Chrysler vehicles with returnless fuel supply systems will have MPGuino fuel economy figures that will be thrown off by an average of 3% from actual, on an otherwise properly calibrated MPGuino. This is currently unavoidable. Tweaking MPGuino fuel calibration on these vehicles will only minimize this error, but will not eliminate it. Ford (and presumably other vehicle manufacturers) returnless fuel supply systems do not have this problem.
If your returnless fuel supply system features a fuel pressure sensor and a fuel pump that is controlled by the engine computer to regulate fuel pressure, then you do not have this problem. If your returnless fuel supply system features an in-tank mechanical fuel pressure regulator, you have this problem.
The reason is this:
In performing fuel delivery calculations, MPGuino assumes a fuel pressure that is a constant value above intake manifold pressure. This assumption is perfectly valid for older vehicles that have a return-style fuel supply system, because they have a mechanical fuel pressure regulator that is controlled by engine vacuum. The regulator will cause a constant pressure difference across the fuel injectors, regardless of engine vacuum, such that the same amount of fuel will be squirted for a given injector cycle time every time.
Returnless fuel supply systems were introduced by the major auto manufacturers as a way to comply with pollution regulations regarding unburnt fuel emissions. Currently, there are two different returnless system designs in use.
The first system design, introduced by Chrysler, uses an in-tank mechanical fuel pressure regulator that does not get connected to engine vacuum. This regulator regulates fuel pressure to a constant 58 psig above ambient atmospheric pressure, instead. This is mechanically a very simple design, but it does require that the engine computer take this into account when performing fuel delivery calculations, as there is now a variable pressure difference across the fuel injectors. This difference is directly related to engine vacuum. For a given injector cycle time, more fuel will be squirted out of the fuel injectors with a high engine vacuum, than would be with a low engine vacuum. Therefore, for this fuel supply system, the MPGuino fuel pressure assumption is not valid, and MPGuino fuel delivery calculations may be in error by as much as 7%, on an otherwise perfectly calibrated unit.
The second system design, introduced by Ford, and also used by other vehicle manufacturers, uses no mechanical fuel pressure regulator. Instead, fuel pressure is regulated directly by the engine computer. The engine computer does this by reading a fuel pressure sensor, then electronically controlling the fuel pump to obtain a desired fuel pressure. This system regulates fuel pressure identically to an older return-style mechanical system that uses a fuel pressure regulator controlled by engine vacuum. This system will cause a constant pressure difference across the fuel injectors, regardless of engine vacuum, such that the same amount of fuel will be squirted for a given injector cycle time every time. The engine computer has simpler fuel delivery calculations as a result, but it does have to have an additional task of controlling the fuel pump to regulate fuel pressure. Therefore, for this fuel supply system, the MPGuino fuel pressure assumption is valid, and MPGuino fuel delivery calculations should not be in error on an otherwise perfectly calibrated unit.
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Original post: I bought an MPGuino a few days ago, and received it in the mail. Impressive-looking little unit.
However, I've had less than stellar luck with having one question answered: Does MPGuino handle returnless fuel lines? That's when either an in-tank regulator or a power-modulated fuel pump supplies constant pressure to the fuel line and fuel rail, regardless of intake manifold vacuum; and there is only one fuel line coming out of the tank.
I've searched EcoModder.com high and low, and this is all that I could come up with:
MPGuino and a Series 3 Supercharged
I strongly suspect that I know the answer already, though... Might have to dust off my microcontroller skills and add this capability.