Advisory for Chrysler Returnless Fuel Supply Systems
 

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.

---

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.

Does having a returnless system mean that different amounts of fuel can be injected with the same length injector pulse? If it's the same amount of fuel for the same length of pulse, then I'm not understanding how the fact that the system is returnless would make a difference to the MPGuino...

Yes. In a returnless system that uses a constant-pressure fuel regulator, fuel injector pulse time is one of two variables that determines the amount of fuel to be delivered. Returnless systems, that vary the voltage of the fuel pump to vary the fuel pressure, are likely not affected, but I don't know for certain.

In a returnless system that uses a constant-pressure fuel regulator, the amount of fuel to be injected is now also affected by the pressure differential across the fuel injector itself. In fact, it will vary as the square root of the pressure differential. Earlier systems used a fuel pressure regulator that was indexed to intake manifold vacuum, thus providing a constant pressure differential across the fuel injector. These earlier systems would reliably squirt the same amount of fuel for a given fuel injector pulse, whether at idle or at cruise or going up a hill or at WOT.

Given that my fuel pressure regulator regulates to a constant 58 psig, regardless of intake manifold vacuum, and that my intake manifold vacuum could vary from 5 to 14 psig, that means that the fuel injectors could themselves see anywhere from 63 to 72 psig of differential pressure. This translates to about a 7% variation of the amount of fuel that may be delivered, for a given length injector pulse. The engine computer knows this and will compensate, but the MPGuino does not (and cannot, in base form).

Interesting! Thanks for the explanation.

Sounds like you'll be taking a reading off the manifold pressure (along with the injector pulse) so you can emulate the ECM?

Any particular reason you didn't go with an OBDII approach (OBDuino, Scangauge, etc)?

I currently use an Ultragauge in the Karen-mobile, and a ScanGauge II in the Fiat Dakota.

Not sure about the OBDuino or the Scanguage, but I know for a fact that the UltraGauge cannot accurately show the effect of the ecomods I have done to the engine of the Karen-mobile. For instance, this post about initial EGR sensor modification results showed a real-world fill-up FE figure of 25.8 MPG. However, the Ultragauge only recorded a per-tank average of 23.9 MPG. This, after I had previously calibrated the Ultragauge to get its average FE reading to within 0.2 MPG of the calculated fill-up FE figure. It's very likely that it has something to do with the way that the Ultragauge estimates fuel consumption. If I am lowering intake manifold via increasing EGR opening, then the MAP sensor reading will reflect that, and the Ultragauge is likely showing that as a artificially lower fuel economy figure.

The Scangauge is likely going to use some version of MAF to estimate fuel consumption. The OBDuino get_icons function actually does do this -> "formula: (3600 * MAF) / (14.7 * 730 * VSS)".

The only way to really get around that, IMO, is to directly measure fuel consumption. The MPGuino is still going to provide an estimate, but it should be very accurate, compared to the estimation formulas used in OBDII instrumentation. I got a couple of MAP sensors laying about, so I can get both ambient and intake manifold pressures with little effort. The JellyBeanDriver version of the MPGuino has hardware provisions for collecting 2 different analog signals, and it should be a relatively simple matter to code in a simple square root function.

My car and many others here with an MPGuino installed have returnless fuel delivery systems. I have no complaints with the accuracy of the guino.

Good data points.

Ford ERFS patent - first used in 1998 Ford vehicles. Varies fuel pressure according to load.

2009 Toyota RAV4 New Features - uses a mechanical fuel pressure regulator in-tank, implying a constant pressure fuel system.

Your RAV4 MPGuino is accurate?

I don't have one in the RAV, only the ZX2. You say the min and max could vary by up to 7%, but what percent of your driving is actually at the ends of that range? The majority of my driving (85-90%) is highway at 55 mph. So the actual influence on the indicated mpg is minimal.

It is an inaccuracy in measurement. For fuel economy purposes, I'm ditching the Ultragauge I have now, in favor of the MPGuino, because of a 6% inaccuracy in fuel economy measurement that cropped up because I performed one ecomod.

Say that I have the MPGuino installed, as is, in the Karen-mobile. I have it tuned for highway driving. There is no compensation for the constant pressure returnless fuel system. Now, if I turn around and do a bunch of city driving, then the MPGuino will now indicate less fuel was consumed, than would actually be the case. This would be because the vehicle would be under less loading than on the highway. This would not be reliable.

I might as well have just relied on the Ultragauge I have now.

Got it... forgot that you'd done some ECM-fooling mods. Since the OBDuino does provide the MAF reading via OBD and can be tweaked to your needs (by modifying the formula and adding a separate injector pulse input, for instance), could that meet your needs? Seems like the OBDuino can give you access to quite a bit of additional data from the ECM that you'd need to manually wire up with the MPGuino.

Not questioning your approach (please don't take it that way), but I am very interested in your thought process... it's an interesting problem.