- Joined
- February 8, 2003
- Messages
- 9,824
- Reaction score
- 94
- City, State
- Sacramento, CA 95827
- Year, Model & Trim Level
- 1992 XLT
Often, when the check engine light comes on, there is a tendency to want to start replacing sensors right and left in hopes it might solve the problem. Too few seem to want to take the time to get the codes read, or better yet look at the other parameters a good code scanner could provide. In hopes of saving some of those folks some money by sparing the replacement of good sensors, I thought I’d put together this little primer on the operation of a computer controlled engine. The principles are generic, and in some cases different manufacturers call certain components by different names, but the items are largely similar in function.
How did we get to where we are today? The simple answer is emissions standards. Remember back in the 60’s with big engines and holley 4 barrels? Well the big engines are slowly coming back to power the behemoth SUV’s, but the carburetors are history. Why? Let’s look at the progression of emissions control devices. The first attempts to reduce emissions was to recirculate exhaust gasses and in some cases to inject air into them to allow more complete combustion. Stricter control over carburetor settings (including the infamous tamper proof cap) arrived, with attendant driveability problems. Ford attempted to improve the carburetor by adding a variable venturi in the early 1980’s, and though an engineering marvel, it’s field experience was less than wonderful. What was needed was better and more precise control of fuel mixture at all phases of engine operation – fuel injection controlled by a computer arrived. The fuel delivery problem was solved by having a computer monitor the engine function and deliver a precisely metered amount of fuel to the cylinder based on that data. It created a near perfect fuel air mix throughout the power curve – something not able to be done using carburetors, so, good bye carburetors.
Although an engine can run acceptably with less than the “perfect” air fuel ratio of 14.7 to 1, that ratio not only produces the fewest emissions, it also happens to the range where the catalytic converter (CAT) can operate at maximum efficiency. The CAT operates poorly, if at all, much out of that range on either side. So to further tweak the mixture control capability of the system Oxygen sensors got added providing even more precise measurement of the process for the computer. First they got added upstream of the CAT for this purpose, later we added them downstream too, to provide some indication of CAT performance, and finally we added them on both banks of cylinders. The continuing trend is to monitor more and more functions in more and more places.
Lets look at the information the system works with and how it gets it. Understanding this can often help narrow or pinpoint failed or out of range components. Mind you some of these sensors can be out of whack and not make much of a problem with driveability, and still set a trouble code on the check engine light, others, well if they are bad the poor engine may barely run.
First off, and rarely a problem, is the crankshaft position sensor. This lets the computer know where the engine is in the cycle, and in some cases provides RPM data as well. This more or less keeps the computer abreast of the mechanical operation of the engine. Next there is the matter of air/fuel. Air coming in is measured by the Mass Airflow Sensor (MAF) there is also intake air temperature (IAT) sensors to measure the temperature of that air as well. Let’s take a minute and think – if the computer is receiving bad data about how much air is being used, what’s the likelihood of the fuel air mix being anywhere close to acceptable? Not high. So when the MAF is bad, you are likely going to experience driveability problems. IAT ? If that’s bad probably you may not notice anything at all in the operation of the vehicle. Similar is the coolant temperature data. On the fuel side, the fuel pressure is measured, and the computer, using data as to engine load and speed and throttle position - from the throttle position sensor (TSP) and using a built in lookup table, provides a specific pulse of open injector time to allow the introduction of fuel. You can imagine what a bad TPS could do. The poor computer will be trying to provide fuel for a non-existent condition of operation. At some point after startup, oxygen sensors come into play. Early ones had no heaters, so it took them a while to become operational. Later ones had heaters added to speed up the process. Once these “wake up” the system switches over to them to “fine tune” the air fuel mixture. It does this by adjusting “short term fuel trim” – changing the injector pulse width. The computer can tell you how much, plus or minus, in percent this is being done. In addition your computer has a set of parameters it has learned and uses to compensate for the overall system, called long term fuel trim. As in short term, this can be plus or minus, and the computer can tell you how much is being applied. Fuel trim numbers are a gem that few DIY’ers ever mine. A significantly high long term fuel trim percentage figure (plus or minus) can give you a clue if you are running chronically rich or lean, and provide valuable clues to system problems before they become really major. Finally, we have kept the EGR on many engines, and added controllers for them (Differential Pressure Feedback EGR sensor (DPFE) and EGR vacuum regulator (EVR)). If these get out of whack, too much exhaust gas can be let into the system at lower rpms, significantly affecting the idle, and the computer’s ability to compensate. Often this will show up on fuel trim numbers in the early phases as the computer tries to fix the problem by adjusting pulse width on the injectors. It is very common for a problem in one part of the EGR system to set other trouble codes as other components try and compensate unsuccessfully for an out of balance system. This is why a code reader alone may not lead right off to a successful diagnosis – it may flag an O2 sensor when it’s the DPFE or EGR or EVR that is the real culprit. A scanner can be a great help in sorting out multiple trouble codes.
There are other systems being monitored, but other than to mention the Idle Adjustment Compensator (IAC) I won’t go into detail on them. The IAC exists to give the computer an ability to add engine speed when a load or other demand has caused it to fall or the computer finds it necessary. Usually this would be something like an A/C compressor load as the AC clutch kicks in, or during cold idle situations.
I’ll end this by saying that for the couple hundred bucks they cost, a code reader/scanner is the most valuable tool you can own. Knowing for example that an oxygen sensor should provide a rapidly varying output between .2 and .7 volts and that it should cross over .45 volts frequently can let you watch and see if you have a bad O2 sensor instead of plunking down $75 or more to replace what may be a perfectly good sensor. Scanners can pay for themselves over time, and make diagnosis much easier. Few people realize that today, the DIY can place in his or her hands, interactive computer capability rivaling that which the dealer has and do it rather inexpensively. In another article, and assuming there is interest, I’ll give you readout numbers that can help if you have a scanner.
Happy Exploring
Chris
How did we get to where we are today? The simple answer is emissions standards. Remember back in the 60’s with big engines and holley 4 barrels? Well the big engines are slowly coming back to power the behemoth SUV’s, but the carburetors are history. Why? Let’s look at the progression of emissions control devices. The first attempts to reduce emissions was to recirculate exhaust gasses and in some cases to inject air into them to allow more complete combustion. Stricter control over carburetor settings (including the infamous tamper proof cap) arrived, with attendant driveability problems. Ford attempted to improve the carburetor by adding a variable venturi in the early 1980’s, and though an engineering marvel, it’s field experience was less than wonderful. What was needed was better and more precise control of fuel mixture at all phases of engine operation – fuel injection controlled by a computer arrived. The fuel delivery problem was solved by having a computer monitor the engine function and deliver a precisely metered amount of fuel to the cylinder based on that data. It created a near perfect fuel air mix throughout the power curve – something not able to be done using carburetors, so, good bye carburetors.
Although an engine can run acceptably with less than the “perfect” air fuel ratio of 14.7 to 1, that ratio not only produces the fewest emissions, it also happens to the range where the catalytic converter (CAT) can operate at maximum efficiency. The CAT operates poorly, if at all, much out of that range on either side. So to further tweak the mixture control capability of the system Oxygen sensors got added providing even more precise measurement of the process for the computer. First they got added upstream of the CAT for this purpose, later we added them downstream too, to provide some indication of CAT performance, and finally we added them on both banks of cylinders. The continuing trend is to monitor more and more functions in more and more places.
Lets look at the information the system works with and how it gets it. Understanding this can often help narrow or pinpoint failed or out of range components. Mind you some of these sensors can be out of whack and not make much of a problem with driveability, and still set a trouble code on the check engine light, others, well if they are bad the poor engine may barely run.
First off, and rarely a problem, is the crankshaft position sensor. This lets the computer know where the engine is in the cycle, and in some cases provides RPM data as well. This more or less keeps the computer abreast of the mechanical operation of the engine. Next there is the matter of air/fuel. Air coming in is measured by the Mass Airflow Sensor (MAF) there is also intake air temperature (IAT) sensors to measure the temperature of that air as well. Let’s take a minute and think – if the computer is receiving bad data about how much air is being used, what’s the likelihood of the fuel air mix being anywhere close to acceptable? Not high. So when the MAF is bad, you are likely going to experience driveability problems. IAT ? If that’s bad probably you may not notice anything at all in the operation of the vehicle. Similar is the coolant temperature data. On the fuel side, the fuel pressure is measured, and the computer, using data as to engine load and speed and throttle position - from the throttle position sensor (TSP) and using a built in lookup table, provides a specific pulse of open injector time to allow the introduction of fuel. You can imagine what a bad TPS could do. The poor computer will be trying to provide fuel for a non-existent condition of operation. At some point after startup, oxygen sensors come into play. Early ones had no heaters, so it took them a while to become operational. Later ones had heaters added to speed up the process. Once these “wake up” the system switches over to them to “fine tune” the air fuel mixture. It does this by adjusting “short term fuel trim” – changing the injector pulse width. The computer can tell you how much, plus or minus, in percent this is being done. In addition your computer has a set of parameters it has learned and uses to compensate for the overall system, called long term fuel trim. As in short term, this can be plus or minus, and the computer can tell you how much is being applied. Fuel trim numbers are a gem that few DIY’ers ever mine. A significantly high long term fuel trim percentage figure (plus or minus) can give you a clue if you are running chronically rich or lean, and provide valuable clues to system problems before they become really major. Finally, we have kept the EGR on many engines, and added controllers for them (Differential Pressure Feedback EGR sensor (DPFE) and EGR vacuum regulator (EVR)). If these get out of whack, too much exhaust gas can be let into the system at lower rpms, significantly affecting the idle, and the computer’s ability to compensate. Often this will show up on fuel trim numbers in the early phases as the computer tries to fix the problem by adjusting pulse width on the injectors. It is very common for a problem in one part of the EGR system to set other trouble codes as other components try and compensate unsuccessfully for an out of balance system. This is why a code reader alone may not lead right off to a successful diagnosis – it may flag an O2 sensor when it’s the DPFE or EGR or EVR that is the real culprit. A scanner can be a great help in sorting out multiple trouble codes.
There are other systems being monitored, but other than to mention the Idle Adjustment Compensator (IAC) I won’t go into detail on them. The IAC exists to give the computer an ability to add engine speed when a load or other demand has caused it to fall or the computer finds it necessary. Usually this would be something like an A/C compressor load as the AC clutch kicks in, or during cold idle situations.
I’ll end this by saying that for the couple hundred bucks they cost, a code reader/scanner is the most valuable tool you can own. Knowing for example that an oxygen sensor should provide a rapidly varying output between .2 and .7 volts and that it should cross over .45 volts frequently can let you watch and see if you have a bad O2 sensor instead of plunking down $75 or more to replace what may be a perfectly good sensor. Scanners can pay for themselves over time, and make diagnosis much easier. Few people realize that today, the DIY can place in his or her hands, interactive computer capability rivaling that which the dealer has and do it rather inexpensively. In another article, and assuming there is interest, I’ll give you readout numbers that can help if you have a scanner.
Happy Exploring
Chris