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Old 02-08-2018, 12:46 PM   #16
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Re: EVAP / LS1 Plastic Tank / LT1 Metal Tank

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Did the tube internal to the fill pipe come out from the tank? That may be a vent that allows the displaced air and fuel vapor to exit the tank as the station pump is putting several gallons a minute into the fill tank. That's what I thought an external hose connected to the top of the fill pipe would be for. Was there an equivalent internal pipe on the 99 fill/tank?

Donít worry about the diameter of 97 fill pipe. It wasnít that important.
I wish I had held on to the tank for a few more days so I could verify, but I bet you are right. I tried to find a pic of inside of the neck with the gas cap off, but I think there was a small opening above the metal flapper behind the gas cap that would have been positioned at the same point as the internal tube. The fill tube on the new tank is 1 1/2" OD, and if the neck on the metal tank was the same size, I would guess that the internal tube was either 1/2" or 5/8" OD.

In any case, disconnecting the EVAP did allow gas via a siphon pump to freely feed gas into the tank. I ran about 2 gallons with the tube disconnected, then another 3 after reconnecting the tube minus the LT1 pressure control valve.

I felt this was good enough to put everything back together in the rear, but won't call it 100% until I can get it to the pump. I plan to be at the DMV this weekend for another moving pass, so I can test the tank. On the LS1 forum, there were a few people posting about this issue, and the solution for a few of them was a plugged EVAP system.

Is shaking the carbon canister and listening for rattle a valid test of it the canister is good or bad?
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Old 02-08-2018, 06:47 PM   #17
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Re: EVAP / LS1 Plastic Tank / LT1 Metal Tank

*** Differences V8 / V6 Tanks***
The first discovery that I made when working on this swap, and the first thing I would look at when attempting the swap is what engine the donor car was equipped with. The V8 and V6 cars all use the same tank, but the fuel bucket and and some components of the bucket are different. I opted to replace the fuel pump during this swap to an Areomotive 340LPH GM pump, so the pump originally in the bucket didn't matter. The V8 has a venturi pump system that pumps fuel into the bucket from the tank that keeps the pump in fuel regardless of fuel level to keep the pump cool as well as a pickup inside the bucket which helps keep the pump from running dry in hard cornering with low fuel. The V8 also has a built in fuel pressure regulator set at 60 PSI, where the V6 had its regulator in the engine bay. The last difference that I am aware of is the mounting of the fuel level sender. Both units send the same signal, by have different clips that hold the sensor to the bucket. There are also 2 styles of connector on the top of the fuel senders, flat and square. I don't know the details between which years used what, but the plug style is only being mentioned because you will need to

I paid $55 for my tank, lines, sending unit, filler neck and bracket. You can get a tank and sending unit for $115 plus shipping at Hawks Motor Sports, so I would not pay more than this at a yard. Additionally, these tanks are non-repairable, thus if your junkyard drills the tank to empty the fuel, the tank is ruined.

*** Pumps ***
If you want to use the Racetronix pump kit, I highly recommend getting a tank from a V8 car, or replacing the fuel bucket with the V8 model. Modification to the V6 bucket for the Racetronix kit more than likely will not work. The Racetronix setup will use the venturi pump to draw fuel from the bottom of the bucket, while the base of the pump pulls from inside the bucket. This system keeps the nice features of the stock pump and provides an upgrade with no modification of the bucket.

While on the topic of the venturi pump, if you plan to install a stock pump, I would not use a V6 fuel bucket in a V8 or vice versa, as the two are functionally different.

If you are going to use an aftermarket pump, modification of the bucket to allow fuel into the bucket to cool the pump, and to the base of bucket to allow the pump to pass through the bottom of the bucket to pickup fuel. Also note that many (including Aeromotive) want upgraded power due to small gauge wire through the car. I had a Racetronix hotwire kit installed from the previous tank. Aeromotive has their own kits, but I prefer the Racetronix hotwire as it is not that much more expensive, and is a direct bolt on to the car where the Areomotive is more generic.
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Old 02-08-2018, 06:49 PM   #18
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Re: EVAP / LS1 Plastic Tank / LT1 Metal Tank

*** Fuel Level Sensor ***
There are two difference signals that GM used between the 1993 and 2002 fbodies. Between '93 and '97 (maybe 98, I am unsure), GM used a 0-90 ohm signal, where 0 represents empty and 90 represents full. The newer cars use a 240-30 ohm style sender. This needs be be changed as the signal is not compatible with each other.

If the destination car is a '98, you will need to test as I did not have a car to test. The plastic tanks are all 240-30, so if your original sensor is this range, no change is required.

For '93 - '97, the gauge in the dash requires 0-90, which means the sensor in the plastic tank must be swapped. Through a lot of reading in 3rd gen forums, it was discovered that the sensor from a 1997 N/A Grand Prix is the same shape as the V8 bucket, but the needed 0-90 ohm range. The V6 bucket has a different clip system than the V8, so if you are using the V6 bucket, there are extra things that you need to know, and be prepared to modify. The parts mentioned will directly fit the V8 bucket.

The GM replacement part for this is $130 plus shipping. I found a company out of Florida (Herko) that sells OE replacement parts at a fraction of the cost, but buy whatever you feel comfortable with. The staff were nice great over the phone and email, but couldn't verify ohm ranges, only reference a specific application or part number.

V8 bucket:
The GM part number is #25319676 and again is for a 97 Grand Prix non-supercharged, roughly $130 plus shipping.

One of the 3rd gen sites which pointed me to Herko, indicated that the GFC13 was the needed sensor. I can verify that this sender is 240-30 ohm, do not buy this sensor
The GFC31 is a 0-90 ohm, and fits directly into the V8 bucket: here $24, free shipping

V6
On the site based on sight, the GFC27 seems to be the correct bracket, but the ohm range is unknown. I have emailed Herko to see if someone would test, but haven't seen a response yet.
If someone wants to buy it and test, I'll update this thread. here

If the GFC27 is the wrong range, some modification is required to have the sensor work.

Option 1: Install an aftermarket gauge and wire into the factory wire which works on 240-30ohm (most guages, but be sure to double check)
Option 2: If installing an aftermarket pump where modification of the bucket is required, you could slip the GFC31 (or GM part) unto the bucket, then install a bolt with rubber washers to wedge the sensor in place. If you take this route, be extra careful to not over tighten the nut so you don't crack the bucket.
Optiin 3: Since the GFC31 slides into the bucket, but doesn't clip, you could use a super glue to attach the sensor. I advise against this, as changing the sensor later would be tough, but it's an option....
Option 4: Switch the GFC31 resistance board into the V6 bracket. I could not remove the resistance board without taking the actuator arm. The arm is held on with a brass fitting that has a small barb, so some fineness is required. Here are the steps I took for the change:
1. Press out the brass retainer holding in the actuating arm in both brackets
2. Remove the resistance boards from both brackets
3. Heat the solder on the connection points of the resistance board to align the wires with the clips in the original bracket
4. Verify the sweep was the same between the resistance boards
5. Press the brass retainer back into the bracket
6. Test that resistance is sent through the harness
Option 5: Leave the 240-30 sensor and use a device to adjust the output inline with your gauge. See this as an example.

A note here, is that the Herko brass retainer had a significantly bigger barb than the GM. When I tried to press out the Herko retainer, the plastic snapped (the GM retained came out without issue). That said, I would not attempt this with two Herko sensors.

Sensor information and credit

*** Filler Neck Bracket ***
Since the filler neck is not solid with the tank, a bracket will be needed to hold the filler neck. I was able to sawzall the bracket from my donor car, but if you have a 90 degree drill bit and a punch, you can most likely drill out the weld points. I then used pop rivets to attach the bracket to the car.

To mount the bracket, I first drilled a mounting hole in each side of the bracket, size matched with my rivets. I then I installed the filler neck cover, placed the filler neck into the cover where the filler looked normal, and the end of the neck pointed directly at the tank, then marked and drilled holes. The bracket was then pop riveted to the frame and test fit with the filler neck.
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Old 02-08-2018, 06:50 PM   #19
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Re: EVAP / LS1 Plastic Tank / LT1 Metal Tank

*** EVAP ***
The components currently on the car will stay connected with the removal of the pressure relief valve from the existing carbon canister. Here are the steps:

1. Remove the carbon canister and EVAP solenoid from the plastic tank and discard (or save them ...). It is held down on the left and right sides. I found the left (when looking at the tank carbon canister closet to you) side easiest to work with. I used a large flat head screw driver to pry back the clip holding the canister.
2. The pressure relief valve in the plastic tank will be connected directly to the carbon canister in the fender well. To do this, the hard plastic 5/8 line on the tank that takes a large U turn from the black circular pressure relief valve on the middle of the tank to the carbon canister on the plastic tank needs to be removed. Carefully cut the plastic hose at the barb on the pressure relief valve and remove
3. Install the 5/8 fuel line to the plastic tank pressure relief valve. Be gentle with the worm clam as to not crack the plastic barb on the tank. Route the hose to the lowest plastic clip on the tank then on a gentle 90 degree turn towards the filler port. Mark and cut the hose about half an inch short of the filler port.
4. Install the 5/8 barb (Summit) to the 5/8 hose (add a little bit of oil to the inside of the hose to ease installation (be careful to not place any pressure on the pressure relief valve. If there is any doubt, remove the hose from the tank to install the fitting)
5. Take the rubber connection from the original hose coming from the tank and install it on the 5/16 barb (Summit)
6. Connect the AN fittings together
7. Cut the plastic line that originally connected the carbon canister in the fender on that tank side, so the newly made line reaches just passed the filler port.
8. Remove the pressure relief valve from the carbon canister in the fender. Depending on the condition of the line, you may want to replace it. I would buy 2', and trim during install. This line will connect to the line made in steps 3-7

*** Fuel Lines ***
All fuel lines from the donor car will match up directly without modification. If you did not obtain the lines from the doner car, you will need to fabricate your own fuel lines. On the sender, from left to right when looking top down, the ports are: return, vent and feed.

Line routing:

Return -> Original return
Vent -> Small port on the tank pressure relief valve (on the same barb that feeds the carbon canister, perpendicular of the 5/8 line)
Feed -> Original feed

*** Wiring ***
The connection to the new cars is different than the '93 - '97. I am unsure about the connectors on the '98, but I believe they are the same as '99 - '02. To make the system function, You will need to cut and solder and heat shrink the plug from the old tank onto the new harness. The new tank has two connections on the sending unit: Fuel pump/fuel level sensor and a pressure sensor. Looking at the sender installed, the pressure sensor is on the left. You can also verify by looking at the underside of the sender, the pressure sensor has no connections. The '93 - '97 does not use the pressure sensor. I opted to leave the plug and wires as part of the harness (to keep the plug connection clean and in case I had a use for the sensor later), so I chopped the wires about 5 inches before the car side plug then taped them down to the harness. For the fuel pump and sensor plug, you will need to solder the pump power, to pump power, fuel sensor to sensor, then combine the pump ground and sensor ground together. Here is the pin out:

Sender Side Pin / Wire Color / Function / LT1 Car Side Pin
1 / PPL / Fuel Level Input / 2
2 / GRY / Fuel Pump Control / 1
4 / BLK / Fuel Pump Ground / 3
3 / BLK/WHT / Fuel Level Sensor Ground / 3
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Last edited by DrewHMS97SS; 02-09-2018 at 12:41 PM.
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Old 02-08-2018, 06:56 PM   #20
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Re: EVAP / LS1 Plastic Tank / LT1 Metal Tank

*** Parts Needed ***

_ Tank
_ Sending unit
_ Fuel Lines
_ Fuel Pump
_ Fuel Level Sender
_ Filler neck
_ Filler hose with worm clamps (connects the neck with the tank)
_ Filler neck cover
_ 3' 5/8" fuel line with 1 worm clamp
_ 3' 5/16" fuel line with 1 worm clamp
_ One 5/8 barb to 10AN male
_ One 5/16" barb to 10AN female
_ One 6 AN male to 1/8" NTP
_ One 6 AN all male tee
_ Four 6 AN female to 5/16" barb
_ 1' high pressure submersible fuel line (might not be needed depending on bucket and pump)

*** References ***
Fuel Level Sender
Fuel level sensor information, wrong Herko part number
More fuel level sensor info

Fuel Tank
Fuel tank info
Swap on a 98

Fuel Bucket
New V8 fuel bucket, but I cannot speak to the quality New V8 fuel bucket, but I cannot speak to the quality

Fuel Pump
My reference for the aftermarket pump installation
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Old 02-09-2018, 12:36 AM   #21
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Re: EVAP / LS1 Plastic Tank / LT1 Metal Tank

Did you ever check the weight difference of the tanks? Good, write up. Got me thinking of doing it for the next project.

Last edited by Robert Bronner; 02-09-2018 at 12:42 AM.
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Old 02-09-2018, 12:37 PM   #22
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Re: EVAP / LS1 Plastic Tank / LT1 Metal Tank

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Did you ever check the weight difference of the tanks? Good, write up. Got me thinking of doing it for the next project.
Thanks for the kind words. The only thing I still need to verify as that the tank will fill without issue at the full flow at the pump. Before figuring out the pressure valve on the carbon canister, I could not even feed the tank with a gas can or siphon pump. That portion works now. I hope to have the car out tomorrow with an answer.

I was more stressed about trying to get the tank in the car due to the limited time on the moving pass, and I have limited space in the garage. I tossed the old tank before I realized that there was so much disagreement with the actual weight differences, otherwise I would have.

Others have posted that both tanks are 25lbs, but I don't buy it. Just from a difference. I had to put effort into moving the metal tank, where I could easily move the plastic tank around one handed using the hole for the sending unit.
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Old 02-12-2018, 10:27 PM   #23
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Re: EVAP / LS1 Plastic Tank / LT1 Metal Tank

Got the car out over the past few days and to the gas station twice. Slow filling. Anywhere from 1/8th to 1/2 gallon between squeezes of the pump. The situation is livable, but not optimal. Running the tank seems solid. It did throw a EVAP flow code, but I have not had a chance to test the solenoid. Maybe tomorrow. More to come as I figure it out.
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Old 02-12-2018, 11:56 PM   #24
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Re: EVAP / LS1 Plastic Tank / LT1 Metal Tank

The air/vapor has to get out of the tank in order to fill the tank. It would appear the metal tank fill pipe had an internal vent pipe that brought the vapor to the top of the fill pipe, so the big rubber bellows on the nozzle, pressed against the top of the tank, could collect the vapor into the station's vapor recovery system. The big rubber boots are/we’re common primarily in urban areas that had the vapor collection requirement. They have them on all pumps here in NJ. When I lived in CA near Los Angeles, they had the boots, and I see them on urban area pumps all over the country.

In 2012 the Federal EPA indicated they were phasing out the rubber boot systems, because “70% of the vehicles on the road are equipped with on-board systems to capture the harmful vapors". Most rural areas I have travelled to do not have the rubber boot vapor collection systems.

Just out of curiosity, would it have been possible to cut the fill pipe off the metal tank, and use is with a coupling to the plastic tank?

If there is no internal pipe in the plastic tank's fill pipe, there has to be some feature in the plastic tank to collect the vapor at a high volume rate. The big black circular device may be part of the "on-board" system. But that is an unknown. The large diameter of the rubber line coming out ot the black device would indicate it is capable of handling a volume of vapor similar to the internal pipe in the metal fill pipe.

How about.... pull the rubber hose off the metal "tee" with the clamp, run the hose up the fill pipe with zip ties to the top, and see if that allows you to fill the tank at an increased rate. If it does, tee the hose into the plastic fill pipe above the nozzle diameter baffle.

I just can’t follow the logic of the vent piping as outlined in red.
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Old 02-14-2018, 05:43 PM   #25
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Re: EVAP / LS1 Plastic Tank / LT1 Metal Tank

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Just out of curiosity, would it have been possible to cut the fill pipe off the metal tank, and use is with a coupling to the plastic tank?
Not for the venting purpose that I can see. If you look at the pic of the tank in the car, you can see the valve in the tank where the filler neck connects. It looks like a cone facing out the neck connection, and prevents anything from coming out of the tank. Additionally, if that was the case, I would need to make a bracket to hold the filler neck in place.


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Originally Posted by Injuneer View Post
If there is no internal pipe in the plastic tank's fill pipe, there has to be some feature in the plastic tank to collect the vapor at a high volume rate. The big black circular device may be part of the "on-board" system. But that is an unknown. The large diameter of the rubber line coming out of the black device would indicate it is capable of handling a volume of vapor similar to the internal pipe in the metal fill pipe.

How about.... pull the rubber hose off the metal "tee" with the clamp, run the hose up the fill pipe with zip ties to the top, and see if that allows you to fill the tank at an increased rate. If it does, tee the hose into the plastic fill pipe above the nozzle diameter baffle.
I think you are right about the larger black tube (5/8 OD) on the tank handling the off gas. When the LT1 carbon canister had the valve, the tank would not even fill as fast as as a siphon pump pulling out of a bucket before it puked fuel out of the filler neck. As it sits now, it does not puke fuel at all, but cuts the pump off.

I can perform the majority of your test suggestion without dropping the tank by disconnecting the tube you mentioned at the pump from the carbon canister. This should allow the connection to be completely open to the air, which hopefully will make the difference.

If so, I think that I am imagining the same solution as you, but I would place a tee near the carbon canisters connection of the 5/8 tube, then drill into and tap the metal filler neck at the top behind the splash guard and add an AN fitting, then run from the tee in the 5/8 line to AN fitting. Thoughts?



I need to burn more gas so I can test pulling the line. The only other restriction is the 5/8 to 5/16 AN reducer. I might be able to unscrew that connection if disconnecting from the carbon canister still does not work to have a full open 5/8 from the tank open. If it gets to that, I will run the 5/8 line all the way to the canister, then tee to the top of the filler and reduce the size after the tee to the carbon canister.
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Old Yesterday, 02:27 PM   #26
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Re: EVAP / LS1 Plastic Tank / LT1 Metal Tank

The fueling issue is resolved. In testing, I pulled the line from the vent and pressure valve coming from the tank off the carbon canister, and fuel pumped normally. There was fairly significant air flow off the line.

I relieve the pressure at a faster rate, I tapped into the fuel neck behind the inner wall where the pump nozzle goes, and made a tee into the vent line. I could have used a brass connection for significantly less money, but wanted to be able to disconnect individual parts of the line should I need to work on the tank or canister. Here is the actual install, along with a Paint diagram (that could have been done by a third grader).









This mod replaced the cut piece of hard plastic line that was used to go from the 10 AN fitting to the rubber line on the canister. The system now has rubber line from the 10 AN fitting, which reduces the line to 5/16", with 6 AN to 5/16" barb fittings. The neck has a 6AN to 1/8" NTP, which connects to a 6AN to 5/16" barb fitting. Parts list updated.

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