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overheating - what actually stops the engine?


freeagent
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My 300Tdi disco overheated no the way home the other night, while doing 70mph on the M25 :angry:

I looked down to see the temp gauge at maximum, I pulled off onto the hard shoulder but the engine died just as a slowed down... I know the head gasket is knacked as there was loads of steam everywhere, including coming out of the tailpipe, thetre was also water splashed all over the engine bay, so I asume/hope it didn't cut out because the pistons were siezed into the block?

So what actually stops the engine? Is there anything that kills the electrics (inc fuel shut-off solenoid) to prevent the engine completely self-destructing?

Only reason I ask is the cabin fan wouldn't work moments before I stopped (i tried to put it on and turn the heating right up to drag some heat away from the engine) the serpentine belt is still intact so the alternator would have been spinnning.... which says to me a fuse has blown somewhere?

The Disco is now at a garage, as I don't have the time/ expertise to fix it at the moment, I know it'll need a head skim and a new head-gasket, but am hoping it hasn't trashed the engine completely...

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Sorry but usually what stops a Tdi is mechanical death - there is no "fail safe" it stops when it can't move any more - I have seen a couple cooked in this way and they were scrap :(

I had a feeling that would be the case.... :angry::angry:

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Coolant overheat causes excessive friction/materiel expansion between the pistons and bores, so piston siezure is normal.

Oil loss causes excessive friction between bearing shells and crank/cam pins. Coolant induced siezure is usually repairable, but lubrication failure can be permanent damage (or at least very expensive to put right). My 300TDi engine suffered from both - which was nice :)

An overheated engine will quite commonly conk out with no sign of siezure when it's stripped down. Sudden no-load on it and a blown head gasket will make it stop usually.

Expect your head to be bent and maybe cracked as well though :(

Les.

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The piston to bore clearances on modern engines is a bit tight compared to earlier designs and are not so forgiving to instances of overheating.At the very least you'd probably require a new cylinder head, pistons and a rebore. The 1959 2 1/4 litre engine in my vehicle by comparison has on 2 occasions due to failed bottom radiator hoses been boiled completely dry at 60mph on freeways, and although a bit low on compression and having a cracked block and cylinder head filled with cold galvanising paint as a result, is still soldiering on and would probably take me round the world if I asked it to.

Bill.

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Coolant overheat cuases excessive friction/meterail expansion between the pistons and bores, so piston siezure is normal.

Oil loss causes excessive friction between bearing shells and crank/cam pins. Coolant induced siezure is usually repairable, but lubrication failure can be permanent damage (or at least very expensive to put right). My 300TDi engine suffered from both - which was nice :)

An overheated engine will quite commonly conk out with no sign of siezure when it's stripped down. Sudden no-load on it and a blown head gasket will make it stop usually.

Expect your head to be bent and maybe cracked as well though :(

Les.

Hmm.. this sounds slightly more encouraging, and makes some sense, as the pistons would expand as they heated up to a point when the resistance would cause the engine to stall... when everything cooled down it may return to normal...

I am expecting at least a head-skim, but was hoping to avaoid a new engine...

We'll see what my garage have to say when they look at it at the end of the week...

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Its likely stopped because the HG let go, rather than it actaully siezing up i would imagine. But you'll really need to take it apart to see what damage there is in the bores etc. One thing i'd say is dont leave it sitting for any length of time, as the water in the bores will corrode them in no time.

I once baught an astra that had blown its headgasket. The wifey had parked it up outside and left it for a couple months before i baught it. Engine wouldnt even turn over, and when i pulled the head off the bores were thick with rust and the pistons (well the rings presumably) were rusted into the bores so badly it wouldnt even turn over.

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Its likely stopped because the HG let go, rather than it actaully siezing up i would imagine. But you'll really need to take it apart to see what damage there is in the bores etc. One thing i'd say is dont leave it sitting for any length of time, as the water in the bores will corrode them in no time.

I once baught an astra that had blown its headgasket. The wifey had parked it up outside and left it for a couple months before i baught it. Engine wouldnt even turn over, and when i pulled the head off the bores were thick with rust and the pistons (well the rings presumably) were rusted into the bores so badly it wouldnt even turn over.

i'm hoping it will be stripped down within the next week, so not too long to wait..

We are currently looking into wheather a vehicle actually needs to be running in order for it to qualify for the governments scrappage scheme... :rolleyes: just in case it is a complete scrapper.

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I have a feeling the engine did not seize. Either an electrical induced problem (with steam everywhere around the engine comp) or from the amount of coolant got into some of the cylinders (given the white clouds on the tail pipe). Nonetheless, the cylinders, pistons and at least the bigends should be inspected.

An engine temperature alarm would have saved you from this. The stupid gauge indicator is useless. By the time it reaches the red zone the temperature is over 135 degrees and the engine is already cooked.

I have a temp alarm on my engine and it saved me once from disaster (given I was on vacation in another country). The temperature reached 105 degrees and the needle from the stupid factory gauge was dead in the same position as when the engine has 75-80 degrees.

This is one elementary warning thing these engines don't have, a plain and simple temperature switch on the cylinder head that would close when the temp is above say 100-105 degrees and trigger a warning light on the dash.

This is something anyone can fit on (ex) EGR tdis in place of the temp sender in the middle of the cylinder head. Couple of wires, a bulb and a buzzer would complete the circuit.

At least this, if not a more fancy and expensive thermometer with alarm and and coolant low level alarm.

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^^^^ if it is repairable and financially worth while to repair it, I will be fitting a proper digital engine temp gauge, and a low coolant level alarm to try and avoid a repeat performance.

It is very important that you have the con rods checked, they normaly bend.

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