FoggyStyle

iTrader: (5)

Hi all,
I'm currently in the middle of a conversion on my '87 Monte Carlo SS. I've got a 5.3 truck engine with a Walbro 255 in-line pump, and I've found there are a couple routes I could go with the fuel system:

1. The G-body has a 3/8" and 1/4" supply/return line for the fuel system, stock. I've read that the LS motors require a 5/16" return line. Has anyone run into this situation? Is a 5/16" line imperitive, or would the system actually work with a 1/4" return? They're on the same side of the car, so I'm asking basically for the sake of convenience.

2. The funny thing is, the car has a 5/16" vapor line (go figure). I'm tempted just to use that for the return, however I'm not sure how these lines terminate in the fuel tank, and whether or not they can be modified.

3. What about eliminating the 1/4" vapor line altogether? I'll have to buy a new sending unit for the tank, regardless. It appears there are two versions available: the 3-outlet, and the 2-outlet. The latter does NOT have provisions for a vapor line. Would it be "wrong" to go this route, and use a vented fuel cap? From the images, it appears the 2-outlet version has a 3/8" supply and a 5/16" return. I need a way to verify this. Just curious, has anyone seen/used this sending unit?

Here's the stock config NAPA #BK 2003449:


Here's the 2-outlet version NAPA #BK 2003627:


If anyone has input, I'd love to hear it. Thanks!

cutlass_455

iTrader: (2)

I have all the same questions. I hope there are some good help that comes to our rescue.

Foggystyle, where did you mount your pump?? Have you mounted the motor and trans yet??

I would suggest that if you wanna change your sender, just go with the 3 port one, it leaves options for the future.

sandking

iTrader: (7)

I am not familiar with the in-tank stuff, but -6 (3/8") is plenty for 500hp supply and return. You could step up to -8 (1/2") if you are worried, but not needed. I am using -6 stuff on my 5.3 conversion.

FoggyStyle

iTrader: (5)

Yeah, I'm not too worried about the 3/8" (-6) supply line. I'm just wondering if the 1/4" return would be sufficient. I suppose it would be at WOT. haha!

I haven't mounted my pump yet, but I plan on placing it inside the frame, just in front of the right rear wheel.

I've done plenty of Mock-ups for the Engine and Trans, and I've decided to use the CTS-V oil pan, with ebay-purchased 1" setback mount adapters. The transmission crossmemeber will have to be fabricated, or heavily modified.

sandking

iTrader: (7)

I am sure the 1/4" will be ok for the return. There is not much volume or any pressure.

G-Body

iTrader: (4)

What model/option package/engine are you putting in to get those sending units??? The look like they are for a carb`ed monte. The carb`ed montes did not have a return line to my knowledge. The EFI monte`s had a return line, but IIRC that was a 5/16 return not a 1/4".

You cannot use the vapor line as a return because there is a baffle system on the end of that line inside the tank that is designed to stop fuel from flowing through it.

80bu

I'm doing the the swap also so i am curious wich sending unit to get from napa?

LS1MCSS

iTrader: (18)

The correct sending unit for an intank pump is ac delco p/n 25004938. Be prepared to pay a lot for it, though.

FoggyStyle

iTrader: (5)

They are indeed for a carb'd Monte, which is what I have. I don't know why it has a return, but it does, and it is indeed 1/4." Every parts website shows those two in-tank units for my application: 1987 Monte Carlo SS, VIN G. If I could find a return line for a fuel-injected 4.3 Monte, that would be perfect.

The fuel pump on the engine block has two lines, one is 3/8" and the other is 1/4." The driver's side has the charcoal canister, which is plumbed with a 5/16" hard line that runs back to the tank. It is baffled, just like you said. I pulled the tank and looked it over.

G-Body

iTrader: (4)

Hmm, I didn`t know they used return lines on the carb`ed montes. Wonder why, my el camino didn`t have a return. Maybe it was something for performance?

Project GatTagO

iTrader: (7)

I would use a Corvette filter/regulator mounted just after your in-line pump. Use the 2 line tank top and run a 5/16" line from the regulator to the tank.

Andrew

cutlass_455

iTrader: (2)

I have a truck style rail and it has that regulator at the rail, is there any way I can use that Corvetter style filter/regulator and still use my rail??

G-Body

iTrader: (4)

I guess you could, just cap off the return on the rail......but why would you want to? The fuel regulator on the rail is better for performance. The only reason that GM went to the regulator in the tank for the f-car and in the fuel filter is due to emissions. Having the regulator back there prevents hot fuel from being dumped back into the tank ad lowers evaporative emissions slightly.

cutlass_455

iTrader: (2)

I might install an LS1 intake in place of the truck intake due to clearance issues and my LS1 intake is a non return style.

Its an added cost, but I might use the truck intake and rail and use a return from there just to get the car running and on the road and get a junker hood and hack it up a bit. Then after that switch to the LS1 intake and change the fuel plumbing to match it.