Cylinders Low In PSI ? (HELP)

[BigDog98]

Hello. I have encounter a problem in my engine this past week. I have run the compression test in my 5.7L 250HP Mercruiser. Here is a link to all the specs of the engine! ( Mercury MerCruiser 5.7L (250-hp) Inboard Engine Test / Reviews Videos, Specs, Fast Facts, Captain reports | BoatTEST.com ). What could be the reasoning for some of the cylinder to have low PSI in them. And should I attempt to try to fix this problem on my own? Thank You :watermelon:

 

[Fun Times]

Re: Cylinders Low In PSI ? (HELP)

Hi Dennis, Per QC's request honesty is going to be helpful here. PS, one post, one thread at a time.

Please explain how you did the engine compression test and what psi numbers did you come up with on each cylinder?

Was the engine warm?

Was the throttle fully open?

And did you crank the engine over anywhere between 4-6 puffs to get your readings?

 

[BigDog98]

Re: Cylinders Low In PSI ? (HELP)

Hi Dennis, Per QC's request honesty is going to be helpful here. PS, one post, one thread at a time.

Please explain how you did the engine compression test and what psi numbers did you come up with on each cylinder?

Was the engine warm?

Was the throttle fully open?

And did you crank the engine over anywhere between 4-6 puffs to get your readings?

The engine was not warm. The throttle was not fully open. Yes i puffed the engine 5 times to get the reading. The way i did the test was buy taking out all the spark plugs. Disabling the distribution. And connecting the gauge to each cylinder 1 by 1 and testing each cylinder! Here is the following data of the engine. (CY1=170PSI) (CY2=180PSI) (CY3=70PSI) (CY4=135PSI) (CY5=120PSI) (CY6=50PSI) (CY7=150PSI) (CY8=180PSI). Hope this info helped! :laugh:

 

[thumpar]

Re: Cylinders Low In PSI ? (HELP)

You can try putting some oil in the low cylinders and redoing the test. If they come up then you know it is cylinder rings or pistons. If it doesn't change much then it would be head or valves. Those numbers are all over the place. I think in the end it will need a rebuild but before you take it apart do as much diagnostics as you can. Once it is apart you can only visually inspect.

 

[BigDog98]

Re: Cylinders Low In PSI ? (HELP)

You can try putting some oil in the low cylinders and redoing the test. If they come up then you know it is cylinder rings or pistons. If it doesn't change much then it would be head or valves. Those numbers are all over the place. I think in the end it will need a rebuild but before you take it apart do as much diagnostics as you can. Once it is apart you can only visually inspect.

I did the wet test to cylinder#4 and the PSI was even lower. It doped down to 130PSI!

 

[jcbache]

Re: Cylinders Low In PSI ? (HELP)

First of all, what prompted you to do a compression test? Was it running rough? Did it ever get hot (overheat)? What did the plugs look like when you first pulled them? Looks like the low cylinders are not on same side of engine so probably not a head gasket. Might be a valve problem.

 

[BigDog98]

Re: Cylinders Low In PSI ? (HELP)

First of all, what prompted you to do a compression test? Was it running rough? Did it ever get hot (overheat)? What did the plugs look like when you first pulled them? Looks like the low cylinders are not on same side of engine so probably not a head gasket. Might be a valve problem.

My boat performance dropped down by a couple hundred rpm. So decided to see it it was a weak cylinder by doing the test. And it was indeed. Yea the motor did overheat a few times but because of careless mistakes.And the engine did seem to run rough and louder then before. Hope this info helped !

 

[thumpar]

Re: Cylinders Low In PSI ? (HELP)

Do a leakdown test.

 

[jcbache]

Re: Cylinders Low In PSI ? (HELP)

Ditto on the leak down test. Sounds like you might need to pull the heads. Check your head gaskets, look for cracks in the heads, burnt valves, broken valve springs, etc. Might as well just drop them off at a reputable machine shop to get checked out once you pull them. Then you can also check the condition of the cylinders visually. Hopefully the bottom end is ok. Good Luck

 

[BigDog98]

Re: Cylinders Low In PSI ? (HELP)

Ditto on the leak down test. Sounds like you might need to pull the heads. Check your head gaskets, look for cracks in the heads, burnt valves, broken valve springs, etc. Might as well just drop them off at a reputable machine shop to get checked out once you pull them. Then you can also check the condition of the cylinders visually. Hopefully the bottom end is ok. Good Luck

Ok i will follow your advice. I will aslo be making a video if the engine and the compression results so all of you guys can have a more visual approach to the engine. So check back this Saturday or Sunday and i will have the video posted up on this thread soon!

 

[banshee owner]

Re: Cylinders Low In PSI ? (HELP)

just curious as to why you did a wet test on cylinder #4 when according to your results cly #3 and #6 where extremely low or was it a typo.

 

[BigDog98]

Re: Cylinders Low In PSI ? (HELP)

just curious as to why you did a wet test on cylinder #4 when according to your results cly #3 and #6 where extremely low or was it a typo.

I wanted to do a wet test on all 3 cylinders that were low and i got lazy to do the rest so i only did one.But next time i am on the boa then i will redo the compression test the right way and will do the wet test on all 3 that are low! Will post the results soon!

 

[Bondo]

Re: Cylinders Low In PSI ? (HELP)

I wanted to do a wet test on all 3 cylinders that were low and i got lazy to do the rest so i only did one.But next time i am on the boa then i will redo the compression test the right way and will do the wet test on all 3 that are low! Will post the results soon!

Ayuh,.... If ya wanta play Mechanic, ya gotta be Methodical, 'n Diagnose the issue, to develop a cost effective Plan,...

Pull All the spark plugs, open the throttle Wide open, pull the ignition wire off the (+) terminal of the coil, 'n Do the compression test,...
Then do All 8 again wet,...

Then do a leak-down test on All 8 cylinders,.....

If the boat's in the water, start it up, 'n warm it up abit 1st at hi-idle,..
Then do yer tests,...

 

[BigDog98]

Re: Cylinders Low In PSI ? (HELP)

Ayuh,.... If ya wanta play Mechanic, ya gotta be Methodical, 'n Diagnose the issue, to develop a cost effective Plan,...

Pull All the spark plugs, open the throttle Wide open, pull the ignition wire off the (+) terminal of the coil, 'n Do the compression test,...
Then do All 8 again wet,...

Then do a leak-down test on All 8 cylinders,.....

If the boat's in the water, start it up, 'n warm it up abit 1st at hi-idle,..
Then do yer tests,...

Ok I will do that.We will see what numbers come up and compare the right way vs the wrong way. Thank you. I will keep you guys updater!!

 

[harleyman1975]

Re: Cylinders Low In PSI ? (HELP)

Just to make sure you are giving us the straight poop cyl 1,3,5,7 are on one bank and 2,4,6,8 are on the other. How does the oil look? is it clean or milky?

 

[BigDog98]

Re: Cylinders Low In PSI ? (HELP)

Just to make sure you are giving us the straight poop cyl 1,3,5,7 are on one bank and 2,4,6,8 are on the other. How does the oil look? is it clean or milky?

Yea thats right thats how the cylinders are stamped.There is no water in the engine aka oil pan but the oil is a very dark black color i would guess from the few times i overheated!But i change it every year aw well as the oil filter and fuel pump ect ect .

 

[achris]

Re: Cylinders Low In PSI ? (HELP)

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 order. 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 ear muffs while running the engine (but it’s not needed for the compression tests)...
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, 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 revolutions).
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 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, using step 6.
11. Record the cylinders compression reading for that cylinder in the ‘wet’ column.
12. 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

 

[BigDog98]

Re: Cylinders Low In PSI ? (HELP)

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 order. 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 ear muffs while running the engine (but it’s not needed for the compression tests)...
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, 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 revolutions).
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 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, using step 6.
11. Record the cylinders compression reading for that cylinder in the ‘wet’ column.
12. 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

Ok. Quick question! Do i have to put the spark plugs back after i added the oil and turned the engine over? It is just a pain to remove them again and do the wet test that way. Can i just add the oil in each cylinder and the connect the gauge to each cylinder right away ? Thanks

 

[jcbache]

Re: Cylinders Low In PSI ? (HELP)

It's the same amount of work either way.

 

[BigDog98]

Re: Cylinders Low In PSI ? (HELP)

It's the same amount of work either way.

Alright i guess ill do it achris's way. Will also make a video of it to make sure that i did it right.