New member here. My future son-in-law has a beat up 96 3.9L Dakota.
Standard Trans. It failed the smog test due to High Nox.

everything else passed. Note that NOX was low at 15 mph

Test Results:
15 mph: 1473 RPM, CO2-15.05, O2-.16, HC-63, CO-.41, NO-0036

25 MPH: 1488 RPM, C02-15, O2-.47, HC-43, CO-.02, NO-1360 FAIL (MAX ALLOW - 859)

186K miles on it.

He has had the truck a couple of years. He "thinks" he put a new CAT on it when he bought it.

I figured it might be the EGR valve, but I've since learned this year doesn't have one (please confirm)

I smell fuel/gas under the hood when it's running, not real strong, but it's there.
There are no stored OBD error codes.

Seems to idle ok, maybe a little rough.
I volunteered to try and help him out, so now the truck is at my house!

Other than look for the EGR that doesn't exist, looking for codes, and smelling the fuel, I haven't done anything yet.

I know to look for vacuum leaks and I'll do that, but in the mean time I thought I would look to the wisdom of the forum.

Anyone else have this issue?
Anyone have any suggestions?
Please don't let it be the CAT

Thanks in advance
Paul

 

IDk how to directly address NOX. I do know as a habit when I lived in Ca before SMOG on something this old I changed the oil, wires, cap, rotor, plugs air filter. Then go straight to the SMOG station.
Rock Auto does sell an EGR for the 96 3.9. That is no guarantee but if you look at it then look at the passenger side of the motor near the firewall at the corner of the intake buried under plug wires. A couple of vac hoses connect to it. If its 4wd you can get a CARB CAT for $260is on Rock Auto but for some reason the RWD CARB CAT is $498! I hope its not the CAT too.

 

is the engine running hotter than normal? high combustion temps create nox so anything adding to combustion temps will add more nox production. typically, when the nox are high the hc are low.

Is the ignition timing adjustable. to far advanced?
brain storming here: I wonder if a fuel with higher octane would lower nox....higher octane fuels have lower latent heat


if there are no other nox control devices...it probably needs a cat. don't cheap out or you will be in the same boat.

 

is the engine running hotter than normal? high combustion temps create nox so anything adding to combustion temps will add more nox production. typically, when the nox are high the hc are low.

Is the ignition timing adjustable. to far advanced?
brain storming here: I wonder if a fuel with higher octane would lower nox....higher octane fuels have lower latent heat

maybe before trying a cat...back off timing 2* and run premium fuel for the next test?


if there are no other nox control devices...it probably needs a cat. don't cheap out or you will be in the same boat.

 

My thoughts exactly, was going to try higher Octane gas. Need to find a test station where you pass or don't pay. The Cats that are certified for California are a little more expensive but are good quality. I tested all of the vacuum lines today, did not find any leaks. I dont believe the timing is adjustable on this model. The truck runs cool, it has a 190 degree thermostat in it and Vince reports that the temp gauge stays way below 200 degrees. So, tomorrow I'll price cats and look for a smog station with pass or dont payy (if those still exist).

 

There is a device on the rear of the engine on the drivers side that has a couple of vacuum hoses going to it, but I dont think its connected to the exhaust. I'll look again tomorrow.

 

I've used this stuff with success when I lived in a state that smog tested.....it actually worked.

https://shop.advanceautoparts.com/p/...E&gclsrc=aw.ds

 

You'll see there is one vac line connection and an electrical connection. The vac hoses on the driver side if it is a unit attached to the intake and up high is probably the purge valve for fuel fumes from the charcoal canister.
https://www.rockauto.com/en/catalog/...egr+valve,4968

 

That's cannister purge. (EVAP system.)

96 *shouldn't* have EGR.

 

I had a new CAT installed. Monitors weren't ready, so I started a drive cycle. A few minutes into the drive, the check engine light came on. Bad upstream O2 sensor. I replaced both sensors. Took a while to get all but one monitor ready. O2 Heater was still not ready but had it tested. Failed again for high NO.

I noticed that the engine temp stays low, around 155 to 160. Wondering if the temp sending unit is bad and reporting wrong. I wouldn't think that would cause the engine to run hot????

Vince, the trucks owner, did recently change both the water pump and the thermostat. Water pump was leaking. I don't think he changed the temp sending unit.
It the sending unit turns out to be bad, does anyone think that could be causing high NO (lean condition).

Not real happy about putting in a new CAT and O2 sensors and it still not passing smog.

Oh, and this is a 1500 NOT a Dakota as I wrote in the title. Sorry, thought it was.