Rover 75 1.8T gasket problem?

Question

I recently replaced the cylinder head gasket for my vehicle after it overheated and blew it. Two weeks along the way the car idling starts to be unstable and the power goes down, only to find that oil and water are being mixed up but the head gasket is still fine, what could be the problem?

Answer 1

It is not at all unusual when a head gasket fails due to overheating that the head or even the block has become slightly distorted from the overheating at well, particularly on alloy heads and blocks. Unlike cast iron, which can handle a great deal of heazt stress before distortion takes place, alloys can warp pretty quickly.

Often this will require that either the head or the block or both will have to be shaved (planed) at the two adjoining surfaces so that they are flat enough to provide consistent support for the head gasket. Otherwise, when you torque it down to spec, the high points on the surface will provide resistance sufficient to give you the proper reading on your torque wrench, while the low points will not be applying sufficent pressure and support to hold the head gasket firmly and flatly between the surfaces.

The resulting voids will easily allow the internal pressures in the cooling system journals, the oil journals and the combustion chambers themselves to blow out the new gasket and permit exchange of fluids between the cooling and lubrication passages.

If you did the job yourself, you will need to pull the head again, and check the two surfaces carefully (a length of round steel rod is good for this, which you can roll along each surface and look for gaps. If gaps exist on the head, and not the block you can take it to a machine shop to have it planed. If the block is also affected you may have to remove the engine and sent that to the machine shop as well. I've never had that happen and I really do not know if the machine shops have a device for planing a block still in the car, but if they do, I have not heard of it.

If, on the other hand, you paid a professional shop to do this work, then they did not exercise due diligence by checking the head and block for warpage before installing a new head gasket. Of course it is one thing knowing this yourself, and quite another convincing them to assume the liability and correct the problem.

Now I should mention one other thing as it relates to this. When the head and or block gets shaved you are necessarily reducing tolerances between the valves and pistons and somewhat increasing compression in the cylinders. If the warpage is too great you will not be able to shave one or the other enough to bring it back to a flat plane. In that event, you will be looking at a new head, block or both.

Answer 2

Get the cylinder head pressure checked. You should have had this done when you had it out, anyway. The head could be warped or cracked.

Answer 3

Thermostat or leaking water pump, freeze plugs or cracked waterjacket.