no fuel pump noise? 1972 280SE 4.5 - PeachParts Mercedes-Benz Forum

douging · #1 06-15-2006, 10:59 PM

Hi,

I've searched the forum but I'm going to throw this out in the hope of getting a straight answer.

On a 1972 280SE 4.5, should the fuel pump make an obvious noise if it's working properly?

My original pump neither pumped nor made noise. A pump that I bought from eBay makes a barely audible hum and I can barely feel something humming with my hand on the exterior. It does not pump fuel through it but fuel flows nicely from the tank with the output line disconnected.

I've owned cars before where the fuel pump made a very audible noise when the key was turned on.

You guys have been very very helpful with my project since I got it in February. I don't want to mooch from you guys too much but many of you are real authorities on these cars and you're a great resource.

Thanks,

Doug

Motorhead · #2 06-16-2006, 12:51 AM

Are you measuring 12 volts at the pump when you are checking operation? I am assuming that you are checking it with the pump installed. Have you checked your fuel pump relay? Obviously with gasoline present.... we don't need to tell you to be careful about a spark. The early injection, I believe, only worked for a few seconds when you turned the key on then ran constant as long as the engine was rotating with the key on.
I can't tell you where the fuel pump relay is on that car. Sorry.... CHris

POST your question on the "Tech help" forum

cth350 · #3 06-16-2006, 07:11 AM

When you turn the key, you should have 12v at the pump. There should be constant and quite noticable hum. You should get lots and lots of gas flowing. There is a gallons per minute spec for the pump. With the system closed, and pressure gauge included in the injector ring, you should see 32 psi and when the pump is turned off, it should stay at 32 psi for a while.

The first thing I'd check is if the tank's outlet is clogged. Are you getting a good gush out of the fuel line when it's open to the air? Is there sediment coming out (bits of rust)? If so, get the tank boiled out before continuing.

It's possible the pump is just gummed up from sitting. The cure (presuming it's curable) is to run fresh gas with lots of cleaner through it. Starting with a near empty tank, put in a big bucket of cleaner and let the pump push it through the system with the engine not running.

-CTH

douging · #4 06-16-2006, 08:39 AM

I have power throughout the car with the key on - interior lights, exterior lights, dash lights, etc - but I have not measured for voltage at the pump's connectors. BUT, I was testing the pump with it off the car and using a 12v source...

Fuel flows great out of the nozzle on the tank (and onto my floor

)

I read somewhere else that knocking it with a rubber mallet might free it up but since I don't know what it's supposed to sound like I might be beating a good horse.

'constant and quite noticable hum' is pretty much the answer I was looking for. The hum I get now is constant but not noticeable unless my ear is on the thing.

brian eiland · #5 06-16-2006, 08:49 AM

Have a look at this thread http://www.peachparts.com/shopforum/showthread.php?t=155631

My experience, pump only pumps loudly for a maybe 5-10 seconds until it builds up the required pressure, then is not audible

douging · #6 06-16-2006, 09:25 AM

That makes sense.

Interesting reports about the junk yard. I've never had great luck with junkyards but I suppose it's worth a shot.

Thanks

Tomguy · #7 06-16-2006, 10:49 AM

The fuel pump on your 4.5 should make a loud buzzing sound as soon as you turn the key to the run position. It should only last for about 1-2 seconds. You can easily hear it inside the car with all the windows shut. If you cannot hear it inside your car with the windows up, then it's weak or malfunctioning.