Cold / Startup Enrichment

by: Michael Decipha Ponthieux
Posted: 2013-11-12
Last Updated: 2015-05-31

Cold Engine Fueling

A cold engine will require significantly more fuel than a stabilized warm engine. There are 4 primary reasons for this

Startup Fuel Enrichment - "Choke"

The ford ecu's can ONLY control fuel with LAMBSE. Thus for cold startup enrichment, the ford ecu's use a Lambse Subtractor Table FN1361. Lambse is the ford nomenclature for Lambda. This table is scaled by ECT and Engine Run Time ATMR1. It is common for most engines to consume 17% - 25% more fuel for the initial 60 seconds of startup when at operating temperature. This is equivalent to demanding between (14.64 / 1.17=) 12.5 AFR and (14.64/1.25=) 11.7 AFR to reach stoich during WARM startup. This is further compounded on a colder engine, it is not uncommon for a cold engine to consume 65% - 85% more fuel on initial fire up at temperatures near the freezing point (32* F). This is equivalent to (14.64 / 1.65=) 8.87 AFR to (14.64 / 1.85=) 7.9 AFR. However, this amount of fuel is typically only required for up to approximately 5 seconds or so after the initial fire off. Fueling can usually be tappered off to approximately 40% enrichment (14.64 / 1.4=) 10.5 AFR after a minute or so of engine run time at that temperature (32* F). Typically, the majority of that fuel for the initial fire up is collected on the port walls as fuel puddles. It is not uncommon for the puddles to trap up to 6x the amount of fuel required for a single combustion event.

Tuning Startup Enrichment - FN1361 / FN1306

Startup fueling CAN ONLY be dialed in once the stabilized warm fuel has been dialed in, otherwise you'll be making incorrect adjustments. Most cold startup surging and dieing issues are fuel related.

The ford ecu's control startup fuel enrichment by subtracting FN1361 from FN1362.
FN1362 / FN1307 - Base Lambse Table (this table remains effective until the ECT_STABL flag is set)
FN1361 / FN1306 - Lambse Subtractor Table (this table remains effective until the ECT_STABL flag is set)
Note: The ECT_STABL flag switches fueling to use the FN1360 - Stabilized Fuel Table (does not exist in older ecu's)

For example:
Lets say at 30 degrees for perloads below 75 we have a value of 14.64 in our base fuel table. And in our lambse subtractor table we have a value of 4.00 at 0 seconds up to 5 seconds. When the engine is fired up at 30 degrees ECT, the lambse will be (14.64 - 4.00=) 10.64 Lambse.

Even though our fuel is dialed in when warm, the observed AFR on our WBo2 will be near 15:1 AFR due to the additional fuel requirements of the engine being cold. The ONLY way to properly dial in this table is to start the engine while cold and adjust the current cell in use so the wideband reports near 14:1 until the engine has reaching operating temperature.

Why 14:1 ?? Because a cold engine will respond much better to a slightly richer AFR.

When dialing in FN1361 the Lambse subtractor table keep in mind that the top row of that table is ALWAYS in use. Changing the top row to anything other than 0 at normal operating temperature will ALWAYS make the engine run rich.

Startup Airflow Requirements

As fuel has the most significant control of idle stability especially when cold, the idle air requiments have an equally important role in maintaining a stabilized idle when cold. A cold engine will require more airflow due to the 4 primary reasons stated above. Just like fueling, the idle airmass is significantly greater for the initial fire off and reduces with engine run time. It is not uncommon for an engine operating near freezing temps (32* F) to require approximately 2.5x as much airflow to maintain the same rpm as it would compared to warm operating temperature. Again, these effects are reduced as engine run time is increased and would typically only require approximately 65% more airflow after the engine has been running for a minute or so at that temperature. A warm engine at normal operating temp typically requires approximately 30% more airflow for a restart reducing to normal airmass requirements after approximately 2 minutes (roughly 120 seconds).

ISC Airflow Multiplier - FN1862

After startup fuel enrichment has been dialed in, only then can the ISC Airflow Multiplier table be properly dialed in. If the startup fuel table is not dialed in, the engine will require SIGNIFICANTLY more airflow to maintain a specific idle rpm. Attempting to dial in this table without having your fueling correct will give you inaccurate corrections.
The ISC Airflow Multiplier table(s) can ONLY be dialed in AFTER the warm stabilized idle airflow has been dialed in.

Reference the Idle Air Write Up for specifics.

This table is a multiplier on the ISC Airmass Offset. The ISC Airmass Offset is the amount of air the ISC (idle air control valve) is actually flowing. This airmass will be the result of the Idle Airmass (FN875) subtracted by the Throttle Body Bypass Airflow (ITHBMA).

For example:
Lets say your normal operating temperature stable idle ISC Airmass Offset is 0.15 lbs/min.

In the FN1862(N) table we have a value of 1.35 at 0 seconds and 210*F ECT (normal operating temp).

The ecu will calculate (0.15 * 1.35 =) 0.2025 ISC Airmass Flow, and will demand the ISCDC that coincides with that airmass from FN800.

Referencing the FN800 - ISC Transfer, we can interpolate that at warm stabilized idle the ISC valve is at (0.15=) 30% Duty Cycle. and for a warm startup the ISC Duty Cycle will be (0.2025=) 36% Duty Cycle.

Now that you know how the multiplier table functions you can choose to use either of the following methods to dial in the table.
Using the information above, you can log the ISC DC from a cold startup and calculate out the airmass requirements at each cell for the ISC Airflow Multiplier Table. However, there is a much easier way which I myself exploit since its much faster.

Dialing in the Startup ISC Airmass Multiplier

This table can only be dialed in by logging a cold startup and adjusting the current cell in use to get the IPSIBR and ISCKAM corrections to remain as close to 0 as possible. The easiest way to do this is by making adjusments and updating while the engine is running during these conditions. Thanks to realtime emulation from the QuarterHorse, we can do so.
Be sure to allow ample time for the corrections to take place whie making adjustments.
If you need more information on what those adjustments should be or how IPSIBR and ISCKAM function, read over the Idle Air Write Up

Keep in mind this table too like the lambse subtractor table will ALWAYS be active. So putting any value other than 1.00 for the top row at normal operating temperature will ALWAYS multiply airflow. Unless you are using the Mechanical Idle Control in which the top cell will have a value of 0.172

Note: The 93+ ford strategies have two ISC Airflow Multiplier Tables, FN1862N (neutral) and FN1862D (drive).
For those with Automatic transmissions that have two Airflow Multiplier Tables, you must log a cold startup while in neutral as well as another cold startup immediately placing the vehicle in gear to properly dial in the tables, all else remains the same just that the process is repeated.

Transmission Engagement Enrichment (to prevent stalling)

It's common for an engine to lean out when the transmission is engaged, the ecu has a function to combat this that conveniently adds fuel when the transmission is engaged. That fuction is FN371 it's usually always neccessary to demand enrichment when the transmission is engaged to prevent stalling or severe idle drop. For more details check out the Transmission Write Up.

Continue reading on to the Idle Air Write Up.

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