Code P0171 Running Lean Bank 1

[David]

I have a 1999 honda civic ex completely stock. about 7 months ago I failed my emissons test and was told my cat was bad which then lead me to do the biggest mistake ever and try scotty kilmers lacquer thinner method to clean a cat, not saying his method is bad buf in my case it didnt work for me… So after I did that I ended up getting a misfire on all cylinders but I fixed that and the problem was a fuel pump so I got that out the way but shortly after I fixed it I get another CEL P0171 which is Running to lean bank 1 code, I change 02 sensor and that wasnt the problem. What steps should I take in fixing this problem? I been driving my car like this for about 5 months…

P.S also when ever The car is idling the rpm gets really low and almost feels like the car is going to stall… And I also hear a sound coming from underneath the car which sounds like a cow mooing … If that helps

Thanks in advance

Topic

[Richard Kirshy]

Almost any sensor that monitors airflow or fuel pressure can cause a lean condition. This includes not just the O2 and MAF sensors, but the manifold air pressure — MAP — sensor, intake air temperature sensor and even the sensor that monitors the exhaust gas recirculation system. An EGR stuck in the open position will act just like a massive vacuum leak, allowing excess air from the exhaust to re-enter the engine in an uncontrolled manner. Any of these should trigger a check-engine light.

Legitimate vacuum happen anywhere intake manifold vacuum has a chance to pull air in from the outside. Any number of hoses and lines could leak, but so could loose air intake hoses and leaking intake manifold gaskets. The old mechanic’s trick is to spray ether starting fluid in short bursts at the suspected vacuum leak. If a vacuum leak is present, the engine will suck the starting fluid in, smooth out, briefly increase rpm and run properly for a few seconds. Be careful with this though — starting fluid is extremely flammable, and doesn’t get along well with electrical sensors and connections.

[David]

Thank you I will try the starting fluid method.. Also how would I go on about checking if my sensors are functioning properly? this is my first car and Im trying to learn and become mechanically inclined

[Steve C]

I had this same problem on my 06 G35 coupe for several months w/same exact code. I swapped out O2 sensors, MAF sensor, and sprayed unlit propane and carb cleaner around everywhere trying to see if I had a vacuum leak – Nothing. So, I broke out the vacuum gauge hooked it up directly to the manifold noticed the Mg and RPM’s would raise and drop like clock work. Next, I started isolating the manifold from the various hoses (ie: disconnecting a brake booster line, PCV line, etc..). I narrowed it down to somewhere in the brake booster line since the engine ran perfectly when that was capped off, but every single hose still looked good with no cracks.

I had to get creative and built my own DIY smoke machine, which consists of baby oil, small paint can, some wire that gets hot, air compressor/lungs, and battery power. Start blowing the smoke into the lines and low and behold an o-ring behind the brake booster was bad and causing this terrible vacuum leak. Ended up costing my 15 bucks to replace, but my vacuum leak went away along with the 171 and gas mileage returned to normal.

I’d start with the hoses, since your car is 99 and look at a vacuum diagram and make sure all those hoses are good.

[Author]

Replies