T/A-Bob

Question as per the title. Just trying to understand what is involved as the engine moves from stone cold to operating temperature.

Thanks.

shoebox

Basically, only the O2 sensors.

ENRKyle20

when its in open loop the engine pours fuel in based on air flow, intake air temp, known displacement, a few other things. it calculated the right ammout of fuel and it does not check it self because it cant. as long as it runs at this point its happy. then, once the engine is heating up and the o2's are hot it can start checking the amout of oxygen left over after combustion (in the exhaust), if it senses alot of oxygen left (not enuff fuel to combine with oxygen to burn it all) then it adds more fuel to combine with that wasted air. if it sensen no air, then it pulls some fuel until it sees a bit of air. it will keep doing this back and forth many times a second. this can be seen with a scope and o2 sensor and is measured in cross counts. 450mv is the magic number.

so basicly, when in open loop, its guessing on how much fuel is needed. when its in closed loop it reads the oxygen sensors and gets feedback and adjusts fuel on that info.

Injuneer

You're sort of mixing together MAF and speed-density functions in your description of open loop operations. The MAF sensor has all the info it needs to tell the PCM how much (mass) air is entering the engine, without refering to inlet air temp, MAP, the volumetric efficiency tables or the engine displacement. The MAF tells the PCM how many # of air entered the engine, the PCM divides by the target A/F ratio, and sets that as the # of fuel to add. Works the same whether the PCM is in open loop or closed loop. The single difference is that in open loop the PCM does not use the short term fuel corrections in the calculation of the number of # of fuel to add. It can't, becasue the short terms are determined by the O2 sensor feedback. Its still using the "learned" long term fuel corrections (BLM's) to calculate the fuel flow. Since its using the stored, historical data from the BLM cells, its going to be fairly accurate, although as you correctly point out, if it is "wrong" there is no way it can correct itself.

In speed-density, the PCM has to calculate the volume displaced by the engine, considering the displacment of the cylinders, the RPM/2 and the volumetric efficiency from the tables. Once it has the volume air flow, it converts it to mass air flow by calculating the density of the air based on IAT, MAP and the perfect gas law. From that point forward, once the speed-density system has calculated the mass air flow, the process is the same as it would be with an MAF-based system.

And as Shoebox already pointed out, the only thing that is ignored in open loop is the O2 sensors.

T/A-Bob

Thanks gentlemen.

black/blue

This is a good question Ive been wondering myself. So what should the o2s say on a scannner on cold start?

Injuneer

When you start the engine with cold O2 sensors, they will read about 450mV. With an AIR pump running, as the sensors warm up, the voltage should slowly (1-2 minutes) drop to 0xx mV. At that point, the sensors are hot enough to be working, the coolant is warm enough, and when the timer (205 seconds) has expired the PCM will shut the AIR pump off and kick into closed loop. Then the O2 sensors should start cycling rapidly over the full range of 0xx-9xx mV.

T/A-Bob

Hmmmm...

A follow-up question... if the difference between open & closed loop is essentially the use of short term vs. long term fuel corrections (which are fairly similar if I'm following this OK), then why does my engine do fine with a quick heavy throttle when cold, but it stumbles all over the place if I do this when warm...?? (which makes it impossible to launch )

Injuneer

One possible explanation - probably indicates the O2 sensors are producing a false reading. When only the long term corrections are used in open loop, it doesn't upset the way the engine runs. The long terms can only add 25% extra fuel, or deduct 15% of the fuel (stock programming BLM limits). Enough to make it run "not so good", but not enough to make it run "terrible".

When you go into closed loop, BOTH long term and the short terms are used. Now the PCM can add twice the "normal" fuel, or reduce it by a huge percent. That much correction, in response to a false reading will be enough to make it run very poorly.

False O2 readings can come from misfires, exhaust leaks before the O2 sensors, contaminated sensors, bad wiring, damaged sensors, etc.

You could observe the long terms and short terms with data logging software, to see if this hypothesis is valid.

T/A-Bob

Thanks, Fred. I have no access to data logging equipment (nor the knowledge to understand the results), but based on your answer - and other posts I've read here - I feel it's time to change out my O2 sensors.

Injuneer

I just went through a log someone posted yesterday. It was pulling out 42% of the normal fuel, 15% with the long terms and the rest with the short terms, and still running rich, based on his burning eyes. The long terms were bottomed out at 108, and the short terms were running at 88. That's a case where there is not enough correction in open loop, and the excess fuel on cold start is probably doing significant engine damage.

T/A-Bob

Fred, are you saying that false O2 readings could cause problems starting the engine when it's cold?