Pressurised water on w124 300 CE

[AlanJ100]

Hello there, I bought a 1993 300 CE with an overheating problem. I removed the head, got it skimmed, new head gasket, replaced water pump and reassembled . The car started almost straight away and ticked over perfectly until it got warm. Once warm it started misfiring and the water was bubbling. it pressurised so much it blew a hole in the radiator! After some research I found out I should have replaced the head bolts, so off came the head again!
It was replaced again with new bolts and another new head gasket.
once put together again it started perfectly and ticked over sweet, until it got warm. It still had the same problem, pressure in the water jacket.
Both times I removed the head there didnt appear to be any blow across the head gasket. I was wondering if these engines have a problem with cracked heads or blocks. Any help would be much appreciated.

 

[Bolide]

Did you have the head crack tested? It seems most likely that exhaust gases are pressurising the coolant so either the head gasket has blown or there is another path for the pressure to escape

I'd have the head off again and have it crack tested. I'd also examine the block and the bores very carefully. Make sure that both the head and the top of the block have perfectly flat surfaces. If there are any marks on the bores check to see if there is a waterway behind them - it is possible for corrosion to eat from a waterway to the bore. You might find that a compression test and a leakdown test will help track down which cylinder(s) have a problem. You might want to do two leakdown tests - one cold and one hot as the problem seems temperature-related. If you know which cylinder(s) are losing compression then a visual inspection should be able to find the problem

As the car almost certainly had problems prior to you buying it I'd expect it's been seriously overheated at some point and that all the problems stem from this and / or from a lack of antifreeze which caused corrosion

Nick Froome
www.w124.co.uk

 

[hmang]

might be more economical to source another cyl head if you are confident of the block

 

[AlanJ100]

Thanks for the replies. I havent had the head pressure tested but it was stripped and cleaned thoroughley and checked visually with no obvious signs of cracks. the block is a lot harder to check so I m not as confident about this being ok. the head was skimmed the first time it was removed and was checked for flatness the second. the top of the block was scraped clean and flat. there were no obvious signs of blowing across the hg both times the head was removed. the water pressurises when hot and there is some water coming from the exhaust(I know condensation causes this sometimes) but the oil isnt emulsified.

 

[brandwooddixon]

Having checked out the head and block, I'd be tempted to check that you actually have coolant flow. If there isn't then you'll get local hotspots around the engine causing steam to pressurise the system.

Mind you on rereading your post you mention that it was misfiring, which frankly smacks of something else.

 

[Legoman]

take the theromostat out and see how she behaves then. should always replace that after a head rebuild imo , as it can be the cause.

 

[AlanJ100]

Thanks again folks, I have run it w/o the thermostat and have checked for flow with a hose pipe and its ok.
Just a thought for last resort attempt, does anyone think it might be worth retightening the head now its been warm?

 

[television]

Thanks again folks, I have run it w/o the thermostat and have checked for flow with a hose pipe and its ok.
Just a thought for last resort attempt, does anyone think it might be worth retightening the head now its been warm?

It cant do any harm, thats what we used to do, did you torque it and the angle it

 

[panason1c]

does anyone think it might be worth retightening the head now its been warm?

Be careful, there is a strict procedure for tightening cylinder head stretch bolts which would initially involve using a torque wrench at a low setting for the first pass followed by further stages of tightening the bolts using an angle guage.......(as refered to by Television)

Any deviation from the set procedure could result in a snapped or weakened head bolt.

Bearing in mind that you said there has never been any sign of blowing across the head gasket then it's even less likely the head needs further pulling down.

I would be inclined to source another engine as it could be the cheaper option in the long run.

 

[mercsman]

Water pump bearing

This might sound slim but I've experienced it a few times (worked on Mercs for 20 years). Check your water pump, even though it may not be leaking or making any obvious noises... if the bearing is bad it can create air bubbles in the cooling system thus losing much of it's "pumping" efficiency.

Let me explain; if the water pump fans are surrounded in 100% water then it can circulate it effectively but if the water has a high percentage of air bubbles (created by pitted/damaged bearings in the water pump) say 50%, then it losses half it's "pumping" efficiency so the engine will overheat due to poor circulation and pressurize due to the air bubbles.

It's good practice to replace the water pump after this type of work anyway.

Hope you get to the bottom of this.

Good Luck.