M52 Diagnosing: slow cranking (no start) after overheat

[bjamham]

Hey Everyone,

I've got an M52B28 with ~200,000km on it. I just purchased the car for a great deal under the assumption that the engine needs to be replaced due to overheating.

The previous owner said the car overheated while driving and he had to get to a doctor's appointment so just continued driving while it was hot. The oil level checks out okay with very minimal signs of coolant in the oil (not that some is okay).

I'm trying to diagnose just how bad the engine is. I started with a compression test, but the engine cranks very slow, even with a booster pack on the battery and spark plugs removed. Could this be a starter issue, or could overheating be bad enough that bearings or pistons are tight? I don't want to spend much time if the engine is blown but will happily use it if I can get it running.

Questions:
- can an overheat "cook" a starter?
- if I'm turning the crank with a ratchet, how much torque should it roughly take with the plugs out?

[Panici]

the engine cranks very slow, even with a booster pack on the battery and spark plugs removed. Could this be a starter issue, or could overheating be bad enough that bearings or pistons are tight?

Battery and booster pack are confirmed good on another vehicle? Grounds from body to engine are good as well? Did you try a set of thick jumper cables from another vehicle with it's engine running?

I'd be getting a borescope down through the spark plug holes to look for rust from a failed headgasket.

You've changed the oil already?
Try putting a little oil in each cyl to lube up the walls and see if you can get it spinning over quicker.
The shadetree mechanic's choice to free up a "stuck" engine is ATF+Lightweight oil just for reference.

I don't have a torque number, but you should be able to spin over the engine by hand with a regular ratchet (if the plugs are out.) Failing this you could take off the belt just in case you have an accessory with something seized dragging it down.


I wouldn't be worried about getting compression numbers just yet.
If you can get it spinning over quick enough to start, let it run for a bit to see how the (fresh) oil looks afterwards.
You can also test the coolant for combustion gasses while it's running.