Any smog experts out there?

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cudamark

cudamark

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A buddy's '95 Dakota 3.9 5speed flunked it's latest smog test. Only 3800 miles since it's last smog test two years ago when it had a new catalytic converter installed. All the HC, CO, and Nox readings are very similar between the two tests, both at 15MPH and 25MPH, except for the 25MPH Nox reading. It went from the 200's to 837 with a 753 max allowed. Engine is running cool, Idles fine (no apparent vacuum leaks), EGR system is working properly, no codes have popped. I need to borrow a scan tool to test the 02 sensor though, as maybe it's on the verge of failing? Anything else you can think of that is a likely suspect for this vehicle? I'll probably just replace the 02 sensor anyway as I think it's the original, decarbonize the cylinders, and maybe put in some real short reach plugs temporarily to lower the compression ratio a bit to help lower the combustion chamber temp. What else am I forgetting?
 
Can't something that old qualify for exemption? Sorry but CA is ridiculous.
 
Make sure the egr is working right and engine temp is 195*
 
I'm probably wrong here but wouldn't a hotter combustion chamber burn the fuel mixture more completely? I agree with changing out the O2 sensor.
 
Maybe you can check to see if it is running in closed loop, or open loop when it is up to temperature.

Oxygen sensor related, regulating fuel air mixture.

Believe the Closed Loop is when it is running in pre set parameters.

Open Loop is when the oxygen sensor is up to temp reading the exhaust gases telling the computer the needed fuel air mixture.

Most oxygen sensors are 3 wire. One of the wires is for the built in heater to keep it up to operating temp so it will read correctly. That heat circuit could have failed, hence replace O2 sensor.

Evap system sealed proper? Can try new gas cap as they fail to seal after a while. ??? Usually get a check engine light with this one.

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Like I mentioned, the EGR system is working fine, and no codes that have turned on the check engine light.
 
Dan the man said:
I'm probably wrong here but wouldn't a hotter combustion chamber burn the fuel mixture more completely? I agree with changing out the O2 sensor.
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That's a bit of a two edged sword. Hotter temps are good for low HC readings, but, not so good for NOx readings if they get too high. That's why the EGR system was developed. Adding a bit of exhaust into the intake lowers the combustion chamber temp, and also helps for more complete combustion. At least, this is what I've been told and have found over the years. I'm just trying to figure out what made just the 25MPH NOx reading hit the roof. All the other readings are all well under the max allowed and haven't changed much from the last smog test readings.
 
Running E-85 gasoline may help. Simple new air filter.

Change Oil and filter, that my help clean up the readings too.

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Can you adjust the timing on that one? If so, retard the timing and try it again.
 
RustyRatRod said:
Can you adjust the timing on that one? If so, retard the timing and try it again.
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Timing is computer controlled, and triggered by the flex plate tone ring The only thing the dist does is a 180* trigger for the cam position, and of course it needs to be close enough that the contacts are in phase
 
67Dart273 said:
Timing is computer controlled, and triggered by the flex plate tone ring The only thing the dist does is a 180* trigger for the cam position, and of course it needs to be close enough that the contacts are in phase
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I kinda figgered.
 
cudamark said:
Like I mentioned, the EGR system is working fine, and no codes that have turned on the check engine light.
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Why would they even test the readings?
Here in Northern CA, they just do a scan of the computer, do the visual check of the equipment, look for smoke from the pipes and never do a sniffer test unless it is ran on adyno.
Dyno testing is only done on pre OBD vehicles.
 
Kern Dog said:
Why would they even test the readings?
Here in Northern CA, they just do a scan of the computer, do the visual check of the equipment, look for smoke from the pipes and never do a sniffer test unless it is ran on adyno.
Dyno testing is only done on pre OBD vehicles.
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They dyno everything here except for big trucks and motorhomes. Some years they even run a smoke test for the evaporate emission system too to check for leaks.
 
Hmmm…. Pulling this out of my butt since I’m not familiar with that engine. Exhaust leaks can schew O2 sensor readings. If it has a MAF, the rule of thumb is that idle reading (g/s) should be about equal to the displacement in liters. Make sure the coolant temp and air intake temp sensor data looks accurate.
 
NOX is usually EGR related, the egr valve could be working properly but if the ports are carboned up it will not flow enough to work correctly
 
Sterling said:
NOX is usually EGR related, the egr valve could be working properly but if the ports are carboned up it will not flow enough to work correctly
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I agree and that's the first thing I checked. I found a strong signal to the EGR valve when I rev the engine, and if I apply vacuum to the valve at idle, it opens and makes the engine die. That shows it works fine to me.....
 
The combustion chambers could be carboned up, this can cause high NOX. You might consider running combustion chamber cleaner through it.
 
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I have a 1988 318 Dodge with fuel injection and a computer. I failed the emissions test and found out that the transducer went bad. It is a vacuum device that controls the egr valve. About $60 from the dealer. Replaced it and all is fine. Mine is OBD 1. Yours should be also.

You should try doing the ''key dance''. Ign. key on, off, on, off, on. Wait and count the flashes. 55 is the end of the program.
 
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Like I mentioned, EGR system works fine, and no codes triggered.
 
This shows the relationship between AFR and emissions before the cat (or non-cat) stock type engine.
It also assumes timing is correct for the mix. (For example late timing at idle can help burn more HC, and reduce CO)
1688312877655.png


I would expect that for emissions purposes, the idle mixture targets something in the 14.1 to 14.7 AFR.
Above idle, under light load at 25 mph, probably similar will produce decent economy power and emissions.

If only NOx has increased between idle and 25 mph, I'd try to figure out what system controls the NOx rather than look at fuel mixture (which would change all combustion exhaust gases)

Just some ways to approach the issue. Don't know a lot about these new systems
There's been some timing maps posted for FI factory 318s or 360s. Maybe also some fueling maps. Those could be helpful baselines to reference.
 
Exhaust leaks will affect your post cat reading, but post cat o2 only meters cat efficiency...the pre cat o2 is what the computer uses for af metering...that one bounces at idle normally then steadies as you accelerate , climbing to rich at wot..900 something, both should 'under wot'. If you have an exhaust leak.. deceleration will show lean spikes on the post cat O2 sensor...but it won't correct for it..only the pre cat o2 will..and the pre is already bouncing when letting off by then. Watch the live data. If there is a exh leak...your post will be lively instead of steady like it should be from sensing too much oxygen.

One thing you always want to do before you go to a smog test is get on the freeway and drive it for 5 minutes and get that catalytic converter nice and hot get those O2 sensors nice and hot and warmed up. You also dont want to run OLD GAS. Octane boost helps old gas.
does this thing idle smoothly?
No chance of a plug wire loose?

Aside from those obvious things you could also have a bad injector/leaking injector o ring.

But I've gone through the California sniffer with a bad plenum gasket before and passed. I've passed with a leaky injector o-ring before. My truck has no EGR but has 314,000 miles on the bottom.
 
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Don’t discount the new cat being bad. Even with only 3800 miles since replaced. The cats are very responsible for lowering nox. If you can read live data look at the switching rates of the pre cat o2s and post cat o2s and this should tell you the cat is working or not. The front o2s should switch fast and the rear 02s should be fairly consistent, the fronts should switch between 0.1-0.9 volts fairly quick, and the rear should be pretty steady at 500-700mv. This will tell you the cats are working.
 
Only one (pre-cat) O2 sensor this year. Wouldn't a bad cat also skew the HC, CO, and 15MPH Nox readings too?
 
****, it’s obd1 sorry I should’ve read more carefully.
Not necessarily, the cat may be able to keep up at lower speeds. Was the cat replaced with an aftermarket 50 state legal, or an eBay special when it was done?
 
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