Fuel Pump Connections

[Pawel66]

I am looking for a picture that would show the correct way of routing wires and hoses to fuel pump.

I am confessing here: I have not finised my homework yet on this. There is 35 pages of posts I have to go through and I went through 7 so far. But I thought some of you may have a picture at hand - then I thought perhaps I would kindly ask to post it.

I have the small parts (clips) etc., but I have no idea how should they be attached to pump brackets and how to route wires through them.

I have the newest pump and its suspension/protective cover.

[Shvegel]

Found this on Motoring Inveatments "Holy Grail"

[Pawel66]

How did I miss it...

Thank you!

[Jordan]

Pawel, the wires for my pump come down through a hole in the undercarriage immediately adjacent to the pump. I also have a return line from the pump back to the fuel tank.  This is tied into the return line from the engine (Y connection).

[Pawel66]

Marcus, thank you! And I guess the wires go first through this rubber sleeve that is NLA in MB and is anchored in the hole in the floor, correct?

[Jordan]

Marcus, thank you! And I guess the wires go first through this rubber sleeve that is NLA in MB and is anchored in the hole in the floor, correct?

Yes there appears to be a rubber sleeve, no doubt to protect the wires where they pass through the undercarriage.  Don't see why a rubber grommet couldn't be used and just use heat shrink around the wires as added protection.

[Pawel66]

Ok, thank you!

[Pawel66]

Almost done there, but I have one more question: I do not see the fuel return line coming out of the fuel pump on the "Holy Grail" picture (enclosed again). Could be it is covered by the fuel inlet line on this picture. But anyway: is it possible that this line (fuel return from pump to T-connector) is not there?

The nipple in my fuel pump is plugged with a screw (PO?). I see on the parts list that the T-connector fits my car.

Should I unplug the nipple in the pump na run a line to T-connector, or better leave it the way it is since it seems to be working ok?

[Jordan]

Pawel, I will take a shot at this and someone can correct me if I am wrong.  Not all cars have a return fuel line from the pump.  In most cases if you have one or not depends on a previous owner (either installed or removed).  I don't know if the cars originally came with returns or they were installed later but mine has one and I like the redundancy of having it in place.  Some just cap off the return from the pump and leave it at that.  So it's up to you if you leave it capped or install a return from the pump.

Perhaps someone in the know can tell us if the return line was an original fit or it came later?  As the pumps are equipped for the return line I would think they would have been an original fit.

[Pawel66]

Marcus,

Thank you for giving it a shot.

From the EPC I foigured out that the T-connector and the return line from fuel pump appeared together with the last generaiton of fuel pumps, as of chassis No. 044 010242. I looked through the BBB ans Haynes. I found a note that the return line to pump was added to minimize the risk of air buffles in the injection system.

All this suggests I should have it.

I have no idea why the "Holy Grail" does not have it - from what I remember it was a quite late car.

[gorgoo]

Hello there,

some pictures of my fuel pump (Oct '68 280SL), it came with the "Y", and I wondered "Y"!  :
As found:

and as is now:

Hope this helps!
Didier

[Pawel66]

Thank you! Very helpful pictures. I see now that the return flow goes straight from engine to pump and to the tank is the "side" passage. Getting back hot fuel straight from FIP - logical...

The EPC tells that the new pump and "T" connector appeared after 044 010242 VIN number, which is April-May '69. If we believe the EPC, your pump was replaced (lower hanger, pump, rubber hangers and cover, T connector) at some stage. I am not sure, but the hoses and clamps may be not the MB replacements.

If the above is true, someone who replaced the pump did a good job hooking up the return lines, T connector, adding original lower bracket, cover - looks like a fairly thorough job.

Thank you for the pictures - they are very helpful, I will get to finish the return line on Saturday.

[A Dalton]

 The reason that the by pass T is used  on the small pumps is b/c that pump is a wet pump. Meaning the windings/ armature  are cooled by the fuel.

This type of design heats the gas and can cause vapor locks under the right conditions.
The T allows this gas to re-circulate to keep the fuel supply cool at the MFI pump, specially when the engine is not consuming high rates of fuel  [ idle/traffic/ high ambiant/ etc./


The long pump had a slide seal between the windings/armature and impeller, so it was a dry pump. No motor heating of the fuel from the windings.

But the long pump  system had a return valve on the MFI that allowed fuel to  return  to the tank regardless of pressure.

The later had a different valve and that is the one they recommend using if one does upgrade to the short pump.


BUT >>>>>>>>>>

Both are long ago  NLA, so no one does that change, but there was a difference in the two valve designs.

Early ones had  a bleed hole  for return , later did not.
So, the later T was added to accomadate the wet style pumps and solving possible vapor lock conditions........

[awolff280sl]

Mr. Dalton or someone please correct me if the following is not accurate:
The original set up when the small fuel pump was introduced did not use a bypass fitting with a Y connector at the fuel tank, Instead, there was a small orifice in the bypass valve (aka overflow valve). This is a vapor hole in the outlet screw fitting of the fuel injection pump. Once removed, the small vapor hole can be seen on the valve disk when looking at it from the bottom.

The latest version uses the by-pass fitting with "y" connection at the fuel tank, after the electric fuel pump, which allows some  fuel to return directly back to the fuel return line . This system uses the by-pass fitting with no orifice at the FIP. If the car has no "y" and no by-pass hose at the tank, the injection pump should have the pressure fitting with the small orifice.

[Pawel66]

I see that this topic is more complicated than it looks on the surface.

While reading your posts and attempting to understand how it all works, I see that it may be somewhat difficult to figure out what type of valve I have on FIP (R25), but it seems to me that:
- if it has the orifice - it is ok, no harm, to install the T or Y connection anyway (and unplug the fitting in the electric pump for it)
- if it does not have the orifice - the T or Y connector should, actually, be there

Am I correct?

[awolff280sl]

According to the EPC, the T connector (114 476 0125) was used from CHASSIS 010142.
At the FIP, the overflow/bypass valve is fitted between the outflow port of the FIP and the hose to the damper. The EPC shows 2 valves there: 000 074 1884 and 000 074 2284. The second valve shows multiple ENGINE breaks, so it's difficult to correlate. Maybe somebody can shed some light on this.
Anyway, I would guess that if the valve has an orifice then it would be ok to go with or without a T connector. You don't want to  not have the T connector setup if the valve has no orifice. This is from my notes after speaking with Hans at H&R Fuel Injection years ago.

[awolff280sl]

I found some more old notes:
With early "large" pump, there is no bypass hose from the pump to the return line, but there is a hole in the injection pump outlet valve (not in the fuel dampener).

When the "small" fuel pumps were first installed, they have the bypass fitting but it was not used, it is blocked with a screw and these also have the hole in the injection pump outlet valve (so the arrangement just like the large pumps).  Note that if a small pump is used to replace an original large pump, the bypass hose does not need to be connected.  (And as noted, the screw in the bypass fitting is a handy way to bleed air out of the pump if it's airlocked).

Later, around the beginning of 1970 (or thereabouts???) the bypass hose was installed, connecting the small pump bypass fitting to the fuel return line.  At the same time, the hole in the injection pump outlet valve was eliminated.
This latest version uses the by-pass fitting with "y" connection at the fuel tank, after the electric fuel pump, which allows some  fuel to return directly back to the fuel return line . This system uses the by-pass fitting with no orifice. If the car has no "y" and no by-pass hose at the tank, the injection pump should have the pressure fitting with the small orifice.


The situation you don't want is a fuel pump with no bypass hose fitted together with an injection pump outlet valve with no hole,  and the other is the smaller pump with the by pass hose connected with  the IP valve  with the hole ..
 This is where you would have two loops  , which will result in lower fuel pump capacity to the IP at higher rpm..

Pawel, I don't know if the bolded info is correct, but it goes to your question. Sorry for any confusion. Some fuel system experts need to chime in.

[Pawel66]

Thank you!

I thought I had it clear...

Is there a way to check if there is a hole in the valve in the FIP from the outside?

Or maybe somebody knows if R25 (which is the last FIP type, I believe), was without a hole in the vlave?

I have not seen the Y connector anywhere yet. I saw the T connector and this is what MB sells today, I just bought it - a metal T-joint. And I see in a couple of places (also in this thread) the return hose going from FIP, straight through T to the pump and from the third T leg to the tank.

[awolff280sl]

I think that the Y connector is the same as the T connector.
If I remember, if you disconnect the hose going from the FIP to the damper at its FIP connection at the valve, you can see the valve disk with the tiny vapor hole when looking from under the car. You might need a mirror.
The vapor hole allows air/vapor to leave the FIP when the fuel pump is not running and the valve disk is shut.
Now why or why not this would result in less fuel to the FIP at high RPMs if there's also a T connection at the tank, I don't know.
Bottom line: if there's a vapor hole in the valve disk, you don't need a by-pass; no hole, by-pass needed.

[A Dalton]

 The reason is they simply want a constant gas flow back to the tank via a  loop [return line] , ....regardless  of pump pressures or flow rates.
Moving gas lessens vapor lock criteria.

 They also want the  venturi action the  return flow has on the swirl pot.
It is this venturi action that helps keep the swirl pot full when gas levels are below the top of pot.  W/O that . you could introduce air into the fuel  loop.


So, bleed hole at MFI valve or T fitting...either  keeps the  flow  recirculation going.

As I stated earlier, the T  mod was to move more flow b/c it was supposedly  needed with the wet pumps design of fuel in the motor section of the smaller pump [ more heat transfer]

I have never had vapor lock problems with newer pump conversions and bleed hole MFI vale combo.
No T  by-pass.
But tropical ambient temps and  load burdens may benefit from the T  by-pass.

 
 

[Pawel66]

Thank you very much for your kind attention and answers! It might have prevented me from doing the wrong thing.

Andy, your bold text might be right, actually.

I had to modify the whle post as I looked at Haynes and BBB again. They talk about the return valve replacement if a bypass (T-connecotr) is installed. I think haynes is mixing return valve with fuel damper on a photo, but they talk about checking the valve for the hole.

So this is the first thing to do - check what valve I have. I am very curious.... GIven what the BBB is saying about changes in FIP, R25 should have no-hole valve. We will see.

[Pawel66]

Again, many thanks for yuor kind attention to this topic and your advice!

I have done this this morning. My car does not have a hole in the return valve in FIP, we have removed the valve and examined it. Full disc inside. My car did not have the bypass at the fuel pump either. No idea why.

We have installed the bypass and the T-connector. The T-connector from MB is not metal like I thought at  the first glance, it is a dark plastic piece.

I hope I have the fuel system as it should be more or less now. Unless there are other scecrets waiting to be discovered.