How to do a Compression test

[jjgag60]

I have searched this site and have found some really good ideas on doing a compression check, but I need some more help. I want to know the sequence for checking the compression on a Gt-40 with electronic ignition and fuel injected (1996 to 2002 year models).

Do you remove all the spark plugs at once and then do one at a time, or leave 7 plugs in and do one cylinder at at time?

What do you disconnect on the motor so you dont fry the electronic ignition. It cant be the coil wire with electronic ignition can it? IS there a relay or fuse you should pull?

I know the smart idea would be to have a dealer check it out, but sometime you have to pull the trigger and get your boat and run. To many war stories about the dealer telling the selling party, oh you should have gotten more money for this then what you got.

[Red57Bird]

I did a Google on this and found info related to testing compression on cars. I don't know if it is different for boats, but I wouldn't think so.

http://www.geocities.com/siliconvalley/2072/carcomp.htm
http://www.automedia.com/Engine_Comp...cr20050801cc/1

Good luck.

[Teleman]

From what I know about engines....This is good info to test. Remember, this only tests the portion of the engine that is holding the pressure. Yes, the cylinders, rings, valves, pistons, gaskets and some other parts are getting tested here. It is certainly not the end-all test. Engines and all the associated devices that make a day out on the water happen need to work too. Like the web sites recommend. It is more important that the values be closer to each other than the book values. If the new pressure from the engine is around 145 psi then I would expect there to be less than 10% variation from each other after the engine is old. Remember too. bad compression will also sound like there is a "Miss" to the engine while at idle too. This is just another factor to consider when checking a old engine. "Just cuz the test is good doesn't mean it will run good".

[2gofaster]

You can do it either way. It doesn't really matter whether you pull only the plug on the cylinder you're testing or all of them all at once. You're not going to damage the ignition at all. Just unplug the coil wire and you'll be fine. Look for a balance between all the cylinders. It's best to do the test on a warm engine, but that may not be possible if you're doing it in your driveway.

[wakeboardin2k4]

Just unplug the coil and ground it. You do not want to leave an open spark in the engine bay. I would leave the engine hatch open for a solid 10 minutes to be safe from any fumes and run your blower. People might say this is excessive but its better than blowing up.

Its best to do it warm. With all the spark plugs out, this keeps from putting excess pressure on your motor since it is not getting any spark.

How many hours are on the motor?

Youre looking for less than a 10% variance between cylinders like teleman said.

Look at each plug and see if its oily. If any one of them is covered in oil then that may tell you exactly which cylinder is experiencing blow by.

Good luck

If the motors not running this is a good initial check. If you have good compression id get an enclosed spark board and check to see if you have spark going to each plug.

[AirTool]

It would be better if you did the test with no power to the engine so there is NO fuel and NO spark. Use a jumper on the starter solenoid to crank the engine.

The instructions on my compression set say to take all the plugs out. I prefer to do the test one plug out at a time.

AirToll

[DanielC]

This is how I would do it. First get the engine warm. Remove all the spark plugs. Disconnect the fuel relay. Ground the coil wire by disconnecting it at the distributor cap. Open the throttle wide open.
The engine needs to be warm, because that is what the specs are measured at.
Removing the spark plugs removes the influence of other cylinders on the one you are testing. Disconnecting the fuel pump relay stops fuel flow, its is safer that way. I think that you might be able to disconnect both relays, that would prevent the engines ECM from getting power when you are testing it.
The ignition system in a modern engine develops enough voltage that it will find a way to get to ground if there is no plugs in the circuit, and it can damage, or weaken the insulation in something else if it is not shorted to ground.
Make sure the engine hits the same number of power strokes on each cylinder.

[JUST-IN-TIME]

all plugs out
if you have a leaking head-gasket, and adjacent plug in engine, that cylinder will give u a false reading

i do
cold
hot
wet tests

then leak-down if i find low numbers

[jjgag60]

Where are the fuel pump relays

Daniel: Where are the fuel pump relays for the fuel rails located?

[Red57Bird]

I believe you can simply disconnect the wire from the fuel pump in the FCC.

[JUST-IN-TIME]

I believe you can simply disconnect the wire from the fuel pump in the FCC.

or remove the safety lanyard
it stops spark and fuel

[DanielC]

GT-40 engines will start and run with the safety lanyard disconnected as long as you hold the key in the crank position. The lanyard disconnection stops the fuel pumps in normal operation only.
There are two fuel pumps, also.
Many service technicians have been fooled by a motor that cranks, starts, and runs only when the key is held in the crank position, and dies when the key is let up.
The fuel pump, and engine computer relay are on the back of the motor, (flywheel end). The two relays that look the same.