Effects of overheating a VP2001

[frilaens]

Over the last three seasons I have experienced several problems with my VP2001. She has never been an easy starter, always requiring 10-20 seconds or more of starter to start from cold (using the cold start procedure.) The first serious problem was oil leaking into the cooling water at high revs but without water appearing in the oil. The following season I had the motor out and replaced the head gasket. Last season I had problems with it overheating. I know that on at least one occasion the motor was run hot for a while as the alarm was not working. The cause was a faulty thermostat and scaling in the exhaust elbow. After this the oil leakage at high revs returned, as well as some leakage from the gasket joint.

My question is what is the effect of overheating a motor? Can I just replace the head gasket again or do I need to do something about deformation of the block? I have heard about "skimming" a motor; can this be done in situ?

Andy

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[ex-Gladys]

Had a Merc 813 truck that boiled over on M25 a gazillion years ago, warped the head... that may be your prob

<hr width=100% size=1>Larry Botheras

Anderson 26 "Amber"

 

[oldharry]

Cooling channels in the head are designed to remove heat quickly from hot areas such as the exhaust valve seats, and to allow the head to operate at a reasonably uniform temprature throughout the casting. Shortage of coolant, either by restricting the flow, or loss of coolant altogether from part or all of the head, will result in hot spots rapidly developing. As a result the head heats up, and overheats unevenly, and particularly with aluminimum heads the metal within the casting is stressed and stretched. These stresses distort the casting, and it only needs a few thousandths of an inch movement to cause the head gasket seal to fail. In severe cases the distortion is enough to actually crack the casting, usually writing off the head.

'Skimming' is simply the process of shaving sufficient metal off the face of the head to ensure it is flat enough to allow the new gasket to seal properly again. It is usually done a special skimming lathe, although mildly distorted aluminium heads can be restored using a fine grit carborundum paper on a flat bed, and simply working the head evenly over it. This is particularly effective with single or two pot heads and I have more than once retrieved damaged outboard heads succesfully this way. Care needs to be taken to apply even pressure at all times, otherwise the head grinds unevenly.

Obviously, and particularly with a diesel, there are limits how often a head can be skimmed, and manufacturers specify the maximum depth permissible. Diesel engines often specify differing gasket thicknesses to compensate for this procedure.

Most engine reconditioners will skim a head for a modest charge.



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[bendyone]

On the 2001 in the head is a small pipe which dirrects the water across the head. In this pipe is a small hole 4mm dia. On our engines the service guys in St Peter Port increased the hole to 6mm which sorted out our overheating problem at full revs.
Its a know fault with 2001 engines!

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[frilaens]

Ah! Thanks for that... I noticed that in the head block there was a much larger hole than in the cylinder block and I couldn't figure out why there should be such a difference. If I try just replacing the gasket (again) I will drill it out.

Andy

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[frilaens]

Do you think any distortion can become permanent? I would imaging if the cooling has been uneven then the blocks would distort when hot. After the block have cooled down then they should return to their original form. If I fix the cooling problem and replace the gasket the motor should work as normal?

Andy

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[oldharry]

Yes, distortion is permanent, which is one of the reasons why skimming is necessary. A local 'hot spot' will expand and stretch or compress the cooler metal around it. When the head cools down again, the overheated area will attempt to shrink back, unfortunately the movement will have distorted the adjacent metal out of shape, and cooling down only makes things worse.

Its very similar to putting a dent in your car door. You can try to pop it out, but if the metal has been stretched, you can never get it precisely back in place again. Welders have to go to great lengths to ensure that the heat does not distort the surrounding metal.

The distortions and stresses set up within the head material only matter if the metal actually fractures resulting in a cracked head, but almost invariably the mating surface for the gasket will have been pulled out of true. Tightening or slackening the head bolts in the wrong sequence, or to the wrong torque will also bend the casting. Commonly the effect of the head overheating is to induce a bow effect across the head, lifting it away from the seals between the cylinders allowing compression leakage. It can also bow from end to end, so that the centre is raised from the gasket.

Unless the head surface is absolutley flat - and we are talking to within thousandths of a centimetre - there is probability oif gasket failure. In a major overheat situation, the block itself may distort as well, so that its surface too has to be skimmed - much more major operation involving total engine strip down.

Skimming is not expensive and does not take long - my local machine shop will often do it while I wait if they are not busy, and it really is the only way of ensuring the head is really OK.

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[frilaens]

Thanks for the explanation; I hadn't thought about the difference between the head and the cylinder block. Of course, the head block is much smaller and more easily distorted. But I can easily take that to a workshop for checking and skimming as I will have to replace the gasket anyhow. I was thinking more along the lines of the whole motor needing to come out again to be able to skim the cylinder blocks surface. As I know very little of the motors history (if it has been skimmed before), am I correct in assuming that skimming the head block will not affect the working of the motor other than needing a re-adjustment of the valve timing?

Andy

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[oldharry]

The only difference skimming makes is that it raises the compression ratio slightly, unless the makers specify a thicker gasket to compensate. There is a marginal increase in power, but this is rarely noticeable in service unless the head has been skimmed beyond limits. No reputable machine shop would do that as they could be in for a replacement engine if a valve hits a piston as a result!

Skimming will make no difference to the engine timing, but a pushrod engine will need the tappets to be readjusted. An OHC engine should be set up to its normal timing marks. The machine shop should be able tell you whether or not it has been skimmed before and how much reserve there is left.

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[oldharry]

The only difference skimming makes is that it raises the compression ratio slightly, unless the makers specify a thicker gasket to compensate. There is a marginal increase in power, but this is rarely noticeable in service unless the head has been skimmed beyond limits. No reputable machine shop would do that as they could be in for a replacement engine if a valve hits a piston as a result!

Skimming will make no difference to the engine timing, but a pushrod engine will need the tappets to be readjusted. An OHC engine should be set up to its normal timing marks. The machine shop should be able tell you whether or not it has been skimmed before and how much reserve there is left.

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