P0171 Driving me Insane!!!

[XJSteven]

Thanks for all the help (and new ideas) so far. I'll attempt to reply to everyone in one go...

The P0101 code I mentioned in my first post was a typing mistake, thankfully I only have the P0171 code.

I had a closer look at the MAP sensor using a mirror and yes there is a small hose coming off it that makes a 90-degree bend directly into the throttle body, so all good there.

I assume this problem isn't related to the fuel pump, fuel pressure or injectors because it is a dual fuel system and I get the code when running either fuel (my LPG system doesn't use the fuel pump or injectors and still has the same problem of running too lean).

I checked the oxygen sensor with the ignition ON and engine OFF and the grounds seem to be ok but I didn't get a 12V reading on any of the wires so this could be the problem...unless it is powered the same as the fuel pump and only comes on for a few seconds as I've asked in the above post???

Would a vacuum leak test show whether I have a leaking intake manifold or any air leaks in the LPG conversion?

 

[XJSteven]

Yes, that is the MAP sensor. And yes, they did relocate it. The change happened with the introduction of OBD!!.

As a 96 you have an OBDII system that doesn't have a 60 way connector at the PCM, you will have 3 connectors at the PCM.

I hate to say it, but you may want to look at a 97 service manual. I own a 96 and I know that aftermarket service manuals don't state that a 96 is different than a 95. It is closer to a 97 on the PCM operation side.

I would suggest checking the fuel pressure first. 50 psi is normal, no return line on this system. Try driving it with the pressure gauge connected and see of the pressure drops significantly.

Check for vacuum leaks at the intake manifold. Get some Carb cleaner and spray it where the head and intake meet. The RPM should change if there is a leak.

Next I would find someone with a computer scan too, not just a code reader. I bet by looking at the information displayed you should be able to find the source of the lean condition.

Hi Bryan, I can't see the 3 connectors at the PCM either - are they up underneath out of sight? Is there a picture somewhere?

My XJ is a 96 but I only have the 95 service manual. Without breaking any forum rules does anyone know where I can get a copy of a 96 and 97 service manual online?

 

[lawsoncl]

I checked the oxygen sensor with the ignition ON and engine OFF and the grounds seem to be ok but I didn't get a 12V reading on any of the wires so this could be the problem...unless it is powered the same as the fuel pump and only comes on for a few seconds as I've asked in the above post???

Check fuse 23 in the power distribution center. That's the fuse for the O2 sensor heater circuits

 

[XJSteven]

Check fuse 23 in the power distribution center. That's the fuse for the O2 sensor heater circuits

Thanks. Fuse 23 is good and there is power getting to the 02 sensor heater after all (13.8V) but it is as I suspected and only with the motor running or for a few seconds after the ignition is turned on. I also put the ohmmeter across the two white wires to the sensor to check for resistance and got approx 5-6 so the sensor itself looks good.

Back to square one. Any ideas?

 

[XJSteven]

Next I would find someone with a computer scan too, not just a code reader. I bet by looking at the information displayed you should be able to find the source of the lean condition.

Might try this next then.

 

[Ecomike]

I would take a high impedance analog volt meter, set for 1 volt scale, DC, and back probe the O2 sensor wires (not the heater wires), to see what it is actually doing.

If your mileage is poor, it is running rich not lean. If it thinks it is running lean, but is not getting good data at the O2 sensor, it will run it rich to the limit.

So either the O2 sensor data is bad (confirm what the sensor is seeing directly with the volt meter), or there is a vacuum leak, maybe in the exhaust manifold (or you may have a warped manifold, or head surface, or poor bolt bevel washer overlap between the manifolds, or it is cracked, again), or the O2 wiring to the PCM is losing signal due to damaged wires....

Might check the O2 signal at the PCM and at the O2 sensor directly next.

Your poor fuel economy says the PCM is over compensating for bad data, lean data.

 

[ktwalker01]

My 96 4.0 aw4 was throwing this code 0171, and it was spelled in plain English "to lean" :eeks1: turns out the fuel pump and filter weren't allowing enough fuel to get to the engine causing it to bog down.
I would replace your fuel filter first, and monitor the fuel pressure at the rail and see where to go from there.

 

[Ecomike]

My 96 4.0 aw4 was throwing this code 0171, and it was spelled in plain English "to lean" :eeks1: turns out the fuel pump and filter weren't allowing enough fuel to get to the engine causing it to bog down.
I would replace your fuel filter first, and monitor the fuel pressure at the rail and see where to go from there.

Does the 96 have a ballast resistor in the fuel pump power circuit?

 

[joe_peters]

Does the 96 have a ballast resistor in the fuel pump power circuit?

No, it went the way of the DoDo 93/94, definitely gone by 95 per FSM.

 

[Ecomike]

The Dodo is gone?

 

[joe_peters]

The Dodo is gone?

Yep, saw it in a documentary called "ICE AGE".

 

[Ecomike]

Yep, saw it in a documentary called "ICE AGE".

That's COLD!

LOL

 

[WB9YZU]

Did you replace the IAC actuator. 99.9% of them are missing from JY vehicles, and it's purpose is to lean the mixture. at idle. If bad, it would lean the mixture all the time.

 

[lawsoncl]

My 96 4.0 aw4 was throwing this code 0171, and it was spelled in plain English "to lean" :eeks1: turns out the fuel pump and filter weren't allowing enough fuel to get to the engine causing it to bog down.
I would replace your fuel filter first, and monitor the fuel pressure at the rail and see where to go from there.

If you read the original post, you'll note that he's seeing this when running on gas or LPG. A plugged fuel filter or bad fuel pump wouldn't explain the lean condition running on LPG.

Did you replace the IAC actuator. 99.9% of them are missing from JY vehicles, and it's purpose is to lean the mixture. at idle. If bad, it would lean the mixture all the time.

Heh? The Idle Air Controller is just there to regulate the bypass air in the throttle body to control idle speed.

 

[larsen]

If this occurs only after driving at speed for an extended period my guess is fuel delivery to the injector rail is marginal. Partially clogged filter or weak/leaking pump.
Check the fuel pressure and delivery volume.

 

[ktwalker01]

If you read the original post, you'll note that he's seeing this when running on gas or LPG. A plugged fuel filter or bad fuel pump wouldn't explain the lean condition running on LPG.

woops, I guess selective reading in oem tech lol

 

[Ecomike]

I am wondering how the computer is controlling the LPG feed rate. How does it know which fuel is being used?

 

[XJSteven]

I didn't mention this earlier but I also replaced the fuel pump and filter but I doubt it has anything to do with fuel pressure or fuel delivery because as I said before it happens on LPG too.

I am wondering how the computer is controlling the LPG feed rate. How does it know which fuel is being used?

Sorry Ecomike but I have no idea how this works.

I would take a high impedance analog volt meter, set for 1 volt scale, DC, and back probe the O2 sensor wires (not the heater wires), to see what it is actually doing.

OK so here's what it is doing:

At idle the voltage is a constant .940 volts. If I create a vacuum leak by removing a vacuum hose it drops to .080 volts, when I plug the hose back in it returns to .940 volts steady. I assume this proves the oxygen sensor is working but there is a problem elsewhere.

At 2000 rpm the voltage fluctuates above and below 0.45 volts, from a minimum below 0.2 to a maximum above 0.8 volts.

So what does this data mean? I'm not really sure...???

 

[Skreed]

That is good...O2 sensors have a voltage of 1 volt. .5v is "perfect" mix so to say, but thats not how the computer works. It will fluctuate between about .2v and .8v to "keep" the mix at or near the target of .5v

A bad reading would be constantly high or low. Lean=low voltage Rich=high voltage

Another bad reading would be if it takes a longer than normal time to add or subtract voltage...that would be a "lazy" sensor.

 

[Ecomike]

This one is puzzling. I am going to say the computer thinks the engine is running lean, and is adding fuel to compensate, but that the computer is not seeing the rich fuel mix data from the O2 sensor. Sounds like a signal voltage drop between the sensor and the PCM to me, bad wiring or connections between O2 sensor and PCM. Check the wires to see if they touched the exhaust manifold and got shorted or damaged.

At 2000 rpm it should lock in close to .45 V +/- .05, so .4 to .5 V range, which is not, indicating the computer has no control.

If the computer was not throwing that code, I would suspect leaky fuel injectors!