Quote:
Originally Posted by olddog
You can change all the start up parameters, if you chip the ECU and get into tuning. There is a whole new world out there.
This is the tuning parameters for a 5.0 Mustang GT with manual transmission, from 1989 to 1993, which is commonly called the EEC 4 A9L Box Code.
Idle RPM is set to 672
The start up idle adder is table based on engine temperature. The maximum adder is at 76 F and that is 432 RPM. If the engine is colder or warmer the adder is less. At 50 F or less the adder is 48 RPM. At 100 F or greater the adder is 48 RPM. When the temperature is between 50 and 100, linear interpolation is used to calculate the adder. So a 63 F (which is half way between 50 and 76) the adder would be half of 432 or 216 RPM adder.
So at most (when the engine is 76 F) the engine will idle at 1104 rpm, at start up.
The start up adder time controls how long it stays at this idle and it is set to 25 seconds.
With my chipped ECU, I have changes all of these parameters.
Now when you first crank the engine the manifold is full (at atmospheric pressure) as if the throttle is wide open. So when it first fires, it revs higher than the idle set point until the air is sucked down (vacuum in the manifold). So for a few seconds it will rev a bit higher.
There are other parameters that might impact this. There is a goose idle that is used to test the IAC valve, which is 250 rpm. There is an idle increase when the AC turns on. Another when the power steering pressure increases. I do not think Ford adds any of these in on start up, but it is possible. If you did have the AC on or was turning the steering, these would add in.
That all said if you are hitting 2000 rpm for more than a few (5) seconds, that seems excessive to me. You might want to have it looked at.
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Thanks olddog. That's really interesting to an 'efi Luddite' like me to know what parameters are being fed in.
What concerns me is that when something stops working on a car, it's likely to be electronics related. To me simple is good, which is one of the many, many reasons that building a car like a Cobra replica is so appealing....if something breaks, I can fix it easily.
Cheers,
Glen