You're making things much worse by randomly replacing things. With every new sensor, the Engine Computer has to relearn the characteristics of it. It does that by comparing its response to the signals of the other sensors and running conditions. Think of buying new tires for your truck. It will handle differently. Add a turbocharger. It will respond differently. Install a camshaft with a different "profile". Engine response will be different. Now do all of those things at once, hop in and drive it and you'll have all kinds of new things to learn. If there's a problem, which modification do you blame it on? With so many changes on your truck, the computer is going to take a while to learn all of the new variables because no two sensors are exactly alike or produce exactly the same signals. Also keep in mind sensors only report what they see. Too many people replace them when they don't like the readings, but that's replacing the messenger when the message is the problem.
The first thing you need to do is determine why it won't pass emissions. Is it because of excessive emissions from the tail pipe or simply because there are stored fault codes? Fault codes can be erased safely on your truck by disconnecting the battery negative cable for half a minute.
I have to add here, for other people reading this, that disconnecting the battery on newer vehicles can lead to a whole pile of problems. It is one of the main reasons I refuse to give up my '88 Grand Caravan daily driver and buy something newer. Disconnecting the battery on some Volkswagens will require dragging the car, still in park, onto a flatbed truck for a trip to the dealer to unlock some computers before the engine will come off of idle or in some cases, start. They designed in a whole bunch of "got'chas" to bleed money from unsuspecting owners after the sale. General Motors isn't much better. So disconnect batteries at your own risk, but be informed first.
Back to your very simple and reliable '90 model. If any fault codes come back, those are the circuits that need further diagnosis. If the reason for not passing the emissions test is the makeup of the exhaust gas, I'm sorry to say that is not my area of expertise, but there will be a lot of people in your state who are.
Sensors only cause a small percentage of excessive emissions without also causing running problems. The oxygen sensor is the main culprit. Some of the more common causes include a vacuum leak, excessive fuel pressure, and misfires. Keep in mind oxygen sensors only measure unburned oxygen in the exhaust, they do not measure unburned fuel. A vacuum leak introduces extra air into the engine that the computer doesn't know about. The extra oxygen is detected and reported by the oxygen sensor as a too-lean condition. In response, the computer commands more fuel to go with that air.
A misfire is easier to understand. When the unburned oxygen and fuel from a single misfiring cylinder enters the exhaust system, only the oxygen is detected. The computer assumes all cylinders are running lean so it commands more fuel to all of them. Now you have five cylinders getting too much fuel, but there's still that unburned oxygen from the misfiring cylinder. No matter how much extra fuel is requested, a lean condition will continue to be reported. You'll likely smell that extra fuel at the tail pipe.
The MAP sensor has the biggest say in how much fuel is needed in the engine. It's normal range of signal voltage is 0.5 to 4.5 volts. Even if it reports the wrong value, as long as it's within that range, no fault code will be set. A tiny vacuum leak in the hose going to the MAP sensor will cause it to read a lower than actual vacuum. That mimics acceleration or higher load which requires more fuel. The computer will only modify fuel delivery about plus or minus ten percent beyond pre-programmed values based on the readings from the oxygen sensor, and that might not be enough to overcome incorrect MAP readings. Almost all of the fuel entering the engine is a result of the MAP sensor's readings. The MAP sensor also reports barometric pressure when the ignition switch is turned on but before the engine begins cranking. A defective sensor can stick in that area causing an incorrect barometric pressure to be stored in the computer, yet it can appear to work properly during engine operation. A higher barometric pressure forces more air into the engine. The computer is programmed to command more fuel to go with that air but small errors should be corrected by readings from the oxygen sensor. The current barometric pressure is stored every time the ignition switch is turned on. If the reported value is incorrect, the computer will make adjustments to the fuel / air mixture later based on the oxygen sensor's readings.
A leak in the exhaust system before the oxygen sensor can cause an increase in fuel delivery and emissions. Between each pulse of exhaust flow, the momentum creates little pulses of vacuum that can draw fresh air in through that leak. The computer will request more fuel which won't burn because there's not enough air in the cylinders to go with it. The unburned hydrocarbons will go out the tail pipe. Some will be burned in the catalytic converter with the air from the leak, but that's after the oxygen sensor detected that extra oxygen. The computer is going to keep on seeing that extra oxygen and will keep on adding more fuel, which still will go out unburned.
The amount of fuel entering the engine is a result of how long the computer holds the injectors open during each pulse, intake manifold vacuum, and fuel pressure. Think of a person jumping out of a perfectly good airplane, (hopefully with a parachute). Three things determine how fast he jumps out; the vacuum created by the air flow, the size of the door opening, and the strength of the guy pushing him. In the truck's fuel system, the computer knows engine vacuum, (MAP sensor readings), and fuel pressure is programmed in, so it calculates from that how much to open the door, (hold the injector open). Fuel pressure is not monitored so unexpected changes affects fuel entering the engine. Chrysler has almost no trouble with their fuel pressure regulators, in fact, I only read about one failure in over 25 years, compared to that being such a common problem on GMs that every mechanic knows about it. What CAN happen is the return hose going back to the gas tank gets blocked or kinked. The pump is capable of producing way too much pressure. It is relieved by the regulator which sends the extra fuel back to the tank. When that hose is blocked, pressure goes up but the computer doesn't know that. That extra pressure pushes extra fuel into the engine. Only the fuel that has fresh incoming air to go with it burns. The rest goes out the tail pipe, unburned, where it causes excessive hydrocarbons, ... And a failed emissions test.
On most fuel systems, the pressure is varied with a vacuum hose connected to the pressure regulator. In the case of my airplane story, there will be more vacuum pulling the sucker out of the plane if it is going faster. That means the guy pushing him out doesn't have to push so hard to get the same result. In the truck, coasting creates a much higher vacuum pulling the fuel molecules in, so pressure is dropped to maintain a constant force on them. More pull and less push. Dropping the pressure eliminates a rich condition during coasting. During acceleration intake manifold vacuum drops so there is little pull on the fuel. Thanks to that vacuum hose on the regulator, fuel pressure goes up to convince that fuel to squirt through the injector nozzles. If that vacuum hose is leaking, the regulator will think vacuum has dropped and it has to increase pressure. More fuel will enter the engine that the computer hasn't calculated, and it won't know about it since it won't be detected by the oxygen sensor.
I mentioned misfires and the unburned oxygen in the exhaust creating what appears to be a lean condition, but that can happen too from a plugged injector. Normally you will feel the misfire, especially on a V-6 engine, but they can be so intermittent that it isn't obvious. Since you replaced the spark plugs and wires already, that leaves fuel and compression as the other two requirements. You'll feel compression problems all the time. Due to the way Chrysler buys their injectors from the manufacturer, problems are almost unheard of related to them. Still, you have to consider what would happen if a speck of dirt partially plugged the nozzle in one of them. It might flow enough fuel at idle to make the engine run smoothly, but not enough at higher speeds. That would again create a lean condition, and the computer would respond by increasing fuel to all of the cylinders.
All of my injector comments pertain to six individual injectors. Many older engines have one or two in the top of the throttle body, and they supply all of the cylinders equally. Symptoms and diagnostic steps will be a little different for those systems.
A lot of engines use knock sensors to detect preignition. I can never remember which ones so I have to look. When that knock occurs, the computer momentarily retards spark timing in an attempt to stop it from occurring. Very often mechanical problems inside the engine cause a similar vibration to be picked up and incorrectly interpreted as knock. The spark timing will be constantly retarded resulting in a huge loss of power, and an increase in fuel consumption to get the vehicle back up to speed. A worn timing chain is a common offender when it slaps against the housing it sits in. Worn, (loose) timing chains also retard the camshaft and valve opening and closing. Think of all "L"s, loose, late, and low. A loose timing chain causes late valve timing and an increase in low-end torque. That's designed into camshafts for motor homes and buses so they have more power to get going from a stop sign. That was also designed into the old "gas mileage" camshafts you could buy from mail order places like J.C. Whitney. I put one in an old 318 I rebuilt and it did indeed improve fuel mileage a lot and I could squeal the tires from a stop sign on that old heavy station wagon. Don't have a clue how it affected tail pipe emissions though. Think of "T"s for the opposite condition. Tight timing chains increase top-end torque by, uhm, advancing valve timing. That's the case in my old 440 police car engine. It goes from 0 to 60 like any 318, but then goes from 60 to I-won't-admit-how-fast so quickly it almost tears the seat off its hinges! THAT was designed in for highway police cars. The point of this story is worn high-mileage timing chains are often overlooked because they result in the engine having uncharacteristically high performance where it's noticed the most, ... Leaving stop signs.
Up here in Wisconsin, a lot of people have trouble caused by ethanol in the fuel too. Besides using ten to twenty percent more fuel to go the same number of miles, the extra oxygen in the alcohol, (oxygenated fuel), creates a leaner mixture. That means the fuel molecules in the cylinder are farther apart so it's harder for the flame front to spread from one to the next. The combustion cycle can die out before all of the fuel is burned. There can be enough of a power pulse to prevent misfire detection, (which is done on all '96 and newer models), but not enough to prevent some unburned fuel from going down the exhaust pipe as increased emissions. We have a gas station in my town that just started selling gas with no alcohol, and for the same price as with ethanol. There are two things I do to tell how well my '88 Grand Caravan is running. After turning a sharp corner in the country, I have a really long steep hill to go over. I could just barely reach 47 miles per hour without it downshifting by the time I got to the top of the hill. Replaced the 12-year-old spark plugs and distributor cap last summer and could squeak out 54 miles per hour. Ran a tank of straight gasoline recently and had no trouble hitting 60! You tell me how ethanol is going to save the world if we have to burn more of it to get less power. I also pull an enclosed trailer that's bigger than my van to the nation's second largest old car show and swap meet. Due to its wind resistance, 55 mph is the best I can do on a flat stretch of highway. I recently dragged it down the same road running straight gas and had no trouble in the left lane passing people putting along at 65 mph. That proves that while our exhaust might be a tad cleaner running ethanol, we're dumping a lot more of that exhaust into our air by burning it. It used to take a half tank of gas for this round trip. I only used 3/8 tank the last time with straight gas. Add to that alcohol cleans the fuel system and knocks off particles that otherwise wouldn't cause a problem and it can deteriorate rubber parts. All rubber parts in the fuel systems of cars today are built to withstand the corrosive properties of ethanol but there has been a lot of damage caused by using it in small engines. Ethanol isn't the answer to our pollution problems; it has just created more problems.
Now we hear about people getting sick from breathing the fumes from reformulated gas in some larger cities, like the idiot politicians think that air isn't going to blow over to the next state. What I'm suggesting is if no other cause of excessive emissions can be found, you might try gas from a different supplier / gas station.
Okay, that's everything I know about excessive emissions pertaining to your truck. Since you have emissions testing, there are going to be shops in your area that specialize in this problem. We have a high-level trainer who comes to town from Illinois once a month. He owns such a shop, and their main customers are other shops. He also networks with other shops and trainers all over the country, including manufacturer's trainers. These guys have figured out some really innovative ways of finding the causes of these problems. In the long run you're going to be money ahead letting them solve this. I love seeing these older vehicles staying on the road. I've been to a chain of two dozen salvage yards, including three of them in Tennessee, and it's nice to see the condition your vehicles stay in when the state doesn't throw a pound of salt onto an ounce of snow! I have a cousin in Nashville too who plays with the symphony orchestra. Wouldn't mind living in TN where it's warmer in winter, but I'll never move to where they have emissions testing or tell me what kind of car I can drive.
Hope you can pick out some useful tidbits or get some ideas on things to look for. Dollar-wise you will be WAY ahead if you can keep your truck vs. Buying something newer with all its unnecessary, unreliable computers.
Wednesday, December 7th, 2011 AT 12:37 AM