CEL code P0171 System Too Lean

glynster · 07-04-2009, 08:37 AM

I got an engine light and I read the code as "CEL code P0171 System Too Lean. Bank 1"

I'm no expert with cars but I believe that I should check the hoses for leaks - which ones?

The engine seems to run fine - no problems but if I was to look for them with a kind of car-hypercondria then I might say 2 things...

1. When first idling from cold the engine does a slight soft wobble every few seconds (just like the car has been very slightly buffetted by a gust of wind) but it's not much, others cant detect it and maybe I am being paranoid and a friend of mine who used to be a bike mechanic says that's normal for injection engines when they are warming up and might just be the autochoke - is that right?

2. The only other thing I have noticed is that when the engine is hot and the ignition is turned off for a few mins and then the car is re-started the fan is very loud and revs high as the engine revs and drives away but it turns off after a few seconds but again because this is a new car to me I dont know if this is normal and just colling the engine which has warmed while the hot engine was turned off for a short while - am I right?

Thanks

Glyn
Venhicular-Health-Anxiety Expert

Baratacus · 07-04-2009, 01:06 PM

Usually A too lean condition indicates an air leak after the MAF sensor. If it's intermittant, it could be a loose air intake coupler in front of the MAF opening and closing with movement and vibration of the engine. Someone had problems with the connection to the air box pulling off and creating a surging and mixture problem. You could have a dirty MAF sensor which is easily cleaned. The problem may be a faulty O2 sensor. The problem could also be from a fuel supply issue. Instead of too much air it could instead be getting not enough fuel. Fuel can be choked off by dirty injectors or a clogged fuel filter.

The engine shouldn't lope at idle. The idle can shift up and down as different accessories load up the engine. When you push the brakes, it puts a small load on the alternator to run the brake lights and the vacuum booster draws some vacuume to assist the brakes. When you turn on the AC the compressor puts a load on the engine. When you turn the steering wheel, the Power steering pump puts a load on the engine. When you turn on the headlights, the Alternator loads the engine. Whenever there is an extra load on the engine at idle, the idle has to kick up the rpm's a little to maintain idle speed. The RPM shift should be constsistant with something being activated to put a load on the engine and it should be an even idle up, not a fluttering idle. This is done with the Idle Air Control Valve. If the IAC valve is sticking open it could be letting in too much air at idle and leaning out your mixture. It can also cause surging and bogging down if its open on acceleration.

glynster · 07-06-2009, 09:22 AM

Thanks for the detailed reply - I appreciate it. I'll start with cleaning the MAF and checking visually for any obvious vacuum leaks and then reset the cel and see if it reoccurs - if not, your last suggestion is worth investigating.

The car has been dealer serviced regularly - do they replace oxygen sensors at any point in the dealer service schedule? I just wondered.

Baratacus · 07-07-2009, 12:49 AM

I think the O2 sensor is around 40K miles... I don't know for sure. Most cars trigger the Check engine light between 40k and 60K miles to tell you to replace the O2 sensors. They last a lot longer in normal operating conditions, but they recommend replacement way before they go bad. Your O2 sensor may be fine but you could have dirty contacts or connectors which would give the same result as a defective sensor.