Fuel Sender/Tank problems

[Longshot]

First, great forum! I just registered and this is my first post, but I have searched many threads in the past and there is a lot of good info.

Here's the problem: Gas gauge was not reading properly on '99 GTP (purchased approx 6 months ago). Fluctuating from full to empty. Removed fuel pump assembly and checked sending unit. Confirmed sending unit resistance was bad...dead spots, etc. Purchased new Bosch assembly (unit taken out was original unit and extremely difficult to re-install (see below), so replaced pump and sending unit to avoid having to repeat in future). Installed new unit and gauge appeared to work properly from full tank down to about 1/4 tank. Now it is stuck at 1/4 tank, even with tank empty or fuel.

I have jumpered 50/100/150/200/250 ohm resistors across the connector to the level sending unit, and I get what appear to be proper gauge readings varying from empty (50 ohms) to full (250 ohms), so I believe wiring/PCM/gauge is ok. I have seen some postings on internet that state this is a 45-90 ohms unit, but I think it is 50-250 ohms.

When installing fuel pump assembly via trunk access hatch, it was very difificult due to the need to compress the springs almost completely to get the lock ring in place. Now, inspecting tank from below, it seems the bottom of the tank is 'sucked in' or collapsed upward. So I'm thinking that's the reason for the difficulty in installing the pump assembly, and I'm wondering if the tank deformation has caused the float arm to get caught and stick at roughly 1/4 tank. I added gas, roughly 1 gallon at a time, such that there were about 2 gallons (starting) to around 9 gallons when stopped, and the sending unit resistance stayed steady right around 70 ohms, and the sending unit voltage remained steady around 1.1 Volts. Per my checking with resistors, this voltage seemed to be right for 70 ohms resistance.

So, any thoughts on the 1/4 tank reading? Could the float arm be stuck? What would cause the fuel tank to 'collapse' to begin with? Any advice appreciated.

Thanks

[BillBoost37]

The springs are required to be nearly 100% compressed. That part is pretty normal.

The only thing I can think of is if the strainer sock is under the float.

[Longshot]

Would the strainer sock prevent upward movement? I can see how it wouldn't allow the last little bit of downward movement (1/4 tank worth????). But unless it snags the float, the fuel sender would register ok above 1/4 tank.....

[BillBoost37]

I missed the sitting at 1/4 part. I think it could have gotten snagged. If so..a few smacks to the bottom of the tank may dislodge it or may be visible on teh gauge.

[Longshot]

I might also add that besides the springs being nearly or totally compressed, the job difficulty was greatly increased because the tank surface was slanted relative to the opening. Therefore, as the springs were compressed, the fuel pump assembly would 'kick' off to one side, resulting in the top of the unit not being parallel to the tank opening.

In reality, I'm not sure we (yes it took 2 of us, me (200 + lbs.) placing all my weight down using the handle of a mallet while my son put the retaining ring in place) weren't actually pushing the bottom of the tank down a little bit.

[BillBoost37]

The kicking out part isn't normal.

I also thought..maybe your float filled with fuel and stopped floating.

[Longshot]

I thought about pounding on the tank to try to dislodge. That's when
I realized how 'collapsed' the tank was...when I slid under the car with a rubber mallet. BTW, several hits with the mallet did nothing.

The float is brand new....still conceivable to fill with fuel??

[BillBoost37]

I think it's a sealed bubble. Either that or a super light plastic that floats.

[mrfreez]

Welcome to the forum!
OK, I just replasced my fuel pump a few days ago. I'm unclear if you replaced the whole unit or just the sending unit itself? I replaced the whole unit. A trick I used to compress the pump was I cut a broom handle to wedge between the fuel pump and the top of the trunk. That held down the pump so I could get the ring in place.

Before taking it apart again, did you check the fuel pump fuses? If you didn't disconnect the bettery when you replaced the pump you could have blown a fuse.

[Longshot]

I replaced the entire unit. I pulled the old unit to check the fuel level sending unit and verified it was bad. I put the old unit back in to keep the car drivable while I ordered parts (fuel pump was still ok) and realized how difficult it was to put in. Decided to order complete new unit to avoid having to replace a fuel pump in a few months.

With the new unit in, the fuel pump is fine. Car is running with no problem, just not a functioning fuel gauge. Resistor jumpers across the sending unit connector also resulted in correct gauge readings, so I don't think I am dealing with any blown fuses.

My son is the primary driver, and tends not to fill the tank most of the time, so it becomes difficult to track mileage and know when the tank is nearly empty.

The problem I have now is that I don't know if the sending unit is bad again (probably not likely to fail with a valid, steady reading of 70 ohms) or if the float arm is stuck. Looks like I'll be pulling the unit in the near future. May have an opportunity to try your broomstick method....although my problem is not so much holding the unit down as getting it pushed down to start with. Thanks!

[Longshot]

I've had some input on the fuel sender unit......so anyone have thoughts/ideas/experience with a collapsed fuel tank?
Can a faulty evap valve either cause too much vacuum to be pulled and/or not allow the tank to properly vent?

The fuel system appears to have 2 valves/solenoids related to the evap canister. It's my understanding that 1 opens the line from the engine to the evap canister to pull the vapors from the characoal canister. I'm not sure I understand the purpose of the 2nd one. Does it open to the outside world (when the 1st valve opens) to provide a vent/purge path for air to come into the canister to 'flush' the characoal? If so, is it possible that if the 1st valve is stuck open, the system could pull vacuum all the time, or if the 2nd is stuck closed, then no 'vent' air coming in might result in a vacuum being pulled on the tank? Is it even possible for the engine to pull enough vacuum to collapse the tank?

I am considering pulling the sending unit to verify it is still working properly. If so, and the float still floats, then I must assume the float arm is not working properly do to catching on something. Maybe it would be
time to replace the collapsed tank. However, if there's a problem that collapsed the 1st tank, I don't want to replace until that problem is fixed.