electric fuel pump on older car (w115) - PeachParts Mercedes-Benz Forum

DieselPaul · #1 03-09-2018, 11:15 AM

Seems to take forever to get my 1972 220 mechanical fuel pump to get enough fuel the carb to start after it sits for a couple of days that you nearly kill the battery trying to get it to start.

I'm sure its been done, has anyone here hooked up universal inline electric fuel pump instead of the mechanical pump? I was looking for some tips as to where you mounted the pump, where you pulled key on hot power to switch a fuel pump relay, etc.

Just looking for some direction from those who have done it first.

My girlfriend likes to daily drive this car when its nice so it would be nice to make it easier to start in the morning.

Mike D · #2 03-09-2018, 02:55 PM

Most of the inline low pressure pumps are "pusher" style. They will work mounted on the fender but not exceptionally well. I mounted my electric pump at the rear of the car in the rubber hose between the fuel tank and the metal line.

Post #38 on THIS thread.

http://www.peachparts.com/shopforum/vintage-mercedes/238292-m115-t-b-i-conversion-3.html

Idle · #3 03-10-2018, 12:41 PM

Make sure you install a low pressure fuel pump. Most FI systems that use an electric pump run at 42 psi. Some are a bit higher, some a bit lower.

But in any event 42 psi will blow your carb gaskets out. So don't grab a junkyard pump from a VW and expect it to work.

But you can install a fuel pressure regulator to limit the carb psi to something like 7 or 5 psi. You can find these on-line or sometimes at auto parts retailers.

These were a big deal back in the 70's. They were sold as a fuel saving device since by cutting the pressure to your carb you were also cutting the flow. (Duh.)

It will not work to save fuel but it will extend the life of your carb since most of them came with an additional fuel filter.

meltedpanda · #4 03-10-2018, 04:18 PM

I had one on my 220 , it worked fine, I went back to original just so it looked like it should LOL

DieselPaul · #5 03-12-2018, 04:07 PM

Quote:

Originally Posted by meltedpanda

I had one on my 220 , it worked fine, I went back to original just so it looked like it should LOL

Any recollection as to how you wired it up?

DieselPaul · #6 03-12-2018, 04:09 PM

Quote:

Originally Posted by Mike D

Most of the inline low pressure pumps are "pusher" style. They will work mounted on the fender but not exceptionally well. I mounted my electric pump at the rear of the car in the rubber hose between the fuel tank and the metal line.

Post #38 on THIS thread.

http://www.peachparts.com/shopforum/vintage-mercedes/238292-m115-t-b-i-conversion-3.html

Cool thanks, I really would like to do what you've done with yours just don't have the time nor the desire to spend the money on this particular car. Your setup is fantastic though.

meltedpanda · #7 03-12-2018, 08:11 PM

basic + to fuse that came on and stayed on with key and ground then in line with fuel line

Mike D · #8 03-12-2018, 08:16 PM

I used the 12V side of the coil wiring (before the ballast resistor) for my initial set-up when I was still using the carburetors.

barry12345 · #9 03-13-2018, 01:23 AM

Certain that is your issue? Shut the car off the fuel bowls are full.

Unless it evaporates too much overnight. There should be enough left in them to start on. I cannot remember if you have electric chokes.
Just think so. If so do they work?

Also famous for air leaks around the base of the carbs. You may just be low on carb suction when you need it the most .

You might also try pumping the gas pedal slowly for say ten times before trying to start as a test. There is a power valve that pumps more gas into the throat of the carbs. I forget the correct term for them. Accelerator pumps?

Another test. If you pull off the air cleaner and pull the linkage. Watch for a gas spray in the throat that you can repeat a few times. Then again there is gas in those carbs.

You should have to pull back the choke plates to observe this carefully. with a cold engine first thing in the day. Been a long time since I played with those engines and in all my cases they had the zenith carbs.

I always found those engines to be great cold starters. If anything harder to start hot. Possibly prone to some amount of vapor lock.

97 SL320 · #10 03-15-2018, 06:22 PM

The mechanical pump could be getting weak with a leaking check valve. I've had a couple over the years that would run fine down the road but would lose prime when sitting for a few weeks.

When installing an electric fuel pump be sure to:

Install an oil pressure switch to shut off power when the engine stops. The switch needs to have at least 2 terminals and conducts when the oil pressure is up. ( Some have a 3rd terminal for the oil pressure light ) GM used these on late 70's / early 80's carburetor cars for the electric choke function. Have a look at a 78 to 82 ish Buick Regal with a V6 for an example.

Installing an inertia switch is a good idea too in case the car crashes, breaks a fuel line and engine continues to run. This switch can be found on fuel injected Ford products from the mid 80's and up to 2000 + or so. They have a round red button or a rectangular white one. Look in the quarter panel trunk area of a car or passenger interior fire wall / kick panel of a pickup.

The down side is you can no longer crash a barricade. On my Ford pickup, with malice aforethought, I've bypassed the inertia switch / anti theft starter cutout / clutch switch to maintain some mobility if things turn dire.

I use the " looks like a fuel filter with 2 wires " type solenoid pump. These pumps make a jiggling noise when pumping especially when priming / flowing fuel so mounting them on soft mounts may be needed. Carter and Airtex make them as do a few others, I'd stay away from an unbranded China Amazon / E bay pump though. Get the " import low pressure " unless the shop manual calls for the USA higher pressure unit.

The pumps should be mounted near the tank as they are better for pushing than pulling. A rear mount reduces vapor locking tendencies as well.

It is possible to use this pump in addition to a regular mechanical pump as both will move fuel if one outruns the other.

Mike D · #11 03-15-2018, 06:33 PM

Do NOT install an electric pump with the original manual pump still connected! If the manual pump diaphragm develops a leak you will fill the crankcase with fuel. Go ahead, ask me how I know.

97 SL320 · #12 03-16-2018, 08:45 AM

Yes, this can occur but it is no worse than a mach only pump failing on it's own.