Engine overheating

[SaveMyZ3]

I have read people have similar overheating problems.
My engine seems fine until left for 10 mins on tick over and then boils over through the expansion tank.
I have fitted a new radiator which remains cold all the time.
I fitted new water pump but nothing wrong with old one.
I removed thermostat hoping to give good flush through but still boils over.
I have read here that it could be simply air locked???
Also that cylinder head gasket maybe leaking or engine block is cracked.
Can anyone help?
It would be nice to think it was simply air locked. The engine has 100k miles and I bought car from new 1997. It's 1.9 engine.

 

[Shelly]

 

[Duncodin]

I have read people have similar overheating problems.
My engine seems fine until left for 10 mins on tick over and then boils over through the expansion tank.
I have fitted a new radiator which remains cold all the time.
I fitted new water pump but nothing wrong with old one.
I removed thermostat hoping to give good flush through but still boils over.
I have read here that it could be simply air locked???
Also that cylinder head gasket maybe leaking or engine block is cracked.
Can anyone help?
It would be nice to think it was simply air locked. The engine has 100k miles and I bought car from new 1997. It's 1.9 engine.

If radiator stays cold, even when engine is over heating then, i would normally say that thermostat is stuck closed. But you said you'd already removed the thermostat so it can't be that.

So maybe the radiator is blocked?

But you also said that it happens when idling. Do you mean that it only happen when idling? ie doesn't happen when driving along at 50. If that's the case then radiator is working and it's pointing at the fan not working. Fan not so important when car is moving because air is being forced through the radiator. But when stationary the fan needs to kick in when it gets hot.

 

[Delk]

Did the plastic impeller or anything else fail in the past? Just wondering if any parts could still be inside blocking the passages.

 

[Zephyr]

To start with the simplest fix: properly bleeding the cooling system to eliminate any air lock.
Park the car on a slight incline or jack up the front end to make the expansion tank the highest point—this helps air rise out.
Ensure the engine is cold. Set the heater controls to maximum heat and low fan speed (but don’t start the engine yet).
Open the expansion tank cap. Locate the bleed screw (usually on the thermostat housing or upper radiator hose for the M44).
Fill the expansion tank slowly with a 50/50 mix of BMW-approved coolant and distilled water until it’s full. Then, with the bleed screw open (unscrew until coolant starts flowing out steadily without bubbles), continue adding coolant directly through the bleed screw opening if needed until no more air escapes.
Close the bleed screw temporarily. Start the engine and let it idle to operating temperature (monitor the gauge closely to avoid overheating—shut off if it climbs too high).
With the engine running, reopen the bleed screw to release any new bubbles, and top up coolant as the level drops. Give the engine a few gentle revs (to about 2,000-3,000 RPM) to help circulate, and squeeze the upper and lower radiator hoses to dislodge trapped air (you’ll see bubbles in the tank).
Run the heater fan at full speed; it might blow cold at first but should get hot as air clears the core.
Once no more bubbles appear and coolant flows steadily from the bleed screw, close it. Top up the tank to the “cold” mark, reinstall the cap, and test drive while monitoring temps. After cooling (ideally overnight), recheck and bleed again if needed—it can take a couple cycles.

If this doesn’t resolve it, test the expansion tank cap for proper pressure relief (it should hold about 1.4-2 bar; a faulty one can let coolant boil at lower temps).
Also, check if the fan is engaging at idle (it should “roar” and feel resistance when spun by hand with the engine off and hot).
For head gasket concerns, look for constant bubbles in the expansion tank with the engine running (indicating combustion gases), white exhaust smoke, or coolant loss without visible leaks— if present, do a compression test or chemical block test.
If this is the issue use “steelseal” that works for closing small cracks and gasket faults.

 

[Zephyr]

STEEL SEAL FOR ALL MODELS - FIXES BLOWN HEAD GASKETS PERMANENTLY - Add to Water | eBay UK

Unlike other products making similar claims Steel Seal contains no fibrous material to seal your blown head gasket. It relies on a thermo chemical bonding process which is a chemical reaction, to seal the leak in your blown head gasket.

www.ebay.co.uk

 

[SaveMyZ3]

I have only had to replace radiator and pump about 10 years ago and new exhaust about 15 years ago. The car drives ok - 30 miles or more without problem. I could not get car MOT 2 weeks ago as engine boiled over before they could do the test. I have filled and drained the cooling system several times and still overhea

 

[Duncodin]

The fact you can drive 30 miles without a problem, in my mind at least, rules out head gasket, engine block cracked etc. Also rules out blocked radiator or air lock. Your water is circulating OK.

My money is on the fan. I have an M44. When the engine is cold I run the engine and the fan spins but it's just freewheeling. I can stop the fan with my fingers. There's a kind of heat activated clutch in the hub of the fan. When the engine is hot the clutch engages and the fan runs faster and trying to stop it with your fingers is not recommended.

Assuming you still have the original fan that is where I'd be looking. I can actually hear the difference in noise from under the bonnet when the fan is engaged. It's pretty noisy.

 

[SaveMyZ3]

Thank you for help. I think fan is ok but still not sure. I will try bleeding through vent in radiator to eliminate air lock theory as someone suggested. My car is almost 30 years old and I don't want to lose it.

 

[Lee]

Hello mate, these zeds love to airlock easy, jack up the nose as high as your jack will safely allow. With the cap off and some good rev's you should clear the airlock once the thermostat opens.

After changing parts on the z3 cooling system its 50/50 weather we have to do this clearing air locks, loves to hold air.

Hope this helps.

 

[SaveMyZ3]

There seems to be 2 main theories. One is the airlock that several people have mentioned an the other is the cooling fan viscous clutch. So will work on both. The fan seems ok but not sure how I can test it. I have been unable to get car MOT as it boils over during test - twice. And it's first time in 28 years?

 

[SaveMyZ3]

Ps. The radiator stays cold the whole time suggesting airlock or blockage in the engine block?

 

[Trevor /chedder]

Yes check out the viscous fan, my looked like it was working but wasn't. It's an easy fix and the forum members recommend changing the fab at the same time as it can brake with age. Fyi I went through similar, changed thermostat, pump and did bleeding.

 

[Duncodin]

Radiator will be cold until the thermostat opens. If the radiator stays cold even when the engine is boiling over then radiator is blocked or airlocked.

Testing the viscous fan. My patented fan testerizer.

Just some paper strips taped up dangling in front of the radiator.

Engine is idling.

The paper mainly hangs. Fan is free wheeling so there's a bit of paper leaning toward the rad but not much.

Short while later. Engine now getting warm. I can hear the fan engaging and feel the air being sucked in. Engine still idling. The paper is being sucked in towards the radiator now.

Crouching in front of the car even without the paper I can hear and feel the air being sucked through the radiator when the fan is engaged. It should be really obvious if the fan is engaging or not.

If you can't hear and feel it then your fan is duff.

 

[SaveMyZ3]

I removed the thermostat til I get this sorted. It is a new radiator and water pump. I checked radiator before fitting with hosepipe and water flows freely inlet to outlet. But radiator stays cold while engine boils over. I read the thread of previous overheat problem and the member had cylinder head gasket replaced - and made no difference? Thank you for suggestions and help

 

[SaveMyZ3]

I have read people have similar overheating problems.
My engine seems fine until left for 10 mins on tick over and then boils over through the expansion tank.
I have fitted a new radiator which remains cold all the time.
I fitted new water pump but nothing wrong with old one.
I removed thermostat hoping to give good flush through but still boils over.
I have read here that it could be simply air locked???
Also that cylinder head gasket maybe leaking or engine block is cracked.
Can anyone help?
It would be nice to think it was simply air locked. The engine has 100k miles and I bought car from new 1997. It's 1.9 engine.

Just as update. I carefully filled the cooling system using expansion tank with radiator vent removed. The system appeared to take at least 1 litre more water. I warmed the engine through with expansion tank cap removed and radiator vent removed. As expansion tank level dropped, I topped up with more water until expansion tank level was correct. As the engine warmed up the radiator began to get hot whereas previously the radiator remained cold.
So I think/hope my overheating problem is solved. I conclude that radiator was air locked???
Hope I am right and thank you for suggestions. SaveMyZ3 - saved!!!