Mysterious fuel usage (loss) - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 03-13-2014, 03:37 PM   #1
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Default Mysterious fuel usage (loss)

Hi everybody, sorry for such a delay! Installed new Bosch wires, cleaned up the icm, and air flow. First time out got lousy fuel mileage; second time out (by out I mean stint between refueling and driving), I got a fantastic 28mpg! Then the strangeness began! I filled the tank, drove about a mile home and parked the car. The next morning, because of bad weather, I drove my wife to work; as soon as we got a few blocks from home I noticed that the fuel gauge was off the full mark. The tank took 2 gals more than it should have taken; I drove 14 miles and consumed 3 gals of fuel. The engine ran great, plenty of power. My conclusion is that someone had stolen 2 gals of fuel during the night; I hate to entertain such an idea but there was no evidence of fuel leakage; I have a locking gas cap (I wonder how hard it is to get one open without breaking it? Master key maybe, lock pick?) The next fill-up produced 24mpg; the next one produced 13mpg; I had just filled the tank and used 2.5gals to go 33.5 miles. I have taken steps to secure my carport with a surveillance camera, but I am stilled nagged by the possibility that there might be another explanation for the loss of fuel. My plugs are clean, healthy and new.
Anybody have an idea? Are locking gas caps easy to break into? Could there be some strange mechanical problem that could cause this?
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Old 03-13-2014, 03:54 PM   #2
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If you can, have someone follow you to the gas station and top off again. The person following you should have both front windows open and the HVAC system on "fresh air" mode, not recirculate. Have them follow you as close as possible to and from the gas station. What they are looking for is the smell of gas coming from your car. Not exhaust fumes, but the raw stench of unburned gasoline. Try to rock the car as much as possible, a few hard brakes, a couple of hard launches. If they smell nothing, then we can eliminate a leak.

Next thing would be to check/replace your FPR.

Here is an excellent video. The mechanic can actually see his fuel trims change all because of his fuel pressure regulator going bad allowing the intake to suck gas from it.

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Old 03-13-2014, 04:44 PM   #3
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Leaky fuel pressure regulator, or a sneaky neighbor.
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Old 03-13-2014, 05:07 PM   #4
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Thanks guys,
how do you remove the old regulator? Just pry it off? Ebay is selling them for $60-$70. I looked at mine when I was working on it but it didn't look like it could be taken apart.
I hope that that is the problem; I just hate thinking that a person is stealing from me.
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Old 03-13-2014, 05:48 PM   #5
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There is metal spring clip you have to remove.
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Old 03-13-2014, 06:04 PM   #6
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No, its held in by a metal clip. Use snap-ring pliers or something suitable to pop out the ring. Then the FPR will pop right out.
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Old 03-13-2014, 06:26 PM   #7
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Great! I see it in the video!
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Old 03-15-2014, 09:09 AM   #8
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Did the test for leakage as shown in the video; after 5min no leaks. Pulled the old FPR and did a search on ebay for the number and found out that it wasn't correct for my car! So, I installed the new FPR. So far, after 28 miles the fuel gauge needle hasn't moved off the full mark which means I'm getting at least 28mpg (I know how many gals are used between marks). Only time and mileage will tell the truth. Thank you all for your help!
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Old 03-15-2014, 10:34 AM   #9
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Your very welcome.
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Old 03-16-2014, 10:27 AM   #10
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Glad to hear your issue may be solved, keep us posted if the issue comes back.
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