1999 Mercury Mountaineer p0171 code please help

[Robdabest2015]

First off hello to everyone I am new here.

The problem I am running into would be the code p0171. I purchased a 1999 Mercury Mountaineer 5.0 v8 4x4 a couple weeks ago from my brother in law's brother. I had several other codes I have cleared and fixed but this is my main concern. The exhaust was replaced and was advised that a straight pipe was put in and told that 02 sensors couldn't be put on the truck. Normally I wouldn't mind but my fuel economy has been terrible. I drove less than 70 miles and burned though almost a half tank of fuel.

Other codes I have had come on but have since fixed
Rear ABS Sensor (Speedometer stopped working but fixed after replaced)
DPFE High Voltage (Replaced and code went away.)

My question is what can I do to improve my fuel economy? Would this require me to get a new exhaust that accommodates 02 sensors? Or is there something I can hookup to bypass those sensors?

Thank you everyone for your time.

 

[Faerco]

This is absolutely the fault of not having o2 sensors. Was the ECU flashed to remove their usage when the headers were installed? I do know that dumby sensors exist that you can buy to trick the ECU into thinking that the voltages are correct, but without those or a proper exhaust your fuel economy is going to tank like that.

 

[Robdabest2015]

This is absolutely the fault of not having o2 sensors. Was the ECU flashed to remove their usage when the headers were installed? I do know that dumby sensors exist that you can buy to trick the ECU into thinking that the voltages are correct, but without those or a proper exhaust your fuel economy is going to tank like that.

Great question, I looked under there briefly the more I look at it looks like just the muffler and back was replaced. I am going to see if I can find the 02 sensor. If I can I am going to look and see if I can hear or find a vacuum leak as I was told this is possible too. Should be able to update that here soon.

Yay cold michigan day

 

[Robdabest2015]

So, I checked and found out I do have 02 sensors installed on my truck. I also want to note that I pulled out my MAF Sensor and Cleaned it. I also did a visual test on all lines that I could see for any noticeable cracks or wear on the lines. I couldn't find any so I reset the code and drove another 20-25 miles then the check engine light popped back on and started blinking. Pulled over turned off and back on, light went away then came back 5 miles later.

 

[Robman]

I don't have a 5.0 but if that's a lean condition code.. it could be o2 sensor(s) or intake gasket no?

Do the code outputs specify which o2 sensor is bad if one is bad?

If an exhaust was preventing a vehicle's o2 from being installed, I don't see why you couldn't drill a hole and weld on an appropriate nut to make your own bung.

Hmm what else could cause a lean code? egr stuff?

Just thinking outloud.

 

[Robdabest2015]

I don't have a 5.0 but if that's a lean condition code.. it could be o2 sensor(s) or intake gasket no?

Do the code outputs specify which o2 sensor is bad if one is bad?

If an exhaust was preventing a vehicle's o2 from being installed, I don't see why you couldn't drill a hole and weld on an appropriate nut to make your own bung.

Hmm what else could cause a lean code? egr stuff?

Just thinking outloud.

Robman,

Unsure but I think I am going to take it into Tuffy's tomorrow he said he would do a free diagnosis and hopefully they can rule out some things hopefully its as simple as a vacuum leak and no major repair needed.
I really hope its not the gasket

 

[Robdabest2015]

Okay, So I ran to Advance today and they pulled another code, P0303 Cylinder 3 misfire, I am going to pull that plug and see what it could be. Hopefully its just faulty as I replaced that when I got the truck. Other codes went away I guess cleaning the MAF sensor cleared that code

 

[imp]

Running "lean" means not getting adequate fuel. That fact is directly in opposition to using way too much gas in matter of fact.

The O2 sensors are actually vital in maintain proper injector "trim" in that they actually "take over", once the engine reaches operating temperature, and relieve the computer of actually controlling the mix, which it does only when cold. This process is called "closed-loop operation". imp

 

[Turdle]

My diagnosis is bad injector on #3 .
This would create a lean condition on bank one and trigger a cylinder 3 misfire.

By the way, your truck should have 4 o2 sensors installed, 2 on each bank.

The bank one upstream sensor ( p0171) is located right in the downpipe-primary cat on the passenger side, tucked in next to the bell housing. The bank 2 primary sensor is directly under the transmission, the remaining catalytic converter function o2 sensors are between the primary and secondary cats.

 

[Robdabest2015]

My diagnosis is bad injector on #3 .
This would create a lean condition on bank one and trigger a cylinder 3 misfire.

By the way, your truck should have 4 o2 sensors installed, 2 on each bank.

The bank one upstream sensor ( p0171) is located right in the downpipe-primary cat on the passenger side, tucked in next to the bell housing. The bank 2 primary sensor is directly under the transmission, the remaining catalytic converter function o2 sensors are between the primary and secondary cats.

So you wont believe this,

I dropped under to look at cylinder 3 and I spotted the problem right away. The plugs that I had replaced previously I had mistaken to push the heat shield from the wires back in place to prevent the headers to burn the wire. The result was a almost burnt thought wire for cylinder 3, ran to advance and picked up a new set and installed in less than 15 minutes. And everything's perfect, no check engine light and no hesitation on the hwy. I also now realize that it was a rough idle I had prior to doing this(I figured it was the truck and the V8) I came from a focus lol. Now it perrrfect! Runs like a champ )))


Edit *Thanks for the info on the 02 sensors I located 2 and assumed the others were there as well ***