Compression Problems - again!
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I did a compression test on my +2 twin cam the other day, and the results were (dry):
1 175
2 155 (oh-oh!)
3 180
4 180
and wet on 2: 175 (more oh-oh!)
The rings are less than 10000 miles old, and number 2 was at 180 after the rebuild.
My question is, with a slight loss in compression like this, is the oil on the piston method still effective in diagnosis? I have a vision of oil sealing the valve as well as the piston rings temporarily if the leakage clearance is small. I can't try the compressed air method without going to a garage.
Another question is, what loss in compression would a broken piston ring give? Can I rule this out or is it still a possibility?
Any help would be appreciated.
Dave Chapman.
1 175
2 155 (oh-oh!)
3 180
4 180
and wet on 2: 175 (more oh-oh!)
The rings are less than 10000 miles old, and number 2 was at 180 after the rebuild.
My question is, with a slight loss in compression like this, is the oil on the piston method still effective in diagnosis? I have a vision of oil sealing the valve as well as the piston rings temporarily if the leakage clearance is small. I can't try the compressed air method without going to a garage.
Another question is, what loss in compression would a broken piston ring give? Can I rule this out or is it still a possibility?
Any help would be appreciated.
Dave Chapman.
- david.g.chapman
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Dave
The change in one cylinder soon after a rebuild while all the others have not changed suggests something substantial has gone wrong.
A broken ring could be a candidate given your oil test. If the oil splashes up it can help seal a slightly leaking valve also so not definite.
Have you had any sign of leakage into the coolant system ?
I would check the valve clearance in the low cylinder and then do a leak down test to confirm if possible whether its valves or rings before deciding what to do next.
regards
Rohan.
The change in one cylinder soon after a rebuild while all the others have not changed suggests something substantial has gone wrong.
A broken ring could be a candidate given your oil test. If the oil splashes up it can help seal a slightly leaking valve also so not definite.
Have you had any sign of leakage into the coolant system ?
I would check the valve clearance in the low cylinder and then do a leak down test to confirm if possible whether its valves or rings before deciding what to do next.
regards
Rohan.
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rgh0 - Coveted Fifth Gear
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Rohan,
I agree a leak down test would be a good idea - a trip to the garage is in order if I don't start a stripdown anyway.
There are no strange sounds from the engine, and no exhaust smoking or cross contamination between coolant and oil.
I am just about to put 400 miles on the car on my autumn break, so I have filled the car up with leaded petrol (at 140p per litre!) to protect the valves and will take the risk for now. I will monitor the compression again half way though the trip and call out the breakdown services if it goes too low. I had a broken ring before, and the compression went down to 120 without significant bore damage. Wish me luck!
Dave Chapman.
I agree a leak down test would be a good idea - a trip to the garage is in order if I don't start a stripdown anyway.
There are no strange sounds from the engine, and no exhaust smoking or cross contamination between coolant and oil.
I am just about to put 400 miles on the car on my autumn break, so I have filled the car up with leaded petrol (at 140p per litre!) to protect the valves and will take the risk for now. I will monitor the compression again half way though the trip and call out the breakdown services if it goes too low. I had a broken ring before, and the compression went down to 120 without significant bore damage. Wish me luck!
Dave Chapman.
- david.g.chapman
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Well, I have been on my 400 mile trip, and the compression (dry) is now
1 145
2 150
3 180
4 180
I an still thinking about a leak down test, but the local garages gave me blank stares. Anyway, I tried removing the breather pipe to the carbs (Strombergs) and with the engine idling, there is just a slight sensation of air pulsing out. I could just feel it on my fingers. If the pipe is sealed for 3 or 4 seconds, a puff of air comes out, but not that much - not a blast of air.
My feeling now is that it's the head gasket between 1 and 2, but I am in a quandry.
The play in the water pump is about 0.5mm when rocked. Do I bite the bullet, remove the sump and poke the pistons up anyway (and also change the water pump) or do I just look at the head?
The pump is about 15000 miles old, but with a slack belt. Any thoughts on how long it might last?
Dave Chapman.
1 145
2 150
3 180
4 180
I an still thinking about a leak down test, but the local garages gave me blank stares. Anyway, I tried removing the breather pipe to the carbs (Strombergs) and with the engine idling, there is just a slight sensation of air pulsing out. I could just feel it on my fingers. If the pipe is sealed for 3 or 4 seconds, a puff of air comes out, but not that much - not a blast of air.
My feeling now is that it's the head gasket between 1 and 2, but I am in a quandry.
The play in the water pump is about 0.5mm when rocked. Do I bite the bullet, remove the sump and poke the pistons up anyway (and also change the water pump) or do I just look at the head?
The pump is about 15000 miles old, but with a slack belt. Any thoughts on how long it might last?
Dave Chapman.
- david.g.chapman
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I went through all the trauma of worrying about compression tests until I had a leak down test after I finally found someone who knew his arse from a hole in the ground. No problem.
Don't mess around with compression tests.
Chris
Don't mess around with compression tests.
Chris
- chrishewett
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Thanks again everyone,
I will try a Jag dealer. My feeling is that the water pump is good for 20000 miles (please feel free to correct me if I'm wrong!) and I can just deal with the head if the test says so.
Incidentally, I will change the f ing exhaust valves anyway with certified 214N stainless valves. They seem to be fine on my Mondeo Zetec engine, so they cannot possibly burn in a twink?
Dave C.
I will try a Jag dealer. My feeling is that the water pump is good for 20000 miles (please feel free to correct me if I'm wrong!) and I can just deal with the head if the test says so.
Incidentally, I will change the f ing exhaust valves anyway with certified 214N stainless valves. They seem to be fine on my Mondeo Zetec engine, so they cannot possibly burn in a twink?
Dave C.
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- Joined: 26 Nov 2003
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