Compression test - how to do-gctid391901

[Guest]

Just a quick question hoping for a very very simple answer. I have read lots of ways to do this and they are all talking about disconnecting all sorts of leads. I am mechanically minded but when I read some of these I think maybe I am not. Anyway about to change my plugs this weekend and decided I might as well do a compression test as well. I have a tester but just want to know how to do it in a very simple manner. I thought just take plug out, put in tester and turn engine over for about two turns and check meter. Obviously I am wrong so what do I do and what should the compression be for a 3.0 L mercruiser, 2004, 175 bayliner. Thanks.

[Guest]

Pull the coil wire, pull all the plugs wires off the plugs and mark as necessary, pull all the plugs which relieves strain on the starter as well as the battery and allows the motor to spin easily. Allow the motor to spin at least 6 cycles. If you end up with low compression in any cylinder squirt some oil in the cylinder and re-take immediately. If the compression for that cylinder comes up significantly you probably have worn piston rings. If the compression only comes up slightly but is still low you likely have a valve problem.

[Chief_Alen]

On a warm engine remove all the spark plugs, advance the throttle all the way forward. Keep track of what plug goes in what hole.

http://www.secondchancegarage.com/public/186.cfm

http://www.ngksparkplugs.com/tech_support/spark_plugs/faqs/faqread.asp

[Guest]

Thanks both of you, that's exactly what I wanted.

[2850Bounty]

Where Wayne suggests pulling the coil wire, he means to disrupt the power to the coil. We don't want to create high voltage with no where for it to discharge. This may damage an ignition coil.

5 or 6 cycles per cycler is correct..... it will be accumulative only to the point of the instrument reaching actual pressure.

You can perform both cold and warm tests.

FYI:

Adding oil to cyclinders is old school from the days of flat top pistons and in-line vertical cylinder engines.

The little in-line 3.0L Merc engine was apparently available with both a dished and flat top pistons.

If fitted with dished pistons, the oil will pool within the dish and without aiding piston ring sealing. The oil volume alone will raise cylinder pressure witout pin-pointing bad valves or bad piston rings. This can become a misnomer while testing.

If fitted with flap top pistons, then this will aid in pin-pointing bad valves or bad piston rings.

.

[Guest]

2850Bounty wrote:
Where Wayne suggests pulling the coil wire, he means to disrupt the power to the coil. We don't want to create high voltage with no where for it to discharge. This may damage an ignition coil.

5 or 6 cycles per cycler is correct..... it will be accumulative only to the point of the instrument reaching actual pressure.

You can perform both cold and warm tests.

FYI:

Adding oil to cyclinders is old school from the days of flat top pistons and in-line vertical cylinder engines.

The little in-line 3.0L Merc engine was apparently available with both a dished and flat top pistons.

If fitted with dished pistons, the oil will pool within the dish and without aiding piston ring sealing. The oil volume alone will raise cylinder pressure witout pin-pointing bad valves or bad piston rings. This can become a misnomer while testing.

If fitted with flap top pistons, then this will aid in pin-pointing bad valves or bad piston rings.

.

Thanks, I must admit and say I probably won't put anything in if compression is poor as I just want to see what it is. I have no reason to beleive that anything is wrong as the engine runs perfectly but was just interested and thought I would do the test whilst changing the plugs as part of routine maintenance.

[Guest]

2850Bounty wrote:
Where Wayne suggests pulling the coil wire, he means to disrupt the power to the coil. We don't want to create high voltage with no where for it to discharge. This may damage an ignition coil.

5 or 6 cycles per cycler is correct..... it will be accumulative only to the point of the instrument reaching actual pressure.

You can perform both cold and warm tests.

FYI:

Adding oil to cyclinders is old school from the days of flat top pistons and in-line vertical cylinder engines.

The little in-line 3.0L Merc engine was apparently available with both a dished and flat top pistons.

If fitted with dished pistons, the oil will pool within the dish and without aiding piston ring sealing. The oil volume alone will raise cylinder pressure witout pin-pointing bad valves or bad piston rings. This can become a misnomer while testing.

If fitted with flap top pistons, then this will aid in pin-pointing bad valves or bad piston rings.

.

Are you calling me old!:arr

[2850Bounty]

Fish-a-Palooza wrote:
Are you calling me old! :arr

NO.... not at all!

[Guest]

Now now I don't want to cause a fight !!! Funny.

[2850Bounty]

Bay Cruise wrote:
Now now I don't want to cause a fight !!! Funny.

Hey....... Wayne and I are just kidding each other.

Besides, he may not be too far behind me in age, so he has no foot to stand on! :kidding

.

[Guest]

2850Bounty wrote:
Hey....... Wayne and I are just kidding each other.

Besides, he may not be too far behind me in age, so he has no foot to stand on! :kidding

.

Of course all in fun Although if Rick thinks he's only a little older than me something is wrong either with his eyes or maybe I should have taken much better care of myself.:arr

[Guest]

I agree with the removal of spark plugs, and wot.

One thing a guy told me once, which I now live by, is to have a buddy hit the starter and you watch the guage. It's not just important to see where it tops out, but how it gets there.

In particular, I'm looking for about 70-80% of max on the first stroke, 80-90% on the second, then small increments thereafter. This tells you about the health of the rings as well.

Then look for difference between cylinders/wet dry etc.

Chay

[2850Bounty]

I'll say this again..... when we do a wet test on a V engine, the oil will tend to pool at the low side of the ring landings.

(I know.... this thread is discussing a 3.0L in-line engine)

Ideally we want an even oiling around the perimeter of the ring landings in order to aid them in sealing...., and this is just about impossible to accomplish on a V engine, or an in-line with dished pistons.

Also, adding 4 or 5cc of oil to any cylinder will increase cylinder pressure during several cycles.

This is no different than if we changed cylinder heads to a smaller chamber, or changed pistons to a smaller dish volume.

If we add 3cc to one cylinder, and if we were to add 6cc of oil to another cylinder, and then test, we will come up with unlike readings.

How much difference.... I don't know.

The wet test is not a perfect science unless we measure the oil volume, and/or unless we can somehow wet the ring landings evenly.

Just say'n!

[Guest]

Thanks guys I didn't get to do it today but if the weather is bad tomorrow and I am not on the water I should get to change plugs then.

[Guest]

Should have asked, do I need to run water in the stern when cranking the engine over.