If I understand the fuel trims. You have a lack of oxygen to the cylinders and too much fuel being added. My original 171 and 174 lean codes were rectified by updating the ECU at the dealer. I’ve gone through a massive amount of work to not fix the problem too. My lean codes have been back for a few months now. I’ve found a ton of vacuum leaks and am still finding some. But I’ve got lean codes where to much unmetered air gets into the cylinders… you have Rich codes.

what kind of air filter do you have? Has the intake manifold been cleaned?

 

Lean codes occur when the O2 sensors detect more oxygen in the exhaust gas than expected. This most typically happens because of a vacuum leak behind the MAF, which lets unmetered air into the intake, resulting in a higher (lean) air to fuel ratio. The computer then adds fuel into the system until the O2 levels balance to 14.7:1 air to fuel. This is the ideal (stoichiometric) ratio for emissions. The extra fuel added results in rich fuel trims, but it is known as a lean condition because the uncombusted oxygen is the underlying factor.

 

thank you Seattle Driver, i am going to take it back in to get a fresh set of eyes on it to try to track down the vacuum leak. we did fix leaks on the intake manifold as well as replaced the plastic thermostat housing (known issue for cracking). we will go back over all other connections and hoses to search for this.... i will keep you posted.

appreciate your insights

 

That explains lean codes, but Buffaloandy has Rich codes. Can Rich codes be caused by the same issues as Lean codes? I ask because, why would there be P0171 and P0174 codes when the P0172 and P0175 codes would be caused by too much oxygen.


Rich means not enough fuel is being burnt and the O2 sensors are directing low oxygen and high levels of unburnt fuel. It’s rare that two O2 sensors malfunction. Right?

 

I was clarifying the lean codes based on your post. But no, the causes of lean codes and rich codes are different. Rich codes would mean there’s too little oxygen being detected in the exhaust, stemming from too much fuel. This could be an issue with timing, ignition, or fuel injector.

OP in post #9 said there is a “tiny leak” shown by a smoke test. It doesn’t take much of a vacuum leak to cause problems. I’m suggesting they correct any leaks, then check trims before going down any further route.

 

Hello Again,

two more things to consider while pondering this puzzle. My one mechanic told me "the only way he could get it to lean down was to induce a leak"....... thoughts on this?

other item.... not sure if it is related or not BUT sometimes the engine stumbles to start when the engine is fully warm. is that because there is too much fuel as well?

thank you!

 

When dealing with fuel to air ratio:

Lean = too much air
Rich = too much fuel

This is when dealing with ratio only. So yes you could have, let us say, a VERY dirty air filter which could obstruct air intake causing a rich condition. So in that case it would read as a rich condition, which is correct, but it is not really being caused by too much fuel but instead too little air. And another example could be a lean condition where there is too much air but the actual cause is a restricted fuel filter on the pump which is in fact causing too little fuel.

This is why its best to not flat out assume a lean condition is caused by a vacuum leak and only chase on that end and why both "ends" should be checked - such as a fuel pressure test or even swapping injectors or testing the installed on the bench for performance. Etc, etc...

Of course you also have the "modern engine" design issues where things like temperature sensors greatly influence the expected ratio (when cold, engines tend to dump more fuel) and a bad sensor can cause issues yet not be bad enough to toss a code.

 

Hello,

Just wanted to provide an update on the progress of the LR3. Found 2 fuel injectors leaking down, EVAP purge valve was stuck open AND the ECU needed to be reflashed....... if you ever need your ECU reflashed, send to Precision ECU, they are terrific and FAST.