. Let us know what you find.

I haven't been running it with the vent line hooked up actually.

No vacuum line to the breathers?

Or vacuum line just not connected to the rest of the system.

If the latter, are you certain you don't have it pinched between the tank and the floor?

Sounds like you have adequate fuel pressure but insufficent flow. Did you check the hoses inside the tank for cracks? If you're sure they're OK, then maybe there's an obstruction in the fuel lines, either from the pump to the filter or from the filter to the fuel rail.

If you have an air compressor you can blow air through the lines to check...

Anak said:

No vacuum line to the breathers?

Or vacuum line just not connected to the rest of the system.

If the latter, are you certain you don't have it pinched between the tank and the floor?

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I had it unhooked at the T-fitting at the tank, it's hooked back up now though

Runnin'OnEmpty said:

Sounds like you have adequate fuel pressure but insufficent flow. Did you check the hoses inside the tank for cracks? If you're sure they're OK, then maybe there's an obstruction in the fuel lines, either from the pump to the filter or from the filter to the fuel rail.

If you have an air compressor you can blow air through the lines to check...

Click to expand...

I did that today actually, seemed free and clear

Is it a Mopar fuel pump or a POS Asian knock off?

Isn't the FPR and check valve in the pump?

Gotta be a hose restriction, bad fuel line internally between the tank and the injectors if pressure falls due to a lack of volume.

Is there more than one filter, one inline and one on the pump suction?

Did you inspect the tank guts? Mine was an awful site, I replaced the tank, hoses and everything. Even replaced the vent-check valves.

run a dedicated power wire to the pump and repeat test, will prove or eliminate electrical issues

Ecomike said:

Gotta be a hose restriction, bad fuel line internally between the tank and the injectors if pressure falls due to a lack of volume.

Click to expand...

This jogs the mind...

Someone had a problem with a fuel pump assembly where the hose between the pump and the top of the assembly was split (or something like that) and was losing fuel. I think they managed to find it by running the pump assembly outside of the tank, like in a pan of fuel or something like that.

That thread is somewhere around here, but I can't recall where.

that usually just creates low pressure all the time though, pump runs at same psi because voltage doesn't change

Pull pump and pickup out to inspect. Had an in tank blockage that was almost identical. Would run ok on startup, then lose pressure after a few minutes.

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