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Old 04-16-2017, 04:45 PM   #1 (permalink)
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Cars with returnless fuel systems

Hello everyone,

Long time lurker here, this is my first post!

I am presently making up my own version of an MPGuino from scratch, using T Vago's latest software - which by the way is amazing and he deserves much praise for doing all that work.

My car is a 1999 Toyota Tercel. I believe (but I am not 100% sure) that my car suffers from the same problem as the Chrysler returnless system. The returnless systems on the Tercel only made a brief appearance in the latter part of the model's existence from 1997-1999. Not having a shop manual, and trying my best to Google information leads me to believe this is NOT a computer-controlled regulator pump, but instead a mechanical in-tank pump and regulator. The pump supposedly cycles on and off to maintain a constant pressure (roughly 48PSI) over atmospheric. Also, it is possible that the fuel pressure changes with fuel / vapour level changes but I am very short on those details. I do not see a fuel pressure readout when plugging in an OBDII reader, instead I have to screw in a gauge to the fuel rail.

If this is the case, this means that as manifold pressure changes, then the pressure differential across the injector will also change, and this will throw off the MPGuino numbers, since the pressure drop will cause less fuel spray in a given amount of time.

My questions are:

1) Does anybody have any bright ideas on how to tell if my car has this issue, or not?
2) Is there a list of cars that are confirmed to have this issue?
3) Is there an additional issue here in that as fuel level changes the fuel pressure also changes which will also serve to throw the numbers out of whack?

Also, an unrelated note, on the MPGuino schematic diagram on the wiki, shows 50k resistors for the VSS and Injector leads, but the Iduino shows 100k. Not sure if that really matters or not (oddly I have seen the same diagram with both values) but I went with the 50k.


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Old 04-16-2017, 10:36 PM   #2 (permalink)
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First of all, thank you very much for the props! I've put in a lot of work into improving MPGuino, and I'm glad to see that people are using the upgraded code.



1). Based on what I saw at RockAuto.com, I'd have to say that your fuel system is indeed a constant-pressure version. You'll need to compile in the returnless pressure option when you compile the MPGuino code, and set a fuel system pressure of 48 psi.

2). There is no list that I know of that has cars that have this constant-pressure regulator. It'd be good to compile one.

3). If the fuel pump does cycle on and off to keep pressure at 48 psi, then there will be times when fuel pressure will be above the pump's setpoint, and there will be times when the fuel pressure will be below the pump's setpoint. Will this cause fuel pressure to vary? Yes. Will it make a noticeable difference? Maybe. You'll have to experiment and let us know what you find.
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Old 04-17-2017, 12:32 AM   #3 (permalink)
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Thanks for responding! What I will do is measure the line pressure at a full tank, and at empty tank, and see if there is a difference. I will let you know the findings, but it may take a while since I need to rent the gauge.

Anywho, I really appreciate the quick response. I actually saw the same regulator and thought it was for the previous models to mount of the rail, but I see I am wrong. Doh.

Thanks again for all your hard work on this, this project rocks
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Old 04-17-2017, 05:57 AM   #4 (permalink)
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Everything that I can find shows the Tercel having a standard Toyota EFI system that has a manifold referenced fuel pressure regulator with a return line. In the post above, you can see the return line and manifold pressure line. On the hidden side, as a part of the flange, is an o ring connection to the fuel rail.

In my >30 years of owning and working on Toyotas, I have never seen or heard of a mechanical in-tank fuel pump.

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Old 04-17-2017, 10:44 AM   #5 (permalink)
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Much thanks for your input!

The Tercel 1997-1999 is definitely returnless, opening the hood shows no return line (I had thought I had a bad fuel pump last year and did a bunch of diagnosis). I am thinking that the linked regulator is either situated in the tank itself (lines for input, output, and to tank), or is for a previous model. My model has a dampener, but no regulator, on the rail.

I have a link but I have too few posts! But anyway, check out tercelreference[dot]com, it is a good site (created by none other than Daox) and shows that 1997+ models have the returnless system.

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Old 04-17-2017, 11:29 AM   #6 (permalink)
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Quote:
Originally Posted by jonzobot View Post
I have a link but I have too few posts! But anyway, check out tercelreference[dot]com, it is a good site (created by none other than Daox) and shows that 1997+ models have the returnless system.


Yah, I don't see anything on that vacuum diagram that would cause fuel pressure to be referenced to intake vacuum.

I am also thinking that this returnless system does the same thing as the Chrysler returnless system does - the fuel pump is a simple electric pump which is energized for a couple of seconds when the ignition key is turned, and is also energized when the engine computer senses that the engine crankshaft is turning. The fuel pump output then goes through a fuel pressure regulator which regulates the pressure down to a constant 48 psig. Pretty simple system, and easy enough for the engine computer to compensate for. Only real difference would be that the Chrysler returnless system is set to 58 psig, instead.

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