Mechanics

CHEVROLET SILVERADO ENGINE PROBLEM

2009 Chevrolet Silverado • 53,000 miles

I just recently had exhuast put on my truck and I put a cold air itake on. Since it has beEn reving up to about 2,000 rpm on start up and when I rev it up and it stays at the higher rpm for a few seconds and while driving it does the same Thing until you get to a stop or slow down. I was told that the sensor just needed to adjust to the new air flow cause of it getting more air. Its been 2 weeks since I installed it and its still doing. I have tried disconnecting the battery to see if the computer would readjust to it and it hasn't. Please help! Thank you. Oh it has the 5.3 litre engine with the v4 mode
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Hoyt
April 6, 2013.




You are not getting more air. The goal of the intake system is to warm the air so the fuel will vaporize more efficiently. Liquid fuel does not burn. That was one of the problems with carburetors. We needed a choke to force way too much fuel into the engine in hopes a high enough percentage would turn to a vapor. The rest burned in the exhaust system where it only produced pollution, not power. Engines start harder in cold weather too. Why do you want to detune your engine and go back to that?

According to the advertising, cooling the air contracts it allowing you to pack more into the engine. That doesn't do anything unless you have more fuel to go with it. That raises engine speed. The Engine Computer knows the desired idle speed and it adjusts that by closing the idle air control valve and reducing how long the injectors are pulsed on. The only place you will potentially gain anything is at wide-open-throttle. You'll have a little more air, and if the maximum pulse width of the injectors hasn't been reached you'll get a little higher speed. That's wonderful, ... On the racetrack.

The computer bases its fuel metering calculations on the weight of the air going into the engine. That is calculated by the drop in temperature of a preheated thermistor inside the mass air flow sensor. That is compared to the reading from the ambient air temperature sensor, sometimes called the intake air temperature sensor. Sometimes the IAT an MAF are two totally different sensors in two different locations. Sometimes the IAT is built into the MAF. One of the many self-tests the computer constantly runs is to compare those two temperature readings. The operation of your cold air system is dependent on air flow. It doesn't cool anything when the engine is not running. Under normal conditions the air temperature drops very little as it passes through the intake air plumbing so the temperature drop measured in the MAF sensor is based solely on the weight of the air. Now you're introducing a new variable with an air temperature that basically goes down as flow goes up. The cold air system needs some type of restriction to cause that to happen. The restriction causes a drop in pressure. That's the exact same thing that makes air tools get cold when you use them. It is the basis by which every air conditioning and refrigeration system works. And it's the venturi in the carburetor that makes it draw in fuel. Now, if you have a drop in pressure, the air expands, so what happened to that extra air you think is going into the engine? It could be colder but the volume is still limited by the position of the throttle blade.

You also have to consider that if there is no restriction in this system, how can it cool anything? If the air is 60 degrees inside the system, it's 60 degrees outside too so no temperature change will take place. Intercoolers are used with turbochargers for a different reason. By packing the air tighter, just like in an air compressor, more the BTUs are squeezed into an area so the temperature rises a lot, just like when you burn your hand on the head of that air compressor. The air in the system is a lot hotter than the surrounding air so it has heat to give up, just like with the radiator.

The last thing you have to remember is the manufacturers spent a REAL lot of time perfecting their fuel injection systems to deliver precisely a 14.7 / 1 air / fuel ratio by weight, not volume. That is where combustion occurs most efficiently and almost no harmful emissions result. If you are successful in getting more air into the engine, (which the rest of us do by pushing the gas pedal further), you are going to get more fuel too. The computer sees to that. Carburetors could only be built to deliver a perfect mixture at two points; idle and high speed. In between anything had to be a compromise but it could not be allowed to dip into a lean condition. That would cause an objectionable hesitation or stumble. To avoid that they always ran a little rich in the mid range. Your fuel injection system is controlled so precisely for any combination of air temperature, engine temperature, altitude, barometric pressure, throttle position, direction and rate of throttle position change, and engine speed and load. There's nothing you're going to do to change that without introducing some running problems, the cause of which may not be detected by the computer, or increasing emissions which will be detected. If the engine runs too lean or too rich emissions will go up. A diagnostic fault code will be set and the Check Engine light will turn on.

You also are not going to reset anything by disconnecting the battery. That's going to make it worse. As you drive the computer is constantly comparing all the sensors to known values and data programmed in at the factory. Since no two sensors are ever exactly alike it learns what is normal from each one when it sees a certain set of conditions from the other sensors. When its memory is erased it has to start all over rebuilding those "data tables". Most of the time you will never notice that is taking place. The most severe thing that some people complain about, particularly on Chrysler products, is the engine is hard to start or it wants to stall at stop signs after the battery was disconnected or run dead. The fix is extremely simple but you have to know what to do while driving it.

The relearns you're referring to also include short and long-term fuel trims. The short-term fuel trims, (STFT) is a set of data points that modify fuel metering right now to achieve a perfect mixture. You will rarely notice when those have been erased because they begin to update as soon as you start the engine. When the computer sees it's always making the same corrections for the same set of conditions it moves those numbers to the long-term fuel trims, (LTFT). Those are what it starts running off of each time you start the engine. As an example of the things it looks at, if you do get more air to go into the engine the computer is going to add more fuel. Now it will see a higher than expected idle speed with no corresponding change in throttle position sensor reading. It will be confused if it can't reconcile the higher speed to the closed throttle.

Your engine uses a separate intake air temperature sensor in the fresh air tube. What you might try is relocating that closer to the mass air flow sensor so both of them will agree on temperature. You might also try removing it from the tube. You'll need to plug the hole and isolate it from engine heat but if it reads outside air temperature accurately you can let the computer rely on the mass air flow readings alone. Also be sure there are no leaks in the fresh air system between the mass air flow sensor and the throttle body. Any air that sneaks in there or through any vacuum leak will not be measured by the mass air flow sensor so the computer won't know about it. It bases its fuel calculations off the air it knows is going into the engine.

Caradiodoc
Apr 6, 2013.


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