Suddenly overheating 280SE

[CraigB]

Big day, leaving me thinking ' what did i do to deserve this!'. The short story is that i picked up the khanacross W116 280SE from having new engine pipes made, drove about 5km to my 92yo mums place to try and sort out some mobile phone issues etc and the car behaved fine and temperature relatively low. I am not running a fan but can switch on a big W126 electric fan if needed. All through the thrashing in competition, I only used the fan in form up after runs to drop the temp to the lower side of normal range before the run.

I took a trip of a couple of km to pick up some 'corona cash' for her (she is sticking to home and needs cash to pay for a few things that come to her) and returned. Temp all still normal and low. Then finally heading home.... after the trials of explaining to a 92yo not to be scared of using the functions of the phone just because it did something odd.... took a while.... and finally getting to head home in darkness.... got a couple of km and sitting at the lights and noticed temp was right at boiling point and turned the big fan on. It held on and got me through the lights to pull over. Heaps of pressure in the hose and no sign of other leaks in the system, but I realised that the fan was not working and when I looked closer, the wire had been knocked out to the fan (was a bit of a bodge by me and something i planned to go back to once I worked out it was useful. I reconnected, let it blow for a bit and then revved it for a bit to try and circulate water past it. Was still pretty hot so i waited for 15 mins or so for it to cool a bit to about 80. I didn't have any spare coolant or water on board.

I then drove on a few more km but it shot back up there, so I pulled over again and waited a bit longer. Keen to finally get home, I gave it another go but pushed it a bit too far and when i pulled over it blew. Rang Jo to rescue me with some water, about 30 mins drive, so chance to cool down a bit more. It still held pressure for quite a while, every time I went to first notch on the cap and rocked it, still wanted to push out water. I then fired it up and trickled water in, so it didn't get hit with a block of cold water, but I was surprised not to see water flowing past the cap.... maybe cool enough to close thermostat I thought.

Took off and got a few km up the steep Shepherds Hill Road - started at about 40 but it just climbed and climbed continuously before boiling again. This time while waiting i had a better check and realised that while there was pressure and top tank felt super hot, bottom tank and the core itself felt super cool..... and I still hadn't seen water circulating past the cap. So my guess is a broken water pump. Belt is tight and bolts in it. A bit dark to check if the shaft is turning. My guess is the impeller has fallen off or something like that.

Unbelievable though - how often does that happen ... could be worse though - Khanacross resumes in a few weeks time and it could have happened at the start of an event and wrecked the fun.

[Nabstud]

Possibly a stuck thermostat too?

[tsharkey]

Possibly a stuck thermostat too?

That's where I headed. I would have thought a broken water pump would have leaked given it's just a cast iron impellor

[CraigB]

Will report back and will of course check the easier thermostat option first before trying to break bolts holding an old waterpump! My reason for not thinking thermostat is that with no flow past it, would you expect the top tank to get so hot? I was thinking that with it open, and everything sitting still, its own little convection currents would pass the heat through to that top tank, sitting up above the head. Also this was a 'black and white' event - car running on the cool side of normal all the way and then 'bang' overheating and reheating. It didn't have a chance to cool down at any point to close the thermostat in that time either. I would have thought some sort of sticking and raising of heat first - not like a thermostat to snap shut within a matter of minutes once engine warm. All will be revealed but thought i would just share for the record my thought process. Also radiator blockage ruled out for similar reasons - weird how it is uniformly cool over all the fins and bottom tank - any water seeping through is getting cooled, but thermostat would have that same behaviour.

[Nabstud]

Well I have a good secondhand water pump here if you need it haha!!!

[T-Modell]

Hi Craig,
that's what happened to my AMG in 2018 at 225,000kms ... No power, no warning and above 120°. My "good old mechanic" said instantly "water pump" and it was the case, no ciruclation. Funnily, the 6.3 AMG has two water pumps, so the heating inside still worked.

The propeller (plastic) went to pieces and I myself stood in the MB workshop for around an hour trying with a water hose to wash the pieces to the front where I could get them out of the motor. Luckily nothing more failed and the car drove another 20,000kms.

No leakage btw.
Regards
Thomas

[CraigB]

Well i am very pleased to say i was wrong and you were right! Pulled the thermostat (and of course sls and you can't get it out without taking the whole housing), boiled it up and it stayed fully shut. Its not seized because you can push it against the spring tension, so i guess the wax has leaked out and that just happened to happen in the dark, at the end of very long day and within a couple of km. Could have been worse I guess!

As for behaviour, I guess even if the thermostat isn't opening, there is a continuous column of water from the head, through the small hole in thermostat and up to that top tank, and so even if there is not flow of water, there is conduction of heat perhaps. But once it gets to the fins of the radiator, they are constantly being cooled.

I haven't test driven it, but with both the old and another used thermostat in the saucepan, one stays shut and the other one opens, so you would expect all to now return to its good behaviour. A new lesson for me though and out of all the thermostats i have tested in my years, never seen one not even move like this one.

[Nabstud]

Rarely does it happen but it's nice when its the easier solution that fixes the issue!!! Normally it's the other way round....

[CraigB]

Absolutely! I tried to drive it this arvo but had trouble starting it. Wet and miserable here so didn't worry too much. The car was always a good starter, right up to the khanacross and in the week afterwards. Then it sat for a while with removing manifolds and broken studs, probably 2 months, and then starting it without any exhaust to get on the trailer, it wasn't keen to start.

What i noticed is that the tachometer is bobbing about, going to redline and back - sort of bouncing. I think i have seen that behaviour before on another car but can't remember what caused it. 1976 kjet with standard electronic ignition. Once started its going like a rocket and totally normal behaviour. It was a used fuel pump I put on it, so i could check fuel pressure. Its not a hot start thing so unlikely the accumulator. I wonder if i knocked a wire off somewhere when doing the manifolds - will study up on what it uses for cold start. Also if i put a bit of fuel down the intake and it fires quicker, that might direct me to fuel or ignition. But this tach might be a hint its ignition related.

[Chai]

The tachometer needle bobbing and occasionally redlining is symptom of an electrical issue with, I recall, the contact breaker to ground circuit.
In my case, it started when the earth wire on the distributor plate, where the contact points are screwed to (and swivels on advance and retard) began to break/fray from metal fatigue.
When all the strands of wires finally snapped after a few months of the bobbing tacho (with the occasional and intermittent loss of engine power), the engine died and couldn't be started because the electronic ignition didn't get the signal from the contact points.

[CraigB]

Thanks for that and it makes complete sense .... oh dear, revealing my slackness. The car fired up so easily when i got it I haven't even had the distributor cap off - I was assuming it was like my 1980 car with the same electronic ignition box and had breakerless ignition. More to investigate tomorrow... I might even lash out and check the timing! I think it is because the car only just got back together before the khanacross and hasn't been used regularly since.

[Chai]

Had another think - it's most likely a loose electrical connection from between the electronic ignition module to the contact point.
The tacho sensor wire is joined somewhere in the engine bay (don't know specifics of the 280SE), between the module and the contact points.

[CraigB]

thanks Chai - I checked today and it doesn't have contact points but has the sensor thing, but that might be what you mean. I can see the wire and the rubber cap at the ignition box end, but not sure about the connection at the distributor - doesn't seem to be a part that moves to twist the wire (like the wire to contact points on a moving plate) but I can see if I can check resistance. The rubber cap on the other plug into the ignition unit also just crumbled when i checked it out. I need to think how to waterproof all that. It looks like the tacho wire comes out of that 4 wire plug, one to earth, one to ignition power and one to coil. So that's all pretty straight forward checks.

I think i said that it was running well all through the overheating etc, when i first picked it up from exhaust, I was redlining and listening to the new exhaust, no ignition break down, but not as easy to start as it had been in the past. I guess it might have been generally dry conditions back when using the car and by the time i restarted, air much wetter. Few things to chase and test.