1stTimer

Hello, I own an 89 jeep cherokee 4.0 with 193,000 miles and it won't pass smog, and unfortunately I live in California. I have been through three tests and failed do to high Nox levels. Here is a list of parts replaced....Cat converter, O2 sensor, muffler, plugs, wires, egr valve, fuel regulator and ran seafoam through brake booster. I am completely stumped. Any help would be great!! Thank you for reading.

SG Mason

Being that I am not from California, I do not know what the rules are there, but here is a list of causes for high NOx. You have already checked a few boxes on the list, but there may be something you are missing.

scooterc

isn't there a law in cali that if it cost more to fix u get a exempt sticker ? I used to live in cali and I used to know places for $200 u pass....if u know what I mean.....

Nick-B

Notice that the very first item listed on that link is "lean mixture". The older XJs are notorious for vacuum leaks. First check the intake manifold to head bolts. Then start checking all the vacuum lines and hoses. There are lots of them and the hard plastic ones get old, brittle and crack. Best of luck.

jay_sco

No. There are programs, depending on your income, that assist in the cost if repairs exceed $XXX (not sure of the exact amount). If it won't pass no matter what there is a "buy back" type program for the vehicle. If it's way off it gets labeled as a gross poluter and can only be taken to certain shops. If it fails at one shop, but magicly passes at another shop, 70 miles away, it raises red flags. We know what you mean, but those shops are becoming rare, and everyone is usually tight lipped about them... Or so I've heard...because I have no knowledge of any such activities.
If it got an exempt sticker because it costs money to repair, we'd all be driving around with exempt stickers.

1stTimer

I signed up for the program. It's a pain in the *** to get approved. First they said it was tampered because I didn't have the flex rube going to my airbox. Fixed it, sent back the smog paper work, took a month for them to say we're missing info. Thought about selling to the state, but it's my only vehicle and I have kids to take to school and work to go to. Right now I feel like I'm stuck.

jay_sco

I'm sure it's a pain, that's bureaucracy for you. And I think the state only gives $1000, and I'm pretty sure it needs to have current registration, which you can't get if it doesn't pass smog.
Back on track...Nick has a good point about all the vacume hoses becoming brittle. Even if they appear to be ok they can develop hairline cracks reducing their efficiency. Don't know why you got a muffler, has nothing to do with smog. I've passed sans muffler in an old truck I had. Have you checked your timing? Not sure if that affects NoX, just throwing ideas out there.

1stTimer

Muffler got replaced cause the old cat converter broke and stuffed up the muffler. Just threw that in.

1stTimer

Any way to check for leaks in the vacuum lines?

PurpleToxin505

start sprayed them with tb cleaner or brake parts cleaner. when it gets sucked in the motor will rev higher or bog down

NorCalJeep

Spray down engine bay with carb cleaner and see if there are any leaks(lines and manifold). I had a manifold leak turned out.

DFlintstone

Vacuum leaks suck. I go around with short little shots of starting fluid, if you hit one the sound of the engine will change. Small little shots! You don't want that stuff to accumulate. It's a fire hazard, but It evaporates right away. Also with a tube, or a section of hose you can listen for it. Don't forget the lower O rings on the injectors need to seal as well. (uppers would leak fuel).
The manifold bolts have a habit of loosening, especially that rear one. On anything like that you never want to tighten only one. It can warp/bend, even crack things. If I found that any that where easy to reach where loose, I'd pull the air cleaner and tighten them all, starting in the middle and working out towards the ends. I go over about three times. It's a bear of a spot to get a torque wrench on them all. Just don't ape on it. If its firm and not turning, no point in going on to break it, a REAL *****. The rearmost bolt underneath takes a little doing, but it can be done with the right extension. For that very back one by the firewall I use way long extensions (18"), with a swivel at the socket. Anyway, that's a good thing to check on any old Jeep. ALSO, there is a large line coming off the intake, rear of the TB that goes back to the firewall, across, then forward down to the vacuum ball behind your right front bumper. You can pinch that off or unplug it and cap the nipple to test if you have a leak there. The line to the break Booster can also leak vacuum.

cruiser54

Click on the link in my signature and do Tips 1,3,4, and 5 along with checking for vacuum leaks.

Jonas

Is there an update to this thread? I am new to this forum (as well as a new cherokee owner - 1993 4.0 with just over 200,000 miles - with smog issues in California) and would love to find out how it turned out. Thanks for any help in my regards as well.

90Cherokee

Basically you need a clean, well-tuned engine to pass. Before going further, do a basic tune-up and check spark plugs for combustion quality before replacing them. If they show a lot of carbonization, Seafoam it (even multiple times) and then change the oil. If you are running rich, check/replace your O2 sensor. Several things can cause high NOx. Lean mixture, timing, exhaust leaks, high combustion temps from carbonization. Take a shop vac and hook up the pressure side to your intake system and spray soapy water on the connections to check for potential vacuum leaks. Do the same to your exhaust system. Any leaks around the engine and upper exhaust system will cause high NOx. You must have ZERO exhaust leaks - period. Timing must be within spec. If you are given a +/- range set slightly to the - side as slightly retarded will lower NOx at the expense of higher other numbers. If you are running hot, lower the operating temp with a cooler thermostat and fan switch sensor if you have an electric fan. Get the test on a cool morning. A new Cat will knock the NOx numbers way down on a properly running clean, cool engine. A new Cat will do the same on a marginal one but maybe not enough. A new Cat will NOT cure an exhaust leak, even on an otherwise clean engine. Sounds complicated and expensive but we are basically talking proper maintenance. Good Luck!