Help! Dtc 55

[Schrade]

I have been trying to get my brain around the closed loop and open loop description in the fsm. The explanation for loop status is as follows

"Closed Loop" displayed indicates that the ECM is controlling fuel delivery according to oxygen sensor voltage. In "Open Loop," the ECM ignores the oxygen sensor voltage and bases the amount of fuel to be delivered on TP sensor, engine coolant, and MAP sensor inputs only.

So until the engine gets up to temp it is operating in open loop and that is the only time the TPS, or the Engine Coolant and the MAP sensors have any input into the ECM. That is contrary to what I would have thought because it would seem to me that the ECM would need to know what the throttle demand was and how would it know that from the oxygen sensor.

Could someone help me understand what is going on here in 3 volumes or less.

3 volumes or less?

Cold / Open loop - input from almost all sensors is ignored. Fuel trim is max'ed, like a 'choked' carbeuretor, to keep it running while cold. Several conditions must be met before Closed loop, EVEN IF FULLY WARMED UP. Coolant temp, exhaust temp are 2; I think there's more.

A blown CTS will keep a warm motor in max fuel trim / Open loop, and will make for hard (flooded) warm re-starts

Warm / Closed loop - ECM takes readings from almost all sensors, to determine how LITTLE amount of fuel is necessary to keep it running.

Sometimes, FSM diagnostic procedure says for testing: Step 1; coolant temp OVER 176, ...

Someone else might explain better, or add some...

 

[Hib Halverson]

3 volumes or less?

Cold / Open loop - input from almost all sensors is ignored. Fuel trim is max'ed, like a 'choked' carbeuretor, to keep it running while cold. Several conditions must be met before Closed loop, EVEN IF FULLY WARMED UP. Coolant temp, exhaust temp are 2; I think there's more.

A blown CTS will keep a warm motor in max fuel trim / Open loop, and will make for hard (flooded) warm re-starts

Warm / Closed loop - ECM takes readings from almost all sensors, to determine how LITTLE amount of fuel is necessary to keep it running.

Sometimes, FSM diagnostic procedure says for testing: Step 1; coolant temp OVER 176, ...

Someone else might explain better, or add some...

"Open loop"...is a literal term, meaning the fuel control feedback loop formed by the engine control's abiliy to sense the percentage of oxygen in the exhaust and use that data to control the amount of fuel metered to the engine, is "open" or disabled. In general, during open loop operation, all the engine's sensors are enabled except the oxygen sensor and all functions of the ECM are enabled except it's ability to trim fuel delivery based on the O2Ses data.

One thing that does not happen in open loop is the fuel trim being "max'ed". In fact, fuel trim in open loop is "zeroed" because it's disabled. You can easily see that with any scan tester. As for cold enrichment, that is a different open loop function that is very much enabled right after a start up while IAT and ECT are low. As they rise, regardless of loop status, cold enrichment is dialed back and then is disabled. Once the engine is warmed up, if the engine controls are working properly, the system goes closed loop. Now, with older systems, the engine drops back into open loop during power enrichment but with newer systems, they stay closed loop all the way to WOT.

In closed loop operation, the feedback loop discussed above is "closed" or enabled. In general, during closed loop operation, all the engine's sensors are enabled and all functions of the ECM related to fuel metering are enabled.

Now one thing which does not happen in closed loop is the ECM determining "...how LITTLE amount of fuel is necessary to keep it running." Not much could be farther from the truth.

Reality is that in closed loop operation the ECM keeps the air fuel ratio at stoichiometric which, in very general terms, means not too little and not too much. "stoichiometric" means perfect combustion with no unburned fuel and low emissions. If the ECM kept the fuel delivery at the minimum necessary to run the engine, the AFR would be way lean with no unburned fuel, significant emissions, poor drivibility, much more tendancy to detonate and less than optimal torque output.

 

[Schrade]

"stoichiometric" means perfect combustion with no unburned fuel .

Yup - just what I said.

As little fuel as necessary to run. Equal number of atoms on each side of the [combustion] equation.





The rest is clear as a bell too.

In general, during open loop operation, all the engine's sensors are enabled except the oxygen sensor and all functions of the ECM are enabled

In general, during closed loop operation, all the engine's sensors are enabled and all functions of the ECM related to fuel metering are enabled.

:confused x 3

 

[Schrade]

And I suppose 'max'ed fuel trim wasn't COMPLETELY accurate, although VERY close.

In open loop, the primary function of the PCM is to keep the engine operating ...

LT1howto.com :: LT1 PCM Tuning for Dummies

Looks ALMOST exactly like what I said... Again.

Anything else you didn't like the 'wording'???

How 'bout this:
Under NO circumstances is Open loop fuel trim LESS than Closed loop fuel trim, same conditions WITHstanding. It is ALWAYS MORE, again, same conditions WITHstanding!

 

[Hib Halverson]

Yup - just what I said.

As little fuel as necessary to run. Equal number of atoms on each side of the [combustion] equation.

I'm sorry, but your understanding of how combustion works is not correct.

Stoichiometric combustion is not the same as combustion using as little fuel as is necessary to keep the engine running. Stoichiometric is perfect combustion. "As little fuel as necessary to run" would result in an air/fuel ratio far more lean then stoichiometric .

And I suppose 'max'ed fuel trim wasn't COMPLETELY accurate, although VERY close.

LT1howto.com :: LT1 PCM Tuning for Dummies

Looks ALMOST exactly like what I said... Again.

Anything else you didn't like the 'wording'???

How 'bout this:
Under NO circumstances is Open loop fuel trim LESS than Closed loop fuel trim, same conditions WITHstanding. It is ALWAYS MORE, again, same conditions WITHstanding!

I'm sorry, but that's wrong.
As I said before, there is no fuel trim function when the engine is in open loop. The fuel delivery comes straight off the fuel tables in the base calibartion.

In closed loop, fuel trim can result in less fuel than base cal or more fuel than base cal, depending on the O2 content in the exhaust and how the closed loop programming is done.

 

[Schrade]

So there IS still wording that you don't like... Instead of Fuel Trim, you call it, "Fuel Base Calibration".

:eyerole

In closed loop, fuel trim can result in less fuel

That is EXACTLY the very last thing I said. Read my last sentence VERY carefully. You SHOULD have that capability if you get paid for writing...

 

[Hib Halverson]

Well....like I said earlier, there is no such thing as "fuel trim" in open loop. I'll add that contrary to your belief, "base calibration" is not "fuel trim". In fact, in closed loop, when fuel trim is enabled it "trim's" the base cal, ie: the fuel delivered in closed loop will be the base cal plus the trim value, which may be positive or negative, depending on what the O2S says.

But...ya know, "Schrade", there's no further point in my trying to help you work through your confusion about how engine controls work becuase this discussion has turned into a pissing contest rather than being of use to the original poster.

I will tell you this....now I understand why you're banned over on the CF. Fortunately for you, stubborness and stupidity are not grounds for being banned here on the CAC.

 

[boomdriver]

If I may,

Open loop is operation from pre-programming (memory) that is as simple as...."if throttle position is 10% and temp is X and timing is Y then fuel will be Z %".

Its like old MS-DOS computer programs. "if this, go to that". Whatever is written and stored. A "go-to" program with lots of variables.

This is managed and calculated by the ECM and the PROM, where those parameters are stored. Later yrs went to a more sophisticated computer with more computing power so it was able to do more calculations, faster. It also became "programable" instead of restricted to reading whatever the PROM offered.

Closed loop is simply the ECM being able to fine tune and adjust/react in real time within those same parameters by way of sensor input from sensors that monitor what the air flow and the fuel is, and where that is in relation to "ideal" which is a known ratio of 14.7:1 air/fuel. Thats used as a target or reference point.

I know that I am preachin to the choir, but you guys are confusing!

Thought I'd attempt to clarify.Pretty sure I did'nt...Just wanted some mental gymnastics this evening.

 

[John Robinson]

Wires

If I may,

Open loop is operation from pre-programming (memory) that is as simple as...."if throttle position is 10% and temp is X and timing is Y then fuel will be Z %".

Its like old MS-DOS computer programs. "if this, go to that". Whatever is written and stored. A "go-to" program with lots of variables.

This is managed and calculated by the ECM and the PROM, where those parameters are stored. Later yrs went to a more sophisticated computer with more computing power so it was able to do more calculations, faster. It also became "programable" instead of restricted to reading whatever the PROM offered.

Closed loop is simply the ECM being able to fine tune and adjust/react in real time within those same parameters by way of sensor input from sensors that monitor what the air flow and the fuel is, and where that is in relation to "ideal" which is a known ratio of 14.7:1 air/fuel. Thats used as a target or reference point.

I know that I am preachin to the choir, but you guys are confusing!

Thought I'd attempt to clarify.Pretty sure I did'nt...Just wanted some mental gymnastics this evening.

OK lets see if I have it.

Open loop is like the old time cars with hand chokes when the car is started the ECM has the choke pulled all the way out and as the car warms up. Then when it gets warm enough (like in the old cars when you closed the choke) the ECM closes all of the enriching down and now it adjusts to programmed on demand parameters with feed back from a multitude of sensos to maintain 10:7:1 fuel air ratio.

 

[boomdriver]

yeah, sumthin like that !