99+ Plastic Tank Swap in 98 Trans Am Write Up
08-20-2015, 07:45 PM
#1
Teching In
Thread Starter
iTrader: (1)
Join Date: Mar 2010
Location: Apex, NC
Posts: 18
Likes: 0
Received 0 Likes
on
0 Posts
99+ Plastic Tank Swap in 98 Trans Am Write Up Hi All,
I've swapped a 99 Plastic gas tank into a 98 and removed the metal tank and though I'd share my experience. I did a lot of research before trying to do this and unfortunately never came up with a write up or exactly what needed to be done.
First off, the reason I went through all of this work was because I had gas coming out my filler neck and running down the side of my bumper, ruining the paint on the bumper. I replaced the gas cap 3 times, filler neck flap looked fine, replaced the charcoal canister, checked the tank pressure regulator, and had no codes. GM techs could not figure it out and I was stuck. Any time I had a full- 3/4 full tank gas would come out the filler neck while going around corners. I've found that other people had this issue but no one seems to know what caused it. See below.
Next, I found what seemed like people doing this conversion but the most common problem was that people claimed the fuel gauge would not work. They claimed that 98 had a 0-90 ohm fuel sender and 99+ used 40-240 ohm. Well so that everyone knows the 98 does use a 40-240 ohm (or close to this) fuel sender. I tested the old 98 fuel sender along with a new 99 fuel sender and both match up so I knew this would work.
Also, if you look at the top of a 98 fuel pump you will see four lines and a 99+ pump only has 3 lines. Well that extra line goes to a pressure regulator attached to the car that can be removed and not used with the new tank. Also the 98 has a charcoal canister by the fuel filler neck where as the 99+ tank has this on top of the tank.
What I did and my recommendation is go to a junk yard and remove the entire plastic tank assembly with all the plastic fuel lines and charcoal canister. You can't use the 98 plastic fuel lines as they are bent differently. The wiring harness could be used but would need to un-taped and re-taped so that the ends would be long enough to go where they need to, again its easier if you can just get the harness at the junk yard. The 99+ fuel lines and harness will all connect to the car as intended.
*The plastic tank along with canister and lines weighs the same as the metal tank and lines. Some people wanted to do this swap because you would think the plastic tank is lighter. Well I weighted them and they were both 25.5lbs. Only thing I can think of is that the plastic tank is slightly larger 16.8 gallons vs 15.5 gallons, maybe GM did not want to mess up the weight of the vehicle. But don't do this swap thinking the plastic tank weights less.*
Here is the top of the 99+ tank, bought a new fuel pump on RockAuto, the 3.8L and 5.7L pumps are different, but everything else is the same. As you can see, here is the charcoal canister, wiring harness, and fuel lines, all taken from a v6 car in a junk yard.
The hardest part of the whole project was removing the metal gas tank. The filler neck is welded to the tank and can't be removed. It goes through a hole and out to the side of the car so when trying to drop it, it gets stuck in this hole. According to the service manual you are supposed to remove the rear end. Well I highly recommend just cutting the filler neck if you know you are not going to use the tank again. We dropped the rear end a far as you can without removing it, and removed the passenger side spring, and still bent the hell out of the filler neck to remove it.
Here is where the new tank will go, you can see where the harness connects and where two lines connect. These are the lines that go to the charcoal canister and vent valve on the tank.
Make sure you get the filler neck from the junk yard car as it is actually removable and makes it a hell of a lot easier to install the new plastic tank. we also removed the tab that is welded to the car so that we had something the filler neck will connect to just like the factory planned.
I pop riveted the bracket on the car, and this is the hole that I was talking about earlier.
Also take the plastic piece from the junk yard car where the fuel cap goes as this is different between 98 and 99+
Its in the car and everything works great. The new 99+ fuel pump works fine and gas guage works great. I get more mileage out of a tank of gas since it is larger and am getting better fuel mileage because I'm not losing gas. The car is having its rear bumper painted as we speak. The only issue is I was never able to diagnose why the fuel was coming up the filler neck and out the cap. The new plastic tank has a flapper valve at the end of the filler neck that closes whenever gas is not going into the tank. I actually cut the filler neck of the metal tank expecting to see a flapper valve that was stuck open, but it does not have one. It has a smaller metal line running in the middle of the filler neck and this smaller metal line empty's into one side of the tank while the gas goes into another part of the tank, but there seems to be nothing mechanical that is keeping gas from coming up the filler neck when going around a corner. Its still a mystery
I've swapped a 99 Plastic gas tank into a 98 and removed the metal tank and though I'd share my experience. I did a lot of research before trying to do this and unfortunately never came up with a write up or exactly what needed to be done.
First off, the reason I went through all of this work was because I had gas coming out my filler neck and running down the side of my bumper, ruining the paint on the bumper. I replaced the gas cap 3 times, filler neck flap looked fine, replaced the charcoal canister, checked the tank pressure regulator, and had no codes. GM techs could not figure it out and I was stuck. Any time I had a full- 3/4 full tank gas would come out the filler neck while going around corners. I've found that other people had this issue but no one seems to know what caused it. See below.
Next, I found what seemed like people doing this conversion but the most common problem was that people claimed the fuel gauge would not work. They claimed that 98 had a 0-90 ohm fuel sender and 99+ used 40-240 ohm. Well so that everyone knows the 98 does use a 40-240 ohm (or close to this) fuel sender. I tested the old 98 fuel sender along with a new 99 fuel sender and both match up so I knew this would work.
Also, if you look at the top of a 98 fuel pump you will see four lines and a 99+ pump only has 3 lines. Well that extra line goes to a pressure regulator attached to the car that can be removed and not used with the new tank. Also the 98 has a charcoal canister by the fuel filler neck where as the 99+ tank has this on top of the tank.
What I did and my recommendation is go to a junk yard and remove the entire plastic tank assembly with all the plastic fuel lines and charcoal canister. You can't use the 98 plastic fuel lines as they are bent differently. The wiring harness could be used but would need to un-taped and re-taped so that the ends would be long enough to go where they need to, again its easier if you can just get the harness at the junk yard. The 99+ fuel lines and harness will all connect to the car as intended.
*The plastic tank along with canister and lines weighs the same as the metal tank and lines. Some people wanted to do this swap because you would think the plastic tank is lighter. Well I weighted them and they were both 25.5lbs. Only thing I can think of is that the plastic tank is slightly larger 16.8 gallons vs 15.5 gallons, maybe GM did not want to mess up the weight of the vehicle. But don't do this swap thinking the plastic tank weights less.*
Here is the top of the 99+ tank, bought a new fuel pump on RockAuto, the 3.8L and 5.7L pumps are different, but everything else is the same. As you can see, here is the charcoal canister, wiring harness, and fuel lines, all taken from a v6 car in a junk yard.
The hardest part of the whole project was removing the metal gas tank. The filler neck is welded to the tank and can't be removed. It goes through a hole and out to the side of the car so when trying to drop it, it gets stuck in this hole. According to the service manual you are supposed to remove the rear end. Well I highly recommend just cutting the filler neck if you know you are not going to use the tank again. We dropped the rear end a far as you can without removing it, and removed the passenger side spring, and still bent the hell out of the filler neck to remove it.
Here is where the new tank will go, you can see where the harness connects and where two lines connect. These are the lines that go to the charcoal canister and vent valve on the tank.
Make sure you get the filler neck from the junk yard car as it is actually removable and makes it a hell of a lot easier to install the new plastic tank. we also removed the tab that is welded to the car so that we had something the filler neck will connect to just like the factory planned.
I pop riveted the bracket on the car, and this is the hole that I was talking about earlier.
Also take the plastic piece from the junk yard car where the fuel cap goes as this is different between 98 and 99+
Its in the car and everything works great. The new 99+ fuel pump works fine and gas guage works great. I get more mileage out of a tank of gas since it is larger and am getting better fuel mileage because I'm not losing gas. The car is having its rear bumper painted as we speak. The only issue is I was never able to diagnose why the fuel was coming up the filler neck and out the cap. The new plastic tank has a flapper valve at the end of the filler neck that closes whenever gas is not going into the tank. I actually cut the filler neck of the metal tank expecting to see a flapper valve that was stuck open, but it does not have one. It has a smaller metal line running in the middle of the filler neck and this smaller metal line empty's into one side of the tank while the gas goes into another part of the tank, but there seems to be nothing mechanical that is keeping gas from coming up the filler neck when going around a corner. Its still a mystery
Last edited by 198firebird; 08-20-2015 at 09:23 PM.
06-06-2017, 08:27 PM
#6
I just did this. Exactly the same thing, as my 1998 was leaking fuel out of the filler neck down the side of the car. Tried two new gas caps that never fixed it. After removing the tank, I tried to pour the remaining old gas out of the filler neck, but it actually would not pour out.
When washing out the new tank, I saw that it also would not pour out, even with the filler neck removed, due to a one way valve. Perhaps this valve is deeper inside the metal tank? The other thought I had, is that the white vent line may not be bleeding off pressure correctly, so the fuel is being "pushed out" by the return line. Unfortunately, GM no longer offers this valve part. It is the one located on the brake line bracket before the soft line(s) go to the axle.
I also put in a 2002 PCM, so I hope I do not have gauge problems. I have read about the ohms range having to be changed by a tuner, so this may be necessary for me and you taws6ali. I won't know until I put the engine back in.
When washing out the new tank, I saw that it also would not pour out, even with the filler neck removed, due to a one way valve. Perhaps this valve is deeper inside the metal tank? The other thought I had, is that the white vent line may not be bleeding off pressure correctly, so the fuel is being "pushed out" by the return line. Unfortunately, GM no longer offers this valve part. It is the one located on the brake line bracket before the soft line(s) go to the axle.
I also put in a 2002 PCM, so I hope I do not have gauge problems. I have read about the ohms range having to be changed by a tuner, so this may be necessary for me and you taws6ali. I won't know until I put the engine back in.
Trending Topics
06-25-2020, 05:33 PM
#10
Either way, it is the same process: swap it. If it works, you're done. If it doesn't work, there is one more step at the end: plug in a programmer to the OBD2 and set the right sender resistance and voltage values in the table. Yes, you have to take it to a shop for this if you do not have software yourself, but I would not let it stop you from doing the swap.
In my case, I have 2001 ECU, so it may work different for you. When I put mine in, the gauge worked backwards. The more I drove, the more full it became. I was going to leave it, but I realized that once it got to full, it would start spilling gasoline all over the road
In my case, I have 2001 ECU, so it may work different for you. When I put mine in, the gauge worked backwards. The more I drove, the more full it became. I was going to leave it, but I realized that once it got to full, it would start spilling gasoline all over the road
The following users liked this post:
White94Bird (06-29-2020)
06-25-2020, 11:05 PM
#11
Launching!
The sender resistance is the same 98 and 99+.
The confusion is when you swap to a newer pcm. The pcm reads the resistance across the sender circuit and sends a signal to the gauge - in the 99+ gauges, the the fuel gauges work backwards on the signal from the pcm, so the pcm needs to be programmed to work backwards.
Look up fuel segment swap.
The confusion is when you swap to a newer pcm. The pcm reads the resistance across the sender circuit and sends a signal to the gauge - in the 99+ gauges, the the fuel gauges work backwards on the signal from the pcm, so the pcm needs to be programmed to work backwards.
Look up fuel segment swap.
The following users liked this post:
White94Bird (06-29-2020)
07-20-2020, 04:33 PM
#12
Teching In
Join Date: May 2011
Location: michigan
Posts: 48
Likes: 0
Received 0 Likes
on
0 Posts
98 ws6 with 99+ pcm/ engine harness 99+ fuel sender/ harness I just did this. Exactly the same thing, as my 1998 was leaking fuel out of the filler neck down the side of the car. Tried two new gas caps that never fixed it. After removing the tank, I tried to pour the remaining old gas out of the filler neck, but it actually would not pour out.
When washing out the new tank, I saw that it also would not pour out, even with the filler neck removed, due to a one way valve. Perhaps this valve is deeper inside the metal tank? The other thought I had, is that the white vent line may not be bleeding off pressure correctly, so the fuel is being "pushed out" by the return line. Unfortunately, GM no longer offers this valve part. It is the one located on the brake line bracket before the soft line(s) go to the axle.
I also put in a 2002 PCM, so I hope I do not have gauge problems. I have read about the ohms range having to be changed by a tuner, so this may be necessary for me and you taws6ali. I won't know until I put the engine back in.
When washing out the new tank, I saw that it also would not pour out, even with the filler neck removed, due to a one way valve. Perhaps this valve is deeper inside the metal tank? The other thought I had, is that the white vent line may not be bleeding off pressure correctly, so the fuel is being "pushed out" by the return line. Unfortunately, GM no longer offers this valve part. It is the one located on the brake line bracket before the soft line(s) go to the axle.
I also put in a 2002 PCM, so I hope I do not have gauge problems. I have read about the ohms range having to be changed by a tuner, so this may be necessary for me and you taws6ali. I won't know until I put the engine back in.
after swapping everything over i couldn’t get the sensor to work at all, also back in 2017 the tech at tuned by frost mail in tune explained to me that he couldn’t tune for it to work “I mentioned swapping van tables for fuel level reading” and as he explained that there nothing on the tuning side that could work and was redirected to check the wires instead, he further explaining that the evap on 99+ pcm run with fuel level sensor and that I would have to delete it for My fuel sensor to work.
this is all somewhat confusing and overwhelming since i don’t have allot of experience with electronics but it’s a learning process and hopefully I can get some of these issues fixed this year.
I have my evap deleted and I am soon to put it my tank so I was thinking of connecting everything with the tank out and I’ll keep you guys posted.
07-21-2020, 04:03 PM
#13
10-24-2021, 08:34 PM
#15
Teching In
Thread Starter
iTrader: (1)
Join Date: Mar 2010
Location: Apex, NC
Posts: 18
Likes: 0
Received 0 Likes
on
0 Posts
Sorry just seeing this now. Hope the swap was successful. I'll be honest don't remember exactly where the pressure regulator was... either on top of the tank or bolted in the same area as the lines/wiring. It was the 4th line from the metal 98 pump/tank that is not used on the 99+ and was discarded and removed, from what I remember.
11-09-2021, 02:51 AM
#16
Does anyone have a solution that doesn’t involve dropping the tank? I have the same issue, but i believe I’m running a 1999+ cap and does not seem to properly tighten as i have to push and spin until it does move freely. Also does anyone know where i can get a 1998 specific gas cap
12-07-2021, 05:56 AM
#17
Staging Lane
Join Date: Aug 2013
Posts: 56
Likes: 0
Received 0 Likes
on
0 Posts
Sorry to bring this thread back up... but does anyone know if the 1999+ Fuel Pump Assembly will fit into a metal 1998 gas tank? I like the 1999+ Pump Assembly better than the 1998 style pump assembly. I seen it was mentioned that the 4th line on the 98 Pump Assembly can be simply discarded? Where does that 4th line go to?
The 99 Style looks like it just has Vent/Feed/Return. Which I have no problem putting into my 98, if I can just discard the "extra" 4th line. Does anyone know if the diameter of the ump assembly's/fuel tank opening at the top is the same across 98 to 99+? Any help is appreciated. I have been going through fuel pumps like crazy on my 98, and I believe it's a combination of poor electrical (which I am tracking down), and poor fuel pump design of the 98.
Thank you all!
The 99 Style looks like it just has Vent/Feed/Return. Which I have no problem putting into my 98, if I can just discard the "extra" 4th line. Does anyone know if the diameter of the ump assembly's/fuel tank opening at the top is the same across 98 to 99+? Any help is appreciated. I have been going through fuel pumps like crazy on my 98, and I believe it's a combination of poor electrical (which I am tracking down), and poor fuel pump design of the 98.
Thank you all!
01-24-2024, 07:29 PM
#18
Registered User
Join Date: Jan 2024
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
Tank line hook up 00 to 95 
Currently installing a plastic tank from a 2000 into a 95Z 28. I figured out the sensor ohms replacement part. Question is the vent circled in blue from the canister. Cut the end off and Does that hook directly to the line on the car circled in blue on the right picture Also the other pigtail circled in red does that just not get hooked up and used for the canister? is it as simple as hooking up the three lines supply, return and vent to the car and just using the one harness ?
