Low compression after engine overheated

OZROB390

MEMBER
2011 MINI CLUBMAN
2.0L
4 CYL
2WD
AUTOMATIC
110,000 MILES

My Mini Clubman Cooper diesel overheated and broke down due to a coolant leak, pulled over a few hundred yards after the warning light came on. Garage has carried out a compression test - 300 on one cylinder, 200 on two others, fourth cylinder could not get the glow plug out as it was just turning. They have taken the cylinder head off but on visual inspection cannot see anything that would cause the low compression, gasket did not look too bad, no signs on the cylinder wall of any cracks or piston ring failure. A few signs of heat damage only to some hoses as they were stripping it down. Valves appear okay. They have quoted me £1750 ($2,249.27 USD) for reassembling the head, replacing the gasket and hoses, but said they cannot guarantee this will resolve the low compression. Their next course of action they say would be to replace the fuel pump at £1100+vat.($1,413.83 USD) Seems to me a huge gamble to instruct this work if they are not sure it will fix the problem. Any advice would be much appreciated!

3 Replies

STEVE W.

MECHANIC
12,963 POSTS

Overheating like that likely caused the piston rings to lose their temper. They are normally machined so that they press on the cylinder walls like a spring. That allows them to seal and retain the combustion pressure. The heat removes that ability. That will give lower compression. As they have already pulled it apart a wet test to verify that cannot be done.
What I would likely do would be to see if you can source a used or re-manufactured engine and drop that in if the rest of the vehicle is in good shape.
Or do a full overhaul on the one you have. You could probably get away with only doing the piston rings and resealing the engine if you wanted to, but it would depend on what the parts actually cost as to which method is lower cost.