Yea, I read somewhere that when it's warming up it doesn't use the MAF sensor--so ASE book i believe

But now i'm not so sure, because when i started my car the other day and it threw the 48 code obd1 (which i still need help on) it threw the code right away, so it must have been reading it or maybe it just knows before it reads it.......beats me.....all i know is the MAF and the stupid codes it throws is all BS---they should have designed them better

 

Could someone please clarify. InjectedSS states that the MAF isn't used in OPEN LOOP so let's say for argument's sake that IT IS used in OPEN LOOP as many have suggested. It was to my knowledge that while in OPEN LOOP the computer always uses the AFR x 10, Open Loop Vs Coolant Temp Vs MAP so if the MAF is supposed used in OPEN LOOP then what does it do if the computer is using the Open Loop table to trim fuel

 

Is it getting hot in here or is it just me?

 

To trim fuel in open loop, just modify the MAF freq vs airflow tables - once you get the MAF calibration tables spot on, the AFR tables will correspond to the actual AFR.

 

Not this again!

Closed and open loop refer to the modes where the PCM either uses the O2 sensors (closed loop) or ignores them (open loop). The MAF is the primary method used by the LT1 to determine airflow. Common engine control algorithms that do not use an MAF can be either a speed/density system or an alpha-n system. Alpha-n is the simplest. The primary inputs are throttle position and rpm. A speed/density system uses primarily MAP and rpm.

The LT1 can operate in speed/density mode, IOW without the MAF. Even in speed density mode, when the conditions for closed loop operation are met, I believe it will go into closed loop. IOW, there the following modes of operation possible.

1. MAF open loop
2. MAF closed loop
3. S/D open loop
4. S/D closed loop

During normal operation, only the first two are used. I think, though I am not sure, that at startup, the MAF is ignored briefly. For that short period, mode #3 would be operative. But otherwise, modes 3 and 4 are never used when the MAF is operating properly.

Interestingly, if my car ('95) is running normally and the MAF is disconnected, it dies and cannot be restarted. The PCM can be made to run in S/D mode, with the MAF disconected, but only with reprogramming. I have seen another LT1 that will continue to run after disconnecting the MAF without reprogramming (though poorly) and don't know why this is true with some and not others. Since I have only tried this on two cars, I can't generalize about it. If anyone knows why, I'd like to know also.

Anway, back to the topic at hand. Talking about the MAF and closed loop/open loop is an "apples n' oranges" kind of discussion. When an MAF is present in a normally functioning LT1 engine control system, the signal is used by the PCM. This has NOTHING to do with closed or open loop under normal conditions. The only possible exception is that at startup, when the PCM is (coincidently) in open loop, the MAF may be ignored briefly. But that still has no dirrect connection to the fact that open loop is always used at startup.

Rich

 

Fred and Rich are correct. The MAF is used in open loop, closed loop, and PE mode unless there's a failure or you explicitly tell the PCM to ignore it.

Open loop/Closed loop only refers to fuel correction from O2 sensor feedback. MAF or SD is the method used to determine airflow.

Rich- The PCM does indeed run closed loop in speed density, when the conditions are met. I've been running mine this way for a couple of years now. It requires extensive tuning of the VE tables for a modified engine to run in speed density. My VE tables are nothing like stock

 

Your confusing two different parts of the fuel calculation. In cold start mode, or in PE mode, the PCM uses a "calculated" A/F ratio, rather than the hard-coded 14.7:1 that it uses for "normal" closed loop operation. But, even though it is calculating an A/F ratio, is has to divide the air mass flow rate by the A/F ratio to calculate the fuel mass flow. It starts with the air mass flow reported by the MAF sensor in all cases (unless the MAF sensor isn't working). Just because the PCM calculates a different A/F ratio, doesn't mean that it can ignore the mass air flow rate, or that it needs to use an alternative method (e.g. - speed-density) to calculate mass air flow.

 

This is all ancient history.

 

OK then so before we put this to rest I know that we have already covered that the MAF is used in open loop but now what I want to verify is that Fred you are saying that for example when the ECM is in open loop with the MAF connected and SD mode UNCHECKED the computer will then use the MAF tables IN CONJUNCTION with the AFR x 10, Open Loop Vs Coolant Temp Vs MAP Table to calculate fuel? The ECM will also do this in closed loop Im assuming?? Is this correct so far?

Also my last question would be, what about while the ECM is in SD mode? What does the computer use then? Does the ECM just use the AFR x 10, Open Loop Vs Coolant Temp Vs MAP table or does it use the VE Tables AND the AFR x 10, Open Loop Vs Coolant Temp Vs MAP Table to trim fuel??

 

The PCM needs to calculate the amount (mass) of fuel to add to the incoming (mass) of air. Only when it knows the gps of fuel to supply, can it set the injector pulse width. In order to do the calculation, it needs (let's keep if simple) the mass air flow, and the "target" A/F ratio. The only MAF "table" involved is the calibration table that converts MAF frequency output to gps of air flow.

gps air / "target" AFR = gps of fuel.

(100gps air) / (14.7:1 AFR) = 6.81gps of fuel

In a mass air system, without someone altering the stock system, and with a functioning MAF sensor, the PCM uses the mass air flow reported by the MAF sensor. Why would it use speed-density to calculate the mass air flow, when it has a very precise instrument that can directly measure mass air flow?

Once it has the mass air flow, it needs to know what the required A/F ratio is. In closed loop, part load/part throttle, steady state, it uses 14.7:1 as the "target" A/F ratio. On cold start, and in PE mode, it knows it has to set a richer A/F ratio, so it uses the tables you mentioned to calculate the richer "target" A/F ratio.

Once it has the mass air flow and the A/F ratio, it can calculate the mass fuel flow as shown above.

Speed-density is just another way to calculate mass air flow. RPM/2 x displacement X VE X air density = mass air flow

(assumes you have all the "units" of measurement correct).

 

I think you're just having a little confusion as to how the whole process works. The PCM's main job is to determine how much fuel to give an engine. For the sake of this conversation, that's it.

The PCM tries to achieve an Air Fuel Ratio. This is simply how much air there is to fuel (by mass). In order to do that it must know how much air there is entering the engine. For example, to maintain an AFR of 14.7:1. If there is 14.7 grams of air entering the engine, it must add 1 gram of fuel to burn. The first step in this process is figuring out how much air is coming into the engine. With an MAF sensor this is done directly. The MAF sensor directly reports how much air is coming in, the PCM can now do some easy math to determine how much fuel to add to achieve a specific AFR.

When in open loop, the PCM uses the open loop AFR vs. Temp and Load table. It looks at that table, finds the corresponding target AFR, and then adds the correct amount of fuel to achieve that AFR based on how much air is flowing into the engine (as reported by the MAF sensor).

In closed loop the PCM tries to mainain an AFR of 14.7:1. This allows the catalytic converters to work, gives the cleanest balance of emissions, and allows fairly good fuel economy. The PCM takes the airflow readings from the MAF, does the math, and adds the right amount of fuel to give an AFR of 14.7:1. It also double checks itself by monitoring the O2 sensors. It will correct itself using the O2 sensors' feedback.

So the PCM's main job is to add fuel based on how much air is flowing. The MAF works by giving a frequency to the PCM. The PCM takes these frequencies and converts them to a an actual airflow measurement with the MAF table. These actual air flow measurements can now be used by the PCM to do its job in finding out how much fuel to add. In speed density mode the PCM has to calculate the airflow (as oppposed to measuring it directly with the MAF). The PCM looks at barometric pressure, temp., and volumetric effeciency (RPM and Load). Using all these inputs it calculates how much air it thinks is entering the engine, and comes up with an airflow calculation.

Whether that airflow number comes from the MAF or is calculated by the PCM, it still goes through the same process after that.

There's a lot more that goes on but this is the basics.

 

I believe you are correct in the first paragraph.

As for the 2nd paragraph. Unplugging the MAF while the engine is running will stall the engine, since measured airflow goes to zero, so fuel goes to zero. But you should be able to start it up again. You probably don't have enough patience. On my car, I noticed you had to press the gas a little to get it to fire in S/D (yes, even on a fuel injected car, pressing the gas DOES make a difference, you're just not "supposed" to nor should you have to, but the PCM DOES add more cranking fuel when the throttle is opened - plus it pulls fuel at WOT during crank for "clear flood mode"). But it will start, run, idle and actually drive remarkably well (bone stock car here). One thing I have been wondering was, when the PCM is in S/D, how does the PCM calculate acceleration enrichment? Are there tables in there that TunerCat just hasn't found yet? It seemed to be quite responsive, so I can't believe there is no AE (as I have heard before).

 

Very good question. Nobody has been able to find an AE table yet and the pros keep saying there is none. Mine seems to work great so it hasn't been a concern for me.

With the fuel injector placement and responsive PCM it may not be needed at all.

 

I understand all the basic stuff but one of my questions that still hasn't been answered is when the ECM is in OPEN LOOP/MAF Mode it uses the MAF Tables in conjunction with the AFR x 10, Open Loop Vs Coolant Temp Vs MAP.... BUT WHAT does the ECM do when the computer is in OPEN LOOP/SD Mode?? What table does it use then since there is no MAF to give feedback. Does it just use the AFR x 10, Open Loop Vs Coolant Temp Vs MAP OR does it use the AFR x 10, Open Loop Vs Coolant Temp Vs MAP AND Some other table??

 

The ECM uses the VE Tables to calculate fuel trims while in SD mode!