73 450sl fuel pump that needs persuation - PeachParts Mercedes-Benz Forum

sparkyt · #1 06-11-2006, 12:56 AM

I have been scouring all the fuel pump treads but have not run across my perticular problem. It all started as occasional not wanting to start after shuting down. I found out quickly that if I rapped the pump with something I could jump back in and it would start. It has gotten frequent and now someone has to be on the starter while the pump gets a wack. I have not done any further diagnostics besides physically abusing my fuel pump. I hope someone can help me. New pump on a 73 is...... ouch.

mark

Richard Wooldridge · #2 06-11-2006, 01:36 AM

Hi there,
Your hitting the pump seats the brushes for a short time. The problem is that the brushes are worn out, and that means a new fuel pump! (brushes are NOT replaceable in this pump.) I had several Datsun 280Z's in the past, and they used a similar pump. I "fixed" the pump on two of them by hitting them with a hammer, they lasted long enough to drive home, but that was about it. It's time to bite the bullet and get a new pump!

mercmad6.3 · #3 06-11-2006, 08:56 AM

I have use 2nd hand pumps from all sorts of things in various cars to get them running,basically any Mercedes EFI pump will do the job,go to a wrecker and get a pump from a car that hasn't done too many miles.The average pump life is 100,000 miles .

sparkyt · #4 06-11-2006, 11:36 AM

Thanks Richard, I did forget to mention that it has never stopped while driving. Only on a start. I thought it was probably a dead spot on the comm. but I guess worn brushes would have the same effect. There is 178M on my rustfree sl - time for new pump.

brian eiland · #5 06-16-2006, 08:32 AM

Quote:

Originally Posted by sparkyt

I have been scouring all the fuel pump treads but have not run across my perticular problem. It all started as occasional not wanting to start after shuting down. I found out quickly that if I rapped the pump with something I could jump back in and it would start. It has gotten frequent and now someone has to be on the starter while the pump gets a wack. I have not done any further diagnostics besides physically abusing my fuel pump. I hope someone can help me. New pump on a 73 is...... ouch.

mark

I had this same problem on my 73 450slc. Eventually it would not run anymore. When I priced a new pump I thought they were weighing out gold bullion.

I went to a mercedes junk yard (Potomac German auto in Fredrick Md) and looked at a fuel pump unit from another model Mercedes. Yes the configuration of filters and the bracket were different but the basic electric pump looked the same. Works like a charm. I think it cost me 50.00 compared to 500! Hung the unit on with strong plastic wire wraps at first but had to substitute steel wire hangers eventually.

nobby · #6 06-16-2006, 11:07 PM

Quote:

Originally Posted by sparkyt

Thanks Richard, I did forget to mention that it has never stopped while driving. Only on a start. I thought it was probably a dead spot on the comm. but I guess worn brushes would have the same effect. There is 178M on my rustfree sl - time for new pump.

178,000 miles on the fuel pump. I would say that you got your moneys worth out of that one. Put your hammer away and go buy one.

sparkyt · #7 07-29-2006, 09:42 PM

Ok, I replaced the pump with a new rebuilt bosch pump. Works fine but will flood the engine in about 30 seconds at idle. When the old pump with the appearent bad brushes would run, the car operated fine.
Is this a coincidence that the regulator could be bad now? Any help would be appreciated. Thanks

mercmad6.3 · #8 07-30-2006, 06:24 AM

On the front of the intake mainfold is the cold start injector.See if you can find a pressure guage that reads to 100 PSI.disconnect the rubber line leading into the coldstart injector,connect your pressure guage into the line and make doubly sure the hose is secure. Turn on the ignition and start the engine and see what pressure you have.Max will be 28-30 PSI.
If it's more ,you have (i) a fuel pressure regulator set to the wrong pressure,look very clsoely and see if the bolt in the center of the diaprhagm has been touched,if so turn it out to reduce the pressure.
or (ii) a blockage in the return line to the tank.
If I reach this point I connect a spare peice of hose and blow down the return line( downstream from the regulator),if I can't get my breath to push the remaining fuel down the line it's blocked.Starting from the engine,replace the rubber hose from the regulator to the hard line.Move along the hardline ,inch by inch inspecting it for squash marks.One squash or bend ,means you should replace that secion with fitted peice of hardlione or replace the whole deal.It depends on how "anal" you want to get .Once you have reached the fuel tank,and the hard line is all OK,replace the rubber lines running into the tank,BUT before you reconnect the line INTO the tank,blow into that pipe using your spare peice of hose,it should be clear.IF NOT... replace the tank.You have no option on that point because the line rusts inside the tank and cannot be cleared out.
and that should clear up a "flooding problem."
One last point...reconnect your cold start injector to the fuel and aim it into a small container.start the erngine and see if fuel comes the injector with the engine Warmed and running.If so ,the injector must be replaced

sparkyt · #9 07-31-2006, 01:54 PM

Thanks for all the great info mercmad6.3!!! Now here's the update. I've taken your above information to my mechanic (who is not a "Mercedes" mechanic but a very good tech) and he had already taken a reading which buried his 100psi gauge. He does not have a gauge that goes beyond so at this point we don't know what the pressure actually is. He will check the return line but at the time he was assuming that at 100psi + it would have probably cleared any obstruction. He did back off the regulator until fuel came out the top with really no change. Being that this problem occured with the change of the new pump, is it possible for a new pump to be over supplying?

mercmad6.3 · #10 07-31-2006, 05:05 PM

It can only be a major blockage and it sounds like your man is on to it.The pump is doing exactly what it is designed to do,so it's fine.
Cheers!

sparkyt · #11 10-08-2006, 05:59 AM

Return line was clear. I replaced the regulator and dampner and everything returned to normal. My only guess is that I had a faulty regulator that worked fine with an underacheiving pump.