Change out your radiator if you think it is old or Original.
 

My car 1987 300D had a near death experience last week. It was turning 400k miles on the way back from San Francisco. A milestone reached so to speak. However, when it was climbing Los Angeles National Forest the radiator neck snapped and lost most, if not all, coolant. It overheat big time. Luckily we were near the top of the climb and limped to

Fort Tejon State Historic Park

https://g.co/kgs/TjZDaU

The break wasn't a clean break and I couldn't attach the upper hose back on. I had it tow home. Ordered a new radiator and car is now back on the road. So check your radiator and change it out if necessary, plastic radiator is prone to breakage.

Yes, plastic end tanks on radiators (and intercoolers) don't make sense for parts longevity.

My new DD, 'Kjetteri', developed a mystery coolant leak with a tiny bit of foam in the coolant expansion tank.
Turned out to be a cracked plastic radiator nipple, and not a head gasket failure.

On Monday morning I bought a 3/4' X 1/2" PVC coupling and 2 oz of epoxy putty to repair the nipple in the afternoon after the car was cold.

On the way to work, steam started wisping out of the left front of the hood, so I did an emergency roadside repair.

The plastic end tank still leaks a tiny bit of steam from a crack between the top overflow nipple and the top hose nipple.
A replacement radiator will be installed...if I end up keeping this 'Heresy'.

If you need a hand with anything I’m not too far away. I can grab a radiator from my 94 and bring it to you.

I just had a coolant bottle explode.
No excessive pressure, possible prior trauma from an overheat long before I got the car, but has been fine for years now.
I went ahead and replaced the overflow tank and cap just in case, car is running a-ok again, incredibly it blew out in my driveway right as I got home one day.

It happened to me when I used to own the w126 1989 300SE.
Luckily right before entering the freeway bridge entrance on Turkey day!
I stopped immediately.
Fetched some water from a near by store.
was able to keep the neck closed with the 'mechanical' hose pressure,long enough to drive back home. (maybe 6 miles).
Did not overheat.
I think I saw the steam just before entering the freeway... long ago....

replaced the thermostat cover with an aluminum one, the original was plastic, and also replaced the radiator because the neck edges were deteriorating.

It happened to me and about a year latter it needed a head gasket and the head flatted. That's one pattern I've seen several times before.

Glad you have it running again, and it wasn't too catastrophic.

For those with W123 300D, new radiators are now affordable. I got one for $130 vs used to be $350. That solved the overheating in my 1985 300D after trying many other things over years, even another used radiator first left sitting with citric acid for a week. That climb on I-5 N out of the L.A. Basin, on a >100 F day, is where I have heard new car coolant designs are verified. If you go up the grade slower, that will reduce the horsepower output of the engine, if fighting an overheating issue.

This.

When needing to verify work I’d done to my cooling system, this was the proving ground.

Uncle Kent sells a reinforcement kit which is quite effective at preventing this exact issue.

MS in Bellingham has issued a warning on this and recommends a copper sleeve inserted inside with epoxy to reinforce it. He sells the components. it is a known problem and why these car companies think that plastic radiators caps are acceptable is beyond me.

You can reinforce it all you want but the breakage is at the base of the neck and took out a bit of the body itself. Basically no 'neck' left. The kit would not help. The breakage is a crapshoot thing so you take your own risk.

Yah. I can remember all my breakdowns on Tejon pass. living between the Bay Area and Socal we drive this all the time. I think we’ve had four coolant system failures there over fifty years. Most were in my childhood because my dad was an optimist driving his dodgy cars over the hill during summer. My MBs never failed there but they did get hot on occasion and required turning on the heat. My wife and kids hate that part. Sweltering day “we are turning on the heater full blast!”

My rad neck popped a decade ago in SoCal I was taking my FIL out to lunch and it just popped. Luckily there was 1/4” left on the neck and I managed to slip the hose and clamp back on and I wired it with safety wire to the bolts on the oil cooler and possibly elsewhere to secure it temporarily. I recall walking into a restaurant and getting some water out of their sink with a milk jug and refilling. Got back on the road in half an hour.

When I got home I watched the uncle Kent videos. He has a funny one where he tells what a great guy he is for grabbing somebody’s rad neck and giving it a shake and then breaking it off. “See I saved you a breakdown.” Interesting how he convinces himself how wonderful he is. I’m sure the person with the snapped radiator is speechless. He does have a point though. Just an interesting psychological response on his part. Reminds me of some friend “hey man you should be thanking me!”

His kit is basically a piece of copper plumbing and some JB weld. There is a size at Home Depot that fits right in there. On prior cars I bought one and glued it in. Maybe I should do it again. I’m sure there’s a thread here about which pipe exactly to use. If you cannot find it, measure it and go to Home Depot and find the tube.

Let me check my notes. I bet I wrote it down. Huh…my notes are here. Brought back some bad memories. Maybe I should reinforce my cars…both have new radiators since I wrote this.

http://www.peachparts.com/shopforum/diesel-discussion/349577-plastic-radiator-neck-cracks.html

A constant tension band should be used rather than hose clamp just like the newer models with the same plastic neck and no issues with them snapping.

On both high passes climbing north out of the L.A. Basin I-5 N over Tejon Pass (4100 ft) and I-15 N over Cajon Pass (3800 ft), you see cars on the side with hoods up every 1/4 mile on a hot afternoon. Seems most common in late Spring after owners didn't check coolant level all winter. Even with proper coolant and pristine parts, new factory designs can barely handle the power output of those steep climbs at the 70+ mph many drivers attempt. There are a few emergency water stops.

A less challenging route to the Central Valley follows the rail lines: CA-14 to Palmdale (3300 ft pass) then US-58 to Bakersfield (3800 ft pass), and is less often snowed-in. Drivers going to SF Bay can take U.S. 101, also a rail-line with just an 840 ft pass to Ventura. Last time I-5 closed for snow, the Tesla Supercharger in San Luis Obispo was overwhelmed, with 6 hour waits to charge. Leaving the L.A. Basin going east only climbs to 2600 ft (I-10).

On both high passes climbing north out of the L.A. Basin I-5 N over Tejon Pass (4100 ft) and I-15 N over Cajon Pass (3800 ft), you see cars on the side with hoods up every 1/4 mile on a hot afternoon. Seems most common in late Spring after owners didn't check coolant level all winter. Even with proper coolant and pristine parts, new factory designs can barely handle the power output of those steep climbs at the 70+ mph many drivers attempt. There are a few emergency water stops.

A less challenging route to the Central Valley follows the rail lines: CA-14 to Palmdale (3300 ft) then US-58 to Bakersfield (3800 ft), and is less often snowed-in. Drivers going to SF Bay can take U.S. 101, also a rail-line with just an 840 ft pass to Ventura. Last time I-5 closed for snow, the Tesla Supercharger in San Luis Obispo was overwhelmed, with 6 hour waits to charge. Leaving the L.A. Basin going east only climbs to 2600 ft (I-10).

Mine blew on my 83 300D after only 3 laps of a track, then I drove it like that for like 3000 miles including a giant roadtrip, before replacing it with the new style with the wonderful baffles and the level sensor blockoff

W123 400K Club
 

Welcome to the 400K mile club!! Your 300 D looks awesome!

My ‘78 300 CD (C123) has 415K miles. It was my daily driver for many years, but is now driven on nice days when I can put the sunroof back and windows down and enjoy pillarless driving!

Jim
‘78 300 CD

My 1985 CA 300D has 380K miles, but the original engine failed at 330K, then found a pristine used engine (>400 psig compression all cyl). The original paint has failed clearcoat from the evil CA sun but about to repaint it. Daughter crashed it (totaled USPS Transit van which hit her side front) so I bent the front rails back and got new fender and hood from a 1983 (given whole car less engine, doors, & glass). Since the 1983 had been recently painted in a modern grey-blue, I also took other fender, trunk, and sunroof, so now only need to paint half the car. If your 1978 is non-turbo, the engine will likely last much longer. I suspect those were the 1M-mile engines.

Bill:

First engine died in 1997 when the timing chain broke and lifted cam tower off of the head. That was 8k miles after the chain was checked and cleared. Got another engine and had it rebuilt at Metric Motors as I planned to keep the car. Still going strong. Over the years, I have rebuilt the car twice due to living in areas with winter road salt and then areas with desert heat (military). The last refurbishment was performed while I was in Japan for seven years. It currently looks very nice for such an old gal (Dick Clark/Christie Brinkley genes!). The only original parts left are the trunk lid, roof, hood, and a few interior pieces.

Jim
‘78 300 CD

Kent has some good ideas and some goofy ones. This copper sleeve in the radiator neck idea certainly is one of the latter as he doesn't consider the reduction of coolant flow, albeit minimal . An acceptable idea, however, if you happen to have a piece of copper tubing during an emergency, but just as a bandaid to get you home.

My 90 w124 diesel had the radiator neck slightly weep , took me a long while to figure out what was going on as coolant loss was minimal and it ran down side of rad hidden from view and the small amount evaporated away before it could even hit the floor.
A combination of old rad and worn engine mounts and a very old stiff top hose tugging at the rad when engine moved was the cause.

We have read here that the upper plastic nipple on the radiator cracked at the base of the neck and another person said it cracked further out. I suspect unless Kent's idea of a copper sleeve/epoxy reinforcement would help if for no other reason than it spreads the stress out all over and around the hose nipple. I decided to do my own version using the MS method. I used powdered copper as an epoxy thickener which gives astounding strength probably better than the powdered aluminum of JB weld. I also cut pieces of graphite cloth/epoxy around the base and up to the flange for the hose, carrying the load almost an inch on to the radiator cap housing all around to a thickness of 3/16-1/4". This gives what amounts to an epoxy gusset. Curious that nobody mentioned failure of the lower radiator hose nipple, perhaps because it probably runs a little cooler?

This is my take. You can reinforce it all you want but it doesn't resolve the fundamental problem of plastic fatigue. The breakage can be at the neck, at the neck base or thereabouts. You may end up with no neck to speak of. This is what happens to me at the most inopportune time. If the car is a keeper then just replace it for not so much money. I have a #14 head on the 87 300D so any overheating is a death nail. If you want to do any preventative maintenance then this is it.

I always wondered what the failure mechanism was. On my car I think I overtightened the hose clamp for starters. I really got over enthusiastic about squeezing that hose on tight for a good seal. When it broke there was a nice dent all around the nipple where I’d crushed it. Second, my engine had a lot of shake. It was a rocker. Like somebody else said above the rocking may fatigue the hose. I’ve since solved my engine shake so my new radiator will hopefully last.

JB weld is filled with steel or iron powder incidentally. I glue magnets to items with it and it makes for some really interesting fluid effects as the magnet pulls the epoxy to all the wrong places.

Maybe the pipe insert is useless. I don’t think it hurts. On my car it broke right under where I had the hose clamp. I want to put a tube in but I stopped when I saw the price of 1” copper tube. Dang has that stuff gone up in price or what!

Problem with this car again. May be the head was weaken during the climb and overheating. Car is old and high mileage so don't want to spend the effort and money to change out the head. I have a spare #14 head from a forum member.

Anyway, I put the money where my mouth is. I just use Blue Devil sealant on my 1987 300D. It has 410k miles. The radiator bursted at 400k while climbing Los Angeles national mountain on I-5. Very steep and long grade. Car overheated but I think I stopped in time. Changed out the radiator and car did another 10k without issues until 2 weeks ago. It overheated and I did the radiator overfill test and found the head is done for. Coolant overflowed whenever car is run. Long story short, use Blue Devil sealant and the cracked head is sealed. It passes the overfill test and did about 300 miles so far. Radiator hose is soft to touch every morning so no build up of pressure. Jury is still out but I will update at 5k, 10k etc.

Wow ah-kay, I am surprised that the red devil sealant did not clog smaller cooling ports inside of engine.

Good luck !!

I am surprised that you are so misguided. Blue devil will be out of business if it clogs anything.

Well, it clogs the leak in the head gasket.

I have bought 2 in the last month . Same here with the neck . I’m considering a new hose first if the ones look fairly old . It might be the rubber hose is so old it won’t seal , then you go to seal it off with tightening the hose and it’s over pressure on the plastic . Servicing the rest of them with the new hoses and come back with any problems if found .