M108 Fuel Pump Problem

[saxplayer123]

I have a 67 Coupe with a 65 M108 carborated engine. I am running through fuel pumps at a fairly regular rate, they last about a year of so. The current pump is sqirting fuel out the vent hole. Others just stopped pumping, no pressure. The problem seems to be a hole or holes in the pump diaphram.
I have installed a fuel filter in front of the pump, so I do not think it is caused by dirt getting into the pump.

I am wondering, does anyone know, does the M108 engine take a different version fuel pump? that is different from the normal ( whatever that is ) carborated engine?

[Arthur Dalton]

That car was originally Fuel Injection, [ 111 chassis/129 eng.] but they put in a carb engine .. so, if the diaph. is leaking by , you need a new pump..that's about it.
But, there may have been a fuel return line back to the tank that never got hooked up with the swap and that may put a strain on the pump....or a plugged line.

??????????

[wbain5280]

Which fuel pump? The coupe used a fuel injected engine, as previously stated, with an electric fuel pump by the rear axle. Shouldn't that be an M180? Anyway, the carbed engine has a mechanical fuel pump on the side of the engine down low and at the front of it driven by the oil pump. How are the fuel lines hooked up? Does the electric pump run when the car is running? Is the return fuel line connected?

The car must be really slow with that engine. It's from a 220S, not the fastest car MB ever made.

[Arthur Dalton]

WB has a valid point .. check to see if they did leave the electric pump hooked up... you just never know what some of these swap guys will , or better yet, won't do..

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I think you will find this engine swap is from a 250S , 108 engine.
Common swap for that car..

[saxplayer123]

Yes, I did take the engine out of another coupe, '65 250S, and I also took out the electric fule pump and changed the transmission along with the engine. The engine is a M108. Yes, it did have a badly worn fuel injected engine. The M108 isn't real powerful, but it gets me around town just fine ... gas millage is bad, about 10 mpg. I had been driving it most every day.

The fuel return line is hoked up, but it could be plugged up, I guess. Tell me more about that please. Thanks for the comments.

[Arthur Dalton]

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Let me see what we have here first.
There is no 250S Couple . Coupes are FI engines. [111 chassis] The 108 engine had to come from a 250s sedan.
...or did you have another Coupe with a carb engine transplant??

[saxplayer123]

I bought two cars. coups. One had the M108 engine in it, but the body was only rust. The other didn't have much rust, but most every moving thing in the car was busted. Both cars, I think, were built for the European market, the headlights were changed to the US style on the car I kept.

I looked at the fuel recovery cell in the trunk. It is a smallish black plastic tank with 3 lines connected to the bottom, and one to the top. Two lines on the bottom go to the fuel filler tube and the other one looks like it goes to the fuel return valve on the first carborator. The one on top has tape over the end of the hose. My Haynes manual seems to say that this last hose was supposed to go back to the engine so vapors woud be sucked into the air intake and burned. I don't see where that connection was to be made though.

Do you really think that a clogged line to the relief valve could be causing fuel pump failures?

[Arthur Dalton]

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Don't know ...might want to blow it out anyway..
I have never seen a 250S in the Coupe version
I thought all those Coupes of that vintage were 111 chassis with Mechanical FI.. that would be an SE model and I am familiar with that system more-so than the carb.

[saxplayer123]

I was wrong when I said the return from the carborator went to the small tank in the trunk. It does go to to the fuel tank. I disconnected the line at the carborator and was able to blow air through it ... could hear air bubbling in the gas tank, so I think it is clear.

Looked in my old parts box. There are four fuel pumps there, this is the fifth in as many years. Somthing just has to be dredfully wrong here. Put on your thinking hats, and give me some leads.

[Arthur Dalton]

Check the strainer in the tank....

[leenart]

Isn't there a pressure release valve on the front carb? I'd like to know what the inside of those 4 pumps looked like after they failed; was it the same on all 4?
Wonder what the pressure is on the sending and return lines. Finding a mechanic that will work on this is going to be just as hard as finding the problem
Leonard

[leenart]

You know maybe the cheapest and easiest thing to do is just replace the pump every year. Its not a hard job to do and the pumps are fairly cheap. Its going to cost $100/hour to troubleshoot this with no guarrantee of finding the problem, figure 4 hours min
leonard

[cth350]

I'd question where you're getting the fuel pumps from. -CTH

[Arthur Dalton]

Get FP specs and go electric..

[saxplayer123]

I was worried about getting dirt from the fuel tank into the pump, so I put a filter before the pump last time. I have never found anything in the little wire filter that is part of the pump.

I've tried several ... the last one came from Mercedes, I paid over $200 for it.
I've got 2 push rode pumps and 3 of the lever type. Have bought them from various places.

When I take them apart, I have found holes in the diaphram on some. Others, the outlet valve seems to have failed. Some squirt out the vent hole, like the current one, but others just don't have any output.

I actually bought an electric pump last time, but had a little trouble with it. Seems like I would have to mount it back by the tank because the pump was not able to suck fuel out of the tank if I had it too high. The one I got was made by Purolator. I have heard of Clark, but havn't been able to find one. My manual says the mechanical pump puts out 2-3 psi. and will pull someting like 230 mm Hg. That is like 12 feet of water. The Purolator put out 3-4 psi. but couldn't pull much of a head, a few inches. Do you have a favoriet brand? I was going to hook it up with the wires that fed the FI pump. Sound like I should try that.

I was thinking, does anyone know what the specs for the push rod that runs the mechanical pump are? Like how long the rod is supposed to be, how far it should stick out of the flange or mounting block, what the throw ( travel distance ) should be? Maybe somehow my engine is pushing too far, or not far enough which is more likely I think.

As far as I can tell, the valve at the front of the first carb is not pressure releif based on pressure ... it is connected to the throttle linkage .. shuts once the throttle is pushed a little. The linkage opens it at idle.