compression test -> leak down test - best practices

[Stuart B]

Hi,

I have a leak down test kit I hadn't got around to using but I got a compression test kit the other day which I will use first.

my plans are to try dry and then wet - the instructions are a little Chinese to me, it explains what the signs may mean. my plan is to do this first and then if one or two cylinder's are playing up use next door's compressor and perform a real leak down to try and identify the fault(s).

is there a quick reference guide for this.

My plan was
1. to remove the fuel pump fuse and unplug coils
2. only remove one spark plug at a time (photo)
3. check dry to see whether the pressure increases or stays the same (valves?) turn engine over for 5 secs?
4. only if too low in 3 - check wet to see if the pressure is much higher than dry (rings?) - turn engine over for 5 secs?
5. replace plug
6. repeat for next cylinder

Any hints - is this wrong dangerous etc

 

[IPG]

What i would do

1. remove the fuel pump fuse and remove coils
2. only remove ALL spark plugs at a time
- less resistance for when you're cranking the engine with just the starter motor
3. check dry to see whether the pressure increases or stays the same (valves?) turn engine over for 5 secs?
- 5-6 cranks. you'll hear how the sound repeats itself like a broken record
- ideally the pressure you get is 180
4. only if too low in 3 - check wet to see if the pressure is much higher than dry (rings?) - turn engine over for 5 secs?
5. no need to replace plug since you've taken them out
6. repeat for next cylinder

After this - if you find cylinder(s) are low THEN you leak down test to determine where it's leaking...
- rings
- inlet valves
- exhaust valves

I know this process all too well LOL

 

[Silva bullet]

Hope I could give you afew hinters but wow this is expert mechanic stuff.
awesome gti did mine and was about 175 on all cylinders after my oem bam head cracked..

 

[desertstorm]

I agree with Ian that's what I did. I never got around to putting any oil in though as mine all came out reasonable fortunately.

 

[IPG]

10% difference or less between cylinders is what i read to be ideal

 

[Stuart B]

Thanks Guys,

do I need to watch the gauge or does it remember the highest value? I read something about it dropping meaning something else?

I have a suspicion that when I looked at the head it was possible to have both cylinder 1 an 2 exhaust valves open at the same time and wonder whether that might be wrong?

 

[desertstorm]

The gauge should have a one way valve on it that will hold it at the highest measured pressure. A little button on the side of the gauge releases the pressure.

 

[Stuart B]

cool thanks, I can see that - perfect, looking forward to potential bad news I suppose

 

[SHUk]

What do you add to wet test the cylinders, and how much?

Thanks

 

[IPG]

What do you add to wet test the cylinders, and how much?

Thanks

Tablespoon of engine oil

 

[SHUk]

Tablespoon of engine oil

Is that the scientific spec is it... =]

 

[Stuart B]

its just to see if that improves the compression if it does then the low compression is rings if not - valves or maybe even head gasket

 

[IPG]

Good little tool is also a borescope. Can look inside to see if you got piston to valve contact.

 

[SHUk]

Hello

Did the compression test on the Agu, 200k engine and it’s was 165,170,170,170 across the cylinders. I feel this isn’t too bad considering the age and mileage.

=]

 

[IPG]

looks pretty consistent.... but lowered as expected on an old engine

 

[SHUk]

I suspect that the rings and stem seals are on there way out as there’s a lot of blow by...