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Lean trouble code question... (Not answered elseware)

>>> UPDATE <<<

I just discovered that the car is throwing both P0171 and P0174, so updated recommendations are welome.

Hi Everyone…

My mom’s 1999 ford Taurus 3.0L 4V is throwing a P0171 code.

Lots of info online about that but I have seen nothing regarding my below question.

So, I am wondering…

Of all the different things that can cause this, does the fact that when you first start the car it surges to about 1500 to 2000 RPM for about 10 to 15 seconds then idles down and runs like crap tend to lead you guys in a certain direction? (not sure about warm or cold startup)

Any ideas about this? Or… if you were working on this car, in what order of possible causes would you begin to check for the issue?

Thank you,

PS - A bit of smoke coming out from in-between firewall and back of engine. Smells like burning oil but I checked oil level and it was full. My mom is 85 and rarely checks anything or has any kind of maintenance done.

There are four primary inputs into the computer when the engine is started cold.

These are the crankshaft position sensor, the coolant temp sensor, the throttle position sensor, and in your case, the MAF sensor.

I would look at the intake hose between the MAF sensor and the throttle body for leaks.

Because if there’s a leak, this can confuse the computer during a cold start. Then once the engine warms up and the O2 sensor(s) come online and detect a lean condition because of a leak in the hose after the MAF sensor, this can set that code.


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When your vehicle was designed, engineers developed fuel curves for various conditions and loaded them into the PCM (computer). These values are used during cold operation before the O2 sensors kick in and take over control of the amount of fuel that is injected into the engine.

For some reason, the vehicle now needs more fuel than originally predicted. Some of that can be due normal wear and tear but other factors such as worn out spark plugs, dirty MAF sensor or any air leak in the intake after the MAF. An air leak can be the gasket around the MAF, any leak in the duct between the MAF and the throttle body, a leaking throttle body or a leaking intake manifold gasket.

Once the engine goes into closed loop, that is when the fuel trim is controlled by the O2 sensors. The amount of fuel needed to maintain the correct air/fuel ratio is compared to those values that were calculated and programmed into the PCM and if the amount deviates by 25% or more, one of the fuel trim codes such as the P0171 is set and the MIL (check engine) lamp is illuminated.

In closed loop, the engine runs good because the O2 sensors are making sure the correct A/F ration is maintained but when cold or before the engine goes into closed loop, it may not run very well because the original data is being used.

I would think a leak there would also set a P0174 for bank 2

Thank you guys for the kind responses. I do believe that the condition is in closed and open loop mode. Immediately after (10-15 seconds) the vehicle could not be in closed loop already, I think. And it runs very poorly right after the initial 10-15 second surge at startup.

Thanks everyone again for your kind help,


PS - I forgot to mention that the car starts just fine :slight_smile:

I’ll just mention that I did have a bad MAF right out of the box. The engine ran fine but would give a lean code. Not saying that’s the problem but can cause a lean condition. The only way I found it was by swapping the MAF from my other car with the identical engine. Normally though when I have had a bad MAF, it acts like its being starved for fuel and won’t hardly run. You could clean it too to see if that helps the lean condition. I don’t think that would be related to the high initial idle though.

The engine rpm surging is consistent with an air leak into the intake manifold somewhere; i.e. vacuum leak. Could be a faulty IAC gadget, or even something as simple as a faulty pcv component or leaky power steering pump diaphragm. Googling might provide a step by step general procedure to find vacuum leaks. Besides the ideas above, there’s lots of possible reasons for something like this b/c there’s lots of reasons the engine has to increase idle speed on purpose to avoid stalling.

  • turning the steering wheel
  • turning on the AC compressor
  • turning on an electrical component (e.g. headlights) that increases alternator load

etc etc

We needed new o rings in the plenum! $500 fix from the dealer! Lean Bank 1

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I had to replace the o-rings for the fuel injectors on my old VW Rabbit every three years, like clock work almost. But I’ve never had to do that job on my 27 year old Corolla. The materials used in O-rings must have improved.

deleted :smirk:

Thank you all…

I’ll still wait to see what what else is posted :slight_smile:


if auto parts store has these on front display you know they fail alot. check the pcv elbow as noted. it might look ok but take it off and check for cracks. SX425

Thank you, I will do that :slight_smile:


Thank you all very much. I suppose I should look at my data again. It could very well be throwing both codes. I’ll update when I have more info.

PS - how do I make a cheap smoke generator? I made one before but only because I found a used stage smoke machine for 5$ at the flea market and made one out of that. Buying one is out as I am on Disability awaiting a Liver transplant.

If you only have a lean code on one bank, that would most likely be due to a leaking intake manifold gasket. You don’t need a smoke test for that.

There are various methods for detecting a leak in the intake manifold gasket. They all involve spraying something onto the gasket seam and seeing if there is a difference in how the engine is running. Some people use propane, some use a spray oil such as WD-40 and some use a carburetor cleaner.

It doesn’t matter which you use although propane can be a safety concern, but once you spray over the leak, the engine will change. It may run rougher, smoother, faster etc depending on what you use, but because the gasket is leaking at that spot, whatever you spray will affect the engine. If there is no effect at all, then it is not the intake manifold gasket.

Thank you Keith. I normally use carburetor cleaner. I will be back around the car tomorrow and will do some more test then. Including cleaning the MAF sensor wire with electrical spray :slight_smile:

The 4v motor is plumbed differently than the 2v motor. I am not sure it has the Pcv elbow issue.

OK… thanks for the update :slight_smile:

I’ll vouch for these failing often. Seen it enough. It’s the first thing I look at any time I open the hood and see one. Even if all I’m doing is checking fluids.

An exhaust leak can cause a lean code as well. Do you have any exhaust leaks?