4.3L XJ

Also, give it some time. These Jeep ECMs are kind of stupid and need time to learn

ElMartillo

Drove the ‘96 2-door all winter, getting this one back on the road for the summer and revisiting this issue, which really hasn’t changed since the O2 sensor and new injectors were installed.

Additional symptoms include intermittently taking about twice as long or more to start as it should, usually when “cold”, fuel smell from the engine compartment.

I noticed the rubber elbow for the vacuum line attached to the fuel pressure regulator was deteriorating and smelled of gasoline. Regulator was not holding vacuum.

I replaced regulator and elbow, now it seems to start right up as it should. I imagine a vacuum leak here could accound for poor mileage, coupled with a bad regulator.

Question: could unregulated fuel pressure also contribute to excessive fuel consumption?

Thanks!

4.3L XJ

Yes, I have had it happen

eugnton

They are cheap, replace them both. It may help if you are running rich.

ElMartillo

Okay, doing some actual testing.

Fuel pressure reads near zero on original fuel pump prime when engine is cold, not running. Second prime of pump brings it up to 10-15psi, and it drops down to zero within a few seconds. Again, cold not running. Fired up fine and pressure while running is at 45psi. I removed the vacuum line from the FPR, pressure went up to 55psi. Applied full vacuum to FPR, dropped down to 45psi. Cut engine and pressure nearly immediately dropped to zero.

What do these symptoms indicate?

Thank you.

cruiser54

Leaky injectors?

Hose in tank from pump to outlet has a hole or is loose?

ElMartillo

Thank you, Cruiser. I did put in refurbed 4-hole injectors from the vendor on here. I know it’s no guarantee they don’t leak. I still beleive fuel economy is poor. If so I imagine that indicates an injector issue? It would certainly be easier to check for leaky injectors than to drop the tank and go fishing around in there...

Is my initial 10-15psi indicated pump prime pressure a sign of something obvious?

cruiser54

There's a test in the manual where you clamp the return line and see if the pressure drops. I'm not totally familiar with it and I'm on my first cup of coffee......

A 91 you don't have to drop the tank BTW.

I think for now it would be best to separate the 2 symptoms as fuel economy can be affected by so many things.

You have a FSM?

ElMartillo

the return line coming off the rail is hard plastic, so I can’t clamp it there. Getting under it today so I’ll see if there is a soft line near the tank I can clamp.

No FSM.

cruiser54

gotta do it correctly. Want me to look it up for you?

ElMartillo

Any info you can provide would be appreciated.

When it’s convenient for you.

Thanks!

cruiser54

be later today or when I take a break from painting before it gets too hot.

cruiser54

Here ya go!!

cruiser54

BTW, ever see a Comanche with multiple carburetors?

ElMartillo

Once again, thank you Cruiser! As I mentioned before, I'm hesitant to pinch that return line as it appears to be hard plastic. Also, I did replace that fuel pressure regulator last weekend, and the pressure drops to zero nearly immediately after shutting off engine.

I'm going to put the gauge back on it today and at least double check my observations and see if I can perform that leak down test without damaging the fuel line.