hi everyone,
i'm planning on changing the water pump in my w123 om617 on saturday. i ordered the graf model part from pelican.
i've been looking into what i need to do, and want to make sure i have it right, and what else to look out for, as i'm pretty sure there's going to be something i'm missing.

i'm changing the water pump because my coolant has leaked all out. i pulled over and refilled it with whatever coolant i was sold at a gas station for my previous toyota, and brought it to a local mechanic, not a german car specialist. they said 100% the water pump needs replacing.

while waiting for the water pump to come, i've been driving locally with distilled water as coolant, refilling every 30 min as it drips out, my temporary solution.

the best instructions i could find were right here on pelican:
Mercedes-Benz W123 300TD Wagon Technical Articles (1977 - 1985) - Pelican Parts
for taking the fan out, alternator belt, and then water pump

my Qs are:
1) i saw somewhere that sealant should be used on the water pump gasket, but it’s not mentioned on the pelican article, is it necessary?

2) how can one tell if the cap for the coolant overflow tank needs replacing?

3) is 11L / 2.9 gallon of coolant accurate? when i’m using distilled water, it gets full in a gallon or less, so if 11L is right, i’ll have to make sure to drain all the remaining water, i thought it was completely leaking out. i plan to use either MB or zerex G05

thanks!

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w123
1985 300D turbo sept 2015 - present
1985 300TD turbo oct 2023 - present
looking for biodiesel in nyc

I recently pulled my W123 coolant pump just to verify that the vanes were in good shape. The gasket remained in one piece, so I used a thin layer of silicone gasket maker to re-seal it when I put it back on the block. I cleaned the mating surface, and torqued the screws in stages.

My take (assuming that you properly diagnosed the water pump leak)

1) - replace the water pump gasket if it comes off in pieces. No need to use sealant with a new gasket. Verify that a new pump comes with a new gasket.

2) - the overflow tank on the W123 turbo diesel is pressurized. To test this, use a rag to protect your hand and squeeze the upper radiator hose when the engine is up to running temperature. The hose should have enough pressure in it to make it impossible to smash it flat. If no pressure, suspect the cap - but also look for loose radiator connections, leaking radiator, leaking booster pump, etc. If you have a good nose, use it to follow the distinctive glycol smell.

3) - I use G05 concentrate and distilled water. There is a drain on the passenger side of the block about about 2/3 back under the turbo. You can get to it from underneath. This will help you get most of the old coolant out. Since you will have the system open, you might as well use the opportunity to do a citrus flush. I used the instructions on the diesel giant website. Once you get it all back together, I recommend using a vacuum fill device to get all the air out of your system while refilling it. 2-1/2 + gallons sounds about right. Even then, it might take a few weeks (or months) for all the bubbles to work themselves out. Look for increasing cooling system efficiency over time. Check your overflow tank level regularly during this time.

2) borrow a cooling system pressure test kit from Autozone.

3) there is coolant remaining in the radiator and engine block below the level of leak point.

While apart, make sure to replace the 2 inch long hose on the thermostat housing.

How long did you drive with 1 gallon low coolant? That is not good for the head, even a cast iron one as you have.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

That 2in hose is a PITA to get to. Take the old one to O-Reilly or another good source. I ended using a coolant hose designated for an 18 wheeler. Cut off the length I needed. 8 years later, that hose is still showing no signs of leaking or bursting.

when I replaced the WP on mine, you could see it drooling coolant from the front of the engine.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

99 W210 E300 Turbo Diesel, chipped, DPF/Converter Delete. Still needs EGR Delete, 232K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Gone and still missed...1982 w123 300D, 1991 w124 300D

It is common when working on the cooling system for bolts to break off. Plan to have a ride to work in case that happens so you have time to deal with the broken bolts.

It is extremely common for the area under the coolant hoses to corrode and pit.

I had deep pitting under the area of the bypass hose. I wire brushed and scraped out the pits down the bare metal and filled the pits with the slow curing JB Weld epoxey the wrapped tape around it so the epoxey would not run out.

After the epoxy cured there was voids in the epoxey that needed filling and I repeated the taping proceedure.

After that cured I removed the tape and filed and sanded the epoxey to the contoure of the part. Have had no issues for over 8 years.

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84 300D, 82 Volvo 244Gl Diesel

thanks for all the responses. I've used about 6 gallons of distilled water so far since Monday night, each time using a gallon or so to fill to the line on the overflow tank, it lasts about half hour each time.

well, it seems like each day I do more research, more tasks get added to the list, that turned me off from doing what I originally read was a pretty simple part change.

I guess there's no chance that just the seal is worn out and not the water pump?

I talked to my mechanic, when they quoted me $620 they were insisting on also changing the thermostat, and using the genuine mercedes pump. they reluctantly agreed to leave the thermostat and use a graf pump, bringing the quote to $385, so it looks like I'm going to bail on the DIY mission to save myself a day of high potential troubleshooting and a simple task expanding to many...

If you change your water pump, and it was fine, you will still lose coolant. I would first verify that the water is leaking out of the water pump. Is the shop that said "bad w.p." also the one that quoted $620 to replace it and T-stat? I'd quit my engineering job if I could get paid like that all day long.

There are 2 holes on the side. If it leaks past the seal, you will see it coming out the lower hole (use a mirror). Not rocket science.

Changing the water pump itself is almost trivial. No sealant needed, but I like a thin coat of Permatex "gasket sealant" since it makes gasket removal later easier (cleans off w/ ethanol).

The hard part is loosening the alternator, because the EGR valve and AC tubing are in the way of where your wrenches would like to be. Even worse in my 1985 CA w/ heat shield on rear of alternator, but I did get rid of the other two.

If you do replace the short, fat hose on the w.p. housing (a different job), consider 16 mmD silicone hose, like I did (ebay). It slips on easier and should last forever.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

no,
the shop that diagnosed it quoted $490.
the mercedes specialist i prefer going to quoted $620.

but yeah it sounds easy until i get all the warnings of things that often go wrong,
parts that need to be taken out, other parts, sealants, or bolts i'll likely need to track down first. i'm underprepared to do it.

thanks though! at least i know more about what i'm paying for.

Ok but confirm in advance about that short bypass hose. Do not assume the shop is going to replace it when the water pump is done, unless it was specifically included in the estimate.

And be forewarned that if the shop starts the "genuine Mercedes" chat again, that short hose is very pricey if purchased from the dealer. Stand your ground, if it comes to that, and ask for a generic.

Voice of experience from the days before I started DIYing.

I don't blame you for bailing on this job. The DIY bar in this forum is quite high and the word "simple" is used in stretched context far too often.

I DIY-ed my water pump once, on my '85. I will in future happily pay someone else to spend three hours getting the alternator out of situ (voice of my bad back speaking). I assume some dipstick had used an impact gun to install it.

__________________


Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Replacing the Bypass Hose is a separate job and when I did it required removing one of the Housing.

Any estimate a shop give is without broken bolts. If any break the shop is likely to tell you to have a automotive machine shop do the extraction. That way they avoid any liability of the bugger up the job trying to drill and extract the broken bolt/s themselves.

I am not sure what a shop would do if they found heavy pitting under the Hose Area. They would likely expect you to get a new or good used one before the complete the job. And, that makes sense if you expect them to warrant the work.

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84 300D, 82 Volvo 244Gl Diesel

well, the mechanic i brought the car to specializes in german cars, including these old ones, they're almost like going to mercedes, but independent. i trust them to be able to handle any situation that comes up, but for me right now, if it isn't broken, or clearly about to break, don't fix it.

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w123
1985 300D turbo sept 2015 - present
1985 300TD turbo oct 2023 - present
looking for biodiesel in nyc

Bad things happen to good people and intentions. I wanted to change my bypass hose, however the bolts were rusted in the housing.

Steve, that is some serious work getting to those bolts.

Steel bolts going through aluminum will rust to the point they seem like they are welded in.
I slather those with Anti-Siez on the shaft before installing them.

The three little bolt holding the two halves of the thermostat housing together, I give them a good dose of it.

It`s always good to grab these housings when in the yards for a back up.


That bottom bolt on the Alternator is another one that will weld it`s self in.
It hangs down low and gets all the crud from the road thrown on it.


Charlie

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there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

When going to a shop, ask about what ifs. Also have them diagnose because you are paying for them to guarantee the work.

I had a shop put a rear main, clutch & trans behind the Cummins. The transfer case was leaking and had to come out. I told them to stop the leaks in the transfer case while it was on the bench. They put new seals in both ends but didn't change the tail housing gasket or replace bushings that were contributing to the seal leak. Had I said "replace the seals" the ongoing leak would have been mine. They misdiagnosed as needing seals only & had to fix it. I paid for the extra parts and labor (not much).

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

yeah, i do wish i asked the 2nd mechanic to diagnose, but i had called them for an estimate instead of going in, since they're 45min away, and i was gauging if i would do it myself. i'm hoping, being the specialists they are, they will diagnose for themselves anyway.

the 1st shop is 3 blocks from my apt, i would trust them to change the water pump too, except for all the potential complications with stuck bolts etc.

the shop doing the work only uses mercedes coolant. if for some reason it gets low, as they say to watch the levels shortly after a change, i should have mercedes coolant handy right? instead of adding zerex G05? mixing sounds sketchy but i also read it's the same stuff...

thanks!