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Dan, ASE Certified Technician
Category: Toyota
Satisfied Customers: 1668
Experience:  Auto Technician
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I have a 2004 SR5 4WD 4.0 164,000 miles. I have been having

Customer Question

I have a 2004 SR5 4WD 4.0 164,000 miles. I have been having issues with P0171 and P0174 codes. I have replaced both AFRs both 02 sensors, and the MAF twice. All parts are DENSO. smoked engine for vacuum leaks and visually inspected for exhaust leaks.Clear code and it comes back on within 20 miles of driving. Car drives ok, although idle is low while in gear (sometimes). Also exhaust sometimes has very rich smell,
JA: Does the engine stall right after starting? And what about hesitation or jerking during acceleration?
Customer: No runs and accelerates well. Gas mileage seems to be in line
JA: What year is your 164?
Customer: ??? 2004 with 164,000 miles
JA: Are you fixing your 164 yourself? What have you tried so far?
Customer: I have done some myself and others at local shop. My original message indicates everything I have done
JA: Anything else you want the mechanic to know before I connect you?
Customer: No I think that is it
Submitted: 8 months ago.
Category: Toyota
Expert:  Dan replied 8 months ago.

Hello my name is***** am an ASE Master Technician and I am glad to help you with your question.

Expert:  Dan replied 8 months ago.

You need a scan tool that can read live data, you will be looking at the short and long term fuel trims at idle.

Customer: replied 8 months ago.
I don't have access to that. Since I replaced the MAF twice do you think it could be something with the wiring harness or something causing the MAF to die?
Expert:  Dan replied 8 months ago.

I would look for a vacuum leak, not all vacuum leaks will show up on a smoke test, have you tried spraying the engine with throttle body cleaner, while the engine is at idle, if the rpm changes then you have a leak. A scan tool is needed to diagnose this, you need to see live data. Especially fuel trims.

Customer: replied 8 months ago.
Yeah tried both smoke machine and carb cleaner a couple of times and can't find a leak. Had 2 mechanics look at it and neither can figure it out so far. I'm going to take it back to the shop this week to have them look again.
Expert:  Dan replied 8 months ago.

If the sensors see too much oxygen in exhaust the computer will add fuel causing your rich smell out the exhaust. You need to see on the scan tool if the computer is actually adding fuel. Look at MAF, fuel trim, and front oxygen sensor data.

Please reply to this if you need more help and we can continue this conversation at no extra charge. This does not have to be my only answer. If you are satisfied with my service, please rate at the top of the screen. Rating does not close out your question. Remember, sometimes it is difficult to completely diagnose your vehicle without being there, but I will try to give you the best answer I can. Hoping to be giving you 5 star service.


Customer: replied 8 months ago.
I would like to see what my mechanic says this week and the run that by you. Probably mid week.
Expert:  Dan replied 8 months ago.

OK Sounds good. Thanks