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Old 10-25-2022, 08:30 PM   #1 (permalink)
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Lowering Idle Fuel Consumption

In my 2003 Jeep Grand Cherokee with the 4.0L, according to DashCommand, Iím using 1.2gph at idle with the A/C on (itís about 0.9-1 with A/C and all accessories turned off). This just seems too high, meaning I would be able to idle for only 16 hours on a full tank. Does anyone have ideas to lower idle fuel consumption or find the culprits? (I run E85, but gph seems to be about the same on 87 octane).

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Old 10-25-2022, 10:09 PM   #2 (permalink)
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Quote:
Originally Posted by FlashStopFall View Post
In my 2003 Jeep Grand Cherokee with the 4.0L, according to DashCommand, I’m using 1.2gph at idle with the A/C on (it’s about 0.9-1 with A/C and all accessories turned off). This just seems too high, meaning I would be able to idle for only 16 hours on a full tank. Does anyone have ideas to lower idle fuel consumption or find the culprits? (I run E85, but gph seems to be about the same on 87 octane).
During my time with Aerocivic, that small Honda D15 idled around 0.10-0.12gph. Part of this was due to a very low idle speed - around 550rpm. E85 does have 30% less energy content per unit volume, so you an expect to use ~43% more fuel under the same conditions. I assume you have a proper E85 tune?

I've also found OBD-based fuel economy instrumentation to be notoriously unreliable. They make a lot of assumptions, giving you the GPH based on displacement, RPM, load, and predicted air fuel ratio. There's a very good chance it idles using less fuel than is being reported, because it doesn't take into account fuel trims, air fuel ratio, or any number of other factors. You simply won't know, unless you can pull data on the injector pulse length, or use instrumentation like an MPGuino.
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Old 10-26-2022, 10:23 AM   #3 (permalink)
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Quote:
Originally Posted by FlashStopFall View Post
In my 2003 Jeep Grand Cherokee with the 4.0L, according to DashCommand, Iím using 1.2gph at idle with the A/C on (itís about 0.9-1 with A/C and all accessories turned off). This just seems too high, meaning I would be able to idle for only 16 hours on a full tank. Does anyone have ideas to lower idle fuel consumption or find the culprits? (I run E85, but gph seems to be about the same on 87 octane).
That does seem quite high, what engine speed is this at?
You might check to be sure your battery and alternator are working properly. My guess is the 4.0 i6 can idle pretty low if you can get the computer to allow it.
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Old 10-26-2022, 11:43 AM   #4 (permalink)
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Quote:
Originally Posted by Ecky View Post
During my time with Aerocivic, that small Honda D15 idled around 0.10-0.12gph. Part of this was due to a very low idle speed - around 550rpm. E85 does have 30% less energy content per unit volume, so you an expect to use ~43% more fuel under the same conditions. I assume you have a proper E85 tune?

I've also found OBD-based fuel economy instrumentation to be notoriously unreliable. They make a lot of assumptions, giving you the GPH based on displacement, RPM, load, and predicted air fuel ratio. There's a very good chance it idles using less fuel than is being reported, because it doesn't take into account fuel trims, air fuel ratio, or any number of other factors. You simply won't know, unless you can pull data on the injector pulse length, or use instrumentation like an MPGuino.
Quote:
Originally Posted by ksa8907 View Post
That does seem quite high, what engine speed is this at?
You might check to be sure your battery and alternator are working properly. My guess is the 4.0 i6 can idle pretty low if you can get the computer to allow it.
It idles at 750, and is properly tuned for E85. I didnít know that about OBDII scanners, Iíll have to get an MPGuino. I canít seem to find where itís sold, could you point me toward a kit/built unit? Are there any worthy alternatives?

Iíll pull the serpentine belt off and run it for a minute to see how it affects idle consumption. Thanks guys.
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Old 10-26-2022, 03:58 PM   #5 (permalink)
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The original MPGuino design needs to be fully assembled from scratch. It isn't too hard to put one together, but it's not exactly compact or elegant.

There's a guy in Australia that sells a color touchscreen version though frankly I slightly prefer the basic one for its simplicity. The touchscreen one is pretty good though, and has features the basic one doesn't.

https://glengineer.com.au/mpguino.htm
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Old 10-26-2022, 04:05 PM   #6 (permalink)
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1.2gph sounds extremely wrong, that's far higher than my V8 Vantage's cold idle.
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Old 10-27-2022, 03:11 AM   #7 (permalink)
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Keep in mind that there's a limitation regarding instrumentation that directly measures fuel injection pulses to get fuel consumption data.

They absolutely depend on the assumption that there are no variations in the differential pressure between the fuel rail and the intake manifold. With the vast majority of cars, this is a valid assumption. Late model Chrysler products, tho? Not so much.

Most cars either have a fuel pressure regulator that is modulated by intake manifold vacuum, or their engine computers perform fuel pressure adjustments by varying the fuel pump. In either case, the fuel pressure is varyed such that there's always a constant pressure differential between the fuel rail and the intake manifold, typically 43.5 psig.

Late model Chrysler engine computers estimate the pressure differential between the fuel rail that is fixed to a constant pressure at the tank-mounted fuel regulator, and the varying pressure in the intake manifold. The computers then adjust the injector pulse widths by that estimation. It's elegant for the car, but it will absolutely throw off any fuel consumption calculations unless you know how to compensate.

My MPGuino code can do this compensation, and I ran it for many years on my Dodge Magnum.
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Old 11-23-2022, 02:40 PM   #8 (permalink)
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Quote:
Originally Posted by t vago View Post
Keep in mind that there's a limitation regarding instrumentation that directly measures fuel injection pulses to get fuel consumption data.

They absolutely depend on the assumption that there are no variations in the differential pressure between the fuel rail and the intake manifold. With the vast majority of cars, this is a valid assumption. Late model Chrysler products, tho? Not so much.

Most cars either have a fuel pressure regulator that is modulated by intake manifold vacuum, or their engine computers perform fuel pressure adjustments by varying the fuel pump. In either case, the fuel pressure is varyed such that there's always a constant pressure differential between the fuel rail and the intake manifold, typically 43.5 psig.

Late model Chrysler engine computers estimate the pressure differential between the fuel rail that is fixed to a constant pressure at the tank-mounted fuel regulator, and the varying pressure in the intake manifold. The computers then adjust the injector pulse widths by that estimation. It's elegant for the car, but it will absolutely throw off any fuel consumption calculations unless you know how to compensate.

My MPGuino code can do this compensation, and I ran it for many years on my Dodge Magnum.
Gotcha! I'm thinking of attaching a fuel rail pressure sensor to the bleed port, so I can monitor the pressure and eventually add it to my fuel calculations.
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Old 11-25-2022, 01:13 AM   #9 (permalink)
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did you check your maf signal and it's sensor temp? usually it's because the temp sensor failed and the car thinks it's -20 outside. by math since my 4.7 uses 0.55gph at idle at 600, i would think an e85 4.0 and 750 would use around 0.8ish on e85. check all of your sensors. one of these has to be wrong. the quickest way is to sniff the tail pipe with a wideband and see if it's rich.
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Old 11-25-2022, 05:21 PM   #10 (permalink)
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I wish I could figure out how much fuel my car uses when idling

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ecu tuning, engine mods, idle, idle fuel consumption, jeep

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