Unusual Fuel pressure

1990 XJ...4.0L

Now here is the issue. I have had considerable hesitation under acceleration as well as some backfiring in the last 90-120 miles or so.

I decided to check my fuel pressure first. I put a rag under the schrader valve (with the truck off and the key out) and depressed the valve to release pressure. I bet it sprayed fuel for 15 seconds. I know that doesn't sound long, but it is when you are watching fuel spray from your fuel rail.

I hooked up the pressure tester, and turned the key over to enegize the fuel pump. I got a reading around 10-15 PSI. When I pushed the plunger on the pressure guage to empty the fuel it ran for another 15-20 seconds into the pop bottle I was collecting it in.

I tried it again, and again got 10-15 PSI and it drained another 15-20 seconds. All said in those 2 tests, I drained almost 1/3 of a cup of fuel into the pop bottle.

Something is not adding up. It has always had a hesitation, but it is getting worse since my last fill up when I used a fuel injector cleaner in the tank. I also replaced the fuel filter at that time.

Any ideas....by the way new items in the last 500 miles:
plugs, wires, battery, cables, injectors, fuel pressure regulator, fuel pump, fuel filter, CPS, distributor, computer, thermostat, coolant overflow bottle, ballast resistor

Thanks

16 views and nothing??

Wow...are we approaching a record?

Did I stump everyone?

Do you know the spec? 10-15 sounds way low to me, and I'd also expect a rock steady number on the gage, not "around x-y". That sucks if you put in a bad fuel pump, but it wouldn't be the first time.

The spec is about 35-38 psi, so 10-15 is low.

The pump should be just fine. I drove the truck for several hundred miles with no real issues leading up to the last 100 or so.

Anyone else???

As I recall, it's 39PSIG with the fuel pressure regulator disconnected from vacuum, and some 30-35PSI above the intake manifold pressure when it is connected.

Might be a fuel pressure regulator - but check to make sure the vacuum line and port are clear before you get into replacing anything.

5-90

Try blocking the return line to the tank. If the pump can't push at least 50psi dead headed, you have an in tank problem. If you have good pressure, the regulator gave up. Assuming the lines aren't damaged, filter isn't plugged, etc.

I'm probably not remembering the max pressure correctly, but it should be more than the running spec by a small amount. :wave:

sgtmack--

Tell me more about this. Step by step if you don't mind. How do I block of the return? After blocking it, do I just energize the fuel pump and see if it reads 50 psi on the guage?

Anything specific you can tell me would be appreciated!!

Thanks...keep it coming!

Cottontail said:

sgtmack--

Tell me more about this. Step by step if you don't mind. How do I block of the return? After blocking it, do I just energize the fuel pump and see if it reads 50 psi on the guage?

Anything specific you can tell me would be appreciated!!

Thanks...keep it coming!

Click to expand...

Just clamp it off with a pair of vice grips.
Whats the pressure with the engine running. It will not run with 10-15 psi.
I don't think it will run with less than 27psi.

IIRC the return line has a rubber section between the engine and frame. Just squeeze the hose in half like you would shut off a garden hose, or gently pinch it with a small needle nose vise grip, and cycle the key. If the pump is ok, you'll get full pressure. If there isn't a soft line at the engine, I'm pretty sure there is at the tank.

edit- Sorry, I type slow. What he said! And a cool picture.

Are you sure your pressure gauge is accurate? Have you confirmed its readings on another vehicle or used another gauge and got the same 10-15 psi readings?


Kyung

My experience with other FI systems (GM, not jeep) is that they do run ok with pretty low pressure. At 10 or so psi the symptoms are excatly what he describes, idle ok and hesitation under load. If you rev the engine with the pressure gage hooked up you'll probably see the psi dip even lower (of course it normally shouldn't move at all.)