So my gf's 93 HO 4.0 XJ failed the emissions test here in Oregon due to excessive NOx. It is originally a CA Jeep, and the previous owner put on an aftermarket air intake, but the motor is otherwise stock, 210k miles. It runs strong and doesn't have any other problems. I know that excessive NOx is due to excessive combustion temperatures and all of that. I was expecting to find an EGR valve, but I guess the 91+ 4.0's don't have one. Did the key on/off etc and no trouble codes came up. I'm sorry but I don't have the numbers from the emissions test showing how much it failed by. Also, I couldn't find any vacuum leaks.

I've read a few posts around the net about people having a hard time eventually finding the cause, even after replacing some parts such as the cat. Anyway, just wondering if any of you had any similar experience or solutions, and if any of those "guaranteed to pass" fuel additives actually work or if they would pertain to NOx problems. Would the aftermarket intake be causing a lean condition? Any help would be appreciated.

I will edit this by saying that the more I read on NAXJA, is that it is typically due to either a vacuum leak, bad cat, or too hot of plugs on a non-EGR 4.0. It needs new plugs anyway, so I'll do that and then use the cat as the last option. Still curious about that $12 snake oil additive though.

What temp t-stat are you running?

at 210k your engine is very carboned up, having a shop do a "Motor vac will most likely cure your NOx.
The cheaper way to get similar results is to add a can or two of BG 44k to your tank.

SD
I can go on for a while regarding emissions and NOx, teyb this for now and let us know what you come up with.

Pulled the plugs tonight, looks like the previous owner put in Bosch Platinum plugs. They're white as can be, with a ton of corrosion/build up. The DEQ station is pretty close to my house, and the test is free if it doesn't pass, so I'll run it again tomorrow to see if it was just the plugs.

As SD says, high NOx is more than likely caused by carbon deposits upping the compression a wee bit and therefore cyl. temp. There is a cheater way for cheap decarbonizing, but if done wrong it can mess the motor up. It also requires some good hand eye coordination. Get the jeep nice and warm, almost hot. Get yourself a spray bottle with water in it and take off the intake tube coming off the TB. You want to hold the jeep at about 1500-2000 RPM. Now, here is the tricky part. Snap the throttle wide open and at the same time give the TB a nice good hard squirt of water. You want the nozzle set on spray, not stream. As the engine sucks the water in, it hits all the hot parts. Normally carbon deposits form on these hot parts, when the cool water comes by, it shrinks down the deposits and cracks them off. This will work on some deposits, but not all. The are some dangers. If you have any cracks, it really accelerates there growth. If you have big thick chunky deposits, then can get stuck or damage other parts (kind of a danger for all decarbonzing). If you dump down too much water (we are talking ALOT of water here, not the normal volume of a squirt bottle) you can hydro lock the motor and bend up a connecting rod. If you do it right, it will take care of some deposits and might get you down to passing levels.


ThatÆs the beauty of our jeeps. We run a speed density fuel injection setup. The computer does not actually know how much air is going into the motor at any given time. It Calculates via MAP sensor TPS and RPM a base Fuel curve to give the motor. Once it is up to temp it uses things like the O2 sensor, coolant temp sensor, incoming air temp sensor... To trim the fuel ratio to get to the desired ratio. These systems work fairly good and are robust as heck. Because there are no sensors on the intake plumbing, there is nothing to set these calculations off.

If it were a Mass air unit, imports and passenger cars love these, intakes can seriously mess up a computer. Mass air systems use a sensor that directly measures how much air is going into the motor (or at least through the sensor). If there is an air leak behind the sensor, the computer can't sense it and more air is coming in than what is expected driving the car lean. things like PCV breather evap purge... ETC are calculated losses and can be corrected for. But some of those intakes don't have the same size holes or just bypass the system completely causing it to run poorly.... Although it still trims itself, it does not do it as quickly or efficiently as a speed density system.


ALSO if it were running really lean, your CO levels would be pretty dang high too.....

You could change the plugs...... or you could do my way cause i'm cooler than any plug....

Thanks for the tips guys. So Sunder, just out of curiousity <img src="/forums/images/graemlins/smile.gif" alt="" />...do you just give one squirt of water, or do you do it a few times?

So I took it back in today and it failed. Nox was 4.59, max allowed was 3.75. The graph shows that it wasn't failing by much. Other readings. THC: 1.06 max 2.40. CO: 6 max 80. CO2: 706.

So what should I do next in your opinion?

It's running fine, CO and HC are good. More than likely carbon deposits upping the compression ratio.

I give it one squirt at a time, but I do the procedure 5-10 times in a couple minutes, let it idle and get nice and warm again and do it again. If the ghetto method doesn't work, I know places like pepboys have a decarbonizing procedure, I think even jiffy lube might have one..

Or you can try the hillbilly method, drain out most of the oil, put on a new filter, fill the motor with a gallon of deisel and let it idle for about 10 minutes. It will sound like a deisel truck rattling so much. Drain that junk out, put on a new filter, fill with new oil. I watched a guy do this on his chevy truck and it cleaned all of the carbon out. But I don't guarantee that method. Try at your own risk if you dare.

You put diesel in the crank and run it you will destroy the engine, the diesel does not lube, and it is too thin to make any pressure, so the lifter will complain. I've seen people do this to there vehicles too... the motor only last another 5K miles....