Fuel Pressure Drop 1998 XJ sport

I have had some hugging and chugging issues in the last month. It started with what I think was heat soak. It was 90+ degrees out and in the morning it was about 80. After work it was about 100 on the temp gage in the Jeep.

All parts listed are from discount auto parts stores. I know, I know.

I checked the fuel pressure and it was erratic (30-50) and would drop immediately after shutoff to about 25. So I replaced the fuel pump.

This seemed to take care of a significant portion of the problem.

ON startup (after sitting for a couple of hours), it starts fine. After about 1 mile, it hugs and chugs and revs, drops RPM, pops and cracks, then it works itself out, and it runs fine all the way to work.

In the last month I have replaced: Fuel pump, distributer, plugs, wires, coil, IAC, TPS, brakes, **fuel injectors**. I have checked the wiring harness around the injectors and the sensors including the O2 sensors. Everything looks OK.

I replaced the fuel injectors with some Chinese ones that I found online. They said "direct replacement". They had 4 holes and seem to flow about 4X the fuel. I am not sucking enough air through to burn THAT much gas. I swapped the stock ones back in.

Last night I decided to do a leakdown test overnight. It visually dropped to 40PSI quickly. Then after about 2 hours it was at 20. Then a bit later it was 15 then this morning, it was at 5 or a bit less. Should the fuel system leak all the way down overnight?

cushdaddy said:

...... ON startup (after sitting for a couple of hours), it starts fine. After about 1 mile, it hugs and chugs and revs, drops RPM, pops and cracks, then it works itself out, and it runs fine all the way to work. ........... Last night I decided to do a leakdown test overnight. It visually dropped to 40PSI quickly. Then after about 2 hours it was at 20. Then a bit later it was 15 then this morning, it was at 5 or a bit less. Should the fuel system leak all the way down overnight?

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I would strongly suspect the O2 sensors for the described running issues. A genuine Jeep or Bosch full pump assembly should not bleed down that far. The fuel pump should hold +30 psi for about 30 minutes. Determine if you have a check valve problem or leaky fuel injector problem causing the pressure loss.

I also have no check engine light on. The jeep has 162K miles. The original fuel injectors are back in it. The fuel pressure loan a tool doesn't look like it has a good way to insert the gage in so I can rule out the check valve or the injectors. Why would the O2 sensor/s cause this at the same time after startup? The way its acting, its like there is vapor in the fuel after leakdown and until that is worked out, it runs badly.

If the check valve is allowing vapor lock, that would clear up in 15 - 30 seconds or 1/2 block of driving, as would excess fuel from leaky injectors. If the O2's are weak, they very likely will cause stumbling, bucking or other drivability issues as the OBD system goes from warm-up open loop operation to closed loop operation.

If the O2's are bad, would this normally give a code? Can you explain the open loop to closed loop? I get they send signals to the computer to adjust the fuel, timing and whatever. Does the computer ignore them until after warmup? Thanks

Faulty O2's seldom give a code, until they are totally and completely FUBAR. O2 sensors are routine maintenance and have a typical service life of about 100-150,000 miles. When you buy new O2's only buy NTK brand sensors. Bosch O2's and the Jeep 4.0L do not play well together.

OBD-II and open/closed loop operation are on the Google and YouTube.

Would it make sense that if I leaked fuel into my cylinders overnight, or after shutting down, then on startup, it pushes a rich mixture into the exhaust system, temporarily causing the O2 sensor to give faulty info to the computer? When this stuttering/stumbling/rough idle happens, the IAC valve opens up like its trying to compensate for being rich.

Then, after the exhaust heats up and I get to steady state, the O2 reading evens out and all is good? I checked the heater for the front O2 and it reads 5.5 ohms. I haven't checked the output yet.

I replaced the fuel pump a few weeks ago and then this started. I am thinking that the check valve in the fuel pump was leaking down so badly that the leaky injectors were the path of most resistance and not much if any fuel leaked past them. After the pump replacement, they became the path of least resistance, allowing fuel into the combustion chamber.

I have no problem changing the O2 sensor, but it may be doing exactly what its suppose to do.