Compression test, what's involved?

[SDHA4SLine]

Had my car tested for compression loss by a crude and a little inconclusive method a few months back and have been waiting for the same garage to purchase some kit that's supposed to be more accurate in testing for compression loss in a none invasive way. However this new kit they're getting seems not to be materialising for some reason.

So looks like im going to have to pay for the full hands on method but I have no idea what's involved. I wanted to be armed with some knowledge before I start asking around for quotes to do so.

Can anyone please advise what the method is for a proper compression test that will tell me accurately what this issue is (valves or rings and which cylinder(s) are at fault if any)? Approximate hours labour would be nice to know so I'm not getting ripped off.

My engine is a 2006 2.0 tdi 170 BRD (PD)

Any advice gratefully received

Thanks
Steve

 

[SDHA4SLine]

By the way the crude test suggested that I did have a small compression leak. Just it can't tell me what is at fault

 

[rushton]

A compression test will involve removing the injectors from each cylinder and using a tool to measure compression.

As my day job is with Mercedes, every compression test i have done has involved this kit (unsure what else is available as its all ive known)
http://www.ebay.co.uk/itm/Mercedes-...739966?hash=item4ae8e287fe:g:pB0AAOSwqxdTqWFK

If you look at the pictures further down that add it gives you an idea of what the tester looks like. Its basically a bung which you hold into the injector hole while the engine is cranked. The tester will then translate that into a reading, (in this case a needle will mark a small chart which is inserted into it)

As far as pin pointing the problem goes if you have a loss of compression, in my opinion the only way to identify the fault is by removing the head. There are tell tails signs which could help you pre-identify the fault, but more often or not it would be nessesary to remove the head to rectify the problem anyhow.

 

[Charlie Farley]

any good garage that deals with diesels servicing should have the correct tools for the test steve, its not rocket science, compresion testing has been around for a fair few years .

 

[SDHA4SLine]

I'm assuming the cam shaft needs to come out to get at the injectors?

I've had vastly different quotes some months ago which kind of put me off as I didn't know who was quoting for the correct/proper job or even worse who may have been trying to have me over a barrel so to speak.

 

[desertstorm]

I would have thought you would be better off removing the glow plug, less hassle as you won't have to break fuel lines and the like.
You would just need to disable the injectors to stop them injecting.
I have never tried doing a compression test on a diesel, done several on petrols usually test all cylinders, you should have an idea what the pressure readings should be. Looking at them if any are low put some oil in the cylinder. If the compression increases then it's likely worn bores / ring issues. If there's no change then gasket or valve leakage.
Not sure you if you add oil to a diesel whether it would likely combust so that wouldn't be good.
Adjacent cylinders that are low could be a gasket issue.
Basically if one or more cylinders are below what the spec is you are taking the head off.
why do you think you have an issue ?

Karl

 

[SDHA4SLine]

I would have thought you would be better off removing the glow plug, less hassle as you won't have to break fuel lines and the like.
You would just need to disable the injectors to stop them injecting.
I have never tried doing a compression test on a diesel, done several on petrols usually test all cylinders, you should have an idea what the pressure readings should be. Looking at them if any are low put some oil in the cylinder. If the compression increases then it's likely worn bores / ring issues. If there's no change then gasket or valve leakage.
Not sure you if you add oil to a diesel whether it would likely combust so that wouldn't be good.
Adjacent cylinders that are low could be a gasket issue.
Basically if one or more cylinders are below what the spec is you are taking the head off.
why do you think you have an issue ?

Karl

Its a long standing issue I've had for many many months where car is blowing out smoke under load. I've replaced just about every sensor on the car... maf, map, cts, fts etc, had new turbo fitted, dpf is in good working order, new egr both n75's replaced etc etc. I've had many vcds scans and always been clear (apart from one egr fault). It was suggested by your good self and a few others that this may be down to a compression leak. I wanted a more accurate diagnosis before I spend big money on the dam thing as I have thrown money at this with no luck so far.

On idle it does stink of burning oil or diesel (not sure which it is) but there is no excessive oil use and mpg is good (average 43mpg) and zero smoke at idle with engine cold or warm. The car always runs fine and I know you should expect a little smoke from diesels but mine can be excessive.

Regards
Steve