[TheNackNick]

Hello everyone,

Have been chasing this P0171 code on my 2016 Mazda 3 (65,000 miles) for months now, to no avail. Hopefully someone could give me some advice.

I have CEL with a P0171 (Lean on Bank 1) code being thrown. Car has an aftermarket Axel-back Exhaust, and HAD an aftermarket CIA, but was swapped back to stock intake. Dealer also said the Fuel Pump was recently replaced.

I have checked for leaks and cannot seem to find anything. Dealer did a smoke test for me and said: "It failed" (whatever that means) but they we're not able to identify smoke leaking out at any spot.

LTFT sits at +20% sometimes dipping to +17% at higher RPMs
STFT sits at +20-+25% and often falls to 0 (sometimes at idle, sometimes at higher RPMs

Here is everything I have done so far:
-Checked hoses for any cracks or leaks (all seem fine)
-Tested MAF sensor (also replaced with a spare I had, both function fine)
-Replaced PCV Valve, and double checked PCV hose
-Replaced Upstream O2 sensor
-Cleaned Throttle Body
-Replaced all Intake Manifold Gaskets (also cleaned intake manifold)
-Replaced Canister Purge Valve
-Checked Oil Cap O-ring & Dipstick O-ring (seem fine, tight fit)

Next on my list:
Check for Exhaust leaks

Any advice would be greatly appreciated, I am pulling my hair out over this.

 

[JoeMini]

Yes - check for exhaust leaks
I had that issue with my P0171
With the lean you are either getting air that is not measured by MAF that is picked up by O2 sensors (both)
or fuel is lean - injectors clogged / not spraying properly - Use a good fuel system cleaner like a Liqui moly fuel system cleaner.
Another possible is fuel pressure regulator and then fuel rail or slow/clogged flow fuel filter

I would go least expensive route first - fuel system cleaner and check exhaust leaks

 

[TheNackNick]

Ah you reminded me, that I have also ran fuel system cleaner through the tank. will do it again though.

Exhaust leaks is definitely not my wheelhouse. any tips on this endeavor?

 

[zero_gravity]

Exhaust leaks are easy to find. Jack up the car (or use ramps) and make sure it's in park with the back wheels blocked. Start it up and use your ears. Look for black soot, a leak will leave this behind.

Your dealer said it failed a smoke test. What did they test? Usually the EVAP system, but this doesn't tell us anything other than a fault was found. EVAP leaks can be a devil to find....

You seem to be replacing some parts at random here such as the O2 sensor, gaskets and purge valve. You need to stop this. It's about as good as throwing a can of bolts at a car and replacing whatever the bolts hit. Diagnose first, and then replace. Your wallet will thank you.

 

[RocketmanKarl]

Firing up the parts cannon is a bad way to fix a car, you end up hundreds or thousands poorer and the car still doesn't work. As others have said, diagnose first, then fix.

Are there any other codes, and does the car seem to run okay? BTW, driving a car that is running lean is a good way to burn up the motor, you want to get this figured out asap.

As for the smoke test, if it fails but they can't tell you where it's leaking from, then you shouldn't take your car there ever again. They either didn't do the test at all (cheating you), or they don't know how to do the test (incompetent), but either way you shouldn't let them touch your car.

You can make your own smoke generator and do the intake leak test yourself, lots of videos on YT. Another thing to consider is a compression/leakdown test.

As for checking for exhaust leaks, there's a better/safer way to do it than climbing under a running car with a hot exhaust. You'll need a shop vac that you can swap the hose to the exhaust port (so it goes from sucking to blowing - here's where you put in the SpaceBalls MegaMaid jokes), and a spray bottle of soapy water. Anyway, with the car off and cold, jack it up (jackstands for safety), and tape the vac hose to the tailpipe. When the vac is running it will be blowing air into the exhaust system, forcing air to blow out of any leaks. At this point you can start spraying the exhaust with the soapy water, and looking for bubbles that pile up or grow in size, just like looking for a leak in a tire. Start where the exhaust manifold bolts to the block, then spray every joint and the flex pipe till you get to the O2's (after the O2's won't matter). It might be hard to spray some of the connections under the car, but keep at it.

 

[TheNackNick]

yes. I understand throwing parts at the problem won’t fix it. I have only replaced the parts that I have seen to have an issue and could use replacing along the way. For example, the dealer installed my intake manifold and caught some of the foam under the seal during installation, which ruined my intake gaskets. They needed replacing.

Basically the dealer has caused more problems and I don’t trust any work they’ve done, including the smoke test.

i did test the vacuum pressure, and everything is at spec. I still want to do a self smoke test for piece of mind, but I am not suspecting vacuum leaks at this point.

I did run some liqui moly through the tank as someone suggested. This has caused my code to randomly clear itself. While monitoring live, the STFT now seems to around 0 more often, but still jumps to +25% occasionally bringing the code back. ( I have not manually cleared the code at any point during this)
LTFT stays at +20% at idle and load.

I found a few obvious exhaust leaks during preliminary inspections today, but they are way after the cat % o2 sensors. Will still address them, but I can’t imagine they are causing the issues I’m dealing with.

still plan to dive dealer looking for exhaust leaks with the shop vac method.

 

[JoeMini]

Would blowing smoke up the tail of the exhaust be the best way to find a leak up to exhaust ports / manifold ?

The area mine happened is in the down pipe connection - that flex pipe piece - between the headers and 2nd O2 sensor
there are several wield that can rust and create a pin hole for air to be sunked in after combustion.

 

[zero_gravity]

Would blowing smoke up the tail of the exhaust be the best way to find a leak up to exhaust ports / manifold ?

Now you're just blowing smoke up his ass

 

[DBarnes]

Firing up the parts cannon is a bad way to fix a car, you end up hundreds or thousands poorer and the car still doesn't work. As others have said, diagnose first, then fix.

Are there any other codes, and does the car seem to run okay? BTW, driving a car that is running lean is a good way to burn up the motor, you want to get this figured out asap.

As for the smoke test, if it fails but they can't tell you where it's leaking from, then you shouldn't take your car there ever again. They either didn't do the test at all (cheating you), or they don't know how to do the test (incompetent), but either way you shouldn't let them touch your car.

You can make your own smoke generator and do the intake leak test yourself, lots of videos on YT. Another thing to consider is a compression/leakdown test.

As for checking for exhaust leaks, there's a better/safer way to do it than climbing under a running car with a hot exhaust. You'll need a shop vac that you can swap the hose to the exhaust port (so it goes from sucking to blowing - here's where you put in the SpaceBalls MegaMaid jokes), and a spray bottle of soapy water. Anyway, with the car off and cold, jack it up (jackstands for safety), and tape the vac hose to the tailpipe. When the vac is running it will be blowing air into the exhaust system, forcing air to blow out of any leaks. At this point you can start spraying the exhaust with the soapy water, and looking for bubbles that pile up or grow in size, just like looking for a leak in a tire. Start where the exhaust manifold bolts to the block, then spray every joint and the flex pipe till you get to the O2's (after the O2's won't matter). It might be hard to spray some of the connections under the car, but keep at it.

Good tip!
I have always used a cheap stethoscope (from Harbor Freight Tools) with 3’ of rubber hose coming off the tee, then electrical tape a 2’ piece of coat hanger along the bottom length of that hose, then stand above, or rack the vehicle and stand underneath, with the engine running. If it’s an automatic transmission, power braking it in gear a little bit makes the leaks much louder. I also use it for belt/idler pulley/pulley bearing noises, or wind leak noises while inside the car on the freeway