1973 450 SEL fuel pump problem

[Neil]

Hello All,

Nice site you have here.

I just recently came into posession of a 73 450 SEL 4.5 8cyl. The car had been sitting since oct/nov of '04. I charged the battery and the car roared to life.

It idled for approx. 5 minutes at about 1600 rpms then it stopped abruptly and would not restart. I does have spark but no fuel to the engine.

I had a friend turn the key one position and listened for any fuel pump activity by the rear right tire. Nothing. I do not have a manual to show the exact location of the pump. But, I believe that I located the fuel pump mounted under the fuel tank. What I believe to be the pump lays underneath a new looking cylinder. Both are pump and new cylinder are mounted in a square bracket.

Maintinence records show that the fuel filter was replaced May 2004. This info helped put me into the general area. Other records showed that a rough idle complaint was noted by the previous owner after filling up with a cheap fuel. The mechanic noted that the tank should be drained and cleaned. I do not know if this was done.

so my questions are how do I diognose if I have a bad fuel pump or gummed up fuel pump from crud in the tank.

The fuel rely does click when I turn the key.

Any help would be appretiated.

Thank you in advance

Neil

[Tomguy]

The "Cylinder" is the fuel filter.

You DEFINETLY should hear the fuel pump buzz when you turn the key. If you don't hear it (from inside the car) then something's wrong. Check and see if it's getting power (about a 1-2 second pulse when you first turn the key). If not, there's your problem, if so, it's probably a bad pump. If it doesn't buzz, I don't know if clogging can cause that or not, but something's amiss with the pump.

[ctaylor738]

The pump is the cylinder near the filter with a power wire and a ground wire running to it. Use a DMM or a test light across the leads and see if you are getting power to it when cranking. If not, the first place I would check is the relay.

If you are getting power then the pump has gone south or you are plugged somewhere.

[Doug Wilson]

On my 75 450SL I had a similar problem. If yours is anything like mine, I took a relay from another location (same part number) and tried it in the fuel pump relay spot which made my fuel pump work. So I ordered a new relay. Otherwise, on the relay itself there should be a wiring diagram which you can figure out which terminals to jump. Relays are inexpensive.

[Neil]

Thanks everyone for your help

I will try to switch the relays and see if this is the problem. But first, does any one know which relay leads to the pump? the diagram card is missing from the fuse box lid so I would like to know which one to look at first.

Neil

[Doug Wilson]

I do not know if they would be in the same location or in the same order. I asked that same question and was told it was on the firewall side (left side looking at them) of the bank (two rows of three). Mine was top on the right side. If you hear the pump humm when you turn the key on, you can just remove a relay and turn the key on to see if it humms of not, then move to the next one if needed. I put 12 volts accross the correct terminals of the relay and then checked continuity on the switch and found that it flucutated, so ordered a new relay.

[ctaylor738]

Look for the purple/white wire from the starter, and the white/black/red output to the pump, assuming it's the same as a six cylinder 116.

[pxland]

Should you find that the pump is getting power do not give up on it as shot. There is a great post about taking these apart and de-gumming them. It worked with mine and saved me a couplahundred bucks. I saved the info on my home computer and would be glad to repost it if you like.

Let me know.

Paul X.

[Doug Wilson]

I would like to see it. Heck it sounds like it is something good to keep for future reference if nothing else.

[Neil]

Pxland,

Same here please post if you have the time

[pxland]

I beilive this was origionally posted by PSFred.

If it's not been run in 5 years, the fuel pump will be stuck, so take it off before you try and drain all the slime out of the tank (detergent and residual fuel oil is all that will be in there). Flood pump with carb cleaner and connect to a battery (spark free at the pump, please!), reversing the terminals every few seconds. Should thump and hum (very unlikely to actually turn at first). Keep reversing and flushing with carb or brake parts cleaner until it runs, as it most likely will eventually.

Re-install, put at least 5 gal of fresh premium in the tank, add a whole bottle of techron fuel injector cleaner (or RedLine if you can get it), and jumper the fuel pump relay or wire pump directly to a battery and let it run a couple hours. This will hopefully flush all the slime out of the FI ring main and injectors. Fill tank the rest of the way wifh fresh fuel (or at least half way, anway), then attempt to start.

Mine had sat for more than two years, and this started it right up. Ran like crap until I burned off that tank, but all in all not bad.

Tranny will be low of fluid since the torque conveter will have drained back, too, so have some fluid handy to top it up.

Peter

[pxland]

Originally Posted by pxland

I beilive this was origionally posted by PSFred.

If it's not been run in 5 years, the fuel pump will be stuck, so take it off before you try and drain all the slime out of the tank (detergent and residual fuel oil is all that will be in there). Flood pump with carb cleaner and connect to a battery (spark free at the pump, please!), reversing the terminals every few seconds. Should thump and hum (very unlikely to actually turn at first). Keep reversing and flushing with carb or brake parts cleaner until it runs, as it most likely will eventually.

Re-install, put at least 5 gal of fresh premium in the tank, add a whole bottle of techron fuel injector cleaner (or RedLine if you can get it), and jumper the fuel pump relay or wire pump directly to a battery and let it run a couple hours. This will hopefully flush all the slime out of the FI ring main and injectors. Fill tank the rest of the way wifh fresh fuel (or at least half way, anway), then attempt to start.

Mine had sat for more than two years, and this started it right up. Ran like crap until I burned off that tank, but all in all not bad.

Tranny will be low of fluid since the torque conveter will have drained back, too, so have some fluid handy to top it up.

Peter

O.K. its not quite taking it apart, but it is de-gumming. I actually did take the plastic cap off of mine (which had sat with fuel in it for 15 years) and cleaned as much crap out as I could. Be careful if you take the fuel pump apart as there are little metal rods that fit into slots (six of them I believe). These could be very easy to loose.

Paul X.

[Doug Wilson]

Thanks, I will put this where I will be able to find it (hopefully) in the future.

[pxland]

Originally Posted by Doug Wilson

Thanks, I will put this where I will be able to find it (hopefully) in the future.

No problem. I keep all of the past and future useful info I find here (I hope this doesn't piss anyone off) on one big word document. The people in the know here could literally write a book about troubleshooting vintage MB's.

Paul X.

[Doug Wilson]

I agree, the info you can get from people on this site is great. It seems as if someone has had a simliar problem and knows how to fix it and of course all are willing to give their input and advice. We can't forget the mechanics and retired mechanics that are here as well. The people here are great!