Need to get NOX down to pass smog, help

So, I'm going crazy over here with this Jeep i bought. It's a 94 Grand Cherokee 4.0 and I recently completed an exhaust manifold replacement since they are extremely prone to cracking and mine was cracked. Everything seemed to go on okay and I'm 99% sure I put everything back in the right place. The registration is almost 2 years past due, so I need a smog check for the wonderful State of California. I did a pre-inspection and the guy told me my Nox was around 1500, maximum allowable is 450 or so.

I was also getting the check engine light on for a lean code, and I had a local muffler shop check for any leaks, he said the flange was way too tight so he fixed that and put in a new gasket. I had another pre-inspection done afterwards and the numbers were still very high.

I really need to get this thing registered, and I would prefer to have it past smog legit rather than find someone shady and pay an arm and a leg.

Has anyone dealt with something like this before, is there an easy way to lower the numbers, or maybe something common that could be causing it?

I bought a new O2 sensor to put in but the muffler shop said I don't need it, but since it seems to be running lean I think I'm going to change it anyway.

 

Need to find (and fix) the root cause of the lean issue

Check fuel pressure
Check for intake leaks like vacuum lines/pcv/intake gaskets

It's OBD1 so it'll be harder to find a scan tool but adaptive fuel trim info would be helpful. Generally speaking, if it's lean at idle look for a vacuum leak. If it's lean while accelerating/cruising look at fuel pressure/delivery and calculations like MAP and Calculated Load

Might have to find a driveability guy to troubleshoot it properly. Don't take it to the "free diagnosis" guy. He'll just throw parts at it and waste your money. Find a reputable shop and expect to pay for a couple hours of diagnostic time. It's an older rig so don't be surprised if nobody particularly wants to work on it

 

Damn, I was super amazing info thank you. And you are correct, most shops I call don't even want to touch it. Since it passed smog before I touched it, I'm assuming I did something wrong when I installed the manifold but I'm not sure what. Vacuum lines were my first guess but they all seem to be hooked up properly, and it runs fine other than the stupid code. I did also have to disconnect and install one fuel hose but as far as I know that's done correctly.

 

The high NOX is a result of a combustion process that is too hot, which is a common side effect of a lean condition. Rich mixtures burn "cooler".

That's why high performance/turbo cars will often back-fire. Since a turbo packs more air into the engine, they're paranoid about getting enough fuel in there too, so they purposely run around on the rich side. The un-burned fuel makes its way into the exhaust and backfires. Also they're protecting their engine internals by keeping the combustion cool. A "hot" burn can melt pistons and cook valves..

I know you just have a regular old 4.0 but that's just some examples.

Be careful driving it until you figure it out.

 

Shit. That's just something else added to the mix, thank you again.

 

Has the Cat ever been replaced?

 

I believe the previous owner replaced it, I've driven it a total of maybe 15 miles since I owned it, body as kind of a project and have just been tinkering. That's why I believe I must have done something wrong when changing the exhaust manifold, because it passed smog with her right before I bought it.

 

Make sure there's no exhaust leaks upstream from the 02 sensor

 

Got it. Well definitely should figure out any check engine lights and get the A/F ratio in check.

I asked because Catalytic converters for OBD1 cars are pretty affordable.