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Q: 98 RAV4 runs excellent but throws P0171 code and gets terrible gas mileage. Why? on 2008 Toyota RAV4

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Just bought and it passed smog easily. No vacuum leaks as far as I can tell. idles smooth and runs great. I realize this is a lean run condition so something is causing it to increase the gas mixture too much. There are so many things to check with a P0171 I don't know where to start. I put fuel cleaner in and it is my first tank. I am actually watching the gas gauge go down as I drive. Air filter is fine. No MAF to clean. Need help please.
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These can be tricky and you have to be careful when evaluating the system. While you say it runs great here I wonder if it would feel the same to a tech. When the system sets a lean (or rich) air fuel code it stops correcting the air/fuel ratio. So if it was out of control to the point that the fuel trim system couldn't correct it, it should run pretty bad when it stops trying. If the system has a defect, for instance a downstream sensor that fails and drops the downstream O2 voltage signal, the system will ignore the upstream sensors information and keep enriching the fuel mixture trying to make that downstream sensor respond. It really will drive the engine over rich, while reporting a lean code but you need a technician who really understands the system to test and prove if that is occurring or not.
Thank you for your response. I am an old car mechanic from the 70's. I don't touch the new stuff anymore (no monitors, gauges, computers, etc.) if I can help it. I just try on my own first to save some $ before giving up to the shop. We have a good shop around the corner but I have read so many bad stories about mechanics (even Toyota techs) throwing parts at these codes and sometiomes not even fixing the problem. The car actually drives great! Plenty of power, super smooth idle too. I cleared the code and it came back after first test drive. Tell you the truth, if it didn't get the terrible gas mileage, I would probably leave it alone. I just bought it for my daughter and she works everyday so hard to get it into shop. My shedule right now making it difficult to trade cars too. I have seen o2 sensors changed, EGR's cleaned out, injectors replaced, air filters, cleaning the MAP, plugs changed, fuel pump/filter, intake gaskets, etc. on the repair boards. I started with Techron fuel system cleaner but don't have too much faith in that fixing it. It looks like it will have to go to the shop I guess. Anything else cheap to try at home first? Thanks again.
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You are correct. It can be many things. The first place I would start would probably be the MAP sensor if I did not have proper computer diagnostic equipment. You will find that technicians at dealers will throw parts at problems more often than independant repair shops. Unfortunately without having the vehicle to look at & test all of us here are just guessing as much as you are. If it will help we would be more than happy to take a look & diagnose the problem for you. You can call or go online to set an appointment if you would like. I hope this helped.
Thank you. I saw your shop on Repairpal and saw all of the great customer reviews. You are not far from me at all but conflicting schedules with me and my daughter (her car) is limiting us from bringing it in now. Also weekdays are the hardest and I think you are closed weekends. I will have to figure something out soon as we are losing $ to gas waste. I will be in touch soon and I look forward to seeing your shop.
Thank you for the suggestions. It looks like I will need to take in as it is beyond my capabilities. I don't think it is a vacuum leak (I could be wrong though) because it idles so smoothly and at the right rpm.
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