*** Updated 2/8/2018 ***

There has been a lot of conversation (mostly a decade ago) about the possibility of swapping the plastic tank from '99 and newer Camaro's and Firebirds into older models to replace the metal tanks with the solid filler neck. The main reason that I went this route was based on cost to replace the metal tank when I discovered that my tank was rusted out. This swap wasn't as difficult as I thought it would be, but does involve some ability to complete. If you are comfortable mechanically with removing the gas tank, confident with checking for fuel leaks and resolving those leaks, soldering and testing wires, and the use of a rotary cutting tool, then this is a swap you can do yourself.

*** Disclaimer ***
The information provided in this post is meant to be a digest of what I went through to get a plastic tank into my car. By choosing to use this reference and attempt the plastic tank swap, you acknowledge that I take no responsibility for damage to your car.

Scroll down for pics and a wrap up. There was too much text to have it all in this post.

*** Original Post ***

EVAP is the last thing that I need to figure out on the swap to a plastic tank. I am wondering if someone could confirm my thought (which is simplistic in nature, so please forgive any incorrect assumptions).

Looking at the differences between systems, the LS looks to have an EVAP solenoid in engine bay that connects to the TB, as does the LT. The LT has an additional vacuum switch that connects to the EVAP solenoid, then connects to the carbon canister via the fuel line bundle.

Is the solenoid on the plastic tank just the newer version of the vacuum switch on the LT? If so, can I remove the vacuum switch, connect the EVAP solenoid to the fuel line bundle, then connect the line in the rear to the new tank? If these components are the same, I could then run wires from the tank to the vacuum switch connection.

I have a 99 V6 (no FPR) plastic tank with all of the connections. I will wire in my 97 harness to the sending unit, and have a replacement fuel level resistance board on order. It seems like all of the lines on the plastic tank will mate up to the existing hard lines. The carbon canister will take a little bit of rework, but not much.

 

As best I know, the vent solenoid added on the LS1 tank/canister assembly was added to isolate the canister or block the canister vent, so engine vacuum via the EVAP purge line could be used to run a vacuum test on the fuel tank, and a vacuum test on the purge line. There is a pressure sensor on top of the 99 sending unit. You want to attach the vent line for your existing EVAP system to the vent line from the fuel sending unit. Don’t use the 99 canister, bypass it completely.

 

Connectionwise, that should be easy enough. Ill plan the routing out tomorrow. I know about the pressure sensor, but I thought 98 and older were not equipped. Are you suggesting I run leads to the sensor and monitor changes while the car is running?

Also, forgive my ignorance, but can you give some detail as to how I would accomplish the vacuum test? Ill try to post a proposed connection map tomrrow, which may make description of the test easier for me to understand.

Thanks sir!

 

This is what I am thinking. Thoughts?

 

Not sure what the plan is? You appear to be going from the sending unit to something called the vent system (have no idea what that circular piece is), via something called the overpressure relief valve, then deleting the canister and the vent solenoid. What does your "cut link to LT1 carbon canister" attach to? How does the vapor get up front to your engine? If you delete the LS1 canister, and appear to be bypassing the LT1 canister, how does the system work?

How do you regulate pressure in the tank, if you bypass the LT1 pressure control valve, which is in the line to the LT1 canister? Is that circular piece labeled "vent system" a vent pressure regulator, in addition to the over pressure relief valve.?

To me, it's most logical to keep the total LT1 EVAP system intact, by connecting the LT1 vent line to the vent Connection on the sending unit. Why connect to any part of the 99 system? But I freely admit I have no detailed knowledge of the 99 system. If you feel you have discovered some feature that allows you to hook it up the way you want, then do it that way. But I'm still not sure what you are doing.

 

So from reading here, the center line from the sending unit and the black circular item are part of the tanks vent system. The center line in the sending unit actually connects in a tee coming from the black round connection below it. I would then cut the line coming from those vent connections where it u-turns into the LS1 canister and run it to the complete LT1 EVAP canister using the same line that connected to the LT1 tank (my arrow made that hard to see). I am thinking to cut the quick disconnect from the canister side, remount it near the cut, then use this to connect the lines together:



The over pressure relief valve seems to be the LS1 answer to the vent line on the LT1 tank that connects on the bracket that the brake lines hit before going to the rear end. It is an open tube that loops over and down to the ground, no connection.

Here is the tank as it sits now:

 

Here is the vent line on the plastic tank highlighted:



Updated progress:


I am currently waiting on a replacement fuel level sender and the fittings to connect the carbon canister.

I found from third gen forums a ton of info on the swap. After seeing someone try to make a sender, then buy a number of units, they narrowed down the 1997 3.8 N/A Grand Prix sender, which is 0-90 ohm and is a direct fit in the LS1 bucket. The GM part cost is high, and worst case, the replacement from GM is $130ish (part number #25319676). In another third gen forum, I found an aftermarket company, Herko, that sells these at a much lower cost ($14). However, the forum recommended the wrong part which had a 240/30 ohm span, the same as the unit previously in the tank. I contacted the company for a return and have and ordered the what they claim is the right module based on the GM part number. The correct module was a bit more ($24), and hopefully will save me $100 over the GM part.

For the wiring harness, the biggest difference is that the sender on the metal tank is 3 wire, and the sender on the plastic tank is 4. This equates to an extra ground for the fuel level sender, which on the newer cars goes back to the PCM. In the metal tank, the pump and sender are attached to the same ground. For the extra pressure sensor on the top of the plastic sensor, I clipped the wires about half way down the harness, then taped the wires down to the bundle to keep the connector clean, and to give me the option to use the sensor later if there was a need. The EVAP purge solenoid wire was removed from the bundle. I then cut the car side connector off, and spliced in the connector from the old tank, connecting the sender and pump ground together.

If this to works, my totals on the project will be significantly less than ordering a tank online. Hawks has a tank with a sending unit for $200, but shipping doubles the cost and then some to $440. The other junk yards that I called had plenty of tanks for between $20-$50, but all had been drilled to quickly drain the fluids.

Cost so far:
Tank, sending unit, lines, filler neck: $55
Aeromotive Fuel Pump: $115
Fuel level sender: $24
Fittings for the carbon canister: $28
Total: $222

Counting a new pump if I ordered the tank from Hawks, this is $330 less. Granted there was time spent on pulling the tank at the yard, figuring out the fuel level sender and the wiring, wiring, and modifying the bucket for the new pump (similar to Walbro 255 install), but over all it hasn't been too bad. The tank definitely weights less than the steel tank. I saw some posts of people saying it was the same, and while I didn't weight my tank before it was disposed of, there was effort expended to move it around where the plastic tank is just a bit awkward. Add the removable neck and extra gallon-ish of gas, I think this is a pretty awesome accidental upgrade.

Do you see any issues with the canister connection? Do you think I will run into anything else?

 

I have all of the lines figured out, but am still waiting for the replacement sensor. Add another $40 in fuel line to the project. $10 line fuel line for the EVAP/vent connection, and $30 for submersible line on the fuel pump. The bucket mod extended the length of the pump a bit which made the line that came with the pump about an inch short.

EVAP fittings and connection






Pump/bucket






Filler neck

 

I received the fuel sending sensor yesterday, and verified that the resistance board is indeed 0-90. Unfortunately, the bracket it was in not the same as the original. It did have the same slots that hold the unit to the bucket, but the clips were about 1/4" out of position.

In hunting for pictures, I have discovered that the V8 and V6 buckets are slightly different. If the tank was out of a V8, the sensor would have fit perfectly.

So the differences I am currently aware of now between the V6 and V8 are:

Fuel Pressure Regulator in tank:
V6: no
V8: yes

Venturi pump to fill fuel bucket
V6: no
V8: yes

Fuel level sensor mount

Here are pics of the two senders:
V8


V6


For me to get this to work in the V6 bucket, I had to swap the resistance board to the original bracket:

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

In retrospect, it would have been easier to glue the bracket in place, but wanted to retain the ability to switch the sensor later if needed. I also could have added a small bolt below the sensor to keep it in place since I already modified the bucket for the Aeromotive pump.

 

Person on ltxtech.com asked about fitting the plastic tank into his 97. Gave him a link to this thread.

 

I will modify the opening thread will all of the detail I have once I verify the tank is in the car and working. I am hoping the tank goes in today as the sending unit is together.

The only thing that I haven't tested is fitment of the feed and return lines. I am confident that the feed line will fit since the position of the fuel filter has not changed, but am unsure about the return line. A quick fit looked like it will be fine, but you never know until you try it. If not, cool AN fittings and hose will be purchased, just pricey.

 

The tank is in, all of the lines were perfect length for stock connections. The fuel gauge is working, and the car fired without hesitation.



The tank installation process is so much nicer than the metal tank. Getting the hose clamps on the filler hose was a bit of a pain, but pales in comparison to getting the solid neck in and out.

There is one issue: it doesn't take fuel at a very quick pace. I spilled about 1/4 a gallon out of 5 when trying to siphon into the tank. Looking more at the setup, the tank had the vent from the sending unit with a direct connection to the carbon canister. This makes me believe that the large black circle in the tank with the 5/8 connection going to the canister is the check valve for the newer system. I believe that the valve seen in the pic that is from the older carbon canister is hindering air flow from the tank as fuel fills. I am going to remove the valve from the carbon canister and connect directly to the tank, then try fueling again.

I wish I could find more detail about the tank components, as this is purely a guess based on what I am seeing. It seem's that all of the people who have done this swap and documented it just remove the EVAP, or are doing a full LS swap.

 

Temporarily bypass the stock 97 pressure control valve, to see if that is the restriction.

On your 97 was there a hose that connects to the 97 fill pipe near the top? If so, where does that hose originate?

How does the diameter of the 99 fill tube compare to the 97 fill tube?

Separate subject.... could you measure the outside diameter of the 97 fill tube for me. Someone asked a question about how much fuel would be in the fill pipe if it was filled to overflowing.

 

No hose connecting to the filler neck at any point.

That was the plan and I should be able to test this without having to remove anything to get to the hose.

The diameter from memory doesn't seem any smaller than the metal tank. A side note here, I cut the filler neck off so it would fit in the trash, and there are 2 tubes in the neck. I would guess the internal tube being 5/8's OD.

I could get you the diameter of the fill tube on the new line, since the old tank is about 4 days gone.

 

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?

Don’t worry about the diameter of 97 fill pipe. It wasn’t that important.