P0172 Running Rich Left Bank, unsolvable? Dodge Dakota 04

2004 Dodge Dakota 4.7L - Two shops and the dealership have been unable to resolve the P0172 running rich left bank code. Here's the background - The engine was rebuilt by the previous owner who is a mechanic. He was unable to resolve the problem and I bought the otherwise clean truck from him for a good price. I was barely running when I bought it, stalling, etc.I took it to a reputable local shop to have it looked at, they replaced the oxygen sensors with factory parts and it ran much better but the code and rough idle persists. I took it to the Dodge dealership and they said to replace the PCM which I did - no change. I believe most of the sensors are aftermarket. The previous owner said he did a compression test, and also swapped injectors from the other side of the engine with no change. Any help would be really appreciated. Thanks.

Pull the Left bank Upstream O2 sensor and check exhaust back pressure.

Thanks for the quick response. How do I check exhaust backpressure?

You put a pressure gauge in place of the O2 sensor.

Thanks again for your quick replies. I'm going to assume at this point that this site is meant for professional mechanics, based on your responses. Your short, to-the-point answers are not for backyard mechanics like myself who commonly wouldn't know what an exhaust backpressure check is, how to do one, what equipment is needed to do it, how to inturpret the test results, what to do with the results, etc.

This forum is for everyone that wants to learn more about automotive diagnostics. Professional and beginners, alike. We are all here to learn, and help others learn.

The purpose of doing an exhaust back-pressure test is to ensure that the catalytic converter is not clogged/restricted. A restricted exhaust will affect engine performance.

In the simplest terms, an engine is an air pump. It breathes air in on the intake stroke, and expels air on the exhaust stroke. If the engine cannot exhale, properly, it cannot inhale properly. The lack of proper air flow will cause the engine to run rich. Each side/bank of the engine has a catalytic converter that helps clean up the Exhaust emissions. If one side becomes restricted, you will have one bank run rich.

The exhaust back-pressure test is very simple to do. It will require a pressure gauge that can read 0 - 15 psi. ( or so). A clear exhaust will have very little back pressure. 1 1/2 psi, or less, is ideal... 2 - 2 1/2 psi. is something to note, but I wouldn't get too concerned... 3 psi is concerning. Over 3 psi is a problem.

To do the test, first identify the catalytic converter. There will be an oxygen sensor in the exhaust pipe before the catalytic converter, as air flows. Remove the sensor and seal the hole with the pressure gauge. Start the engine and read the gauge, at idle. Then, raise the RPM to 2500-3000 and hold it steady. Read the pressure gauge. That's it. The test is done. If either one of those readings were 3 psi, or above, you have a restricted catalytic converter, and that is why that bank is rich.

If you do not have the capability of doing this test, find a shop knows what the exhaust back-pressure test is, and what the pressure should be. If you ask them and they don't know, find another shop.

I really appreciate the info. I'll do the testing and see what the results are.