Smog failure with no code?

Tiny
PAUL JUNGNITSCH
  • MEMBER
  • 1995 DODGE DAKOTA
  • 3.9L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 237,000 MILES
California smog test failures with no computer error codes. High NOX at 25 MPH indicates running lean. Numerous parts replaced for troubleshooting, done or suggested by confused shops. Catalytic converter, plugs, distributor cap and rotor, oxygen sensor, EGR control valve, fuel pressure filter/regulator, fuel pump. EGR tested with vacuum showing up at EGR valve when exhaust tailpipe covered, and engine shuts off at idle when vacuum applied. Injectors test fired on removed fuel rail. Fuel pressure and capacity great, with no leak down. Compression like new 140-150# all 6 cylinders. Runs and sounds normal with normal power. Three different shops failed to diagnose the cause. Remaining suggestions are bad gasoline, carbon in cylinders, and PCM failure.
Monday, June 5th, 2023 AT 6:13 PM

9 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,071 POSTS
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.

Image (Click to make bigger)
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Monday, June 5th, 2023 AT 9:47 PM
Tiny
PAUL JUNGNITSCH
  • MEMBER
  • 5 POSTS
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.
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Tuesday, June 6th, 2023 AT 9:41 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,071 POSTS
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
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Tuesday, June 6th, 2023 AT 8:32 PM
Tiny
PAUL JUNGNITSCH
  • MEMBER
  • 5 POSTS
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.
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Friday, June 9th, 2023 AT 1:29 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,071 POSTS
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
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Saturday, June 10th, 2023 AT 12:10 AM
Tiny
PAUL JUNGNITSCH
  • MEMBER
  • 5 POSTS
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.
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Saturday, June 10th, 2023 AT 12:38 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,071 POSTS
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
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Saturday, June 10th, 2023 AT 7:59 PM
Tiny
PAUL JUNGNITSCH
  • MEMBER
  • 5 POSTS
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.
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Monday, June 12th, 2023 AT 4:41 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,071 POSTS
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
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Monday, June 12th, 2023 AT 9:41 PM

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