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Mechanic replaced knock sensors, wireharness, coolant hose,new gaskets, plugs, PCV, Thermostat. Now getting codes174 and 171 triggered at highway speed. Car runs good. Some loping at idle. No obvious vacumn leaks. 2003 Avalon XLS with 150K miles. Wondering if using propane to detect leak is still appropriate-feel this could be a little dangerous. Any help would be appreciated.
 

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2000 Toyota Avalon XLS
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Wondering if using propane to detect leak is still appropriate-feel this could be a little dangerous.
I don't recommend it. But if you choose that path make sure the engine is cool and you can connect a long hose to the propane tank. Know the risks before you go down that road; have a fire extinguisher on the ready, eye protection, thick gloves, work in an open area etc. Your spark plugs, injectors or sensors could be arching and well...🔥 The starting fluid method is probably the most dangerous IMO though. I ended up using a smoke machine to find my vacuum leak. More expensive but I'm not a fan of the risk related to the other methods. Good luck(y)
 

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2001 Avalon XLS w/ JBL Audio 215K mi.
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Before you blow yourself up, triple check the boots on both sides of the throttle body. a tear in one of these is easy to do, hard to see, and would explain both banks running lean when sucking a lot of air at highway rpms.

HTH
 

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Running lean at higher RPM normally indicates fuel delivery problem or MAF faulty.
Running lean at idle also.?
Can you post fuel trim numbers and MAF at idle and at 3000RPM?
 

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2001 Avln, 2009 Taco
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My '01 Avalon with 91K miles threw the P0174 code. I cleaned the MAF, cleared the code, and checked for vacuum leaks. A week and 200 miles later it threw the code again. Frustrated, I filled the car with NON-Ethanol gas, looked for vacuum leaks (found none), cleared the code again, and have had no issues since (over 2K miles).
 

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Discussion Starter #7
I don't recommend it. But if you choose that path make sure the engine is cool and you can connect a long hose to the propane tank. Know the risks before you go down that road; have a fire extinguisher on the ready, eye protection, thick gloves, work in an open area etc. Your spark plugs, injectors or sensors could be arching and well...🔥 The starting fluid method is probably the most dangerous IMO though. I ended up using a smoke machine to find my vacuum leak. More expensive but I'm not a fan of the risk related to the other methods. Good luck(y)
 

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Discussion Starter #8
Yes, very very hesitant using that method. If needed, where do you get a smoke machine-can it be rented or do you have to buy one?

Thanks
 

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Discussion Starter #9
Yes, plan on looking at the hoses much closer to see if any cracks. Thanks
 

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Discussion Starter #11
Not throwing those codes at idle or in town non highway driving. Wife reported the engline light coming on at 60 MPH on the highway. Plan on taking for a ride to see if can get codes to trigger and will post idle and 3000 rmp fuel trims and maf . Thanks
 

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Discussion Starter #12
Before you blow yourself up, triple check the boots on both sides of the throttle body. a tear in one of these is easy to do, hard to see, and would explain both banks running lean when sucking a lot of air at highway rpms.

HTH
Plan on checking boots alot closer. Did a cursory review and did not see anything obvious but could be on underside of hoses.
 

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Discussion Starter #13
My '01 Avalon with 91K miles threw the P0174 code. I cleaned the MAF, cleared the code, and checked for vacuum leaks. A week and 200 miles later it threw the code again. Frustrated, I filled the car with NON-Ethanol gas, looked for vacuum leaks (found none), cleared the code again, and have had no issues since (over 2K miles).
Installed a new MAF not long ago but will check to see if needs cleaning. Where do you get the non-ethanol gas? Does premim have ethanol? Thanks
 

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2000 Avalon
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If you can look at freeze frame data and see when the codes set, it may be helpful. Vacuum leaks tend to set at idle, high fuel trims at idle. Dirty MAF sensors tend to set while driving and under load, high fuel trims. Telling us the fuel trims, long term especially and the engine rpm and vehicle speed would tell us a great deal.
 

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2001 Avalon XLS w/ JBL Audio 215K mi.
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Installed a new MAF not long ago but will check to see if needs cleaning. Where do you get the non-ethanol gas? Does premim have ethanol? Thanks
cj1's and Frankiemenz's post makes me want to ask if your recently replaced MAF sensor is O.E. Toyota, or Denso from a reputable auto parts store. It's easy to read it right off the sensor near the air cleaner. Failed knock sensors affect ECU ignition timing, and now fixed other issues may be revealed.

If you didn't have this problem before you had your Avie serviced, then of course you are correct to carefully look over everything that was touched during repairs. After replacing my knock sensors (tore an air boot in the process) I found that an upstream A/F sensor was bad. Since then, my 220K 1MZFE engine has run cool and efficiently on a steady diet of Denso, Aisin, Pennzoil Platinum HM, and Shell gasoline.

Best of luck with your sleuthing.
 

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Discussion Starter #17
If you can look at freeze frame data and see when the codes set, it may be helpful. Vacuum leaks tend to set at idle, high fuel trims at idle. Dirty MAF sensors tend to set while driving and under load, high fuel trims. Telling us the fuel trims, long term especially and the engine rpm and vehicle speed would tell us a great deal.
[
Attached is the data from my Autel OBDII tool when the car running at idle and when I an increased the RPMS. Car was fully warmed up. Drove on highway and no codes triggered but when I parked triggered both lean codes at idle. Have not had a chance to fully inspect hoses yet. Did inspect MAF sensor which looked Ok but cleaned anyway. The MAF is a DENSO OEM I replaced about a year ago.
 

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You have a vacuum leak, most likely. Codes set at idle, vehicle speed at zero.get some carb cleaner and spray around vacuum connections and intake manifold. Keep in mind carb cleaner is flammable, use a bit and wait for it to evaporate , don’t spray and let it build up in puddles on the engine.
 

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Discussion Starter #19
Removed the engine cover and using soapy water was able to find the vacumn leak (little nervous using the carb cleaner method). The ventilation hose that runs from valve cover to the air intake box was cracked at the valve cover connection. Hose was hard as rock. Applied some electrical tape to seal of leak and tested using OBDII and fuel trims back to normal! Ordered a new hose and replaced today. Had a difficult time replacing until I figured out that my mechanic had reinstalled backwards after they replaced the knock sensors etc. Suspect it may have cracked when they reinstalled the hose. So far no codes at idle or highway driving. Also runs better at idle no up and down of RPMS. Will keep a watch. Thanks.
 
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