Need help to perform a compression test

[clinche]

hi guys..i need to perform a compression test on my 5.7ltr V8 small block engine (carbureted 1998-2001) and would like to do it myself to save some money..i know that i have to remove all spark plug wires and spark plugs..attach a compression test to each cyclinder..crank engine 5 times and record highest reading..am i missing something here? shall i disconnect some wired before doing this?

 

[Don S]

Re: Need help to perform a compression test

Read this Merc service bulletin on the subject.

http://www.boatfix.com/merc/Bullet/97/97_25.pdf

 

[futz]

Re: Need help to perform a compression test

nope maybe crank 7 times till the gauge stops moving higher.

 

[45Auto]

Re: Need help to perform a compression test

You've got it pretty well figured out. Only thing you missed was to make sure the throttle butterflies are FULLY open while you're doing the test. If they're closed, the cylinder will be pulling against a vacuum and it will give you low readings.

 

[futz]

Re: Need help to perform a compression test

should be performed on a warm motor.

 

[a70eliminator]

Re: help in doing a compression test

Re: help in doing a compression test

Yes throttle should be wide open so engine can breathe, 4 or 5 compression strokes. Have a pencil and pad ready to record numbers don't go on memory.

 

[Fun Times]

Re: help in doing a compression test

Re: help in doing a compression test

lower the drive down

You might want to unhook the power going to the fuel pump if it is easy to get to.

 

[clinche]

Re: Need help to perform a compression test

Read this Merc service bulletin on the subject.

http://www.boatfix.com/merc/Bullet/97/97_25.pdf

thats some great info Don S thanks 2 quick questions..

do i have to ground the ignition wire to the engine ground? when i had the mechanic perform the test im quite sure he didnt do this...and im not quite sure i can manage to do this myself

engine has to be in normal operating temperature when removing the spark plugs right?

thanks once again

 

[clinche]

Re: help in doing a compression test

Re: help in doing a compression test

lower the drive down

You might want to unhook the power going to the fuel pump if it is easy to get to.

otherwise i would have fuel spilling out of the empty holes ?

 

[Bondo]

Re: Need help to perform a compression test

do i have to ground the ignition wire to the engine ground?

Ayuh,... It's Safest to pull the center hi-tension lead from the distributer cap,+ Ground it to the motor Block....

engine has to be in normal operating temperature when removing the spark plugs right?

Just warmed up abit is Fine,...

otherwise i would have fuel spilling out of the empty holes ?

Nope,... It won't be Pouring out,... I wouldn't worry about it,...
As noted above,.. Hold the carb Wide open for the testing...

 

[Fun Times]

Re: help in doing a compression test

Re: help in doing a compression test

otherwise i would have fuel spilling out of the empty holes ?[/QUOTE]50/50 chance of it, Or just to keep from flooding the engine with fuel in case the carb is not doing it's job right.

if it is a mechanical fuel pump then leave it as is and do not unhook it.

 

[Don S]

Re: help in doing a compression test

Re: help in doing a compression test

do i have to ground the ignition wire to the engine ground? when i had the mechanic perform the test im quite sure he didnt do this...and im not quite sure i can manage to do this myself

All you do is pull the coil wire out of the distributor and touch it to the engine. If it doesn't reach clip on a jumper wire to the coil wire end to a ground.

engine has to be in normal operating temperature when removing the spark plugs right?

The engine does not have to be at operating temperature. Read the bulletin. But an engine that has set for a long time is also not a good choice.
Starting and running for a minute will do the job, it gets things moving and working that haven't worked for a while.

otherwise i would have fuel spilling out of the empty holes ?

The fuel pump only pumps fuel to the carb. Vacuum from the cylinders with spark plugs in them pulls in fuel. Without spark plugs, no vacuum on the carb to pull in fuel. So no reason to unhook an electric fuel pump. Never heard of disconnecting a mechanical fuel pump for a compression test did you.

Now awaiting all the Newbe experts to chime in

 

[clinche]

Re: Need help to perform a compression test

Ayuh,... It's Safest to pull the center hi-tension lead from the distributer cap,+ Ground it to the motor Block....


Just warmed up abit is Fine,...


Nope,... It won't be Pouring out,... I wouldn't worry about it,...
As noted above,.. Hold the carb Wide open for the testing...

u have to ground the lead to avoid any sparks from the spark plug wires which can cause a fire if in contact with gas?

i have no idea where the hi-tension lead is and where to ground it and i really want to avoid having to call in a mechanic..can i avoid this by making sure spark plug wires are in a safe place (away from gas)..

thanks

 

[Don S]

Re: Need help to perform a compression test

i have no idea where the hi-tension lead is and where to ground it and i really want to avoid having to call in a mechanic

I really don't want to go to the doctor at times, but since I am not a doctor, I may not know enough to fix what needs to be fixed. Using a forum is not the best way to become a mechanic either. There could be lots of things that you don't see, feel, hear or know about that could effect the outcome of a compression test.
You need to understand what, why and how things are done on engines. Not just the steps to go through.

 

[clinche]

Re: help in doing a compression test

Re: help in doing a compression test

All you do is pull the coil wire out of the distributor and touch it to the engine. If it doesn't reach clip on a jumper wire to the coil wire end to a ground.]

the coil wire is the one that goes in the centre of the distributor cap? i just need to disconnect it from the cap and touch it to the engine block? anywhere? u disconnect it for safety?

im very new to this so please excuse my ignorance in this matter..

 

[clinche]

Re: Need help to perform a compression test

I really don't want to go to the doctor at times, but since I am not a doctor, I may not know enough to fix what needs to be fixed. Using a forum is not the best way to become a mechanic either. There could be lots of things that you don't see, feel, hear or know about that could effect the outcome of a compression test.
You need to understand what, why and how things are done on engines. Not just the steps to go through.

i just want to avoid the cost of a compression check when i know that i can do it myself..eventually i will still end up calling in a mech if readings arent good..is a compression test a big of a deal? i am planning to have my wife cranking the engine while im taking in the readings...

 

[Bondo]

Re: Need help to perform a compression test

and i really want to avoid having to call in a mechanic..can i avoid this by making sure spark plug wires are in a safe place (away from gas)..

You need to understand what, why and how things are done on engines. Not just the steps to go through.

Ayuh,... No disrespect intended,.... But,.......
I gotta Agree with Don,... It sounds like this will put you in, Way over your head....
For the price of a Decent compression tester,... You can Have it done....
Especially if combined with a Tune Up....


edit;

eventually i will still end up calling in a mech if readings arent good..

Do you have reason to belive they Won't be Good #s,..??
Tell Us just Why you need to pull this Test,....

 

[clinche]

Re: Need help to perform a compression test

Ayuh,... No disrespect intended,.... But,.......
I gotta Agree with Don,... It sounds like this will put you in, Way over your head....
For the price of a Decent compression tester,... You can Have it done....
Especially if combined with a Tune Up....


edit;


Do you have reason to belive they Won't be Good #s,..??
Tell Us just Why you need to pull this Test,....

i just managed to solve an overheating issue i had this weekend by changing the impeller. mind you impeller i replaced was brand new but nevermind problem fixed. no more overheating however im faced with another prob. while returning to the marina i felt some loss of power at 4K revs. thats y i was planning to take this test. i just wanna make sure nothing serious.

My mech took this test a month ago and all cyclinders had good compression..

 

[clinche]

Re: help in doing a compression test

Re: help in doing a compression test

otherwise i would have fuel spilling out of the empty holes ?[/QUOTE]50/50 chance of it, Or just to keep from flooding the engine with fuel in case the carb is not doing it's job right.

if it is a mechanical fuel pump then leave it as is and do not unhook it.

hi boat tech..it is a mechanical pump so less things to worry about ..

i need some clarification on the ignition coil..i need to remove the ignition coil entering the centre of the distr cap and with a junper wire ground it to anywhere to the engine block..can i ground it directly to the -ve temrinal of the battery?

thanks for your help and patience

 

[proxyx]

Re: help in doing a compression test

Re: help in doing a compression test

compression test explained here:
http://www.youtube.com/watch?v=9U8iNb21arw
EASY!!! all you need is a test gauge ($40)
- compression doesn't usually disappear overnight - trust your mechanic here - if ok look elsewhere

you can ground it anywhere, engine block, engine hangers (best if not painted fully) or battery "-" terminal, get the spark plug tester like this
http://www.ecrater.com/product.php?pid=3274031 - MUST HAVE