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MG Midget and Sprite Technical - 1500 Engine Problem

A fellow Black Country MG Club member is having a problem with his rebuilt 1500 engine. After the rebuild it started first time and ran well for about 5 minutes until he replaced the cap on the expansion tank - at which point the engine slowed down and stopped and was very tight to turn over (although eased off after cooling). A few days later, after checking everything (even dismantling the bottom end to check clearances etc.) he restarted the engine and it ran well for around 15 minutes until suddenly getting very hot and shooting water out of the expansion tank. He cut the engine for fear of damage at this point. He is not sue what is causing this.

I know little about the quirks of a 1500 engine (I'm a 1275 person) but I'm at a loss to see what is going on here and I said I'd post something to see if anyone with knowledge of the 1500 can fathom out what is going on here. Is this an air lock in the coolant problem or what. I'll pass on any suggestions.
Chris Hasluck

on my phone so will be brief, but afraid it sounds like overheating then head gasket now gone and pressuring the coolant.

will explain all when I find a computer. I had a similar issue a few months back with my rebuild.

Malc.
M Le Chevalier

As I said, it sounds similar to the problems I had when I first started up my engine.

When filling the coolant you must follow the handbook procedure: fill from the top of the engine through the thermostat housing with the coolant expansion tank hose off and held above the engine. Refit the thermostat plug, then the hose and then top up the expansion tank.

As you fill up the coolant flows down the top rad hose and "in theory" pushes air up and out through the two openings. However it is quite easy to trap air in the system. To combat this I drilled a small 2 or 3 mm hole in the face of the 'stat to allow air to bleed out past the stat (and to allow a small flow through the system when the 'stat is closed.

If the coolant is not filled correctly, or a big air bubble works its way to the top of the system, it can leave the top of the engine "high and dry". As the temperature probe is at the top of the engine, it will look like your engine is fine (i.e. "zero" temperature) whilst it is actually cooking itself!

(Additional note: if coolant drops low, the 'stat will also not open as it is left above the coolant level and coolant then doesn't circulate. Low coolant is not easily noticed from the expansion tank as it is not the highest point in the system)

As it overheats it will start to run like crap. Perhaps this is why "the engine slowed down and stopped" and the replacement of the cap was just coincidence. Or maybe it was some other interrelated problem? Or perhaps it began to tighten up as various hot components beginning to seize up a little?

As it over heats, the head gasket blows and the cylinder compression starts to pressurise the coolant. The high pressure coolant is then squeezed out of the tank like a beautiful fountain! :-)

Been there, done that have the wet t-shirt from my own coolant volcano! :-D

Malcolm
M Le Chevalier

Addendum... it's not all doom and gloom btw!

I replaced HG with new Payen version. Ran up to temperature, ensured I re-torqued it hot (there are a lot of contradicting opinions on doing that but I did it anyway) and all has been well since. Although my car hasn't been far (i.e. < a mile, not getting out of 1st gear!)

Cheers,
Malcolm
M Le Chevalier

Malcolm
Thanks for the quick comments. I'll pass them on to Adrian. All that start up process sounds very complicated. On my 1275 I just filled up all the fluids and away we went - never thought about any complicated procedures!!

Chris
Chris Hasluck

Payen fixed mine at 3rd attempt. As Malcolm says, make sure its got coolant in the head. Air in the head and around the thermostat and temp sensor registers as cold on the temp gauge even when the engine is very hot. Payen gasket much more robust than ordinary one.

To get mine right I have flushed a lot so the coolant can flow to all the recesses of the system OK. Clean out the waterways in the head and the deck while the head is off.

Good luck, Dave
Dave Squire

When I have done radiator work or following engine engine work that has required draining the cooling system, I have found the 1500 very difficult to charge coolant. It inevitably starts and runs with very low temperature on the gauge until the thermostat pops open in a boiling fit. No damage has been done the the engine but it is very difficult to get the bubble out of the system.

Then the system will take an extra 2 quarts or so of coolant.
Glenn Mallory

Chris,
tell your mate to follow the refill instructions in the relevant Driver' Handbook - it's very straightforward, as Malcolm has outlined
Nigel Atkins

This thread was discussed between 13/11/2014 and 16/11/2014

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