Smog failure with no code?

Hi,

NOx gases can be caused by several different things. For example, High engine combustion temperatures, an excessively lean fuel mixture, if the catalytic converter is damaged, worn out, or not functioning properly, it can result in higher NOx levels, and the list goes on.

Based on what has already been done, there isn't much left. However, I do have a question. Has anyone thoroughly checked for an engine vacuum leak? If there is a leak, it will lean the fuel mixture.

Also, has anyone tested the manifold absolute pressure sensor (MAP)? The MAP sensor helps the ECU determine the engine load. The engine load is a crucial parameter used by the ECU to calculate the appropriate fuel injection timing, air-fuel mixture, and ignition timing.

Let me know. If you can, follow the directions in this link to see if you are able to find any engine vacuum leaks:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Also, if you look at the pic below, it provides the directions for a basic MAP sensor test. Let me know if you are comfortable performing it.

Let me know.

Joe

See pic below.

Vacuum at the EGR supply is 18" at idle. I expected the two repair shops to find any vacuum leaks. But the modern plug and play mechanics have shown a skill vacuum when encountering my truck. I will do a comprehensive search with carb cleaner. Thanks for the video. The 1995 dakota has the original OBD system, with the old interface, so my OBDII tester will not work for testing the MAP sensor. So vacuum leak testing and manual MAP sensor testing next.

Hi,

If you still have the OBD1 system, you can check for codes using the ignition key.

Here is a link that explains how to do it:

https://www.2carpros.com/articles/retrieve-trouble-codes-for-chrysler-dodge-plymouth-odb1-1995-and-earlier-car-mini-van-and-light-trucks

I've owned Dakotas most of my life. LOL It started with a 1989 and I've been through them all. I still have a 2009 that I've maintained and is still like new. I'm not sure if it will be the truck or me that goes first. LOL The 3.9L is a great engine when maintained. You've proven that.

By the way, if you have 18" vacuum, chances are there are no leaks. Also, based on that the engine still sounds like it is healthy.

Let me know if you find any codes.

Joe

MAP sensor output voltage varies properly with vacuum at stopped, idle, rev, and decel. Intake air temperature and coolant temperature sensors have correct resistance and vary with temperature. Computer has code 12 - less than 50 starts in memory, and code 55 - done reporting. Driven to another shop, who gave up immediately when they found no trouble codes. Another plug and play shop. Carburetor spray all around, with several pauses to let the spray evaporate. Shaded from radiator fan air flow and wiggled hoses too. No vacuum leaks found.

Hi Paul,

I have a question. When you went for the inspection, did they tell you it only fails because of excessive NOx gases at a specific speed? I ask because it should be related to engine speed which can be the same at any speed.

Also, by chance does the engine run hot?

Let me know.

Joe

The NOX fails at the higher 25 MPH speed. I went back and started over with troubleshooting. I found that the replacement EGR control valve put in by the first repair shop appears to be bad. I didn't test it because I thought they were professionals. A new one will be obtained and tried tomorrow.

Hi,

That certainly can cause a leak. If it is faulty, that is the likely cause.

Let me know what you find. I'm interested in knowing.

Take care,

Joe

The factory service manual diagnoses kept going back to the EGR components. So, I replaced the EGR valve. The old original Dodge EGR valve appears to have a bent stem from the factory. Corrosion on the stem finally got bad enough to cause sticking. Passing cold functional component testing but sticking closed when hot during smog checks. The first repair shop said they removed it and cleaned out the ports, but they failed to examine it close enough. I watched it while applying vacuum slowly and compared the results with the new valve. Smog test soon at a sixth place.

Hi,

Thanks for the update. I'm glad to know you found what sounds like the problem.

If you have a chance, let me know how things turn out for you.

Take care,

Joe