JeffP

Please?

Vader

JP,

In a nutshell, Open Loop mode means the ECM is controlling the engine with essentially no feedback from the engine sensors - the control loop is said to be "open". Closed Loop mode means that the ECM sends control signals to the engine, scans the sensors for feedback to determine further corrections, then adjusts accordingly. The process is continuous, and the control loop is said to be "closed" since the sensors effectively communicate back the the ECM regarding the effectiveness of any adjustments made.

For a more complete look:

Open Loop Mode

When the starter is engaged and the coolant temperature is less than 100°F, the cold start injector provides a spray of fuel or the cold enrichment subroutine allows longer fuel injector pulses on the service injectors. The cold start or cold enrichment subroutine is of eight seconds duration at a maximum. On a cold-start injector system the extra fuel is distributed to each cylinder via a air distribution system built into the bottom of the intake manifold. If the engine temperature is greater than 100°F, the cold start injector/subroutine is disabled by the cold start switch or ECM. Upon startup the ECM utilizes information in the PROM to establish the initial pulse rate for the injectors and the engine starts. At this time the engine is operating in Open Loop mode and will continue to do so until the engine warms up.

Closed Loop Mode

After a warm up period of at least ten seconds the ECM scans the sensors, if all sensors are operating and within their proper range, the engine then goes into Closed Loop operation. This means that the sensors are dynamically controlling the engine. In the event the information received is higher or lower than the normal range, a code will set in the ECM, and the Malfunction Indicator Lamp (MIL) or Service Engine Soon (SES) lamp will light. The ECM receives information on air flow, engine temperature, air temperature, exhaust gas oxygen content and throttle position. This information is used to calculate the proper pulse width for the injectors and fires the injectors for the calculated period. This procedure is repeated continuously in very rapid sequence to maintain the optimum fuel air ratio. The electronic spark control components provide maximum advance, if engine knock is detected the spark is automatically retarded. This too, is a continuous process. It should be noted that the following components are matched for each engine type - Distributor EST module, ESC module, knock sensor and ECM PROM. These components are not interchangeable between 5.0L - 5.7L engines.

The factory ECM/PCM has a Learning capability which allows it to make corrections for minor variations in the fuel system to improve performance and driveability. There are two learning features. The Integrator and Block Learn (INT and BLM) and Block Learn Memory (BLM) cell. The INT and BLM feature is normal with a value of around 128. If this value is higher than 128, it indicates that the ECM is adding fuel to the base fuel calculation because the system is running lean, a value lower than 128 indicates that the ECM is taking out fuel because the system is running rich. The integrator is a short term corrective action while the BLM is along term correction. The BLM value will change if the integrator has seen a condition which lasts for a longer period of time. There are from two to sixteen different cells which the ECM modifies, depending on RPM, airflow or manifold air pressure and other conditions such as AC on or off, etc. The ECM learns how much adjustment is required in each cell, retains it in memory, and applies these adjustments when the engine operates in that cell or RPM/Load Range. These features of the ECM allows the system to adjust itself constantly to your engine and assure peak performance for stock and other than stock engines.

NOTE: When the vehicle power is disconnected for repair or to clear diagnostic codes, the learning process has to begin all over again.
To reinitialize the ECM, drive the vehicle at operating temperature with moderate acceleration and idle conditions.

Performance PROMs typically change the parameters for fuel flow, fuel cut-off and spark advance-timing and will allow increased fuel flow and modify the spark advance curves during rapid acceleration.

Field Service Mode

On the OBDI ECMs, you can jump 'A' and 'B' terminals on the ALDL while the engine is running.

WARNING! This must be done after the engine is running. This is called "Field Service Mode" and will not harm the ECM.

If the ECM is in Open-Loop mode, the SES light will flash rapidly, about 2½ times per second. If it's in Closed-Loop mode, it will flash about once per second. When in Closed Loop mode, flashing less than once per second indicates the ECM is enriching the mixture above the 128 count base line. Flashing more than once per second indicates the ECM is leaning the mixture below the 128 base line.

Backup Fuel Mode

A 3.9K resistor installed between ALDL terminals 'A' and 'B' forces the ECM into Backup Fuel mode. This mode forces the ECM to use predetermined fuel calculations in the ECM PROM instead of the learned inputs in active RAM. This is more commonly referred to as the "Limp Home" mode of operation.

Conditions for the ECM to enter Closed Loop mode are as follows on a typical TPI:

CTS < 14.7°C (58°F) for 75 seconds;
CTS >14.7° (58°F) and < 40.7° (105°F) for 51.4 seconds;
CTS >40.7°C (105°F) for 12.5 seconds;
O2 >0.699V & O2<1.99V for 10 seconds;

Coolant Temperature Sensor Related Parameters
BLM enabled between 50°C (122°F) and 140°C (284°F)
Cold spark advace disabled above 56°C (133°F)
Hot spark retard begins above 116°C (240°F)
Highway Mode spark advance > 59.8°C (140°F)
Knock sensor disabled below 66.5°C (152°F)
Power enrichment at base A/F ratios > 56°C (133°F)
Target IAC idle RPM >80°C (176°F)
IAC multiplier at 1.0 (base) > 32°C (90°F)
Knock Control enabled > 67°C (153°F)
EGR Duty-cycle enabled at 56°C (133°F)
EGR Duty-cycle at MAX >80°C (176°F)
TCC lockup enabled >50°C (122°F)
SHIFT light enabled >50°C (122°F)
Diagnostic communication enabled at 70°C (157°F)
DTC 43 enabled > 90°C (194°F)
Cooling fan #1 enabled at 107°C (226°F)
Cooling fan #1 off at 104.7°C (220.5°F)
Cooling fan #2 enabled at 115.2°C (239.5°F)
Cooling fan #2 off at 110°C (230°F)
Cooling fan duty cycle at 100% at 80°C (176°F)
Acceleration enrichment multiplier at 1.00 below 80° C (176°F)
Acceleration enrichment multiplier at 0.75 above 80° C (176°F)
Acceleration enrichment multiplier at 0.50 above 104°C (220°F)
Acceleration enrichment decay factor at 25% below 80° C (176°F)
Acceleration enrichment decay factor at 40% above 80° C (176°F)
Acceleration enrichment decay factor at 50% above 104° C (220°F)
Acceleration enrichment decay factor at 60% above 128° C (262°F)
Fuel limiting factor timer at 217 counts below 80° C (176°F)
Fuel limiting factor timer at 169 counts above 80° C (176°F)
Code 13 (oxygen sensor fault) enabled above 70°C (157°F)
Code 14 (CTS high fault) enabled above 130°C (266°F)
Code 32 (EGR fault) enabled above 30.5°C (87°F)
EVAP canister purge enabled above 70.3°C (158.5°F)
Hot closed-loop timer enabled above 70.3°C (158.5°F)
Rich/Lean O2 offest at 16 counts between 20°C (68°F) and 92°C (197.5°F)
A/C clutch disabled above 150°C (302°F)

I hope that answers your questions.

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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0

rsilver

Vader,you mentioned that certian modules and sensors are matched to the engine type as is the EPROM. i understand I have to change the PROM if I swap from a 305 to a 350 but do I have to change the ECM too? God, I hope not. I checked a site on the prom board and
It looked like my ECM was the same one used
on some cars that had 350 TBI's.(89 caprice) Do I have this right?
Thanks...bob

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91 camaro RS convert
t-5
open element air cleaner
TBI spacer
Flowmaster muffler

BLACK Z

When I swapped in a 350 for my 305 I changed the module and the knock sensor for a 350 one, but not the est, do I need to change that? where is it and what is it/ thanks

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JeffP

Excellent information as always, Vader. Thank you for your replies in all of my threads!

Jim85IROC

iTrader: (2)

Ok, time for me to ask more questions too. I also did a 305-350 swap, but I have an 85, which we all know uses a fairly retarded ECM. Anyway, other than injectors, all I changed was the knock sensor. My car seems to drive like a sack of crap, and I'm under the impression it's because my ECM is retarding the motor to keep it from knocking. I know that I've got to get a new PROM, but what about the EST and ESC?

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Vader

RS Bob,

No. The ECM is the same for the same families of fuel systems. An ECM for a TPI w/MAF is the same part number from 1986-1989. A TPI w/SD is the same part number from 1990-1992. The same is true for the TBIs. 1985 had a different situation, since the model year saw an ECM change-over. Earlier carbed engines had a couple of different ECMs depending on year. Some of them had 8-bit processors and some later ones were 16-bit.

You should be fine with the ECM you have as long as the PROM and knock sensor are correct for the displacement and fuel system type.

The EST is actually a system, not an individual part or single component. The major parts included are the PROM, ESC module (on the firewall), HEI amplifier/switch module in the distributor, and detonation (knock) sensor. Sorry if I confused the issue by listing the "EST" as if it were a component part.

Most engine exchanges can be done successfully with a PROM and detonation sensor change. To my knowledge, the ESC module is the same for both flavors of TPI engines, but may be different from the TBI and carbed engine controls.

If you're simply increasing displacement, your major concerns are those stated above and providing adequate fuel at all ranges. Oh, and keeping tread on the tires

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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0

rsilver

Vader, thanks for your patience and for clearing this up!! You have done a fine job explaining this and I really appreciate it. ...bob

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91 camaro RS convert
t-5
open element air cleaner
TBI spacer
Flowmaster muffler