So here's something no shop can figure out. Started driving one day and the engine seemed to be surging a little bit, then the MIL light came on and threw me a P0171 (Bank 1 mixture too lean) code. Alright, makes sens so far. I recently had a new exhaust system installed, (cat., muffler, new upsteam O2 sensor from the dealer). This happened about 800 miles after the new exhaust and O2 sensor was installed.

So I get on the highway an notice I just burnt half a tank of gas driving about only 40 miles distance, and anyone who drives a D knows that half a tank of gas is worth like 5 pounds of gold right there haha. Anyways it seems my MPGs vary a ton with load, a little more load will make them fall faster than the stock market. Highway cruising at 60MPH gets me an even 9MPG at best. Accelerating normaly from a stop gets me 2MPGs.

So far I've checked for vacuum leaks, there are none, cleaned the MAP sensor so that's all good. Unplugged the O2 sensor, reset the computer, still always getting 9MPG on the highway, so it's probably not the O2 sensor.

Any ideas? I just don't get how I can be getting a too lean code and everyday I have to put $20 of fuel in it to drive to work and back which is only 30 miles both ways.

 

unplugging the 02 sensor does not prove that it is good, if you got an 02 sensor code it usually means that the sensor is bad, especially the one you got, the other options are low fuel pressure causing it to be lean, im sure hydra, or white widow will chime in with more ideas, but it sounds to me like you got a bum o2 sensor or you have a wiring issue btwn the o2 sensor and the pcm

 

Ah, everywhere I went said the O2 sensor is fine, even the dealer. But I'll be taking it back to them and getting another one here soon. I heard something about the O2 sensor always saying the mixture is too lean, but it wasn't crazy enough to throw a voltage code meaning a bad sensor.

It was working perfectly for about 800 miles then just all of a sudden happened, but I'll get that sensor replaced with a new one and let you guys know what I find out.

 

Well here's an update, Just went to the dealer and bought a new upstream sensor. They wanted $120 for that thing, what is this world coming to. They have to overnight it so I'll get it tomorrow morning and install it.

On a side note it was pretty funny pulling up and parking my D right in between two new 2011 Ds.

Anyone know if the downstream sensor is only for monitoring the CAT. or does it do any tuning of the fuel mixture like the upstream sensor does? Because that's an aftermarket sensor and it's doing fine not throwing any codes but I only want dealer sensors to control my fuel.

 

post cat is only for monitoring, does not affect fuel mixture

 

My opinion is the O2 sensor is not the issue but in fact you are running lean. Toucan take it to a shop and have them plug it in to do a diagnostic on your air/fuel mixture. I believe your O2 sensor is dead, now. Running too rich or lean for too long will burn up your O2 sensors.

Get that new sensor and put it in and see if that changes anything.

The most common reasons I've seen this erected from is due to the MAP sensor. Since you've checked vacuum lines, if replacing the O2 sensor doesn't help, look into replacing the MAP sensor.

Also how long has it been since you replaced the PCV valve? A stuck PCV will cause it to suffocate and run rich.

 

I replaced the PCV valve during the motor rebuild, the old one was in the stuck open poisition. It is running pretty rich though, when I go to start it cranks for like half a second and fires right up, and all I smell out of the exhaust is unburnt gas. It could very well be a MAP sensor and I'll replace that next if this O2 sensor doesn't do anything, quickly though before this new sensor gets ruined too. I asked the parts guy how much is a non-gold plated one, these things shouldn't cost that much.

Just got a new cat. installed not even 2 weeks ago so it would suck if this cat. was ruined too, the last one I got replaced not even half a year ago either.

If the O2 sensor and MAP sensor don't do it, then I'm just going to push my D off a cliff, I already have one picked out, and cut my losses and get a new 2011 D. I think mine is getting to the point where it's just rediculous and more money than it's worth to maintain it anymore. With all the money I spent in gas just in 3 days driving mine how it is, it would cover the first two payments on a new D easily.

 

Sounds like you've been running rich for quite a while then....

 

if you put too small a cat on i have heard of that causing problems it should be the same size as the origional

 

I just want this whole nightmare to be over already. I remember a time when i was getting 21MPG on the highway, or at least that's what the lie-o-meter was telling me, now it's telling me 9MPG. I noticed the engine has really low torque untill I hit 3000 RPMs then it all of a sudden just pulls me like it should when accelerating down a road. The replacement cat. is a universal one for a truck, the housing is larger than the stock so it should be alright.

I've come to realize over the years if I just shell out the money at the dealer for any sensor, it will pay itself back pretty quickly in the fuel savings, not to mention last longer.

I'm taking my D into a shop today to let them figure it out, I have the new O2 sensor still sealed in the bag the dealer gave me so I'll give that to them and tell them to have fun.