DIY - Double to Single Fuel Pump Conversion

[mach schnell]

Is there a DIY on how to do this with factory BMW parts, including the later fuel pump setup?

[marsgorski]

transfer pump current draw

After a few days it doesn't appear to be leaking. All is well. I measured current draw at the pump as 6.47 amps. Using online calculators, 17 gauge can support 6.5 A upto 6ft or so. We just have to determine exactly how long that wiring is.

Edit, interestingly the ETM for 89 M3 shows that the main pump and transfer pump use the same 1.5mm square wiring.

I'm currently working on a problem my car has (described here) and I tried testing the current draw on my transfer pump. I got 0 amps, but I suspect I may have done something wrong. Can you describe the process by which you measured your? That would be greatly appreciated!

[reelizmpro]

Thought I would update this thread since I am doing this again on my 85 323i. It's been a few years and all is well with the conversion on my M3. The shortened stock reducer hose has not split. Here are some pictures to show you what needs to be done.

Note the size of the outlet on the brass holder vs the new TRE fuel pump.


I installed the TRE pump high enough to where the outlet is practically inside the brass tube and the connection is secured with the shortened reducer hose and clamps. The height also keeps the pick up in the ideal location. Make sure you orientate the filter at the proper angle and the level sensor clears the pump as well. For power, I desoldered the stock wiring and soldered the new wiring to the + terminal. For the ground, I chose to install a ring terminal on the ground wire and used the factory ground screw to ground the new pump. Here it is all assembled next to the original.

[Gaboer]

Bit of a bump, but I'd like to say it's important that you do not cut the original wires for the external pump. I did so and it turns out that positive side is made of 2 wires connected at the pump, one of which is the only power lead that goes to the internal pump. I soldered both positive wires back together and my internal pump started functioning again

[Coupelx]

I shortened the tube on the hanger and brazed a smaller nipple on it so I can use the special hose that comes with most pumps, I didn't know how the factory reducer would have liked the higher pressure of the newer pump.

[reelizmpro]

Yeah, it seems be working just fine. It just worries me possibly having a fuel leak in front of the rear muffler. I've been keeping an eye on it and it seems okay. Another option is to use a 1/2" to 3/8" barbed reducer and 3/8" to 5/16" barbed reducer to step the hoses down. A 1/2" to 1/4" barbed reducer would work too...if you can find one for fuel. This is another option that will get the job done and cost about the same. If I had to do it again, I'd probably do it this way just to be safe.

I have not found a step down submersible hose that will work for the intank pump but I cut my stock reducer as short as possible and clamped them. Hopefully, it won't split. Still looking for the perfect solution.

[McGyver]

Just did this today and 3/8" is definitely too small especially if the hose has swelled a bit. Hopefully it doesn't leak in the meantime. I'd say go 1/2" or 5/8".

I did notice that, but wasn't able to get a better size from local parts stores. I'll add info to the OP so people know to look around a bit more if they want a better fit. However, I've been DDing the car since the write-up with no issues or obvious leaks.

[reelizmpro]

After a few days it doesn't appear to be leaking. All is well. I measured current draw at the pump as 6.47 amps. Using online calculators, 17 gauge can support 6.5 A upto 6ft or so. We just have to determine exactly how long that wiring is.

Edit, interestingly the ETM for 89 M3 shows that the main pump and transfer pump use the same 1.5mm square wiring.

[reelizmpro]

Just did this today and 3/8" is definitely too small especially if the hose has swelled a bit. Hopefully it doesn't leak in the meantime. I'd say go 1/2" or 5/8".

[Coupelx]

This is more or less how I did mine except I used a 86-93 moostang pump from the parts store, which is basically what most after market pumps are modeled after. I also moved the fuel filter to the back as mine was up under the brake booster and about impossible to get to.

It is possible to use a standard hose clamp to mount the pump, you just need to grind a a bevel on the head of the screw so it fits in the hole of the tank.

[McGyver]

17 gauge is smaller than I was hoping and I assumed it split where the main pump is for the shortest possible length. From what I've gathered that's going to be marginal. I would probably run the 13 gauge from the main pump to the tank to be on the safe side especially if you have any mods that require more fuel. It wouldn't be too hard to do either. Is your engine stock? running well after the conversion?

I've done this on both of my 1987 verts with bone stock engines. The manual has been running great and seems to have the correct amount of power. Other than than the reducer splitting (as I said in the edited OP), it's been running great. The auto is running a little rough, but that's probably because the gas is more than 2 years old at this point.

The TRE pump makes perceptible noise at idle at a stoplight, but isn't that noticeable when driving or with music on.

I agree, it would probably be a good idea to run a thicker wire to the in-tank pump (especially if you have any engine upgrades), but so far it seems to not be necessary on a bone stock engine.

[reelizmpro]

17 gauge is smaller than I was hoping and I assumed it split where the main pump is for the shortest possible length. From what I've gathered that's going to be marginal. I would probably run the 13 gauge from the main pump to the tank to be on the safe side especially if you have any mods that require more fuel. It wouldn't be too hard to do either. Is your engine stock? running well after the conversion?

[McGyver]

Does anyone know the gauge of the transfer pump wiring?

If it's the same diameter of the wiring at the pump, then it appears to be 16 gauge and at around 5-6 ft it should be okay for at least 200lph which is more than the stock main pump. The tre 340 is actually 255lph at just under 13A.

According to the ETM I have in the in the OP, the wiring from the fuse box all the way back to the external pump has a cross-sectional area of 2.5 mm^2, so thats ~13 gauge. The wire for the in-tank pump splits off somewhere between the fuse box and the external pump and has a cross-sectional area of 1 mm^2, so that ~17 gauge.

http://www.engineeringtoolbox.com/awg-wire-gauge-d_731.html

[reelizmpro]

Should be the same for all cars with the dual fuel pump setup ie 84-87 325e and 88-91 M3's as well. Still not sure what the vert cutoff is...was it when they facelifted it in 91 or before?

Anyway, many years back I dumped about $300 + to get the 2 OEM pumps but if I had to do it again, I'd probably go this route. What concerns me though is the part about the wiring being much smaller than the main pump wiring. This is because the original intank pump is only meant to be a transfer pump. The transfer pump's smaller wiring may be sufficient but it depends on how much current the aftermarket pump draws for your needs. I would not just assume it is with something as critical as this.



Does anyone know the gauge of the transfer pump wiring?

If it's the same diameter of the wiring at the pump, then it appears to be 16 gauge and at around 5-6 ft it should be okay for at least 200lph which is more than the stock main pump. The tre 340 is actually 255lph at just under 13A.

[dnguyen1963]

I can't believe that you left all that rust on the holder. You can clean all that off then coat it with plastidip to make sure that it will never rust again.