J-tube Bypass- for using an aftermarket high flow fuel pump.

[shaeff]

a common modification to the fuel system is a larger pump, usually coupled with larger injectors, possibly removing the fuel pulsation dampener, and an aftermarket fuel pressure regulator. when uprading fuel pumps, it's essential to bypass, or drill out the stock j-tube.

the J-tube is a restriction in the return line of the fuel system, located after the fuel pressure regulator, and is bolted on to the side of the block below the intake manifold. the purpose of the j-tube is 1) cover the hole in the side of the block from the previous M series motors that used a manual fuel pump, and 2) to serve as a sort of second fuel pressure regulator. in this case, if the fuel pressure regulator on the fuel rail malfunctions and will not retain presure, the j-tube's restriction (the pin hole) will provide enough backpressure in the fuel system to allow you to "limp" home, or to a garage.

if you look at the following picture, you can see the end of it only has a pin hole to let fuel through. with this restriction still in place, the line cannot flow enough fuel to satisfy an aftermarket high flow pump (such as the MKIV TT pump, the walbro 255lph, and others) the pin hole provides TOO MUCH back pressure on the system, thus causing a rich running condition (too much fuel), black smoke, and bad over all performance.

to drill it out, all you have to do is get a bit that's slightly smaller than the inside diameter of the pipe, and drill it out. blow the remnants of metal flakes out with compressed air, or something similar.

simply bypass it, just get some fuel hose from your local autoparts store, and run it directly from the fuel pressure regulator (follow the stock line), right to the under-body hard line. (make sure you clamp it!)

or to remove it completely, you could go on summitracing.com and get [edit: thanks, bryan!] a big block chevy block off plate, (it bolts right on, and comes in pretty colors), or you could simply pull yours off (two nuts), and chisel the j-tube off, grind off the slag, sand, and paint (which is what i did).

then use rubber line (or in my case, SS braided from an aftermarket AFPR) to the under-body hard line.

-shaeff

so, we need to bypass it completely, or drill out the restriction.

Edit: Here's another diagram to use if you just want to bypass it altogether:

 

[Slow66]

Just wanted to make an edit that it would be the Big Block Chevy blockoff....

 

[supra90turbo]

Here's some links
click the pictures

 

[Adjuster]

Man, I need to get me a red one! (Mine is the stocker, J tube cut off, and the plate flipped over and painted black..) Ok, so I had some left over sealant... it's not like you see this thing very easy when it's in the car..

 

[dugums]

Sorry for bumping the old thread, but I figured it better than making a new one on the same subject.

I have been doing some reading up on the j-tube, but all the posters seemed to have larger than stock injectors.

I currently have a Walbro fuel pump and an Aeromotive AFPR installed. I have decided that I will be sticking with 440's The AFPR is routed through the j-tube and everything right now is functioning well.

From what I have read the need to bypass/drill the j-tube is simply a result of adding the higher volume fuel pump - is that correct? If so, I guess I need to bypass it, but I just wanted to see if there was any different advice based on my setup.

 

[shaeff]

From what I have read the need to bypass/drill the j-tube is simply a result of adding the higher volume fuel pump - is that correct?

correct. the j-tube is a fail safe device in case the stock FPR dies. the pinhole in the exit of the j-tube will (in theory) provide enough of a restriction to allow the pump to keep pressure in the rail, thus allowing you to limp home.

when you exceed the "regular" amount of fuel flow (and i use that term loosely), you must bypass the j-tube to prevent the possibility of running rich due to excessive fuel pressure.

 

[dugums]

when you exceed the "regular" amount of fuel flow (and i use that term loosely), you must bypass the j-tube to prevent the possibility of running rich due to excessive fuel pressure.

I almost hate to do anything at all to the car (even though I know this won't affect it) - I've never had any of my cars locked into great AFR's like this one is now.

I guess it's going on my list for tomorrow.:icon_bigg

 

[IwantMKIII]

Well....you can see my mods in my sig. they include 550's and a walbro and AFPR. I had the j-tube attachted for well over 3K miles. No problems. FP at idle was about 40-42PSI and 56-59ish full throttle. Soon enough i bypassed the J-tube. No difference TBO. No difference in FP at all.

 

[shaeff]

it's hit or miss. sometimes people experience an extreme rich condition, sometimes they don't. i call it preventative maintenance. i'd rather not hook up my fuel system, then realize i have to pull the j-tube off or drill it out.

 

[Adjuster]

Most people with adjustable fuel pressure regulators, they don't need, or even have this part of the fuel system anymore.

For a few years, I ran with mine completely bypassed. (It was on the motor, but no hoses ran to it anymore...)

Fuel goes from the pump, out of the tank, to the fuel rail via the fuel pressure dampner (If your still stock there.) and out of the rail to the fuel pressure regulator, then back to the tank. The only thing changed is the small hole in your return system is removed, but with higher flow pump, and other variables, the "J tube is not needed, and there really is no way to guess at what size restriction you would need if the AFPR did fail with non-stock fuel systems.

If you are running two pumps, you definately don't need this, and might actually need a larger return line than stock.. But, I have not heard of anyone finding that two walbro pumps over flow the ability of the Areomotive AFPR and the stock return line ID's ability to allow fuel back to the tank that is not needed by the injectors. (At idle would be about max flow back to the tank.... Or during higher rpm and closed throttle conditions where your going to have max voltage to the pumps, but really not need any fuel at all due to the fuel being "cut" to a bare minimum during coasting/no load.. Closed TPS signal..)