Last month we discussed Cadillac's problem with their air conditioning strategy and the EPA. Why did Cadillac get gigged when they passed the Federal Test Procedure (FTP) fair and square? Well, I found out there is a provision in the rules called the "defeat device policy." It states that under no circumstances will an emissions device be defeated. The problem came from different interpretations of this rule.

New rules are being written and debated that will go into effect in the year 2000. It was interesting to note that part of the new rules include specific testing with the air conditioning on.

I wrote the last article as an introduction to a short series on special strategies that affect fuel delivery. I started writing the next article in this series and it soon became two articles. After nearly completing both, I realized I had never covered the foundation of fuel delivery. So, please bear with me when the ideas start to flow they rarely occur to me in a logical order.

I've mentioned sensor authority before; now here are the details. Remember, these are general rules that may not apply to all cars.

Base pulse width: This is programmed by the factory into the programmed read only memory (PROM) and serves as a foundation that the other inputs modify. This is dependent on engine size and design.

Coolant temperature sensor (CTS): The CTS adds fuel to the base pulse depending on the engine temperature. It is one of the parameters that determines open and closed loop control of the O2 sensor. This sensor has a broad range of authority.

Manifold absolute pressure sensor: The manifold pressure or manifold vacuum is a direct indication of engine load. The higher the manifold pressure (lower vacuum), the greater the load, and therefore, more fuel must be added.

RPM: The engine is basically just an air pump; the faster it turns over, the more air it pumps. The more air it pumps, the more fuel it needs to maintain that 14.7/1 air fuel ratio.

Volumetric efficiency: This is an internal calculation based on engine testing and design. Volumetric efficiency (VE) is very dependent on engine speed and load. Most standard engines (not high performance) have a cylinder head, and valve and camshaft design to create maximum VE at 2500 RPM where most driving occurs. As a result of the compromise to 2500 RPM, higher and lower engine speeds suffer. The advantage of variable valve timing is to help eliminate this compromise.

The weakness of the speed density system is that it is based on the assumption that
the volumetric efficiency stays the same over time. When large carbon cones develop on intake and exhaust valves, the ability of the engine to breath is reduced, however, the computer is still delivering fuel based on the assumption that the engine is working like new. This is why speed density systems tend to go rich over time. This is fine as long as the O2 sensor has ample authority to compensate. Another assumption is that the proper amount of EGR is flowing based on the commanded opening of the EGR valve. GM engines command the EGR valve open and just assume that it happens, and that EGR flows at the proper rate. Ford went one better and had an EGR valve position sensor to feed back to the computer. This confirms the valve opens and how far, but if the EGR passages are plugged as often happens, then too much fuel is still supplied. As you can see, both systems suffer from the assumption that the EGR is flowing.

MAF: A mass air flow (MAF) system measures the actual amount of air flow into the engine and compensates fuel for this amount. So, a MAF system does not go rich when confronted by less air flow or no EGR. The only assumption a MAF system makes is that there is no pirate air getting in behind it. This is the weakness of the MAF system. Detecting vacuum leaks in this system is critical, even crankcase air leaks.

Throttle position sensor: Now we are dropping lower on the critical sensor authority list. The TPS does three main inputs. It is usually used to show idle (eliminating the idle tracking switch on Fords and the nose switch on GMs). It shows rate of change in the throttle opening, making it important as the replacement for the accelerator pump in a carbureted system. Also the TPS tells the computer when the engine is at wide open throttle (WOT). WOT is important at cranking speeds to put the computer in "clear flood mode," another mimic of the carburetor days. Also the TPS tells the computer when to drop out of closed loop under WOT at higher than cranking RPM for full throttle power enrichment. Some systems depend on the MAP, the accelerator pump enrichment and only use TPS switches for idle and/or WOT.

Air charge temperature: ACT is usually a minor pulse width change, however, it is important to prevent hesitation. This is a more important input in colder climates. It is often used in a rationality check with the CTS. They should read nearly the same on a cold engine. It should have been used to set codes for a nonfunctional heated air intake system, but I have never seen this done.

Exhaust gas recirculation: All of the inputs above add fuel to the base calculation; the EGR is a subtraction. The exhaust gas that is recirculating the cylinder is inert to the combustion process, therefore, it slows the combustion by getting in between the O2 and the HC.

When the EGR is introduced into the intake manifold, less air flows into the cylinder at the same RPM. With less air in the cylinder, we need less fuel to maintain the same fuel mixture. The more EGR, the less fuel.

Wildcards: These are the ones that I see most techs forget about and when they fail they can eat your lunch. So, here is a quick list. The air conditioning adds about .5 ms to the pulse width usually and changes idle air counts, more on small engines and less on large ones. Power steering pressure switch is about the same as the AC. PRNDL switch ñ when you drop into gear on an automatic, you need more fuel and throttle to maintain the same idle speed. Alternator load and voltage ñ when voltage is low from cranking or a bad charging system, the injector must be held open longer to deliver the same amount of fuel. This is because it opens slower with less power. The fuel and idle air must also be increased when the alternator is supplying a heavy demand, to compensate for the load. All the special strategies fall into this category and I'll get to them in a future article.

The O2 sensor: This is certainly an important input, however, it is last in authority. After all other sensors have had their say, only then can the O2 take over and trim the fuel mixture. Notice I said trim. The O2 sensor has limited authority within an operating window. Considering idle speed for most vehicles at 750 RPMs, 18" of vacuum and 190 degree coolant temperature, the injectors open for 2 to 3 milliseconds. If you enrich the mixture by adding propane, the O2 signal will go high indicating the rich mixture to the computer. On most engines the computer will respond and shorten the pulse width to 1.5 ms and no lower. The reverse is also true. Leaning the mixture out or sending a false low signal to the O2 circuit will usually increase the pulse width to 5.5 ms and no more.

The manufacturers know that the engine should run at idle between 1.5 and 5.5 ms pulse width. If the O2 sensor died and produced no voltage giving the computer a false lean signal, we don't want it commanding so much fuel that the engine will stall or hydrolock. On the other hand, if another sensor has gone wacko and is causing the pulse width to go over 5.5 ms, the O2 will never have a chance to trim the fuel because it is out of the O2 authority range.

The final formulas are:
Speed density: Base + CTS + [(MAP + RPM) x VE] + TPS + ACT - EGR +- O2 + wildcards.

Mass air flow: Base + CTS + MAF + TPS + ACT - EGR +- O2 + wildcards.

Mark Warren is the owner of Mark's Auto, an Automotive Service Association (ASA) member shop in Tucson, Arizona. Warren discusses underhood diagnostics and other service considerations monthly in Tech to Tech. If you have a comment for Mark Warren, you may write him c/o AutoInc., P.O. Box 929, Bedford, TX, 76095-0929, for fax to (817) 685-0225. Mark's email address is 76727.3631@compuserve.com.

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