cdm747

I am replacing my fuel pump and sending unit on my 88. The kit came with a large O ring about 1/4" in diameter and maybe 4". What the heck is this for?

WVZR-1

Sounds as if you've purchased a very generic pump that's used also for for other GM's and they just toss in the seal for the conventional tank also. If tank sender and pump were serviced in one package then the pump is likely used for multiple install options.

Honordebt

Good Afternoon,
Just a little more detail...
In some cases, the fuel pump is mounted on a stalk inside the gas tank. This "tank unit" has the pump, sock/screen and float/sending unit all on 1 fixture.
The pump can sometimes be serviced or replaced separately from the float/sending unit.
This "O" ring seal is for the mounting point of the entire tank unit to the gas tank itself, yours may not be bad or have this type of "tank unit"
If it does ... use the new one and carefully clean the mating surfaces before re-installation.
I have an 87 and have not had this particular joy yet but I've seen these across a bunch of GMs.
Hope this helps and Good Luck??

cdm747

It came with the pump which was separate from the sending unit when it came in. I figured that was the case. At any rate I put it all back together and fixed two problems. My fuel gauge works correctly now and the car starts right up where before I would have to turn it over a little more than I thought it should before it would start. It also holds pressure for a while at the fuel rail which tells me that the check valve was not holding. I think this was the original assembly that I replaced since it had the pulsator on it and there was a parts tag on the outside of the tank lines that looked like a factory parts tag.

JustinAUS