22kart22

When my stang is in neutral and off the clutch I am idling at around 750-800 rpm, but once i roll at about 5-7mph, rpm drops a few hundred. Once I stop, rpm goes back up.


This is the cpu right??? Why does it do this?

owero01

Thats because the engine is doing work to move the car. Thats just like you lifting your hand when its empty or when you have a 40lb weight in it.

spdbrake

Remember idle is not cruise control.
Try it on a steep hill going up.

22kart22

no guys you are not understanding this.

The tranny is in NEUTRAL. Idling down driveway.

once the car starts rolling quickly, the rpms drop. REMEMBER IT IS A MANUAL, AND IN NEUTRAL SO THE ENGINE IS NOT CONNECTED TO THE DRIVETRAIN!


does this make sense?

hammeron

interesting. i wonder if you could duplicate this scenario,
with your hood up and blowing a fan into your engine bay

22kart22

it is really weird.. i mean Its VERY noticeable when it happens.. and once i slow down, vroom, they increase.

22kart22

alright. I put a stupid little sportcam in my cupholder and got a video, ill email it to you if you are interested.

thanks everybody. You guys are the bestest

spdbrake

It may be normal. Take a look at the "learned" idle speeds (Brake must be applied).

Idle air trim is designed to adjust the idle air control (IAC) calibration to correct for wear and aging of components. When the engine conditions meet the learning requirement, the strategy monitors the engine and determines the values required for ideal idle calibration. The idle air trim values are stored in a table for reference. This table is used by the PCM as a correction factor when controlling the idle speed. The table is stored in keep alive memory (KAM) and retains the learned values even after the engine is shut off. A diagnostic trouble code (DTC) is output if the idle air trim has reached its learning limits.

Whenever an AC component is repaired or cleaned, or a repair affecting idle is carried out, it is recommended that the KAM be reset. This is necessary so the idle strategy does not use the previously learned idle air trim values.

To reset the KAM, refer to Diagnostic Methods Resetting the Keep Alive Memory. It is important to note that erasing DTCs with a diagnostic tool does not reset the idle air trim table. Once the KAM has been reset, the engine must idle for 15 minutes (actual time varies between strategies) to learn new idle air trim values. Idle quality will improve as the strategy adapts. Adaptation occurs in 4 separate modes. The modes are shown in the table.

Idle Speed Control Closed Throttle Determination (applications without Electronic Throttle Control)
One of the fundamental criteria for entering RPM control is an indication of closed throttle. Throttle mode is always calculated to the lowest learned throttle position (TP) voltage seen since engine start. This lowest learned value is called ratch, since the software acts like a one-way ratch. The ratch value (voltage) is displayed as the TPREL PID. The ratch value is relearned after every engine start. Ratch will learn the lowest, steady TP voltage seen after the engine starts. In some cases, ratch can learn higher values of TP. The time to learn the higher values is significantly longer than the time to learn the lower values. The brakes must also be applied to learn the higher values.

All PCM functions are done using this ratch voltage, including idle speed control. The PCM goes into closed throttle mode when the TP voltage is at the ratch (TPREL PID) value. An increase in TP voltage, normally less than 0.05 volts , will put the PCM in part throttle mode. Throttle mode can be viewed by looking at the TP MODE PID. With the throttle closed, the PID must read C/T (closed throttle). Slightly corrupt values of ratch can prevent the PCM from entering closed throttle mode. An incorrect part throttle indication at idle will prevent entry into closed throttle RPM control, and could result in a high idle. Ratch can be corrupted by athrottle postion sensoror a circuit that drops out or is noisy, or by loose/worn throttle plates that close tight during a deceleration and spring back at a normal engine vacuum.

Fuel Trim

Short Term Fuel Trim
If theoxygen sensorsare warmed up and the PCM determines that the engine can operate near stoichiometric air/fuel ratio (14.7:1 for gasoline), the PCM enters closed loop fuel control mode. Since an oxygen sensor can only indicate rich or lean, the fuel control strategy continuously adjusts the desired air/fuel ratio between rich and lean causing the oxygen sensor to "switch" around the stoichiometric point. If the time between rich and lean switches are the same, then the system is actually operating at stoichiometric. The desired air/fuel control parameter is called short term fuel trim (SHRTFT1 and 2) where stoichiometric is represented by 0%. Richer (more fuel) is represented by a positive number and leaner (less fuel) is represented by a negative number. Normal operating range for short term fuel trim is +/- 25%. Some calibrations will have time between switches and short term fuel trim excursions that are not equal. These unequal excursions are used to run the system slightly lean or rich of stoichiometric. This practice is referred to as using bias. For example, the fuel system can be biased slightly rich during closed loop fuel to help reduce NOx.

Values for SHRTFT1 and 2 may change significantly on a diagnostic tool as the engine is operated at different RPM and load points. This is because SHRTFT1 and 2 reacts to fuel delivery variability that changes as a function of engine RPM and load. Short term fuel trim values are not retained after the engine is turned off.
Long Term Fuel Trim
While the engine is operating in closed loop fuel control, the short term fuel trim corrections are learned by the PCM as LONGFT1 and 2 corrections. These corrections are stored in the KAM fuel trim tables. Fuel trim tables are based on engine speed and load and by bank for engines with 2 HO2S sensors forward of the catalyst. Learning the corrections in KAM improves both open loop and closed loop air/fuel ratio control. Advantages include:

[*]Short term fuel trim does not have to generate new corrections each time the engine goes into closed loop.[*]Long term fuel trim corrections can be used both while in open loop and closed loop modes.[/ul]
Long term fuel trim is represented as a percentage, similar to the short term fuel trim, however it is not a single parameter. A separate long term fuel trim value is used for each RPM/load point of engine operation. Long term fuel trim corrections may change depending on the operating conditions of the engine (RPM and load), ambient air temperature, and fuel quality (% alcohol, oxygenates). When viewing the LONGFT1/2 PID(s), the values may change a great deal as the engine is operated at different RPM and load points. The LONGFT1/2 PID(s) will display the long term fuel trim correction that is currently being used at that RPM/load point.



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AmericanMuscle4.6GT

i know exactly what your talking about.

like, if u put it in neutral and get on the brakes when cruising/slowing down at a stoplight, the revs go down, but as soon as your stopped, the revs go back up.

22kart22

exactly. is this the cpu????