compression test

[japimport]

hi there im going to buy a compression tester as my car is lumpy on idle i can feel it through the whole car ive already tested the ignition with a spark plug tester and changed the ignition coils so im going to give it a go with a compression tester what i need to know is how to exactly to do it and what compression should i be geting my car is a c200 year 95 any pointers would be great thanks for your help

 

[Ian B Walker]

You should get (dry, explain later) about 10 bar. A variation of 1.5 bar between cylinder is acceptable. Ok what to do :-

Warm engine and remove spark plugs.
Inset compression tester into no 1 cylinder and hold the throttle open. crank the car but count the strokes, 4 is about right.
Read gauge and note figure.
Do the other cylinders and record figures.

If a cylinder is low on compression pour in a tablespoon of engine oil and do the test again. (Wet test) If the compression rises then the piston rings / bore are suspect. If the figure stays the same then there is a valve problem.

 

[japimport]

thanks ian will any brand compresion tester do i seen a draper one in halfords its 30 euro

 

[Ian B Walker]

Draper will be ok but I use a Sealey one. Sealey

 

[japimport]

thanks very much ian ill let yous know how i get on

 

[japimport]

hi there me again ive ended up buying a gunson compression tester to do the test it says i have to disconnect the ignition system how do i do that do i just take the spark plug of the coils im not to sure what to do i dont want to mess things up i know ian told me what to do but i was just reading the instructions and im just double checking thanks in advance

 

[japimport]

heres the results of my compression test can anyone tell me if this is ok


4.12 bar 165psi
3,just over 12 bar 165psi
2,13 bar 170psi
1. just over 12 bar 165psi

from the front of the car

 

[lynall]

Well as they are nearly all the same i wouldnt think anything was wrong.



Lynall

 

[Ian B Walker]

They look fine to me. Mercedes say that a difference of 1.5 bar between cylinders is acceptable.

 

[wemorgan]

it says i have to disconnect the ignition system how do i do that do i just take the spark plug of the coils im not to sure what to do

Sorry to revive an old thread.

But is it necessary to disconnect the injectors before doing a compression test?
With the HT leads disconnected the fuel will not ignite anyway.

This link suggests I do need to:
Evilution - Smart Car Encyclopaedia

Thanks.

 

[japimport]

i let the car heat up to running temp, and disconected the leads when doing mine,and put the compression tester in to plug hole, cranked the engine for 10 seconds and took the reading

 

[wemorgan]

I was concerned that without disconnecting the injectors that the cylinders would get flushed with petrol and clean the cylinder bores, reducing the compression reading.

Did you take a single reading of each cylinder or 2-3 to take the average?

 

[japimport]

i took a reading from each cylinder,ian gave me some advice on how to do the compression test,everything looked ok thank god

 

[Number_Cruncher]

If you're concerned about the fuel, consider removing the fuse for the fuel pump - the fuel rail pressure will drop very quickly as you begin to crank the engine.

However, I've never had an issue with fuel messing up compression testing.

If your car has a fuel clearing mode, whereby no fuel is injected if you crank with a fully open throttle no fuel will be injected anyway - I know most Vauxhalls do this, but, I'm not sure if MB use this ECU feature as well. [To check, see if your car will start with your foot to the floor]

I would be more concerned about making sure the spark has a good route to earth, and so, I would probably fit the spark plugs back into the leads, and allow them to earth against the cylinder head.

 

[wemorgan]

I would be more concerned about making sure the spark has a good route to earth, and so, I would probably fit the spark plugs back into the leads, and allow them to earth against the cylinder head.

Are you saying that there is a risk of damaging the electrics by turning over the engine with the HT leads disconnected?

 

[uhayat]

Question: What about the compression tester in built in the Star Diag machine? is that any good? It resets the ECU and then you have to hold the key for a certain amount of time - the starter turns but it doesnt start the car. Are the readings good? anyone tested with both methods?
Usman

 

[Number_Cruncher]

Are you saying that there is a risk of damaging the electrics by turning over the engine with the HT leads disconnected?

Yes, the worry I have is that given nowhere to go, the spark voltage will just build until it finds a weakness. This can't be a good thing.

Most mechanics won't bother with this, and will say that it doesn't give a problem - the truth is more likely they've not really thought about it in detail.

I tend to use something like ebay item 190298540630 to give the spark somewhere safe to go.