250 engine burning coolant

[Novamonte]

I had to install a new thread insert into the number 4 spark plug hole some time ago. The mechanic apparently caused an air pocket (he had to remove the top of the WRD to get access), which in turn led to overheating. After that the engine started to burn coolant (white smoke, had to top up coolant level frequently). I suspected the head gasket and have spent the last weeks replacing it. I could not see any apparent problems with the old gasket. The head was checked for warping and was milled very little (.15 millimeters) to get it straight. It was also vacuum tested before it was reinstalled.

I just got everything together again and the engine fired up and runs well, jus as before. It still burns coolant however... The white smoke does not start coming immediately when the engine is started (even if it is warm), so I suspect that the water does not enter the combustion chamber until the coolant is pressurised. The pressure in the system disappears almost immediately after the engine is stopped which also seems to indicate a leak somewhere.

Does anyone have any ideas on where to look next and if there are ways to diagnose the problem without removing the head again? Perhaps pressurise the cooling system with a pump (engine off) and listen for leaks?

[ja17]

I had some problem over the years with the 250 cylinder heads leaking because of internal corrosion of the cooling jacket in the head near the combustion chambers. The metal thickness of the aluminum casting in the head is very thin after corrosion takes its toll. Eventually it can break through into the combustion chambers. I had on head off three times until the problem was discovered.

You can test the combustion chambers by removing the radiator cap, pressurize the cooling system to about 14psi with the spark plugs removed. If the pressure does not hold look for moisture in any cylinder. You'll need to borrow or buy  cooling system checker or have someone do it for you.

Keep us up to date.
Good luck!

[stickandrudderman]

I concur.

[Novamonte]

Thank you. I will borrow a pressure tester and proceed as you propose. I will keep you posted. I also consider buying a stetoscope so I can hear where any leak iscoming from.

[mdsalemi]

...cylinder heads leaking because of internal corrosion of the cooling jacket in the head near the combustion chambers...

…and THIS is why regular coolant change is mandated. 

[Novamonte]

Yes-I am aware of that and change all fluids regularly, more often than recommended. Ihave also replaced all coolant, fuel and brake hoses since I purcased the car two years ago. Not so sure about the PO though...

[Novamonte]

I have purchased a cooling system tester and was immediately able to verify that there is a leak in the system, although no coolant visible from the outside. I went on to remove the spark plugs and check the cylinders. I put about a meter of thick candle wick yarn into each plug hole and pressurized the system a few times. No sign of any moisture. I went on to remove the intake manifold and checked for moisture around the top of the intake valves. Same result. I then removed the exhaust manifolds and tested there and - bingo! Found traces of blue MB coolant at the top of the #5 exhaust valve.

Next step will obviously be to remove the head (again...) and locate and fix the leak. Does anyone have any experience from coolant leakages in this area and where I should look for weak spots?

[ja17]

Set, the engine at 0 degrees (TDC) compression stroke of #1 before head removal. You will be able to keep everything in time. Otherwise you may get everything off 180 degrees during re-assembly. Make sure you are on compression stroke  #1. The distributor rotor should be pointing to #1.

[Novamonte]

Thanks Joe. I had the head off just a month or two ago to replace the head gasket which I then believed was the problem. I managed to get everything right but the chain slipped two teeth on the disty/FIP sprocket because I had taken the "special bolt" at the side of the chain gallery out (thought it just held the steering servo reservoir in place) . Won't make that mistake againg. If anyone wants tips on how to remove the FIP on a M129 with the oil cooler right under the pump just let me know! There is a separate thread on this issue.

[Leadfoot]

I'm assuming you went through the proper procedure for re-torquing the head? I had a couple of issues with aftermarket head gaskets and re-torquing of the head. Dealer head gasket, warm engine and then re-torque, no more leaking.

[Novamonte]

Yes, dealer gasket. Haven't retorqued yet ad it has only done 20 km since the replacement. But I have found the leak - small crack in the head, coolant enters above no 5 exhaust valve.

[mdsalemi]

But I have found the leak - small crack in the head, coolant enters above no 5 exhaust valve.

When my engine was professionally rebuilt by Metric Motors, part of the process and procedure for all their rebuilds is a detailed crack inspection including the head. They don't need to know or care about what condition the head is in when it arrives, but only when it leaves. Sure enough, on mine they found a crack. Luckily it was small and repairable. They welded it all up and did whatever they had to do after that in their process. It's been about 13 years now and all seems to be well.

Others I know have sent their engine in for rebuild, too--only to find out that yes there is a crack, and no--the head is either too far planed already, warped, cracked or whatever. Sometimes these things are not obvious except to the pros with the tools of the engine rebuilding trade.

Hope your crack can be easily repaired!

[GGR]

If you brought your head to a machine shop for milling, pressure testing the head before anything is done is usually standard procedure.

[mdsalemi]

If you brought your head to a machine shop for milling, pressure testing the head before anything is done is usually standard procedure.

Not "usually" but should be. There are a lot of terrible stories out there of heads decked too far, cracks not repaired, heads not flat that to assume it is SOP is a huge assumption. I've been reading posts on head issues for many years and if every shop followed the proper procedure we wouldn't have some of these stories.