Fuel tank interchangability and a fix vs upgrade question

[zero cool]

I have an 86 K20 and I'm going to be doing some serious upgrades over this next year but I'm in a position where I need to do fuel tank work now and I'm wondering if an upgrade rather than a simple fix is in order.


The eventual plan for the truck is to run a 5.3 out of a 2000 Tahoe. I'm going to be upgrading my fuel system to support this upgrade.

The trouble I'm running into right now is that both of my fuel tanks have enough crap in them that I need to pull them and clean them so the truck will run worth a crap with the stock motor (running a carburetor).



My first question is about fuel tanks. I know there have been changes in the trucks over the years and that the fuel tanks changed be baffled as well as running in-tank electric pumps for the fuel injection. I have dual saddle tanks in my 86 and I'd like to upgrade to newer tanks that support the in-tank pump for the upgrade. My question is this: What year model range tanks will fit in my 86 and give me the option of running the in-tank fuel pumps? I can do some fabrication to make things fit but I'd prefer to find something that bolts in.

Also, do you think it would be better to just do the fuel tank upgrade, with all the upgraded return and vent lines, etc. now since I'm already dropping the tank or would you just clean the stock tanks and run them until the new motor goes in (probably next fall)? I know I can run a pressure regulator to drop the FI pressure down to run in a carburetor while I prep for the motor swap. I'm thinking that doing the tank swap now would keep me from having to drain and swap the tanks again later but if there's really a compelling reason to keep the stock tanks with the carburetor, I'm willing to do that swap twice.

 

[dyeager535]

You should be able to get tanks from an '87 truck that will do what you need it to do. Only thing that really changed are the baffles, and '87 replacements will have them, if made right. Obviously ask before committing to the purchase. The stock K5 EFI tanks are junk since the baffles are plastic, (edit: the saddle tank baffles are surely plastic as well, but since the tank shape is different, they may or may not hold up better than the K5 ones...impact from accidents seem to break these quite easily) and I suspect the size/shape doesn't help them last. Saddle tanks might hold up better. I paid around $120 for an all-steel replacement EFI tank for my K5 a few years back.

The return/vent lines are done through the sending unit, so you'll need the sending units from a truck with the same size/style tanks as you've got now, or that match up with whatever tanks you end up using. Sounds complicated, but put it like this: you should be safe using 1987 tanks and sending units.

The fuel pump will need to be high pressure, most GM pumps from that era (say, 1989 Camaro TPI) will fit in the TBI sending unit, so that's easily handled.

Depending on emissions, your truck should already have the return lines and what not, so I'd say wait until you start the swap, because the lines from the tank switch forward will certainly have to be cobbled together/fabricated.

It won't hurt to put the new tanks in now, using the older senders. You might be bests served to simply buy 1987 tanks that are identical to yours in size now, checking if the internals are good. A good price will make it worthwhile for the sending units alone (around $100 each, new, if you can find them) and if the tanks are internally in good shape, bonus. When the baffles break, they don't do the sending units any favors, so it's best if the tank baffles are intact.

 

[zero cool]

Thanks for the info.

I knew 87 tanks would fit but I guess 88-newer changed somehow and they don't fit? Or is it that the 87 is known and good enough so why not just use them?

I'll look into the sending units/fuel pumps. Am I correct in assuming that the top of the tank is a "universal" size and I just have to make sure I have it set up for the proper tank depth with the pump/sending unit?

Emissions aren't a legality issue since it's an '86 with 8,600 GVWR so it's emissions exempt, at least partially. The vent line goes to the charcoal canister and I can probably keep that. But it does have a 5/16 return line that will have to be replaced with a 3/8. I've had trouble running a 5/16 return line with the TBI system I put on my '77 Jeep. It tends to back up a little at idle and artificially richen the mixture unless I run a 3/8 return line.

I'm thinking that I'd rather not drop the tanks twice. So I think I'll get everything for the higher pressure fuel system that I need. It'll be a lot less work to just put a fuel pressure regulator on for the carburetor and take it off when I put the new motor in.

 

[dyeager535]

I would expect the '88+ C/K tanks to be substantially different, I wouldn't waste time even looking at them. '87 should be drop in AFAIK since the body and frame should all be the same.

Top of the tank is universal in size at least in regards to our body style. I have no idea at what point GM changed it, if they ever did. You are correct on depth, but of course also the fittings up top. I suspect by '87, all TBI vehicles had very similar fitting arrangements up top, so in essence, universal in regards to how many lines it accepts, etc. I know for instance the Suburban sending unit has a different part number than the K5 sending unit, even though the depth of the tank is the same, which makes no sense to me. Could simply be the angle of the fittings.

You'll probably want to keep the charcoal cannister, it's hard to vent the tanks cleanly and still do a decent job of keeping humidity out of them. The charcoal cannister is about the best way to do it, and I'm sure the newer setups had similar, so it should work as is.

The return lines, I have no idea how you would handle those with the fuel tank switching solenoid. You may need to compare an '87 piece with your '86, and see if the size was changed on it? I never bothed to compare the return-line sizes, the carb and TBI are different, stock?

I'm assuming you'll just wait and swap everything at once?

 

[zero cool]

I don't know what size the actual lines are on a TBI equipped truck but the two fittings on the back of the throttle body end up being the same ID. Putting a TBI setup on a truck with a smaller return line causes enough flow restriction to raise the pressure on the supply side. The throttle body uses a bypass pressure regulator and when you don't have a free flowing bypass line, you get too much pressure. Of course, I may have some blockage in the return line in my Jeep but I haven't looked beyond swapping in a bigger return line.

Though, a cursory look at catalogs shows that the stock return line didn't change from 86-87, even for TBI. So maybe I just have an obstructed return line in my Jeep.

Meh, I'll just spend some quality time digging around and finding all the specifics I need for the lines. I'm swapping to a 5.3 which may have different needs than a TBI anyway. I may end up fabbing some hard lines but I already have a tubing bender and flare tool so I'm good on that once I get that far.

And looking at the difference between the 73-87 tank and the 88-98 tanks, I'm not sure if the in-tank pumps will interchange. I may just use the TBI pickup/sending unit with an external pump. There's a few aftermarket options for tank switches that I can try out.

But yeah, doing all of the swapping at once means I only have to do the work once. It may cost a little more up front but dropping tanks more than once is a lot more work than I want to do.

 

[dyeager535]

My pump runs something like 60 or 70PSI, with stock TBI return lines, so whatever size they are, it's enough. 13PSI is about as much as TBI ran stock, I'm way over that. There are a few running Vortecs here, I doubt they re-did the fittings on the sender for the return line.

Regardless though, you'll probably end up making new lines, flaring and what not may be the only option. Not sure what later used, the TBI/TPI Saginaw fittings are a pain, as long as they can be cut off and flared at the sender, and you can somehow adapt them at the fuel rail, you'll be fine.

You need to know what the 5.3 needs in fuel pressure, and simply get a GM pump that's like the TBI one to solve that issue. Like I said, mines 60 or 70PSI (some Cadillac application) and I doubt the 5.3's are higher than that. I don't like external pumps, noisy, seem comparatively prone to failure, and prime should be worse with them, but it's personal preference.

You can find GM pumps for quite cheap on ebay sometimes, as long as you know what they are, and no one else does.