low compression on one cylinder

[dave friday]

Afternoon all,as above;
cylinders.
1,370
2,360
3,260
4,370
We are thinking of buying an air compressor as a diagnostic tool! the idea is to put number 3 cylinder under pressure and listen for air leaks at the inlet/exhaust and block breather.
Any advice? and what compressor is a good one?
Ta.

[kevtherev]

Afternoon all,as above;
cylinders.
1,370
2,360
3,260
4,370
We are thinking of buying an air compressor as a diagnostic tool! the idea is to put number 3 cylinder under pressure and listen for air leaks at the inlet/exhaust and block breather.
Any advice? and what compressor is a good one?
Ta.

As it's a diesel there's a simpler method.
There's a wet cylinder compression test, which will tell you if it's ring wear.
If the compression remains low then the valves/guide are the problem.
If the compression is higher then it's the rings

Squirt some engine oil into the bores and do your test as normal

[dave friday]

Thanks Kev,if it was a petrol engine I would do what you suggest
I thought that squirting oil into a Diesel was a bad idea as it could ignite!

[MGP]

I would start with checking the valve clearances as i have seen them close up before.
Leak down tests are ok but after checking the valves are closing there are only two places you can lose compression and both of them require removing the head to rectify.
Once the head is off put it face down on a soft surface and pour a little diesel down the ports you will soon know if its the valves not sealing. If the valves are sealing it can only be rings or a hole in the piston although with compression as low as that is should be quite obvious where its escaping.
If you are only getting a compressor to find this fault personally i think you would be wasting your money.

[dave friday]

Thanks MGP,the reason I want to do a pressure/leak down test is so the van will be unusable for the minimum amount of time.
The compressor will be used for air tools ect.
The valve clearances were check 10,000miles ago when I replaced the rings,but I'll check the clearances before anything else
Also most parts take 3 weeks to arrive so..

[MGP]

Ok regarding the compressor question look at the cfm requirements of any air tools you have/want this will give you your minimum then look at the power supply as this will be your limiting factor and you will want a tank as large as possible. I have a 50lt tank 14cfm in the back of my van which just about keeps up with my die grinder however it runs constantly so have to stop and let it cool often.

[dave friday]

Thanks again,my daughter said " I don't think the one at work was much bigger than that" she was a Skoda mechanic for a few years!

[dave friday]

So.. I borrowed a little compressor,no leaks from the valves so it must be the rings!