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Rebuilt a 2003 Mustang 3.8L engine. At idle, engine has a…

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Rebuilt a 2003 Mustang 3.8L...
Rebuilt a 2003 Mustang 3.8L engine. At idle, engine has a slight backfire, at higher rpm the engine runs great. I'm getting two codes P0141 and P0161 even after I replaced all the O2 sensors. Where do I go next?
Submitted: 2 months ago.Category: Ford
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Answered in 3 minutes by:
6/30/2018
Ford Mechanic: Pete, Auto Service Technician replied 2 months ago
Pete
Pete, Auto Service Technician
Category: Ford
Satisfied Customers: 33,905
Experience: Over 17 years diagnosing and repairing automobiles various makes and models with some training and certifications.
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Hi, my name is ***** ***** I will be happy to assist with this today. In some cases, I may need more info to assist better. I apologize for any delay.
I try to be here most weekdays and some weekends. If you respond and I do not respond back right away I will do so as soon as I can.

Has anything else been checked or tested for this issue here at all?

Let me know,thanks Pete

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Customer reply replied 2 months ago
Other than looking for vacuum leaks -no!
Ford Mechanic: Pete, Auto Service Technician replied 2 months ago

Is the backfire from the exhaust or the throttle body?

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Customer reply replied 2 months ago
Exhaust
Ford Mechanic: Pete, Auto Service Technician replied 2 months ago

Does it do this backfire from idle if you were to push the gas pedal to fast or on take off from idle? Does it feel like a hesitation then backfire issue?

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Customer reply replied 2 months ago
When I push the pedal the engine ramps up with no hesitation. When I release the pedal, the engine goes to idle then starts the backfiring. I have not driven the car yet, I will do that shortly and let you know the results.
Ford Mechanic: Pete, Auto Service Technician replied 2 months ago

This sounds like the engine is running lean caused by low fuel pressure or a vacuum leak.MOst common is a vacuum leak and can be a hard to see leak like the intake gaskets leaking. It may also be an EGR fault as the egr can open partially on decel and if it is leaking allowing air past it this back fire will occur.

I don't see this as a fuel pressure issue since it runs o.k. but I wouldn't rule it out. If you have not had the fuel pressure tested this should be done as well to see if there is a pressure issue coming from the fuel pump itself.

If this is o.k. you may have to smoke test the intake injecting smoke into the intake to locate any hard to see leaks like intake gaskets or throttle body gaskets leaking.

The other thing to check is to make sure the timing was installed and set up correctly. If off a tooth this can also occur here.

And one other thing to do is perform a crankshaft relearn. This is usually done with a scan tool that has the function to perform this relearn and it can resolve an issue like this as well.

These are just some ideas that can cause this fault since I am unable to see the vehicle to help diagnose it. But these do need to be checked into further.

The codes that are set are for the heater operation of the oxygen sensors. If there is a blown fuse this can cause the heater codes to set as well as damaged wiring leading to the sensors or from the computer itself. But the heater issue here on the sensors won't cause the backfire your seeing here.

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service, your 5-star rating is appreciated ( should be a link to click on here or above). You can rate at any time and we can continue to work on your question as this will not close out your question. I work on delivering the best answer and help here but in some cases, it can be difficult to do without seeing the vehicle.

Thanks, Pete

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