fuel pump failure?

[dseretakis]

I was driving home today in my '71 280SL when car stalled and would not start again.  Fuel pump fuse was burned out.  I replaced it but car still would not start.  There is power to fuel pump as it makes a buzzing sound.  With ignition on, I also noticed fuel dripping from delivery side of fuel line.  Any help would be appreciated.
Thanks,
Dimitri

[ja17]

Hello Dimitri,
If the electric fuel pump is working it should be making a noticable noise when it is running. You will have to check and see if fuel is being pumped to the engine. Turn the ignition on and open the line at the cold star valve on the intake manifold. Fuel should squirt out if the pump is working.

A fuel pressure and volume check will be the only way to tell if the pump is working well.

Joe Alexander
Blacklick, Ohio

[dseretakis]

Thanks Joe,
I'll start with the cold start valve.
Dimitri

[dseretakis]

OK, so there is fuel at the cold start valve, so if fuel pump is working why did the fuse burn out?  Also car started this am and ran for a couple of seconds until it stalled and could not be restarted.  I guess I could check out the ignition system, but that still wouldn't explain the blown fuse.  I'm at a loss here.
Dimitri

[A Dalton]

Need more info........
  Fuse failure indicates excessive current draw.  Test is with amp meter across fuse terminal w/fuse removed. If draw is high, suspects are pump brushes, poor connections, dirty pump housing/impeller, or clogged suction side screens.  If reading is normal, suspect is fuse  fatique or bad fuse connection at ends [ very common on these type fuses]

 Do the JA PSI/Flow test ..can be done at same time as current draw test.

[dseretakis]

Hi Joe,
What is the JA PSI/Flow test?
Thanks
Dimitri

[Cees Klumper]

The JSA PSI/flow test I think refers to the Joe Alexander fuel pressure & volume flow test - if you do a search on this site using some of these key words, you will find the instructions on how to perform this. Essentially, it will tell you whether the capacity of the fuel delivery system is within specifications.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[rwmastel]

quote:Originally posted by dseretakis

What is the JA PSI/Flow test?

Do you have any manuals for the car?  Mercedes shop manual (BBB)?  Haynes manual?  These would describe the procedure.

If you use the proper terms, you can get results using the Search tool on this site.  I've noticed it's getting difficult to use the Search tool because there is SO much information that you have to be very careful with your search terms.  You can get lots of info returned that is not relavent to your query.

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[ja17]

Hello Dimitri,

The test will measure the fuel pressure and the volume of fuel delivered to the engine's injection pump. Both are critical and create different problems when not right. Here is a photo and text which should help. Note that the return line should be plugged or pinched closed for the test with engine off and ignition on. You do this at the rear rubber fuel line coming from the injection pump.

Download Attachment: pressure check.JPG
37.29 KB
Joe Alexander
Blacklick, Ohio

[A Dalton]

To add to JA page info..

 Another way I use to check these systems in an isolated way  is to first start by taking the feed line off at the engine filter . [pre- filter inlet line].  You can then  slip a section of fuel hose over that lines fitting [ it is a ball/bubble flare fitting, making this easy and snug].  You then turn the key ON and you can take both a PSI reading and the flow test right from this point. The advantage here is you do not have to go and crimp the return line back to the tank and this will give you the pumps absolute Max feed flow and PSI directly [ pre filter and CSV line/return line/pump check valve] , verifying pump condition and feed  screens flow rate as you are taking readings directly from the pumps output port.
 If the  other test [ CSV access] is much lower after the engine filter , the filter is restricted and should be changed.
..and , you still want to test the return line for blockage.
 
  Also, looking at JA jpg., you will notice  illustration index #5.. this is a glass tube that allows the tech to look for air in the fuel flow [ there should be none].
 A trick I use to accomplish the same thing here is to simply put the end of the fuel flow test line into the very bottom of  your flow  rate measure container.[ so the end is submerged in fuel]
 This allows any possible air in the fuel flow to be observed when doing the flow rate test. If air is seen [ bubbles], check all hoses and clamps on the suction side  of the pump..
this is an important test as air will affect the pump plungers metering volume/pressure.

[mdsalemi]

JA, ADalton, and others:

As my engine-dying woes are pointing to fuel at the moment--with the most obvious ignition offenders ruled out (still need to check the optical sender in the distributor and the ignition switch) the fuel pressure issue is of interest to me.  The problem I have is that the testing setup mentioned is really only good statically; you can't run fuel lines into the passenger compartment (at least I hope nobody tries it) and I'm looking to hook up a sender unit to an electric gauge so I can see the pressure dynamically, while moving, and see if my engine dying is related to a drop in pressure.  The engine dying won't happen in a garage, thus a static test probably won't tell me much.

Now, the book and all make an issue of volume flow--minimum of 1 liter in 15 seconds.  Is there a maximum?

I'm no physicist or chemist or engineer, but from what I remember, if our "piping" (that's all the fuel lines, etc) is fixed, a steady pressure will produce a uniform flow rate.  So, if 14 psi gives 1 liter per 15 seconds--about near the minimum, any drop in pressure will yield an [unacceptable] drop in volume below the minimum, and increases in pressure will raise the volume.

I can't immediately envision a scenario with fixed plumbing, as it were, where your fuel pressure is absolutely fine--say 14-16 psi, but your flow rate is not.  Even if your measuring point is after the fuel filter, a clogged fuel filter would act like an orifice, and cause a pressure drop after the filter.  Pressure and volume are directly proportional, are they not?

What am I missing here?

Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored

[ja17]

Hello Michael,

Yes fuel pressure and volume are directly related. You may be interested to know that even though these systems are designed to and do run at around 14psi, these electric fuel pumps will put out from 30 to 40psi if the return line is plugged! In other words the system has a constant "leak" or pressure release back to the fuel tank via the return line. So volume is very critical. the fuel pump is continuously circulating fuel and only using part of the volume. The return fuel line is smaller than the fuel delivery line from the fuel tank. This in itself can create some back pressure in the system if the volume is sufficient. The orifice or check valve  at the return line of the injection pump are mainly resposible for creating back pressure in the system.

It is safe to assume that if fuel volume is lacking, then something in the system is restricted or the fuel pump is weak. And this restriction can cause running problems at some point. These engines will run on 5 psi with very little volume, however the engine will have a variety of bad running and starting characteristics which may imitate many other electrical or timing problems.

We'll your correct in that you should use a sending unit and wire instead of a fuel line to plumb a fuel gage into the car. However for test purposes I do the fuel line and gage hook up temporarily. Not the safest method. Good clamps and fittings along with higher pressure injection fuel line make it safer and it only take a few minutes to plumb in. but sending unit, wire and gage is safest. Try your local hot rod shop for an electric fuel pressure gage and sender.

Joe Alexander
Blacklick, Ohio