Primer pump problem? No start. - PeachParts Mercedes-Benz Forum

muleears · #1 09-30-2010, 11:40 AM

I had a new primer pump (black knob) installed a couple weeks ago. Its has leaked a little since then, I'm guessing I need to replace/re-seat the copper washer. But now the car won't start. It has run well for several days, today (after sitting for two days) all of a sudden it just cranks and sputters a little. When I first attempted to start it, it did and ran for a few seconds then died. Since then only sputters. I tried pumping the primer, as it acts like it is starved for fuel. When I pump I just see bubbles between the pump and the filter. The primary filter is full and the bubble in that barely moves when I pump. I checked the secondary and it was not quite full. I put the bolt back on the secondary and pumped with it loose and got some bubbles around the bolt. It seems to be sucking air somewhere. Any suggestions? Bad lift pump maybe?

tangofox007 · #2 09-30-2010, 12:03 PM

You could have a sticking suction or delivery valve in the fuel pump. If either valve sticks open, fuel just goes back and forth with every stroke of the pump. If you disconnect one fuel line at a time at the pump, you can check easily with your thumb over the ports. Look for suction on the intake side and pressure on the discharge side. Pressure on the intake side or suction on the discharge side would indicate that the respective valve is sticking open.

Replacing the valves is fairly easy. I have a spare repair kit if you need it.

muleears · #3 09-30-2010, 12:12 PM

This 123 is new to me, it's my son's. I know where the lift pump is on my 124, but where is it on the 123? Is the primer pump mounted on it?

Right now it is pouring outside (they forecast 5 in. of rain for today) and I can't move it into the garage. So working on this is no fun.

Thanks for the offer of the parts kit, I'd like to take you up on that even if I don't need it right now. I like to have stuff like that on hand. Can I paypal you the $?

tangofox007 · #4 09-30-2010, 12:20 PM

Quote:

Originally Posted by muleears

This 123 is new to me, it's my son's. I know where the lift pump is on my 124, but where is it on the 123? Is the primer pump mounted on it?

The lift pump is on the side of the IP. The suction and delivery lines are on either side of the primer pump. The suction valve is on the underside of the pump behind a plug with a 19mm head. The delivery valve is under the discharge hose nipple.

muleears · #5 09-30-2010, 12:22 PM

To test it I should pull the hoses and put my finger on the nipple to see if there is suction or pressure when I crank it?

cirrusman · #6 09-30-2010, 12:23 PM

It sounds to me like you got some air in the lines... It can help if you keep your pedal pushed down while cranking, it should smooth out once you take care of your air leaking in.

tangofox007 · #7 09-30-2010, 12:30 PM

Quote:

Originally Posted by muleears

To test it I should pull the hoses and put my finger on the nipple to see if there is suction or pressure when I crank it?

Just operate the hand pump.

toomany MBZ · #8 09-30-2010, 01:28 PM

Quote:

Originally Posted by muleears

This 123 is new to me, it's my son's. I know where the lift pump is on my 124, but where is it on the 123? Is the primer pump mounted on it?

Yes.

muleears · #9 09-30-2010, 01:42 PM

The right side nipple (inlet? nearest the primary filter) has a fuel hose with a clamp on it. That one I can easily take off to test. The left side (output? nearest the secondary filter) is the clear plastic hose with no clamp. Does it just pull off or should I remove the whole fitting, nipple and all?

tangofox007 · #10 09-30-2010, 01:52 PM

Quote:

Originally Posted by muleears

The left side (output? nearest the secondary filter) is the clear plastic hose with no clamp. Does it just pull off or should I remove the whole fitting, nipple and all?

You have to loosen the nut on the lower end of the clear line. (An old line might be brittle; be careful.) A crow's foot will come in handy. (SAE 7/8" fits well if you don't have a 22mm.) Once the line is removed, a thin-wall, deep 19mm should work on the nipple. But there is no need to remove the nipple just to test.

nhdoc · #11 09-30-2010, 02:00 PM

People have been known to install a new primer and leave the old copper crush washer from the old one on the pump housing and land up with two washers which won't seal well at all...you might want to double check that you didn't do that. It is easy to do because you need to pick the old washer out sometimes with a dental tool and if you don't then it is left behind and causes trouble...just a thought.

muleears · #12 09-30-2010, 02:19 PM

Update.
I took the inlet side off and pumped the knob. I couldn't notice any suction while pumping, I assume it would be easy to tell. While I had it off I noticed that the short fuel line between the primary filter and the inlet nipple was pretty old and stiff. I replaced it with new line. During this process about half of the fuel came out of the primary filter. When I reconnected the lines and began pumping, it did suck some of the air out of the primary filter. But it still doesn't seem to want to move fuel.

Marty: Good point, I guess I should take the new primer pump off (I didn't put it on) and check those crush washers. Is it just the nut at the bottom of the pump knob I need to loosen to get it off?

nhdoc · #13 09-30-2010, 02:23 PM

Quote:

Originally Posted by muleears

Marty: Good point, I guess I should take the new primer pump off (I didn't put it on) and check those crush washers. Is it just the nut at the bottom of the pump knob I need to loosen to get it off?

Yes, a crows foot tool is the best - I don't recall the exact size, maybe 17mm?

I only thought to suggest this because you just replaced the pump.

Is this a sedan or wagon you are working on? Sometimes the W123 fuel system problems manifest themselves only when the tank is low - how much fuel do you have in the tank now?

muleears · #14 09-30-2010, 02:29 PM

I was at harbor freight day before yesterday and held a crowsfoot set in my hand... and put it back. Next time I'm there I'll pick it up and hold on to it.

nhdoc · #15 09-30-2010, 03:45 PM

You might be able to use a stubby open end too...

Is your tank low right now?