Compression test, hot or cold?

[Simoniz]

Hi, Before I go mad and pul the head off my Mercruiser 3.0l year 2000, Eng No OL 634881, Im going to do a compression test on it.

Ive looked around on here and seen various figures for compressions, but no officlal numbers, so what shopuld they be please, are they measured with engine hot or cold? Assuming throttle wide open?

 

[alldodge]

Best checked hot, and new manuals just list 100 as minimum. A new motor would be 140 and so long as any one lowest cylinder is within 70% of the highest your good. I like no more then 10% between all cylinders

 

[achris]

How to compression test.​

What you need to do is the following. These are the correct and complete procedures for checking compressions. All steps are important and there for a reason. Each must be followed completely, and in the order below. Missing any step or doing them out of order will render the whole test as useless. Read through all the steps below (a few times) before doing ANYTHING, so you are familiar with what you will be doing.

Before you start, make sure you have the following. A fully charged battery (of the right size for the engine), a compression tester, a spark plug wrench and either a remote starter switch or another person with you and some light engine oil (2 stoke oil is good for this) and a way of getting a small amount of it into each cylinder. You also need to know the cylinder numbering of the engine. Not all engines number the same way!

1. Run the engine until normal operating temperature is reached. VERY important! And remember to put water on the muffs/flushers while running the engine...
2. Remove all spark plugs. It’s a good idea to keep the plugs in the order they came out, so you can ‘read’ them and know which cylinder had what plug. It’s also a good idea to double check they are the correct plugs for that engine at this point.
3. Set the throttle to wide open, and leave it there until ALL checks are complete.
4. Remove the centre lead from the distributor cap and put it on the engine block, securely. This is so you don’t have any sparks flying around while you are testing.
5. Using either a ‘remote starter switch’ hooked to the low current side of the starter solenoid or another person operating the ignition switch, turn the engine over a good half dozen times to clear any residual fuel from the cylinders and intake manifold.
6. Put the compression tester in one of the spark plug holes. Turn the engine over until the reading on the gauge is steady (usually about 5 or 6 compressions).
7. Record the cylinder compression reading for that cylinder in the ‘dry’ column.
8. Repeat step 6 and 7 for the rest of the cylinders.
9. Once all readings are done, squirt a small amount (around a teaspoon) of oil into each cylinder. Replace the spark plugs (but only finger tight is needed) and turn the engine over another half dozen times on the starter (throttle still fully open and the distributor lead still grounded).
10. Remove all the spark plugs and start checking the cylinder compression pressures again, like in step 6.
11. Record the cylinders compression reading for that cylinder in the ‘wet’ column.
12. Repeat step 10 and 11 for the rest of the cylinders.
13. When all tests are complete, move the throttle back to closed, clean the oil from the spark plugs, check their gap and refit them to the cylinders. Reconnect the centre distributor lead.

You should finish with a table that looks like this...

Cylinder #	Dry	Wet
1
2
3
4
5
6
7
8

 

[Simoniz]

Thanks Chris, very useful, to others as well, Im sure.

 

[AKJohne]

Chris, Good post, can you expound on the reason for adding some oil to each cylinder.... and performing a 2nd test?

 

[achris]

Chris, Good post, can you expound on the reason for adding some oil to each cylinder.... and performing a 2nd test?

If you find a cylinder low, you have no idea if it's because of worn piston rings, a head gasket or bad valves/seats. Adding oil to the cylinder helps isolate that. If the reading comes up well, then it's likely the rings. And that means a full engine strip to fix, and that means engine out of the boat. If the reading stays low, it's valves or gasket. Both those problems require the head off, and once the heads off, you'll see which it is. And doesn't require the engine to be pulled.

The oil helps seal up bad rings better than a gasket or valves...

Chris.........

 

[AKJohne]

If you find a cylinder low, you have no idea if it's because of worn piston rings, a head gasket or bad valves/seats. Adding oil to the cylinder helps isolate that. If the reading comes up well, then it's likely the rings. And that means a full engine strip to fix, and that means engine out of the boat. If the reading stays low, it's valves or gasket. Both those problems require the head off, and once the heads off, you'll see which it is. And doesn't require the engine to be pulled.

The oil helps seal up bad rings better than a gasket or valves...

Chris.........

Ok, 2nd test to further diagnosis if one or more cylinders are low.

 

[achris]

Ok, 2nd test to further diagnosis if one or more cylinders are low.

:thumb: