ETC are calculated losses and can be corrected for. 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. Mass air systems use a sensor that directly measures how much air is going into the motor (or at least through the sensor). If it were a Mass air unit, imports and passenger cars love these, intakes can seriously mess up a computer. Because there are no sensors on the intake plumbing, there is nothing to set these calculations off. These systems work fairly good and are robust as heck. To trim the fuel ratio to get to the desired ratio. Once it is up to temp it uses things like the O2 sensor, coolant temp sensor, incoming air temp sensor. It Calculates via MAP sensor TPS and RPM a base Fuel curve to give the motor. The computer does not actually know how much air is going into the motor at any given time. We run a speed density fuel injection setup. Would the aftermarket intake be causing a lean condition? If you do it right, it will take care of some deposits and might get you down to passing levels. 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 have big thick chunky deposits, then can get stuck or damage other parts (kind of a danger for all decarbonzing). If you have any cracks, it really accelerates there growth. This will work on some deposits, but not all. Normally carbon deposits form on these hot parts, when the cool water comes by, it shrinks down the deposits and cracks them off. As the engine sucks the water in, it hits all the hot parts. You want the nozzle set on spray, not stream. Snap the throttle wide open and at the same time give the TB a nice good hard squirt of water. You want to hold the jeep at about 1500-2000 RPM. Get yourself a spray bottle with water in it and take off the intake tube coming off the TB. It also requires some good hand eye coordination. There is a cheater way for cheap decarbonizing, but if done wrong it can mess the motor up. Still curious about that $12 snake oil additive though.Īs SD says, high NOx is more than likely caused by carbon deposits upping the compression a wee bit and therefore cyl. It needs new plugs anyway, so I'll do that and then use the cat as the last option. 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. Would the aftermarket intake be causing a lean condition? Any help would be appreciated. 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. 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. I'm sorry but I don't have the numbers from the emissions test showing how much it failed by. Did the key on/off etc and no trouble codes came up. I was expecting to find an EGR valve, but I guess the 91+ 4.0's don't have one. I know that excessive NOx is due to excessive combustion temperatures and all of that. It runs strong and doesn't have any other problems. It is originally a CA Jeep, and the previous owner put on an aftermarket air intake, but the motor is otherwise stock, 210k miles. So my gf's 93 HO 4.0 XJ failed the emissions test here in Oregon due to excessive NOx.
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