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Checking Cylinder Compression - oops 

 

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 Post subject: Checking Cylinder Compression - oops
Posted: Tue May 29, 2007 1:34 pm 
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This post is a reply to the Checking Cylinder Compression post from Ilivittar. Clicked on 'New Topic' it seems
Supercheap had 3 models of tester when I went in. The cheapest unit did not screw in so I got the mid priced one at $29. The most expensive unit just under $40 is the one to get. It has an extended plug fitting so you won't be twisting on the hose trying to tighten it up and because you cannot make it tight you will be wondering how good a seal you have.

1. Warm the motor up, pull out all the plugs
2. If you can get the plug out of the back of the coil (not the HT lead) do that. OR pull the centre lead from the distributor and earth it.
3. Screw in the tester. Position it so you can see it from the drivers seat.
4. Hop in the car.
5. Floor the accelerator (Wide Open Throttle).
6. Crank the motor.

You will see the guage jump up in one or two big steps then creep up on the the next few rotations. The rotations are easy to pick from the sound as the motor turns as you only have 1 cylinder (the one with the guage) developing pressure. I found it required about 5 turns (about 4 seconds) to get to the maximum pressure reading.
Write the results down as you go.

I tried it after by just removing one plug at a time and screwing in the guage then starting the motor. Mine idles on only 5 plugs. I got similar readings although I did not touch the throttle (idle only).

Good luck!

C
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Posted: Tue May 29, 2007 4:49 pm 
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So when you say earthing the leads, i'm assuming the lead resistance prevents it from being a short circuit and wont fry anything. Also, if you use a remote starter switch, instead of getting some1 to flaw the throttle, just pull the throttle under the bonnet.

 

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Posted: Tue May 29, 2007 5:20 pm 
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There's also wet and dry testing, wet seals the rings, that way you know if compression loss is from rings or valves/head.

2 or 3 pumps from an oil can into the cylinder will do the trick.
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Posted: Tue May 29, 2007 5:47 pm 
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Only earth the HT lead from the coil - the energy that was intended to be the sparks will not heat the coil enough to worry it, and yes any resistance in the HT lead will soak up some too.

If you remove the multiwire plug from the back of the coil you should not earth anything.
I did mine (an EF) with out worrying about earthing anything but those early coils are flakey and the higher voltages due to no spark path may tip a borderline coil over the edge and cost you $60 or more. If my coil failed like that I would change it anyway but you may not want the grief - or the potentially lethal sparks. :)
I hope this is clear now.

If the car is running ok now, what will you do if a cylinder or 2 is down a bit?

Cheers,
C
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 Post subject: Re: Checking Cylinder Compression - oops
Posted: Tue May 29, 2007 6:02 pm 
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Commando wrote:
This post is a reply to the Checking Cylinder Compression post from Ilivittar. Clicked on 'New Topic' it seems
Supercheap had 3 models of tester when I went in. The cheapest unit did not screw in so I got the mid priced one at $29. The most expensive unit just under $40 is the one to get. It has an extended plug fitting so you won't be twisting on the hose trying to tighten it up and because you cannot make it tight you will be wondering how good a seal you have.

1. Warm the motor up, pull out all the plugs
2. If you can get the plug out of the back of the coil (not the HT lead) do that. OR pull the centre lead from the distributor and earth it.
3. Screw in the tester. Position it so you can see it from the drivers seat.
4. Hop in the car.
5. Floor the accelerator (Wide Open Throttle).
6. Crank the motor.

You will see the guage jump up in one or two big steps then creep up on the the next few rotations. The rotations are easy to pick from the sound as the motor turns as you only have 1 cylinder (the one with the guage) developing pressure. I found it required about 5 turns (about 4 seconds) to get to the maximum pressure reading.
Write the results down as you go.

I tried it after by just removing one plug at a time and screwing in the guage then starting the motor. Mine idles on only 5 plugs. I got similar readings although I did not touch the throttle (idle only).

Good luck!

C


Hmmm How do I earth the centre distributer lead?

It sounds like my best bet since I doubt I can get behind the back of the distributer.

Just for clarification crank the motor means start it with they key and hold it down for 5 rotations?
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Posted: Tue May 29, 2007 7:22 pm 
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dam my tester costed me $140 but it a top of the line abw one , to earth the centre lead for dizzy ignition just pop it off the cap and rtest it on a stud or simialar within reach

 

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Posted: Tue May 29, 2007 10:37 pm 
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i'm afraid to test the compression of my motor, had some previous bad experiences :lol: As long as it drives good i'm not worried.
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Posted: Tue May 29, 2007 11:40 pm 
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i can relate that,,the last time i did a compression test was on my 78 kingswood,,i set the engine bay on fire..not good

 

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Posted: Wed May 30, 2007 12:32 am 
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Woah how'd you set it on fire?
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Posted: Wed May 30, 2007 12:35 am 
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horis wrote:
dam my tester costed me $140 but it a top of the line abw one , to earth the centre lead for dizzy ignition just pop it off the cap and rtest it on a stud or simialar within reach


So a stud just a bolt?

I never got this whole earthing thing.

So by earthing it do I make it touch anything metal in the engine bay including the engine itself or do I not let it tiuch anything?
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Posted: Wed May 30, 2007 11:36 am 
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"So a stud just a bolt?

I never got this whole earthing thing.

So by earthing it do I make it touch anything metal in the engine bay including the engine itself or do I not let it tiuch anything?"



Ok a stud is a threaded rod without a head - a bolt has a head. The head is the hexagonal (or possibly round) fatter bit on the end of a bolt that you normally turn it by.

The nominally 12 volt battery in our cars has 2 terminals called positive and negative. For a number of reasons in most cars including Fords the negative (-) battery terminal is connected to the chassis of the car. You may find this has been done at a couple of places typically one connection to the bodywork near the battery and another to the engine. The metalwork of the car is then called earth or ground. The negative of the battery is now available everywhere there is metal that is part of the car. You only need to run one wire from the positive battery terminal and connect your load (say a light) between the positive and the metalwork of the car. You can test this with a test lamp. Connect one lead to positive and poke around the metalwork of your car it will come on everywhere you hit metal.
Now look at a spark plug. It only has one obvious terminal. The other terminal is actually the threaded metal part that connects to the engine (earth) when the plug is installed so again you only need to run one HT wire to the terminal on top to get sparks happening. So grounding the lead means to touch it onto the metal. Do this where there is no fuel or fuel vapour. I suggest on the inlet manifold away from the injectors.
Crank the motor with the key like normal. I think when you crank the motor with WOT there is no fuel injected so the fire risks are reduced considerably.

Have fun!
C
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Posted: Wed May 30, 2007 2:04 pm 
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Thanks again.

I think I got it now.

I just touch the center lead from the distributer cap to anywhere metal on the engine (as long as it's away from fuel).

I thought if I done that therre would be a high amount of current running thruogh the whole engine and if I touched anything on the engine I would get shocked.
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Posted: Wed May 30, 2007 7:26 pm 
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thanx commando for explainin that saves me doin all that typing :D :lol:

 

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Posted: Fri Jun 01, 2007 8:09 pm 
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Hmmm took a look in the engine today and the center distributer lead connects to something towards the back of the engine under the inlet manifold.

I I pull the lead off at that thing at the back of the engine instead of the distributer, will that mean I wont have to earth it.

I'm not confident enough with earthing and I dont want to get shocked or set the car on fire
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Posted: Fri Jun 01, 2007 8:50 pm 
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2 things.
1. Why not unplug the fuel pump relay, on the EF it is under coolant expansion bottle and should be green.
2. Unplug the crank position sensor (EF) or TFI module (dizzy).
Start the engine after removing fuel relay to relieve fuel pressure in the system then there will be no fuel pumped into cylinders during test (fire hazzard).
With the CPS (EF) or TFI (other) unplugged the coil will not fire.

 

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