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PostPosted: Sun 07 Jan 2007, 3:13 pm 
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Joined: Tue 11 May 2004, 3:39 pm
Posts: 1930
Location: Melbourne
I've had a browse through the FAQ's and also done a search and I couldn't find a good guide on doing a compression test. There is a brief guide in the 'mega thread' but it's not comprehensive enough for my liking.

Please move this to FAQ section if you think it is good enough!

What is a Compression Test?

A cylinder in an engine needs to be able to compress air/fuel in order to operate. If a cylinder has poor compression (is leaking air) the cylinder will not be operating efficiently or cleanly. The compression test determines how much pressure a cylinder is 'holding' (or how much pressure a cylinder is loosing) during the compression stroke.

When should I do a Compression Test?

a) When diagnosing engine problems:
-Blowing Smoke
-Running Poorly
-Lacking Power
-Blowby (popping dipstick, oil in cooler piping/catch can, pressure buildup under rocker cover)
-Oil in Coolant or Coolant in Oil (white substance in engine oil or coolant)
-Running on 3 Cylinders
(This is not a definitive list, and depending on what is wrong there may be other things you should check for first.)

b) When buying a car (if the owner allows you)
I always take my comp tester and some basic tools when checking out a car I'm thinking of buying. If you are looking at a cordia to buy and it has a compression related problem it is probably not worth buying unless you enjoy changing pistons and head gaskets!


Compression related problems are generally quite serious, it means that there is something internally wrong with your engine. When tracking down a problem, it's a good thing to check that the compression is alright before you try anything else. Here is a quote from a Haynes manual regarding engine compression:

Quote:
The first step in any tune-up or diagnostic procedure to help correct a poor running engine is a cylinder compression check. This check will help determine the condition of internal engine components and should be used as a guide for tune-up or repair procedures.

For instance, if a compression check indicates serious internal engine wear, a conventional tune-up will not improve the peformance of the engine and would be a waste of time and money.


What does a compression check tell you?

It will determine the condition of the upper end of the engine
-Valves
-Valve Seats
-Pistons
-Piston Rings
-Head Gasket

What do I need to do a compression check?

There are two options:

Option one is to take the car to a mechanic and ask them to do a compression test. This will typically cost between $50 and $100 and is recommended if you think the procedure below is too hard.

Option two is to buy a compression tester, they typically cost between $30 and $50 and is a very useful tool which you can keep for the rest of your life. I recommend the ones with a flexible rubber hose rather than the slot-in testers (the slot-in testers will work for sohc cordia motors, but they don't work on alot of dohc engines). You will also need the appropriate tools to remove the spark plugs

How do I do a compression check?

It's generally good to have a well-charged battery to do this test, as a dying battery will crank slower and give you lower readings.

1. Make sure the engine is warmed up when doing the compression test

2. With the car running, disconnect the electrical plug in the boot of the car for the fuel pump. Alternatively you can look for a fuse for "Fuel Pump" or "EFI" or "EFI Pump". The car will splutter and die, you do this in order to relieve fuel pressure from the system and to stop fuel from being injected into a cylinder when doing the test. Leave the plug disconnected until the test is complete

3. Disconnect the plug to the E101 Knock Control (black box on firewall). This prevents the ignition module from sparking. Alternatively you can disconnect the wiring to the coil, the plug on the dizzy or the high tension lead from the coil to the dizzy (and ground it).

4. Remove the spark plug leads and all 4 spark plugs (it might be a good idea to replace your spark plugs if they are looking a bit cruddy since you will have them all out when doing this test)

5. Screw the compression tester into cylinder 1 spark plug hole

6. Press the accelerator pedal to the floor and hold the ignition key to start, let it crank for 4-10 revolutions at WOT (look at the needle on the gauge, crank until it 'stabilises' at a specific value or until 10 cranks whichever comes first). You'll notice the compression get's slightly higher with each crank.

7. Record the compression reading from the gauge (the comp tester will have a 'peak hold' so get the reading then reset it by pressing the button on the gauge)

8. Repeat the procedure for the each cylinder

Interpreting Results

This is known as a "dry" test, have a look at the compression results. The dry test should be within the following:

Turbo engines:
165psi standard
120psi service limit
no more than 14psi difference between cylinders

Non-turbo engines:
192psi standard
140psi service limit
no more than 14psi difference between cylinders

Check haynes manual for your car for exact compression values as they vary from engine to engine depending on the compression ratio.

On a healthy motor, all results should be within service limit, and roughly even across all four cylinders. If there is one cylinder which is specfically low, this indicates a problem with this cylinder.

If everything is within limits and you are happy with the values you can probably leave it there, plug everything back in and leave it at that.

If something was out of limits, the next step is to do a "Wet" test in order to narrow down the problem. Repeat the compression test, but this time add 1-2 tablespoons of oil to each cylinder just before you do the test. Record the results

Compression is roughly the same for the wet test and the dry test
This indicates that the piston rings and pistons are alright, problem most likely lies in the valves and valve seats or the head gasket.

Remove the head and sent if off for a head service, and replace head gasket (or replace head itself). If it's just the head gasket (there will be other signs such as oil in water, water in oil, bubbling coolant when car running and radiator cap off, white smoke from exhaust), you may get away with just replacing the head gasket, but it's recommended you get a head service because head gasket usually means the car has been overheated and the head may be warped or cracked.

(before you take the head off, take off the rocker cover and check that everything up here is working when you turn the engine, also check the cam timing and the valve clearances because these could be causing a valve to stick open, or maybe you have a cracked rocker arm or something).

Compression increases when doing the wet test
This indicates that the leaking is from the piston or piston ring, or the cylinder bore is worn, you should do a full rebuild.

If it's down in all cylinders it usually means rings are worn across the board, if it's only down in one cylinder it will usually be a cracked piston, cracked ring or cracked ring land, it's quite common on cordia's to crack ring-lands

An Example

Tim notices his cordia is blowing blue smoke, has poor power and won't hold more than 3psi of boost, this could indicate that Tim has a problem with his turbo or his piston rings. Some people might assume this is a turbo problem, but Tim has a compression tester so rather than assuming it's the turbo and replacing, he decides to do a compression test.

He did a compression test and it revealed the following results (compression results in psi):
Dry: 95, 95, 165, 165
Wet: 165, 165, 165, 165

There were two cylinders which were significantly low on compression and compression increased when adding oil to the cylinder. This indicates straight away that the top end of the engine is fine and that compression is leaking through the pistons or piston rings in cylinders 1 and 2.

After removing the head and taking out all 4 pistons, Tim discovered that a piece of the ring land between rings 1 and 2 on cylinder 1 had cracked off, and that piston 2 had a hairline crack on the ring land, and also a cracked ring.

It turns out the previous owner of the car had ran unlimited boost, piston ring lands on cordia's are also a common problem. Tim replaced both pistons, and reassembled the engine with new rings, conrod bearings and head gasket. Compression is now 158psi across all cylinders.

(this story was actually what happened to me in my current cordia).

What a compression test won't tell you

It won't determine the condition of the bottom end (ie bearings, rods etc) and any of the sensors, or ancilliary engine components.

It should be used as a guide only, if the compression is fine, then it's good news since all of the above solutions are major repairs.

Conclusion

I hope this guide was useful, I'm sick of seeing people randomly replace stuff when looking at a problem, and the fact is, if you have bad compression replacing stuff won't fix it! Always do this first so you can rule out an engine problem.

A compression tester is something I think any ameatur or professional mechanic should own, I have personally used my comp tester on countless occasions, and it has saved me alot of money in the long run by helping to dianose problems and also helping me make purchasing decisions for various cars me or my friends have considered buying.

_________________
now a certified cordia owner....


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