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Old 12-30-2009, 09:10 PM   #1 (permalink)
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Coasting fuel problem

Me and my friend Zach took his brother's 2002 GMC sierra to Simi valley to my sponsor to get his truck tinted since he was coming back from the army. We were on our way back and the truck had been doing great on gas since it's a V6 (vehicle was reading about 23MPG) I decided to try coasting down a few hills.
Coasted from 60MPH everytime i would let it coast in nuetral the fuel gauge dropped significantly once i was to the bottom of the hill. This doesn't happen when it's in drive though, i will say i forgot to check if the avg fuel economy on the trip went up or down from doing this but does anyone know whyy this truck would do that?
It'd be much apppreciated

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Old 12-30-2009, 10:05 PM   #2 (permalink)
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Because the level sensor changes the fuel level in the tank slowly while you're on the hill, and then the level drops when you're on the level again. Or, the other way around, depending on which direction the fuel level meter is installed.

The situation can be remedied by re-installing the fuel pump so that the lever is located to the side of the tank, rather than the front or rear, but the pump won't always fit in this way.
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Old 12-30-2009, 10:20 PM   #3 (permalink)
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Quote:
Originally Posted by Christ View Post
Because the level sensor changes the fuel level in the tank slowly while you're on the hill, and then the level drops when you're on the level again. Or, the other way around, depending on which direction the fuel level meter is installed.

The situation can be remedied by re-installing the fuel pump so that the lever is located to the side of the tank, rather than the front or rear, but the pump won't always fit in this way.
thanks but why does it only do this in Nuetral?
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Old 12-30-2009, 10:23 PM   #4 (permalink)
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No idea. My wife's car does the same... I'm assuming I've only noticed it in neutral, since in D, you tend to forget about looking at the gauge cluster.

Why not check and see if it happens in D, too?

Maybe I'm completely wrong, and it's a ploy by GM to make you think your car will do stupid stuff if you put it in Neutral...
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Old 12-30-2009, 10:26 PM   #5 (permalink)
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Quote:
Originally Posted by Christ View Post
No idea. My wife's car does the same... I'm assuming I've only noticed it in neutral, since in D, you tend to forget about looking at the gauge cluster.

Why not check and see if it happens in D, too?

Maybe I'm completely wrong, and it's a ploy by GM to make you think your car will do stupid stuff if you put it in Neutral...
Haha thanks i'll see if we can take it out next week and see if it does it in drive
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Old 01-02-2010, 01:53 PM   #6 (permalink)
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Not sure, but I think as the rpms drop while coasting in neutral, the voltage level from the charging system is likely dropping also. In my Ranger, at 60mph, the rpms drop from around 2200 to 1000 when coasting(idles around 750). So even though there are various voltage regulators in the system(alternator, instrumentation, etc) the voltage to either the sending unit or the gauge could be changing. This is just a possibility. I've had the regulator to an instrument panel go bad before and make it look as though the engine was burning up because the temperature gauge was pegging.
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Old 01-02-2010, 04:18 PM   #7 (permalink)
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I don't know about that... the GM fuel level sending unit is a resistance based unit, and I don't think it's resistance changes with voltage change. I've seen them used on 6V systems successfully, so I don't see it being a problem, although I could have been missing something.
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Old 01-02-2010, 06:46 PM   #8 (permalink)
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I believe I can answer this. What seems to be happening is when you are coasting with the engine on, the ECU is commanding the evap purge solenoid closed. This raises the vapor pressure in the tank and forces the float to sink slightly. I have seen this condition in vehicles with faulty purge solenoids, most notably with digital readout fuel gauges. I couldn't confirm this without consulting a manual for your truck, although you could verify it with a scanner while driving.
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Old 01-02-2010, 06:48 PM   #9 (permalink)
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So you're saying the increase in pressure above the fluid makes the float less buoyant?
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Old 01-02-2010, 07:27 PM   #10 (permalink)
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Yep, I recently repaired a 2008 F450 doing the same thing, the purge solenoid was the fault. The fuel vapors are denser than air and forced the float to sink lower. This truck has a miles to empty readout and it would show considerably less miles to empty when the tank was pressurized. When I was diagnosing it, it would have excessive pressure when removing the gas cap. I wouldn't have thought that it would have been that significant on the gauge, but live & learn.

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