Compression Problems - again!

[david.g.chapman]

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.

[rgh0]

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.

[david.g.chapman]

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.

[david.g.chapman]

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.

[chrishewett]

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

[tdafforn]

I am coming around to the thoughts that leak down is better..
My +2 had 180 compression across all cylinders, and yet the vent from the head to the air box had enough blow to dry your hair, and the engine blew oil out of every orifice !
Tim

[hatman]

I an still thinking about a leak down test, but the local garages gave me blank stares.

Try taking it to a Jaguar main dealer - they're used to doing leakdown tests on Nikasil engined V8's.

[david.g.chapman]

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.