- Jun 11, 2014
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Reading this topic has everyone jumping around, so I'm attaching some historic technical information for the typical C4 owner. Individuals need to understand when dealing with C4 engine issues, there are 2 operations "Open Loop" and Closed Loop" C4 Engine Information! All EFI systems rely on sensors to report engine conditions to the ECM so it can make intelligent decisions based on the engine’s performance. Each of these sensors reports on a particular engine condition. It is then up to the ECM to gather all this data and condense it to deliver the proper fuel and spark to the engine. The cool thing about this is that current automotive computers can make these decisions thousands of times per second! Try that with your hand calculator. For example, let’s take a look at a typical late-model TPI- or LT1-style fuel injection system. We’ll deal with only the sensors that have direct input for fuel injection, omitting the emissions-oriented sensors for now. To start with, all EFI systems need an ignition trigger. This is fairly simple since it’s virtually the same as the hookup for a normal analog tachometer. TPS: The throttle position sensor (TPS) hooks directly to the throttle blades to tell the ECM via a voltage signal not only how far the throttle is open or closed but also the rate at which the throttle opening is changing. MAP: The manifold absolute pressure (MAP) sensor reads the intake manifold pressure (or vacuum), converting this pressure reading into a voltage signal that the ECM can use. In speed-density systems, this is a critical sensor. If the MAP sensor fails, the system usually reverts to a limp-home mode. MAF: Certain TPI, LT1, and LS1 Chevy production engines use a mass airflow (MAF) sensor to measure the amount of air the engine is using. This sensor is located upstream of the throttle body and uses what is called a hot-wire sensor. A very small wire strung across the airstream is electrically heated to a certain temperature. As air passes over the wire, it cools. The amount of current required to maintain the wire’s temperature then represents the amount of mass airflow. O2: Oxygen sensors (O2) are often employed in EFI systems to report the amount of free oxygen in the exhaust. This is a convenient way to measure rich or lean air/fuel ratio conditions in an engine at part-throttle. EFI systems employing an O2 sensor are generally referred to as operating in closed loop. If the engine does not use the sensor, the system is said to be operating in open loop. Closed loop refers to the fact that the O2 sensor readings allow the computer to measure the result of combustion. Low oxygen readings indicate a rich condition, and higher oxygen content in the exhaust is a result of lean air/fuel conditions. Even MAF-equipped engines operate in closed loop, since the O2 sensor’s feedback allows for very tight control over part-throttle air/fuel ratios. Unfortunately, these factory-type O2 sensors only operate accurately within a very narrow range of the air/fuel ratio scale. They are most accurate when reading ratios hovering around 14.7:1, which is the “ideal†ratio for emissions where hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) are at their combined lowest levels. Coolant: This is a fairly simple but important sensor. For the engine to run properly at low temperatures, it’s critical that the ECM be aware of the coolant temperature. Low-temperature input, for example, will trigger fuel enrichment outputs from the ECM to act like a choke for cold-start and warmup situations. MAT: Inlet air temperature affects the air/fuel ratio, so a manifold air temperature (MAT) sensor is needed to help the computer compensate for changes in air density based on temperature. All EFI systems offer compensation tables for inlet air-temperature changes. Knock Sensor: All late-model GM engines are also equipped with a knock sensor that identifies engine knock or ping. When the knock sensor informs the ECM that it senses knock, the ECM can retard timing to prevent engine damage. Several aftermarket EFI systems also employ a similar knock sensor system. Cam Position Sensor: Sequential fuel injection systems require a sensor to tell the ECM where cylinder No. 1 is in relation to top dead center in order to trigger the fuel injectors to deliver fuel at the proper time. This sensor is usually some type of magnetic pickup located either on the camshaft drive or on the damper (similar to a racing-style crank-trigger sensor People need to understand there are two different engine management functions: The engine starts out as Open Loop, and then if all conditions are met, it will switch to Closed Loop! OPEN Loop is when it is fueled direct from the fixed tables in the tune Closed Loop is when the ECM has control of the fueling and adjusts it to engine requirements That usually happens once the engine is up to operating temperature(more than 160 degrees) and it is getting a signal from the O2 sensor. The closing of the loop causes the ECM to go into Enrich/Enlean mode that constantly changes the fuel injector pulses to lengthen or shorten to satisfy the O2 sensor. When you have engine related issues, you need to identify if your running Open Loop or Closed Loop before trying to diagnose any engine related problems! The next question is, are there any stored error codes??? [h=2]Open Loop Operation Basics:[/h] o Before anything else, check to make sure that you don't have any vacuum leaks and that your throttle position sensor and idle air control sensor are working and properly adjusted. Check your ignition timing to ensure that it's within factory spec and to ensure that you have a constant spark. These failures can easily cause a rough idle and loss of performance, and they're common enough to warrant checking before anything else. [h=2]Fuel Problems[/h] o A clogged fuel filter won't necessarily cause a serious drop in pressure, but it will cause a reduction in fuel volume. Ultimately, it is this reduction in volume that causes the engine to drop power, and the rapid fluctuations in pressure that can result from a clogged filter will cause a loss of idle quality. Of course, you may also have the opposite problem; your engine could be getting too much fuel if one or more of the injectors are hanging open and constantly leaking fuel into the engine. This latter will result in a constant or intermittent miss and a raw fuel odor in the exhaust. So, you may want to ensure the engine is running OK, and with-in spec before the engine switches to closed loop! ChiTownAL ____________________________________ | |
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davep85c4 Former Member - Send Private Message Joined: 4/4/2009 Posts: 254 | tbirdsps said: My 90 L98 idles very smooth. |
davep85c4 Former Member - Send Private Message Joined: 4/4/2009 Posts: 254 | tbirdsps said: My 90 L98 idles very smooth. Not sure why all L98s can't. Not all L98's are equal. The following differences between a 90 and an 85 will all affect emissions / tuning, and therefore idle quality. 1985 utilizes a one year only, slow baud rate ECM. While certainly adequate, it's processor speed is substantially slower than the "165" ECM used in the 86-up. ( I can't recall the actual baud rates, so I won't guess at the differences. But it's like 4X faster for the 165). The P/N changes for 90 because of the under hood location of the ECM, but the internals and programing are the same as the 165 ECM. Faster up-date and processing rates are more stable. This is a major difference between 85's and all other L98's. Aluminum vs Iron heads. And chamber design. Timing can be more advanced for Aluminum heads, because chamber temperatures are lower than iron heads at idle. The heat is carried away faster. Advanced timing improves idle quality and off-idle response. 90 has pre-cats that light-off a 3-way catylist. 85 has a two-way catylist, and no pre-cats. System can enter closed loop sooner after cold start, and idle richer in general as main cat will stay effective at idle because of the pre-cats. Closed loop is more stable (usually) and richer idle is smoother. And another big one in favor of the 90: The 90 and 91 L98 are speed-density systems. 90-91 have a MAP sensor that measures intake manifold vacuum. The 85-89 have a Mass Airflow Sensor. The MAF system is not as stable at idle as the MAP system. This too is a big difference. The early MAF systems (on all GM's that have MAF) don't idle very smoothly. There's some reasons why "all L98's can't idle smoothly". Even though the engine RPO is common to both 85 and 90, there are some significant differences in the hardware. |
oxrageous;n242905 said:Cooter, I've tried to address my floppy center console lid problem. When in doubt, cover it up!