Fuel Sys Open/closed Loop
ern2112
01-22-2006, 11:12 PM
Hey guys.....while monitoring my car with a break out box.....I can put the car under heavy acceleration and the fuel sys loop changes from closed to open status....soon as I let off gas...it goes back to closed. 1st off....is this normal....2nd, what does this meen....I would have thought that there was an injector causing this or something...any thoughts?
BNaylor
01-23-2006, 07:24 AM
An odb-ii emissions system operates in two modes....open and closed loop for fuel control. It is supposed to switch depending on driving conditions and engine parameters.
Acceleration and idle until warm up to around 145 degrees is classified as open loop operation.
It should operate in closed loop when in idle after proper warm up and at cruising speeds.
I would be concerned if it doesn't switch between open and closed loop modes.
PCM Basics 101:
Executive summary: The PCM controls the fuel injectors by using signals from the engine sensors, like MAP, MAF, RPM, ECT, and others to pick out the right value from the right lookup table to use in the fuel injector pulsewidth calculation. The PCM injector pulsewidth calculation also uses the INT and BLM correction factors based on O2 sensor feedback when it is in closed loop. O2 sensor correction is not used in open loop.
Note that there are 2 kinds of open loop. Right after a cold or hot start, a scan tool will report that the PCM is using open loop fuel control. This version of open loop is described in the next section. Once the PCM decides that conditions are right, it switches to closed loop fuel control, which is described below the open loop discussion. But even after this switch, if you lean on the throttle hard enough, the PCM will switch to power enrichment (PE) mode, which is another form of open loop. In PE mode the scan tool may report that the PCM is in closed loop, but it should also report that learning is disabled. Learning disabled means that the PCM is ignoring the O2 sensors, which is the same as saying that the PCM is using a version of open loop fuel control.
--------------------------------------------------------------------------------
Open loop
When you fire the beast up, whether the engine is stone cold or fully warmed up, the PCM starts out in open loop. This is because cold (or just warm) O2 sensors don't work very well, and they take a little while to get good and hot. Most LT1 O2 sensors have built-in heaters to speed up the process, which is why they have 4 wires coming out of them. Also, cold engines are happier when they are run richer than the fixed closed loop air/fuel ratio target. Since the O2 sensor signal is not reliable right after the engine is started, the PCM has no way to find out whether the engine is running too rich, too lean, or about right. But it still needs to take a stab at the right injector pulsewidth.
When the engine is running in open loop at light to moderate loads (idling, city driving, highway cruising), the PCM uses the AFR table for the pulsewidth calculation. The AFR table uses the ECT (colder engines need to be richer than warm engines) and MAP (higher loads require a richer mix than light loads) sensor inputs to point to the desired AFR for those temperature and load conditions. As always, the MAF sensor tells the PCM how much air the engine is inhaling. The PCM knows how big the injectors are (the amount of fuel is delivered per millisecond of pulsewidth), so with the MAF sensor output (the amount of air inhaled by the engine) and the AFR from the table (the desired air/fuel ratio), it can estimate its best-guess injector pulsewidth. Remember that the PCM has no idea whether or not this guess-timated pulsewidth is anywhere near right. It could be causing the engine to be very rich, or very lean, or anywhere in between.
Remember that engines like a richer air/fuel mixture when they are cold (richer than the fixed 14.7:1 ratio used in closed loop), and when they are asked to make more than just a cruising level of power. So the AFR table values for high MAP (heavy load) and low ECT (cold engine) are lower (richer) than the values for low MAP and normal operating ECT. This means that "cold engine enrichment" and a bit of "power enrichment" are built right in to the table. This quasi-power enrichment may be used in open loop when you maintain your freeway speed up a steep hill in high gear.
When is the engine ready to switch from open loop to closed loop? The current theory is that the PCM decides to switch based on some combination of ECT and engine running time (3 minutes?).
--------------------------------------------------------------------------------
Closed loop
When the engine temperature and time-since-the-engine-started timer agree that the O2 sensors are warmed up and ready, the PCM switches to closed loop (at least this how I think it works). Just as it did for open loop, the PCM calculates the closed loop base pulsewidth using numbers programmed in lookup tables.
While the target open loop air/fuel ratios are contained in the AFR table, the closed loop air/fuel ratio is hard-coded in the PCM to be 14.7:1. The MAF sensor tell the PCM how much air the engine is inhaling, the injector constant and injector-offset vs. voltage values tell the PCM how much fuel the injector will flow for any particular pulsewidth, so the PCM just does the math to calculate the appropriate injector pulsewidth.
Closed loop operation may refine the injector pulsewidth calculation using the 2-dimensional VE tables, which use MAP and RPM as the 2 inputs. In the 94-95 f-body (OBD-I) PCM there are 3 different VE tables, one for cranking/starting (0-340 rpm), one for low rpm (400-2000 rpm), and the third for high rpm (2000-7000 rpm). If used, these tables would account for the fact that an engine does not necessarily use all the air it inhales for combustion. For example, engines with long duration cams pass some of the inhaled air right out the exhaust valve during overlap.
Even in closed loop, the injector pulsewidth calculation is really just a guess based on the MAF sensor, the injector constant and offset, and (possibly) VE table numbers. If the pulsewidth is not right, the fuel/air mixture will be too rich or too lean. It's very important to remember that the PCM tries to keep the AFR at the "ideal" ratio of 14.7 while it's in closed loop.
This is where the O2 sensors come in. They generate an electrical signal for the PCM that represents the amount of oxygen in the exhaust stream, which is proportional to the air/fuel ratio. This signal, which is the feedback that "closes the loop", tells the PCM how good a job it's doing on the fuel mixture. The PCM uses this feedback to calculate a correction number, sometimes called the integrator, or INT for short, or (more accurately IMHO) the short term fuel offset. The PCM uses the short term fuel offset to adjust the pulsewidth calculation so that the air/fuel ratio homes in on 14.7:1.
Acceleration and idle until warm up to around 145 degrees is classified as open loop operation.
It should operate in closed loop when in idle after proper warm up and at cruising speeds.
I would be concerned if it doesn't switch between open and closed loop modes.
PCM Basics 101:
Executive summary: The PCM controls the fuel injectors by using signals from the engine sensors, like MAP, MAF, RPM, ECT, and others to pick out the right value from the right lookup table to use in the fuel injector pulsewidth calculation. The PCM injector pulsewidth calculation also uses the INT and BLM correction factors based on O2 sensor feedback when it is in closed loop. O2 sensor correction is not used in open loop.
Note that there are 2 kinds of open loop. Right after a cold or hot start, a scan tool will report that the PCM is using open loop fuel control. This version of open loop is described in the next section. Once the PCM decides that conditions are right, it switches to closed loop fuel control, which is described below the open loop discussion. But even after this switch, if you lean on the throttle hard enough, the PCM will switch to power enrichment (PE) mode, which is another form of open loop. In PE mode the scan tool may report that the PCM is in closed loop, but it should also report that learning is disabled. Learning disabled means that the PCM is ignoring the O2 sensors, which is the same as saying that the PCM is using a version of open loop fuel control.
--------------------------------------------------------------------------------
Open loop
When you fire the beast up, whether the engine is stone cold or fully warmed up, the PCM starts out in open loop. This is because cold (or just warm) O2 sensors don't work very well, and they take a little while to get good and hot. Most LT1 O2 sensors have built-in heaters to speed up the process, which is why they have 4 wires coming out of them. Also, cold engines are happier when they are run richer than the fixed closed loop air/fuel ratio target. Since the O2 sensor signal is not reliable right after the engine is started, the PCM has no way to find out whether the engine is running too rich, too lean, or about right. But it still needs to take a stab at the right injector pulsewidth.
When the engine is running in open loop at light to moderate loads (idling, city driving, highway cruising), the PCM uses the AFR table for the pulsewidth calculation. The AFR table uses the ECT (colder engines need to be richer than warm engines) and MAP (higher loads require a richer mix than light loads) sensor inputs to point to the desired AFR for those temperature and load conditions. As always, the MAF sensor tells the PCM how much air the engine is inhaling. The PCM knows how big the injectors are (the amount of fuel is delivered per millisecond of pulsewidth), so with the MAF sensor output (the amount of air inhaled by the engine) and the AFR from the table (the desired air/fuel ratio), it can estimate its best-guess injector pulsewidth. Remember that the PCM has no idea whether or not this guess-timated pulsewidth is anywhere near right. It could be causing the engine to be very rich, or very lean, or anywhere in between.
Remember that engines like a richer air/fuel mixture when they are cold (richer than the fixed 14.7:1 ratio used in closed loop), and when they are asked to make more than just a cruising level of power. So the AFR table values for high MAP (heavy load) and low ECT (cold engine) are lower (richer) than the values for low MAP and normal operating ECT. This means that "cold engine enrichment" and a bit of "power enrichment" are built right in to the table. This quasi-power enrichment may be used in open loop when you maintain your freeway speed up a steep hill in high gear.
When is the engine ready to switch from open loop to closed loop? The current theory is that the PCM decides to switch based on some combination of ECT and engine running time (3 minutes?).
--------------------------------------------------------------------------------
Closed loop
When the engine temperature and time-since-the-engine-started timer agree that the O2 sensors are warmed up and ready, the PCM switches to closed loop (at least this how I think it works). Just as it did for open loop, the PCM calculates the closed loop base pulsewidth using numbers programmed in lookup tables.
While the target open loop air/fuel ratios are contained in the AFR table, the closed loop air/fuel ratio is hard-coded in the PCM to be 14.7:1. The MAF sensor tell the PCM how much air the engine is inhaling, the injector constant and injector-offset vs. voltage values tell the PCM how much fuel the injector will flow for any particular pulsewidth, so the PCM just does the math to calculate the appropriate injector pulsewidth.
Closed loop operation may refine the injector pulsewidth calculation using the 2-dimensional VE tables, which use MAP and RPM as the 2 inputs. In the 94-95 f-body (OBD-I) PCM there are 3 different VE tables, one for cranking/starting (0-340 rpm), one for low rpm (400-2000 rpm), and the third for high rpm (2000-7000 rpm). If used, these tables would account for the fact that an engine does not necessarily use all the air it inhales for combustion. For example, engines with long duration cams pass some of the inhaled air right out the exhaust valve during overlap.
Even in closed loop, the injector pulsewidth calculation is really just a guess based on the MAF sensor, the injector constant and offset, and (possibly) VE table numbers. If the pulsewidth is not right, the fuel/air mixture will be too rich or too lean. It's very important to remember that the PCM tries to keep the AFR at the "ideal" ratio of 14.7 while it's in closed loop.
This is where the O2 sensors come in. They generate an electrical signal for the PCM that represents the amount of oxygen in the exhaust stream, which is proportional to the air/fuel ratio. This signal, which is the feedback that "closes the loop", tells the PCM how good a job it's doing on the fuel mixture. The PCM uses this feedback to calculate a correction number, sometimes called the integrator, or INT for short, or (more accurately IMHO) the short term fuel offset. The PCM uses the short term fuel offset to adjust the pulsewidth calculation so that the air/fuel ratio homes in on 14.7:1.
richtazz
01-23-2006, 09:40 AM
Nice explanation Bob, but now my head hurts, :cwm27:
ern2112
01-23-2006, 09:40 AM
Bob,
Wow.....I really appreciate the time you take to explain things.....I totally understand what is happening now...and obviously, my car is doing the right thing.
Don't know if you read the other thread.....but you and Dad helped out big time......did the switch we spoke of on the injectors.....no codes are popping up now....and I expected to move the problem to cylinder 4!! But...just cleaned the injector with gumout....and it solved the probem
The car seems to still have a little shuttering now...not like before obviously.....so...what do you think about me pulling all the injectors out and soaking them in sum carb cleaner for a while to clean them out too......the plastic be ok for that? I would assume so based on them being gas resistant.....
I read your thread on the 02 sensors too...gonna swap mine out.....even if the light is not on....gonna try and get more milage....cause damn...the price of gas is killin' me. I drive 750-800 a week.....
Wow.....I really appreciate the time you take to explain things.....I totally understand what is happening now...and obviously, my car is doing the right thing.
Don't know if you read the other thread.....but you and Dad helped out big time......did the switch we spoke of on the injectors.....no codes are popping up now....and I expected to move the problem to cylinder 4!! But...just cleaned the injector with gumout....and it solved the probem
The car seems to still have a little shuttering now...not like before obviously.....so...what do you think about me pulling all the injectors out and soaking them in sum carb cleaner for a while to clean them out too......the plastic be ok for that? I would assume so based on them being gas resistant.....
I read your thread on the 02 sensors too...gonna swap mine out.....even if the light is not on....gonna try and get more milage....cause damn...the price of gas is killin' me. I drive 750-800 a week.....
BNaylor
01-23-2006, 10:32 AM
Bob,
Wow.....I really appreciate the time you take to explain things.....I totally understand what is happening now...and obviously, my car is doing the right thing.
Don't know if you read the other thread.....but you and Dad helped out big time......did the switch we spoke of on the injectors.....no codes are popping up now....and I expected to move the problem to cylinder 4!! But...just cleaned the injector with gumout....and it solved the probem
The car seems to still have a little shuttering now...not like before obviously.....so...what do you think about me pulling all the injectors out and soaking them in sum carb cleaner for a while to clean them out too......the plastic be ok for that? I would assume so based on them being gas resistant.....
I read your thread on the 02 sensors too...gonna swap mine out.....even if the light is not on....gonna try and get more milage....cause damn...the price of gas is killin' me. I drive 750-800 a week.....
No problem and you are welcome. Thats what we're here for. If one injector was in the condition then the others could use the cleaning too. It is something you may not have to do that often once it is done.
I ran into this procedure a couple of years ago. If you like doing DIY it is a good procedure to dynamically clean the injectors while on the car without having to disassemble the injection system. It will use the fuel pressure generated by the system and will be more efficient. It is shown for an L67 but it will work on L36 GPs.
Depending on the age and mileage changing out or cleaning the 02 sensors is a good idea especially if you have a drop in highway fuel economy and all diagnostic tests pass. If you replace them make sure they are OEM AC Delco and not aftermarket such as Bosch. If you have a 3800 the front is AC Delco PN: AFS109, rear AFS141.
http://www.dans-website.com/cleaning_injectors.htm
Good luck and glad you're getting the problems resolved.
Wow.....I really appreciate the time you take to explain things.....I totally understand what is happening now...and obviously, my car is doing the right thing.
Don't know if you read the other thread.....but you and Dad helped out big time......did the switch we spoke of on the injectors.....no codes are popping up now....and I expected to move the problem to cylinder 4!! But...just cleaned the injector with gumout....and it solved the probem
The car seems to still have a little shuttering now...not like before obviously.....so...what do you think about me pulling all the injectors out and soaking them in sum carb cleaner for a while to clean them out too......the plastic be ok for that? I would assume so based on them being gas resistant.....
I read your thread on the 02 sensors too...gonna swap mine out.....even if the light is not on....gonna try and get more milage....cause damn...the price of gas is killin' me. I drive 750-800 a week.....
No problem and you are welcome. Thats what we're here for. If one injector was in the condition then the others could use the cleaning too. It is something you may not have to do that often once it is done.
I ran into this procedure a couple of years ago. If you like doing DIY it is a good procedure to dynamically clean the injectors while on the car without having to disassemble the injection system. It will use the fuel pressure generated by the system and will be more efficient. It is shown for an L67 but it will work on L36 GPs.
Depending on the age and mileage changing out or cleaning the 02 sensors is a good idea especially if you have a drop in highway fuel economy and all diagnostic tests pass. If you replace them make sure they are OEM AC Delco and not aftermarket such as Bosch. If you have a 3800 the front is AC Delco PN: AFS109, rear AFS141.
http://www.dans-website.com/cleaning_injectors.htm
Good luck and glad you're getting the problems resolved.
ern2112
01-23-2006, 10:52 AM
Bob,
Excellent info....my buddy just telling me about this setup he has in his garage. I am going to use it next weekend. SHould I just sit tight on puling the other injectors out then until I run the cleaner direct to them? Maybe jsut fix the whole shabang by doing that only?
Excellent info....my buddy just telling me about this setup he has in his garage. I am going to use it next weekend. SHould I just sit tight on puling the other injectors out then until I run the cleaner direct to them? Maybe jsut fix the whole shabang by doing that only?
BNaylor
01-23-2006, 11:08 AM
Bob,
Excellent info....my buddy just telling me about this setup he has in his garage. I am going to use it next weekend. SHould I just sit tight on puling the other injectors out then until I run the cleaner direct to them? Maybe jsut fix the whole shabang by doing that only?
I would see what your buddy has and hold off pulling the rails and injectors, especially if you have it all back together and it is working with no misfire caused by injectors. The process explained is similar to what the Pros do to clean the complete fuel injection system. Also, it may save you from any possible complications and further parts replacement, if any. Also, once injectors are removed a new o-ring seal should be put on. Good luck!
Excellent info....my buddy just telling me about this setup he has in his garage. I am going to use it next weekend. SHould I just sit tight on puling the other injectors out then until I run the cleaner direct to them? Maybe jsut fix the whole shabang by doing that only?
I would see what your buddy has and hold off pulling the rails and injectors, especially if you have it all back together and it is working with no misfire caused by injectors. The process explained is similar to what the Pros do to clean the complete fuel injection system. Also, it may save you from any possible complications and further parts replacement, if any. Also, once injectors are removed a new o-ring seal should be put on. Good luck!
Automotive Network, Inc., Copyright ©2025