'94 R129 SL 500 coolant leak

[E55BOF]

Or perhaps "It never rains but it pours Pt II"...

The wretched thing has now sprung a leak! It is a very pleasant car to drive, but if ever a car needed a swift kick up the jacksie, this one does...

Engine runs fine; gearbox shifts fine; no sign of oil/bubbles/scum in the coolant; no sign of mayonnaise in the engine oil; no sign of anything amiss in the transmission fluid. There is no plume of steam following me down the road, no sweetish smell to the exhaust gas, and no sweetish smell in the car either. The footwell carpets are dry. It is not losing coolant through the pressure cap overflow.

It was fine for my first 300 miles or so, but it now seems to be losing about a third of a litre in forty-five minutes' driving. It loses somewhat less cruising on the motorway, but in this cold weather the temperature doesn't get much above the 60 degree mark on the gauge (thermostat thread coming soon...). Trundling around in traffic, it gets up to and a little above 80 degrees. With the coolant temp indicating 80, there was virtually no pressure in the system when I squeezed the top hose (and the pressure cap was on tight).

Please God, if it's not just a weeping hose joint it will be the water pump rather than the radiator (or - horror of horrors - the heater matrix, which I assume would need major dash dismantling ), though there are no visible puddles in the engine undertray. Weather permitting, I'll put it up on blocks on Saturday and take said undertray off, then run it up and see if there are any drips on to paper. In the meantime, does anyone have any views on any/the most likely culprit? All advice gratefully received.

 

[Charles Morgan]

Heater matrix? Footwells would be damp.

I had a leaky duo valve on my 210 following a DIY attempt at diagnosis, on a long journey to Germany it did eat up a lot of coolant.

Low temperature related but that sounds thermostat issue.

 

[Trickythemerc]

A pressure test should show up any visible leaks.

 

[dinoy]

changed water pump before

 

[E55BOF]

changed water pump before

Or you asking if I have done so, or saying you have done so?

If a question, not on this car, but I've done a few (not on Mercedes) in the past, and I don't foresee any difficulty; that sort of basic spannering is well within my comfort envelope.

If you've done so on an M119, was there anything unusual about the procedure?

 

[E55BOF]

A pressure test should show up any visible leaks.

It certainly should reveal visible external leaks, but not having a test kit, I'll go with jacking it up and looking for drips.

 

[E55BOF]

Heater matrix? Footwells would be damp.

I had a leaky duo valve on my 210 following a DIY attempt at diagnosis, on a long journey to Germany it did eat up a lot of coolant.

Low temperature related but that sounds thermostat issue.

My thinking exactly. I've no idea whether this one even has a duo-valve; it's W140-era climate control technology.

 

[Charles Morgan]

Oh yes, they have them, as do W140s.

 

[E55BOF]

I think I've found it... Just in front of the brake servo; can anyone confirm this? It appears to be a mono- rather than a duo-valve, though. No sign of coolant leaking there, though.

 

[BlackC55]

Do a pressure test with a cooling system pressuriser. It will help locate the leak.

 

[dinoy]

sorry changed by my trusted indi cost over 200 fitted

 

[E55BOF]

Do a pressure test with a cooling system pressuriser. It will help locate the leak.

If I had one, I would...

 

[hotrodder]

Mono valve vs duo valve is usually an automatic climate vs dual zone heater deal

Pressure testing gear is fairly easy to cobble together for under a tenner if you can't beg or borrow a 'proper' kit... with the older/conventional bayonet type rad cap cut the pressure stage off an old cap and pressurise through the overflow hose, a presta valve from an old bicycle innertube is often about the right diameter. With the newer plastic screw caps can block the pressure relief valve with something (some aluminium foil tape does the job and isn't permanent) and again either use the overflow or bodge a schraeder valve through a sacrificial cap on header tanks that don't have an overflow hose

With the coolant temp indicating 80, there was virtually no pressure in the system when I squeezed the top hose (and the pressure cap was on tight).

Normal and a good sign. The pressure stage in the cap generates ~20psi tops, not enought to make the hoses feel 'hard' even when there isn't a coolant leak and/or the pressure stage in the rad cap is still working properly- they tend to get noticably weaker with age

As said if the heater core was leaking then wet carpets aside the sweet smell of anti freeze should've given the game away

assuming i picked the right part number then https://infopart.org/mercedes-benz-1295000103-part lists alternative #s for different brands of rad. That said if it's not marking it's territory on the ground or the undertray when parked up my money's on the waterpump. I know the link is for 124s rather than 129s but Mercedes Benz Model 124 - M119 Maintenance Manuals covers the m119 engine and some of the pdfs have 129 specific details too

 

[Doodle]

ECP supply OE type Behr rads - you can see where the little stamped star used to be.

 

[E55BOF]

Hotrodder, thank you for that. The pressure cap looks pretty recent, so the car may have had incontinence problems not too long before I got it. Thanks particularly for the idea of pressurising the system through the overflow; I have some reinforced fuel line which is a nice tight fit on the spigot, and some araldite and an old valve in the other end should be easy to rig up, then pressurise the system cold and start the engine, and with any luck I should see something pretty quickly. Fingers crossed...

Drew, ECP don't list a radiator for my car, but if it is the rad, if all else fails I should be able to get it refurbished, and if all else fails, a second-hand one would be better than nothing. A new pump is no problem, so I'm hoping it's that.

 

[Doodle]

I guess they're no longer stocking, I got mine from them a couple of years back.

 

[RichardF]

Pressure testing gear is fairly easy to cobble together for under a tenner if you can't beg or borrow a 'proper' kit... with the older/conventional bayonet type rad cap cut the pressure stage off an old cap and pressurise through the overflow hose, a presta valve from an old bicycle innertube is often about the right diameter. With the newer plastic screw caps can block the pressure relief valve with something (some aluminium foil tape does the job and isn't permanent) and again either use the overflow or bodge a schraeder valve through a sacrificial cap on header tanks that don't have an overflow hose

I reckon you could do something similar with tyre pressure and an old ezy-bleed kit.

How are you getting on OP? Any joy?

 

[E55BOF]

I thought of my Easibleed too, but it doesn't have a suitable cap. In the end, I didn't even need to sacrifice an old filler cap. A rummage in my washers box revealed an old motorcycle gearbox sprocket lock washer which, with a bit of filing, fitted perfectly in the filler neck. Cut a slot in it to stop the filler cap sealing, replace and tighten the cap. Ordinary car tyre valve, rubber surround cut down, clamped in one end of a short length of reinforced oil/fuel hose, push the other end of said hose on the overflow spigot, and hey presto! It works. I owe a big Thank You to Hotrodder, who put the idea of jury-rigging something like that in my head.

15 psi in the system soon found the leak; when I replaced the coolant temperature sensor, I didn't tighten it enough, probably because I was using a box spanner rather than a socket and ratchet. Coolant was oozing out from there, and of course evaporating on the hot engine, so there was nothing to see afterwards. A bit more torque, and (cue Yoda voice) "sorted it is". Unless there's another leak I haven't found yet...

It's all Doodle's fault; if he hadn't urged me to replace the sensor, I'd probably have been too lazy to bother...