Mechanical question regarding piston not working

[Marina Bay]

Hi every body,

I have MErcruiser 5.7L 1999 and I have tested the engine pressure today and I got the following:

Piston one:175
Piston two: 150
Piston three: 100
Piston four: 200
Piston five 150
Piston six: 150
Piston seven: 165
Piston eight: ZERO no pressure at all.

Can any body advise me with what shall I do next with this problem? somebody said I have to replace the spark plugs then check it and another one said I need to open the whole head block, and now I got lost what to do.

Please help.

 

[tpenfield]

Re: Mechanical question regarding piston not working

OK, this is a compression test result (right ?)

#4 looks like it has fluid in it (200 PSI), maybe the same with #1.

2, 5, 6, 7 you could consider 'normal'

as for #8, well 0 is 0.

you could do a cylinder leak test to find out more about #3 and # 8

'somebody' probably does not know too much about engines . . . and . . . 'another one' may be more on the right track. (it would be called the "cylinder head")

Can you fill us in on what prompted you to perform a compression test . . . That will help in providing some advice.

 

[Don S]

Re: Mechanical question regarding piston not working

Remove the head and look. With 0 compression, the problem will be obvious.

 

[Maclin]

Re: Mechanical question regarding piston not working

If you want to know more about the problem without tearing it down first then a cylinder leakdown test is what you need to do next. I will give a sumary of the procedure below, but with ZERO compression you might as well take the head off because something is very wrong, something that cannot be fixed with the head on. It will be very obvious once the head is off and the valves and piston are exposed.


The problem cylinder is brought to TDC on the compression stroke. A special air fitting is screwed into the spark plug hole, and compressed air is introduced into the cylinder that way. The air hissing is what you listen for, if you hear it in the crank case then you know the rings are (very) bad or the piston has a hole in it. If you hear it thru the exhaust manifold then the exhaust valvl has a problem. If you hear it thru the intake manifold then the intake valve has a problem.

 

[Maclin]

Re: Mechanical question regarding piston not working

I see Tpen and Don S had the quick fingers again!

 

[Silvertip]

Re: Mechanical question regarding piston not working

If your local auto store has a tool rental you could rent a bore scope and take look through the spark plug hole. You could also do this investigation in a more organized manner such as:

1) Bore scope. Chances are a piston with a hole in it would be visible as would presence of water
2) Remove valve cover and check for broken valve spring(s)
3) Then do leakdown test if things look normal to this point.
4) If leakdown fails then remove the head. Even if the leakdown is normal at this point the head still needs to come off to see why there is no compression (as in blown head gasket).

Something has caused you to begin this investigation. What happened. Noises indicate something broke. A sudden engine miss and perhaps a single "pop" would indicate a blown head gasket.

 

[Don S]

Re: Mechanical question regarding piston not working

Leaky valves, head gaskets, worn out rings, etc. will not give you ZERO compression, 20 to 80 probably. The head of a valve missing might give you 0 compression, and a big hole in the piston will. So why spend all the time looking for a little leak with leak down tests and bore scopes. Pull the head, something big is wrong and will be obvious even to an un-trained eye.

 

[tpenfield]

Re: Mechanical question regarding piston not working

One of my buddies had a similar situation this summer. . . 0 compression in one of the cylinders. The mechanic was ready to pull the head off, but thought to check a few more things first.

It turned out to be a stuck valve. It was remedied without removing the cylinder head.

So, you never know, and it is worth checking out the possibilities before opening the engine.

i would check to see if there is a stuck valve on #8 before you open it up

 

[achris]

Re: Mechanical question regarding piston not working

...It turned out to be a stuck have. ...

What is a 'stuck have'?...

 

[tpenfield]

Re: Mechanical question regarding piston not working

darn you auto-correct !!! Remember that thread ???