What's wrong with my overheating engine?

Post Mon Nov 02, 2015 8:29 pm

Hi all,

I've got a problem with my M20B25 (1989, low comp.), that I'm not sure I understand.

Some background: I think I've possibly cracked the head, but I've not tested anything yet. The oil is very, very viscous, and the coolant keeps disappearing (I assume into the oil, although they don't look like they're mixing). I've been topping the water up, and running around like this for a while (I've got a spare M20 I'll build into a B28, and I wanted to get some mileage out of the car this year...).

Anyway, today, after coming out from a blood donor session (sign up to a good cause people!), I noticed there was a lot of steam out of the exhaust. I assumed it was just cold. I did about a mile, at tops, and hit traffic (traffic, in Oxford? whatever next!), and noticed it was very very steamy still, with clouds billowing around the side of the car. I then noticed the temp gauge was a little above 12, and heading towards the next mark around. Revving the engine cooled it down again, I assume due to the fan speeding up?

I pulled over once I could make it to a side road, and checked for coolant. There was none. I then limped it a couple of hundred yards to a friend's house, who gave me some water. We filled it up so the header tank was about 75%ish full, and I started the car. Usually, it will run just fine with the tank off. This time, after starting the car, pressure slowly built up, and you could see the level rising until all the water I'd added boiled up and out of the tank. (Like it was being purged/plunged, not flash boiling.)

Now, I don't much care if this engine/head has had it (although it would be great if I can get an extra 40 miles out of it so I can take it back to the body shop that bodged a load of it up), but what I'm trying to work out is; what's caused the water to react the way it did? I've never seen a car do that before...

Cheers!
Joe

Post Mon Nov 02, 2015 10:36 pm

Sounds like you have a head or headgasket problem causing your water loss.

The mad reaction when you topped it up was probably due to trapped air - when it emptied its coolant tank completely, it will have started to suck air into the engine, coolant hoses, radiator etc. You added the water on top of the air, causing a trapped bubble effect.

The water cant circulate past the air bubble, so heats very rapidly in situ, rather than circulating.

If you bleed the air out the system and add more coolant, you MAY be able to drive it again for a few more miles.

Alternatively, if you added Cold water to the hot engine, that can actually cause aluminium parts to crack due to thermal 'shock', which may have just finished your head off!

Post Tue Nov 03, 2015 5:53 am

I had an e34 535 that did a similar thing, it had blown a head gasket between the water coolant feed hole (corroded) and one of the cylinders and blew steam and used water and overheated. Pull the spark plugs one at a time and check them out, you can clearly see the one using water.
Reason why she pushed the water out is that it was being pushed compression wise and sucked by one of the cylinders feeding pressure and vacuum into the cooling system so can't get to right pressure and will boil at a lower temperature, hopefully it's a blown head gasket and not a crack in the head, cheers.
Don't drive it, get it towed or trucked, if you put water in and drive it, it won't go far at all and over heat and maybe warp the head. not worth the risk.

Post Tue Nov 03, 2015 8:57 am

sounds like a blown HG..possible a cracked head. dont drive it.

Get it stripped and pressure tested before you spend any money.

Post Tue Nov 03, 2015 9:58 am

Cheers

Cracked head was my first guess too - I've assumed it's the head anyway, but I'll strip it down once it's out of the car and see. If the head is savable and it's just the gasket, then that's a bonus.

The trapped air theory makes sense - all the coolant pipes were empty, so I guess that all ties up.

It can stay safely locked up until the stroked M20 is ready to go in, save risking any more damage to this engine. Gives me a good chance to clean up the engine bay and have a go at fixing some of the small issues in there while the old, presumed dead, engine is out of the car.

Post Tue Nov 03, 2015 10:03 am

I'd guess at a crack across an exhaust valve seat. Take out the spark plugs and see if one is exceptionally clean.
If any water gets into a cylinder it will effectively steam clean the plug.

Post Sat Nov 21, 2015 7:02 pm

i had the same problem, although i have a 318i(planning to upgrade to m52b28 when i hit the jackpot) but steal seal fixed the problem and 4000miles later no overheating and no sign of winning the jackpot...