How to replace "Bypass Hose" for 1990 300 SEL
 

Is this a big job?

My bypass hose has a leak. I can't just replace the hose, it looks like I have to take somethings off such as the thermostat or the water pump.

Does anyone know what I need to do?

I have no idea why Mercedes engineers made this so difficult.

Thanks,
Matt

You need to remove the waterpump to replace this hose.

Not a simple job but is doable - it's just time consuming and you need to have all the correct tools.

There is a DIY on this in the resources section under DIY articles.

Been there done, that. It's not hard just time consuming. May as well replace the water pump while in there, any other hoses, new belt, and flush the system.

thank you.

This may sounds like a dum question though, what would be the harm to taking a hacksaw and cutting a 1/2 inch off the pipe that connects to the block? This would allow me replace the hose in 2 minutes.
Is there any reason why the bypass hose has to be so short? Any reason why the thermostat is so close to that pipe? My hunch tells me that the reason why it is so close is because of the pressure from.the water pump may cause a leak if not so close.

I doubt it generates enough pressure to rupture a hose. Its probably designed that close because the German engineers like to have metal between connections more than rubber. :D I'd leave it alone and repair it correctly. Hacksaw = metal particles into cooling system, not good.

Don't know, let us know how it turns out. :D

Thanks.

My other thought that this is how mercedes gets people to go to the dealership for repair$.

Well, it's bound to happen sometime...

Just about everyone here isn't the first, second, or even third owner of their cars. So knowing what has been done and what hasn't is an issue.

You could be on your second one and second water pump. That's pretty good for over 300K.................

I would guess that MB timed it so if this hose needed replacing then so did the water pump. There is also an O ring between the water-pump and engine block that gets hard.

AS the vehicle is nearly twenty years old, you should replace water pump, thermostat, tensioner, belts and hoses. (flush radiator)

I replaced it...
 

without removing the water pump in my 300e. Yes, I cut the hose and installed the new one through the gap. The secret is buying the hose from the dealer as it is more flexible than after market. You will need lubrication and a lot of patience but, it will get in there. Worse comes to worse, you will need to remove the water pump. By the way, if you need to cut the pipe, make sure it is on the water pump side.:D

Good luck!

thanks.

by the way, why on the pump side? i was thinking of cutting the pipe on the block side. seems long enough.

i second the line on replacing the water pump.....as you are replacing the hose.

i was out of town...and my wife started driving on a water pump that was weeping slightly she didn't notice..

cost me a water pump and a head gasket.... 2k. if i had been in town i could have done the water pump for 150 dollars... oh well.

Pump side...
 

Worse comes to worse, you can replace the pump. You do not even need 1/2 inch, may be a 1/4 inch. Do a search...I am sure I posted something about it.
However, if you are keeping the car, buy a new pump.

Overall these are easy to work on, but some things are difficult, as with most other cars. I think they do it deliberately just to annoy people. ;)

Seriously? You managed to slip the hose into that 1/4" gap? I thought even god can't do that. What kind of lubricant did you use (engine oil?). Can you give us some details? I think sooner or later I would have to replace that hose on my W124.