Short fuel pump only works when dry?

[Mollox]

Hi,

New member here - and I hope you don't mind a 4 door W108 interloper asking questions. I've been reading the forum on and off for years but now need your help.

My father's 1972 280 SE (M130) hasn't run for a few years. The fuel pump stopped working so he pulled it off - it looked to be just a broken impeller tooth, back when impellers were hard to come by. In hindsight it was very dirty, tired and worn. Some life happened and suddenly it's 7 years later. Recently I got hiold of it, ultrasonically cleaned it and rebuilt it with the full VEP kit (inc impeller and armature) - great kit and very enjoyable to bring the old and dirty pump back to life with it. It spun up on the bench fine but critically i didn't flow test it....

I also bought a bunch of parts and spent several days refreshing various items, servicing it and getting it ready for a very nostalgic first start (he's had it 40 years...) in a very long time, in the hope of halting its recent decline. And that obviously didn't go well...

In short the pump will spin any time except when it's on the car and has fuel in it.  After mounting and when first trying it will work intermittently for a few tries then it won't work at all. I've measured 11.3 - 11.5v at the pump and also powered it directly with a spare battery. As soon as i take it off the car and drain whatever fuel is in it it will spin up again. I took it apart (again off the car) and tested each stage individually (dry!) - and all good. Right until it gets on the car and we try and start it, then it rapidly returns to doing nothing.

Am sure this must be a basic thing - can anyone point me in the right direction please?

The car is also in another country so i get to spend 2-3 days with it every 3 or 4 months so really want to get it fired up next time as getting so close was torture.

Thanks in advance

M

[dirkbalter]

Welcome.
I have had the issue that the car (108) would randomly shut down and start back up due to the fuel pump not running (which I can hear running or not). As it turned out, it was the fuse connection in the box being oxidated.
That may not be your issue but is easy to check and eliminate.
Good luck.

[BobH]

Also, check that's it's not running in reverse, see this post, if the pump case is assembled incorrectly it will run, but fuel delivery will be low

https://www.sl113.org/forums/index.php?topic=38947.msg284521#msg284521

[Mollox]

Hi

Thanks for the suggestions, but don't think it's these.

@dirkbalter I cleaned all fuse and fusebox connections but also put external power to the pump when mounted on the car to rule out power delivery. It wouldn't spin.

@BobH It's possible that its assembled incorrectly - I did follow the instructions/video carefully. I can't check until I'm next with the car but also we tested with the connections reversed adn no joy. Its not that its spinning the wrong way - it's just not spinning when it is on the car - so its either the fuel or the potential load. I replaced all fuel lines around the pump but can't speak for the main delivery/return pipes to/from the engine. If they were blocked then you'd think the pump would still be spinning or trying to spin? It's easy to hear when it does.

I have a question/theory. Is the top section (containing brushes and armature) meant to run dry? I'm guessing yes. Perhaps fuel is getting into there and stopping the pump from working?

All I have to do is disconnect the fuel lines and it will spin again!

[BobH]

Couple more suggestions:

Perhaps the impeller is on back to front, under load with fuel maybe it won't spin?

Perhaps the system needs priming, purging of air, see this post:

https://www.sl113.org/forums/index.php?topic=35193.msg257358#msg257358

Your "short" pump is designed to run wet, so fuel flows through the brushes and armature to cool it, don't run it for too long on the bench without fuel or it may overheat

[Pinder]

Have you tried running it while mounted in the car but with the fuel lines disconnected? and if it works then connect the inlet fuel line and still see if it pumps (outlet line put into a gas can to catch the fuel.  Basically, try to see if the lines are not clogged.

[Mollox]

Hi,

Thanks for the further suggestions.

Pinder - yes, I’d forgotten that I’d also removed the outlet hose from the pump and it still wouldn’t spin.

BobH - the VEP video was very clear on the thin part of the impeller vanes pointing back into the housing and I can remember double checking that. Unfortunately I won’t be with the car for a couple of months to triple check.

The purging/priming link was interesting thanks.

Re purging, for most of the time testing I had the hose to the fuel filter canister disconnected and feeding into a clear bottle as just wanted to prove fuel was reaching the engine bay. In all the faffing I saw two pulses and about a thimble full of fuel reach the bottle. But again the issue seemed to be a silent pump rather than clogged metal lines - and the test with no outlet hose on the pump should have mitigated clogged lines?

Re priming, I found the tank bone dry and so added 20 litres which is a smidge over 5 US gallons. Could that be it? I’m stuck on the logic here as if it spins dry but not “half dry” then how could adding more fuel fix that? Stranger things have happened though!

I’m also working on the assumption that the pump should spin constantly with the ignition on position 2 - and that’s my dad’s recollection of how it was. I’ve read posts about the pump priming for a second only and then resuming on cranking - anyhow, the pump didn’t spin or pump when cranking either.

Here’s the setup: I renewed all lines to/from the pump, replaced the tank screen/filter and also added an inline filter between tank and pump - and double checked filter direction!

Euro cars tee the return line to the tank back to the pump but have a feeling US cars don’t. That’s how it always was.

I also bought some swanky zinc screws for the bottom cover plate and eventually noticed they were quite a bit longer than the originals so swapped the dirty old ones back in wondering if the cover was clamping the impeller. I then backed off the old screws until the cover was leaking but it still wouldn’t spin.

Still confused! Unless it’s simply not enough fuel in the tank??

(Struggling to add a pic, trying without)

[Mollox]

Pic...I really liked those zinc screws….

[wwheeler]

The in line filter could be preventing the pump from getting fuel. The pump has some suction but most of the fuel draw is from gravity. There is a good fuel tank screen and then a very fine filter in the engine bay. That system is more than adequate for filtration. The added filter is not needed and possibly your problem.

[Pinder]

there is supposed to be another filter on the inlet of the fuel pump. (from what I can remember). 

[Kevkeller]

Measure the voltage at the pump while it’s installed and trying to run. It almost sounds like you have a high resistance somewhere along the wiring that allows it to run without gas in it but as soon as a load starts it’s not enough current to keep it running. 

[Pawel66]

Any idea may be good here...

You mentioned fairly low voltage on the pump in the car. It is likely, as Kevkeller says, it goes even lower under load and there is simply not enough juice...

Does it work in the car when you power it separately from 12V source (not a weak battery)?

I am not sure where the wires on the picture are coming from... Is it the correct arrangement for 230SL?

Also (it is not the reason for pump not running) you probably should not be using the T connector for return in 230SL if you did not replace the return fitting.

[BaronYoungman]

Undo the bleeder screw fully have someone else turn the key to  position 1 you should have the gas come with some pressure out of the bleeder.
Also keep in mind running the pump while dry will cause it to heat up very quickly and melt glue holding armature together.

Bob
Baronyoungman@hotmail.com

[Mollox]

Thanks again for the ideas guys - I can't see how to multi quote so will reply to each individually.

The in line filter could be preventing the pump from getting fuel. The pump has some suction but most of the fuel draw is from gravity. There is a good fuel tank screen and then a very fine filter in the engine bay. That system is more than adequate for filtration. The added filter is not needed and possibly your problem.

I like this one. There really isn't much of a height difference between the tank outlet and the pump inlet and the filter does seem to be raising the centre of the feed pipe a little. Also only 20l/5 gallons in the tank.  And of course it is a significant change from the proven original setup. Tank screen filter was replaced with new and the filter in the pump inlet is glistening as it was ultrasonically cleaned but the tank had sat dry for several years so I added the inline filter as a temporary measure to protect the refurbished pump just in case there was any residue in the tank around the first starts. Ironically I endoscoped the tank and it looked much better than i feared.

The bit I can't resolve in my head is - if it spins when dry but not when say "half filled", why would it spin if fully filled. Unless there's some vacuum/gravity magic at play...

Also really frustrating that the car is in another country as it would take me 5 mins to whip out the filtered line and replace with a straight line and test.

[Mollox]

Measure the voltage at the pump while it’s installed and trying to run. It almost sounds like you have a high resistance somewhere along the wiring that allows it to run without gas in it but as soon as a load starts it’s not enough current to keep it running.

I did a couple of times and got 11.5 and 11.3v. Insofar as "trying to run" it just wasn't making a sound at all so can't say if it was "trying", it just wasn't.

[Mollox]

Any idea may be good here...

You mentioned fairly low voltage on the pump in the car. It is likely, as Kevkeller says, it goes even lower under load and there is simply not enough juice...

Does it work in the car when you power it separately from 12V source (not a weak battery)?

I am not sure where the wires on the picture are coming from... Is it the correct arrangement for 230SL?

Also (it is not the reason for pump not running) you probably should not be using the T connector for return in 230SL if you did not replace the return fitting.

Thanks Pawel.To test  I also powered it directly from wires from a new Bosch 12v battery whilst on the car - and it wouldn't spin either. The wires on this car seem to come from above or beside the fuel tank but its a W108 280SE M130 not a 230SL. Same for the T connector on the return line - that's how its been for the 40+ years he's had it and it was always fine.

[Mollox]

Undo the bleeder screw fully have someone else turn the key to  position 1 you should have the gas come with some pressure out of the bleeder.
Also keep in mind running the pump while dry will cause it to heat up very quickly and melt glue holding armature together.

Bob
Baronyoungman@hotmail.com

Thanks Bob. It's a euro car so the bleeder isn't tapped, its connected to the return line via a Tee as shown in the pic above. I can't remember why we're (you're!) different - seem to recall its evap regs. If you think it would help, I could block off that return line and test with the bleeder open. In 2 months time when I'm with it...

Re testing, I never spun it dry for more than a second at a time and really not that much.

ps that's a nice-looking pump, is it for sale?! 

[BobH]

It doesn't appear to be an electrical problem, don't forget the suggestion by Bob (BaronYoungman), he's the undisputed expert on these pumps

Why not prepare to run the pump off of the car with fuel, if that works then it has to be something on the car, blocked pipes, air in the system or the position of the pump and filter