Hello all,
My wife and I just moved to Victorville. After driving up the 215 in blazing hot weather, I drove up the 15 (Cajon Pass) to Victorville. About half way up the last steep stretch, I noticed a coolant smell and water hitting the windshield. I pulled into the slow lane and made it up the hill. We pulled off on the first exit and the engine died. Opened the hood and steam and water was shooting out of the overflow. I pulled the rad cap- it was whistling- and steam and coolant came out- real high pressure. Let it cool for 10 minutes, started it and drove it 500 feet to the shell station. Filled it back up with water and went and got dinner- cameback an hour and a half later and started it up. After idling for two to three minutes, the overflow started whistling,turned it off and pulled the cap- steam and very high pressure. I had to get it home, so I leap frogged from gas station to gas station, letting it cool for 30 minutes at each one, for the next 2 miles- three gas stations. Got it home, pulled the water outlet to see if it was the thermostat- andsee that the previous owner never reinstalled the thermostat.
Now, after the motor idles for about 3 minutes or so, the cooling system pressurizes to the point that steam and water whistles out of the overflow and water drips from the heater valves on the firewall next to the brake booster. I'm totally lost here- no water in oil, no oil in water, no water in tail pipe, no steam in tailpipe- please tell me it's not the head gasket.
This is my wife's commuter up and down the hill everyday, so I have to get it up asap. Please help me!!! I'm so SOL right now...

-Michael

So you are saying that there was no thermostat in the engine? If that was the case, installing one will likely return things to normal. That engine has a bypass thermostat. Without it being installed, there is nothing to close the bypass, so much of the coolant flow bypasses the radiator.

It will run fine without a T stat. Sounds like a blown headgasket. If it was a blockage it would still run fine under light load. It also wouldn't happen all of a sudden.

This is a long shot but how old is the water pump? Maybe it failed suddenly in some way.

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

The 617 Service Manual appears to disagree.

"At coolant tempertures above 94 degrees C, the bypass line is closed by the bypass valve. The entire coolant volume should flow through the radiator. It is therefore wrong to remove the coolant thermostat for better cooling of the engine."

I think that we have come to the conclusion that the service manual is incorrect.

The recent thread by Jason Mills confirmed it.

If the thermostat is installed, there is a certain flow rate through the middle of the thermostat. If it is removed, the flow rate is significantly higher. Even if some of the flow heads off toward the bypass, the flow rate to the radiator is unlikely to be less than the flow rate with the thermostat installed.

Now, the testing with Jason's vehicle did not involve very high ambient, high load conditions.........so........I reserve the right to be wrong on this.

Nowhere in your explanation of the situation is there any discussion of the water temperature of said engine........either during the event.......or in your driveway.

If you have observed the gauge at any time during the operation of this vehicle, it would be helpful to provide those readings.

If the cap to the overflow tank is intact and the engine overheats immediately in your driveway, the first thing to do is to ensure that the head is full of coolant. This requires you to fill the upper radiator hose with coolant........allow the engine to sit for a bit........or start it up for a minute or so.........and repeat the process..........until the head will take no more coolant.

These engines love to trap air in the head and will immediately overheat unless you take specific procedures to get the air out of the head and displace that air with coolant.

I would replace the cap on the overflow tank and put a proper 50/50 mix of coolant in the engine before continuing with diagnosing its issues.

I ran my SD for 6 months without a t stat. It only got to 60C if I ran it hard. 40C is about what they settle at around town. If I was running it this summer I'd remove it just for fun to prove this point.

The only downside to running this cars without a T stat is they run to cool thereby increasing the chances of the cylinders and pre chambers carboning up. I'd rather run a 617 at 100C then 60C.

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Same symptoms on my 300 SD to make a long story short the problem was a cracked head not noticeable until the head was off very tiny crack at piston #2 but with the high compression of this engines and at operating temp they boil in about 2 minutes, sorry to say but you are looking at a replacement head..

__________________
2000 430 ML
2005 S500
87 300 D
61 300 restoring

3 minutes before the resivoir tank blows sounds more like overheating from trapped air in the head, than a crack, or blown head gasket.
try this:
with the engine COLD, fill the overflow with water, to the top. now with the cap off the resivoir start the car, if it bubbles immediately, start budjeting for a rebuilt motor/donor motor, or a head gasket replacement.
if you get no bubbles, and I mean AIR bubbles, not water sloshing around, within about 45 seconds, shut off the motor and go on to the next test.
put the cap back on, verify the upper radiator hose has no/little pressure, and start the car back up,
BE CAREFULL OF THE FAN, squeeze the upper hose while the car runs, if it pressurizes in less than 60 seconds, to the point that you cannot squeeze the hose, Start budjeting for above repairs...
if neither of these happen, you are most likely in luck. you said water was spraying out the heater hoses near the firewall, get these hoses replaced, or repair the leaking parts they connect to, and PROPERLY fill the block and purge the air from the engine, replace the thermostat with a proper replacement, then drive the car and see if it overheats above 110C, keep an eye on the motor for a few days, if all is still well, drain out the water and fill it with the proper amount of the proper coolant and Pray you got spared..
good luck,
John

As far as the coolant temp, the cluster gauge is inoperable, the PO put on an aftermarket but broke the line to the gauge at the sensor, so I'm going to put an electric gauge on before I do anything else. I just got the car a month or so ago, so history as far as age of components, etc, is unknown.

I'm not going to be home until the weekend, but I'll do all the tests mentioned and report back. It's a w115- worst case scenario, I have to get a new head or entire motor- can I use a donor from a 126, or do I have to find a 300D motor from a 115? (Scary thought if I do, as it was produced for two whole years... )

Unless I fall into a good engine or parts car, I don't have the cash to do anything extensive, so I may have to sell her. Any idea what a '75 300D is worth with a blown motor? Please, anyone who's religious, pray it's an inexpensive repair- I don't want to get out of the MB club a month after I get into it!

Oh, BTW, if it helps as far as diagnosis, the Glow plug relay was gone when I got it- you had to connect a wire manually to the battery to start the car- so I wired up a switch in the cabin. My wife decided it would be a good idea to leave said switch on for 2 hours or so- at least that's when I found it, but I don't know how long it took for the battery to die. I charged the battery and she started up okay the next day- glowplugs worked fine. I even tried it again at night so I could see the connecting lines glow- yup, no probs. THis happened three days before the problem I'm having now. Could there be any correlation?

Also, I wanted to expound on the water coming out of the rad- after I fill it up, I leave the radiator cap off, start the car, and within about two or three minutes, it starts shooting fluid out the filler neck, in spurts- about every three to five seconds, not in time with the engine, and revving it has no effect. If I close the cap and let it idle, that's when I have steam and water shooting out the overflow. Hope maybe this helps for a more accurate idea of what could be occuring.

Thanks a heck of alot,
Michael

Forgot to mention a few months ago with a 123 it was a clogged radiator, at the radiator shop the guy did this “test” with the engine off but engine at operating temperature he squished his hand between the fan and the middle part of the radiator he just touch the middle part and the sides, it was cold at the midle and hot at the sides, I did the same “test” and he was right on his verdict a partially clogged core so he rod it out and problem fixed, try this “test” and see what happen.

__________________
2000 430 ML
2005 S500
87 300 D
61 300 restoring