How long would it typically take if a disco had a cracked block to overheat ?

 

If it was pressurizing the cooling system 10min at idle there abouts.

 

Would you see bubbles right away in the coolant if that was the case ? Why would it be so fast you think ? Coolant would just disappear into engine that quick or is it no circulating properly ?

im going to look at a 04 that guy spent tons on new head gaskets thermostat rad fans etc and says it overheats after 15 min.

I just wondering if it’s a thermostat or something that’s stuck.
Any easy quick way to tell if block is cracked

 

If the block is cracked coolant would likely be seeping into a cylinder and burning off, fouling that plug in the process.

I would inspect the plugs looking for the odd fouled one...

The best way to tell would be to get the block warmed up, say 10 minutes, so it doesn't overheat, and then pull the fouled (or all) of the plugs and inspect inside the combustion chambers with a USB boroscope. You should see the coolant entering one of them in the form of bubbles around the firing ring where the head meets the block. The coolant will make its way either up or down to get around the cylinder to get out. Usually up. Possibly down but no way to inspect for that without much more disassembly.

 

so say it was block cracked and I let it run for 10 min the coolant level would drop quite a bit in the expansion tank right ? In theory if you kept adding collant into expansion tank it would never overheat right ?

 

The point of letting it run is to build up pressure in the cooling system. I don't think you'd lose any coolant through a cracked block until things heat up enough to expand the cracks and pressure builds up enough to force coolant through them. That is the place you are looking to get to. In my experience with a cracked block the coolant level did not drop rapidly like you describe. I got misfires before I got to the point of overheating.

Either this block has much larger cracks or it is something else leaking. If it has much larger cracks the owner must be aware of misfires and fouled plugs long before he got to the overheating point.

 

With my slipped liner, or cracked block, coolant was dumping into my sump. Got fed up and invested in a Turner engine. Happy ever since.

 

so if it is pressuring the coolant system how does it overheat ? Would coolant not flow or would it just start coming out of the overflow tube

 

Open the reservoir and if it smells like exhaust fumes, or fuel = chances are it's pressurizing the system.

I knew a guy that bought an 04 without telling me (I would have gone with him). He said he drove it and all was well minus a coolant leak/cracked radiator tank. When in fact the seller loosened the secondary air temp sensor. System would never build pressure so to the novice all looked ok minus a small coolant leak.

When in fact once I tightened up the sensor, and let it run within 5-10min coolant was blowing out the reservoir cap as the coolant PSI was well beyond 16psi due to exhaust gases getting into the cooling system. It was also coming out the exhaust as well. Reservoir smelled like exhaust fumes/fuel, and he figured it was just HG's. He had it torn down and he had a bunch of stuff going on.. It overheated so bad many many times the heads were warped, oil had mixed with coolant, slipped sleeve, along with a crack in the block on the cylinder with the slipped sleeve. D2 is just lawn art now as he spent nearly 3k on that 04 and doesn't even have a title or an engine for it.

Sneaky seller had changed the oil, loosened the sensor, and the reservoir cap to fool the novice and it worked! Engine was only good for a coffee table if that.

 

The bottom line is it could be that the coolant system has leaks elsewhere or he didn't bleed it correctly (or at all) after all that work.

I doubt it would overheat in 15 minutes from a cracked block unless as mentioned above the coolant was dumping into the sump.

You should be able to see that on the dipstick as it would be milky and way over the full line, and the overflow tank level would be way down.

I'd bet the truck has a cooling system problem rather than a cracked block, but check the condition of the plugs anyway.

Water pump should be new with HG's but you never know... Thermostats have been known to be bad out of the box... I would think he bought a new one but which one? you should try to ID it.

Of course... It could be all of the above.