17mpg on a 1000 mile trip, high tank pressure, fuel boiling, gas smell
08-25-2011, 03:25 AM
#1
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17mpg on a 1000 mile trip, high tank pressure, fuel boiling, gas smell
Just got back to San Diego from a 1000 mile round trip to AZ. Averaged 17mpg, before it was nearly 25mpg on the same trip. So I know something is up. Almost ran out of gas to Gila Bend so I slowed down to 60mph, turned the AC off and rolled the windows down and gas fumes were entering the car. Thats not normal. Took the gas cap off and a 35 second long hiss with drops of fuel or condensation spitting out. At first it sounded like it was boiling!!!
Now its like 150* outside and on near the pavement from Yuma to Phx/Metro so obviously there is a performance decrease from that. But in San Diego it was totally fine.
I'm going to check the Evap Canister Purge lines and the lines to the charcoal canister http://shbox.com/1/evap_system.jpg.
The pump was supposedly replaced last year. We verified that the fuel pressure is fine and the injectors aren't leaking as fuel pressure gauge was still sitting at 20psi the next morning.
Was the fuel seriously boiling and evaporating during the trip? Like I said two months ago, the exact same trip, I averaged 25mpg using AC and going the same speed. 70 in Cali and 75 in AZ.
Now its like 150* outside and on near the pavement from Yuma to Phx/Metro so obviously there is a performance decrease from that. But in San Diego it was totally fine.
I'm going to check the Evap Canister Purge lines and the lines to the charcoal canister http://shbox.com/1/evap_system.jpg.
The pump was supposedly replaced last year. We verified that the fuel pressure is fine and the injectors aren't leaking as fuel pressure gauge was still sitting at 20psi the next morning.
Was the fuel seriously boiling and evaporating during the trip? Like I said two months ago, the exact same trip, I averaged 25mpg using AC and going the same speed. 70 in Cali and 75 in AZ.
08-25-2011, 08:18 PM
#3
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Re: 17mpg on a 1000 mile trip, high tank pressure, fuel boiling, gas smell
No exhaust leaks. But it is definitely temperature based. Its acting normal. I think the excessive AZ summer heat was gumming up the old decayed rubber hoses and the tank wasn't able to vent properly. Its fine for now....
***EDIT: 2 hoses to the charcoal canister weren't even connected. Both (feed and control) hoses to the canister purge valve were broken and one was all gunked up. I can see that being a problem with the gas smell and all, but wouldn't the FUEL TANK VENT VALVE be my prime candidate for the gas tank pressure?***
***EDIT: 2 hoses to the charcoal canister weren't even connected. Both (feed and control) hoses to the canister purge valve were broken and one was all gunked up. I can see that being a problem with the gas smell and all, but wouldn't the FUEL TANK VENT VALVE be my prime candidate for the gas tank pressure?***
Last edited by Gtpguy; 08-26-2011 at 01:25 AM.
09-26-2011, 02:59 AM
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Re: 17mpg on a 1000 mile trip, high tank pressure, fuel boiling, gas smell
I cant say I have ever tested it but the gas tank should not vent positive pressure to atmosphere; think EPA and emissions for reasoning.
Drop in MPG can be a couple things and I would point to temperature and Elevation. Cant say I know the difference in elevation from your home location to where you experienced the problem but look into it. Higher elevation = thinner air / less fuel reqd. Hotter temps also require less fuel to a point. Air that hot is going to drastically raise your cylinder temps. If the Cylinder temps are hot enough there will be some fuel vaporizing upon entry into the CC. You can also think about the scavanging affect and how that is affected. Cold air entering the CC is less likely to flow past the Exhaust valve due to the resistance of the dense air. Hot 150* temp air flowing into a 1200* temp cylinder is more likely to flow past the exhaust valve at a faster rate during the overlap of the Cam lobes.. Air temps that hot def wreak havok on your ECU abiltiy to tune.
Gas fumes could very well have been cuased by the emissions hose off the charcoal canister. Remember the emissions system will pull fumes from the gas tank and introduce them into the intake aleviate the possability of positive pressure in the gas tank. If you have hoses off or missing then that will affect how the system works or not. Did you have a check engine light? I am thinking the ECU should have detected a leak from the hoses off and should have disabled the EVAP system.
As for boiling fuel.......doesnt sound right. Perhaps its just the sound you heard.
In your shoes I would be too concerned if its back to normal under AVERAGE temps....lol.
Drop in MPG can be a couple things and I would point to temperature and Elevation. Cant say I know the difference in elevation from your home location to where you experienced the problem but look into it. Higher elevation = thinner air / less fuel reqd. Hotter temps also require less fuel to a point. Air that hot is going to drastically raise your cylinder temps. If the Cylinder temps are hot enough there will be some fuel vaporizing upon entry into the CC. You can also think about the scavanging affect and how that is affected. Cold air entering the CC is less likely to flow past the Exhaust valve due to the resistance of the dense air. Hot 150* temp air flowing into a 1200* temp cylinder is more likely to flow past the exhaust valve at a faster rate during the overlap of the Cam lobes.. Air temps that hot def wreak havok on your ECU abiltiy to tune.
Gas fumes could very well have been cuased by the emissions hose off the charcoal canister. Remember the emissions system will pull fumes from the gas tank and introduce them into the intake aleviate the possability of positive pressure in the gas tank. If you have hoses off or missing then that will affect how the system works or not. Did you have a check engine light? I am thinking the ECU should have detected a leak from the hoses off and should have disabled the EVAP system.
As for boiling fuel.......doesnt sound right. Perhaps its just the sound you heard.
In your shoes I would be too concerned if its back to normal under AVERAGE temps....lol.
09-27-2011, 05:38 AM
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Re: 17mpg on a 1000 mile trip, high tank pressure, fuel boiling, gas smell
There is a pressure/vacuum relief valve on the tank to protect it from imploding (excessive vacuum) or exploding (excessive pressure). It's the white valve in Shoebox's photo:
http://shbox.com/1/fuel_pump_wiring.jpg
Gasoline blended for San Diego has a higher vapor pressure than the fuel used at higher elevations. It's going to boil easier at higher elevations. But something has to be adding heat to the system. That's what is producing the high pressure and the high vapor load.
As the fuel pump fails, it may overheat the fuel. Exhaust leaks have already been mentioned. An oversize fuel pump (and lower fuel demand due to the increased elevation) returns a lot of fuel to the tank. That fuel has picked up heat as it passes near the exhaust and through the engine compartment.
http://shbox.com/1/fuel_pump_wiring.jpg
Gasoline blended for San Diego has a higher vapor pressure than the fuel used at higher elevations. It's going to boil easier at higher elevations. But something has to be adding heat to the system. That's what is producing the high pressure and the high vapor load.
As the fuel pump fails, it may overheat the fuel. Exhaust leaks have already been mentioned. An oversize fuel pump (and lower fuel demand due to the increased elevation) returns a lot of fuel to the tank. That fuel has picked up heat as it passes near the exhaust and through the engine compartment.
09-27-2011, 02:15 PM
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Re: 17mpg on a 1000 mile trip, high tank pressure, fuel boiling, gas smell
Okay, heres a hypothetical. Back in march I moved to San Diego and got 25mpg temps were cooler. In may I took a trip back to AZ and got 25mpg. Temps were still cool. Took this last trip in the thread and got 18mpg. Hotter than hell. All of these were with the car in the same condition, meaning the charcoal canister wasn't ever connected for all these trips. Now lets say I go back in December and get 25mpg again with the charcoal canister disconnected ( <---keep the variables the same) wouldn't that point to weather being the major factor and nothing is really wrong with my car?
Injuneer, do those white relief valves go bad? I've always wondered about that valve since I started this thread.
Injuneer, do those white relief valves go bad? I've always wondered about that valve since I started this thread.
09-27-2011, 10:01 PM
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Re: 17mpg on a 1000 mile trip, high tank pressure, fuel boiling, gas smell
I don't know much about the relief valves. The factory manuals of all 4th Gen years indicate it is a vacuum and a pressure relief valve.
But in the Fuel & Ignition forum I posted about the valve being there, and someone ripped me a new butt-hole for not understanding the system, and how it works. He insisted he had tested several of the valves, and they are only for vacuum relief. He claimed no amount of pressure would cause them to vent.
In response someone on another forum tested his, and this is what he reported:
I trust this guy's testing, because he is another engineer.
Since its a spring and a valve, and it could get gummed up, or the spring could weaken, it would appear it could fail.
More heat in the system will cause the fuel to vaporize more. That vapor has to go somewhere. Without the canister connected, its just going to dump vapor out the rear of the car. The other problem with the fuel getting hot is the possibilty it will evaporate in the suction line of the pump, and cause vapor bubble in the fuel line to the rails. I suggested this to someone who experienced a loss of power while climbing a 4,000-ft hill, and he tracked it down to excessive heat in the tank, and vaporization in the pump suction line, so I know this is possible. I wonder if that was what caused you to feel you were running out of gas.
But in the Fuel & Ignition forum I posted about the valve being there, and someone ripped me a new butt-hole for not understanding the system, and how it works. He insisted he had tested several of the valves, and they are only for vacuum relief. He claimed no amount of pressure would cause them to vent.
In response someone on another forum tested his, and this is what he reported:
Follow-up (as hinted) on the white relief valve under the car. It IS a combined pressure/vacuum relief valve. Pressure relief level is 1.5-2.0 psi (3-4" HG). Vacuum relief is about 1" HG (13.5" H2O or .5psi). There is a spring and valve for each function that shares the same shuttle. The vacuum is high enough at 13" H2O to not interfere with normal Evap testing (at about 10" H2O).
Since its a spring and a valve, and it could get gummed up, or the spring could weaken, it would appear it could fail.
More heat in the system will cause the fuel to vaporize more. That vapor has to go somewhere. Without the canister connected, its just going to dump vapor out the rear of the car. The other problem with the fuel getting hot is the possibilty it will evaporate in the suction line of the pump, and cause vapor bubble in the fuel line to the rails. I suggested this to someone who experienced a loss of power while climbing a 4,000-ft hill, and he tracked it down to excessive heat in the tank, and vaporization in the pump suction line, so I know this is possible. I wonder if that was what caused you to feel you were running out of gas.
09-28-2011, 12:12 AM
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Re: 17mpg on a 1000 mile trip, high tank pressure, fuel boiling, gas smell
Makes sense. But I'd have to replace the valve on the 2nd leg of the trip back home to see if theres a difference. Theres no telling until I make that trip again in cooler weather. Unfortunately I get horrible gas mileage here on the hilly highways so I can't even compare things that way.
12-18-2011, 03:40 PM
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Re: 17mpg on a 1000 mile trip, high tank pressure, fuel boiling, gas smell
Went back to AZ this past week. 75mph max.
From San Diego to El Centro w/ a half dozen good climbs (120 miles) 17.8mpg.
From El Centro to Maricopa Pretty much flat with some climbs outside of Yuma (220 miles) 22.8mpg.
From Chandler to Tucson and back ALL FLAT!!! with rain (240 miles) 17.8mpg
Maricopa to Yuma, mostly flat with some climbs (180 miles) 22.8mpg.
Yuma to San Diego, good amount of climbs (180 miles) 17.8mpg
I am so confused by this. It seems sporadic with flat distance to get either 17 or 22mpg. There was no tank spitting this time.
I'm seriously wanting to buy a laptop and scan but what should I start looking at?
From San Diego to El Centro w/ a half dozen good climbs (120 miles) 17.8mpg.
From El Centro to Maricopa Pretty much flat with some climbs outside of Yuma (220 miles) 22.8mpg.
From Chandler to Tucson and back ALL FLAT!!! with rain (240 miles) 17.8mpg
Maricopa to Yuma, mostly flat with some climbs (180 miles) 22.8mpg.
Yuma to San Diego, good amount of climbs (180 miles) 17.8mpg
I am so confused by this. It seems sporadic with flat distance to get either 17 or 22mpg. There was no tank spitting this time.
I'm seriously wanting to buy a laptop and scan but what should I start looking at?
08-25-2013, 12:33 PM
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Re: 17mpg on a 1000 mile trip, high tank pressure, fuel boiling, gas smell
Had a very similar experience on my last trip from Los Angeles to Arizona. Then the fuel pump quite in sedona. Try replacing it with an AC Delco pump.
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