Low Compression
Posted: Sun May 22, 2005 2:39 pm
Doing a bit of head scratching today. Here's why -
Was regularly fouling No4 plug. Tried changing to hotter plugs but no improvement. So did a compression test (actually 2 or 3) the results were:
Nos 1 to 3 cyls - 190 lbs/sq in.
No4 - 100 lbs/sq in.
Checked cam clearances - within tolerance
Took off head to see what the gasket was like - it looked OK. Also the oil was still black and there was nothing unusual with the coolant.
Cleaned up the head and did a petrol test on the valves. There was some weepage past the valves but nothing much. All valves weeped the same amount.
The valve seats look OK.
Aha, must be a broken compression ring methinks
So engine came out this morning and I pulled no4 piston. Piston looks OK and Rings were all fine :huh:
Cylinders look OK. No scratches.
The only thing I can see is that the gap in the compression rings is about 18 thou when I put them in the cylinder. (a bit wider that the book says, 9 to 14 th) Is this extra 4 thou enough to cause the problem?
Any other ideas as to what could be the cause? The engine was rebuilt about 12 thousand miles ago, before I got the car.
Regards,
Hamish.
Was regularly fouling No4 plug. Tried changing to hotter plugs but no improvement. So did a compression test (actually 2 or 3) the results were:
Nos 1 to 3 cyls - 190 lbs/sq in.
No4 - 100 lbs/sq in.
Checked cam clearances - within tolerance
Took off head to see what the gasket was like - it looked OK. Also the oil was still black and there was nothing unusual with the coolant.
Cleaned up the head and did a petrol test on the valves. There was some weepage past the valves but nothing much. All valves weeped the same amount.
The valve seats look OK.
Aha, must be a broken compression ring methinks
So engine came out this morning and I pulled no4 piston. Piston looks OK and Rings were all fine :huh:
Cylinders look OK. No scratches.
The only thing I can see is that the gap in the compression rings is about 18 thou when I put them in the cylinder. (a bit wider that the book says, 9 to 14 th) Is this extra 4 thou enough to cause the problem?
Any other ideas as to what could be the cause? The engine was rebuilt about 12 thousand miles ago, before I got the car.
Regards,
Hamish.