Cylinder Compression Test Values

N

nigels

Seaman
Joined
Oct 2, 2011
Messages
54
For a 1999 4.3 the manual says 100psi on any one cylinder is about as low as it should be before a overhaul is due but the manual does not say where the starting point is. In other words if you did a compression test on brand new engine what psi would you get?
 
Bondo

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,525
Re: Cylinder Compression Test Values

nigels said:
For a 1999 4.3 the manual says 100psi on any one cylinder is about as low as it should be before a overhaul is due but the manual does not say where the starting point is. In other words if you did a compression test on brand new engine what psi would you get?
Click to expand...

Ayuh,.... 150psi, 'n up is a rock solid motor,...
120 to 150psi is probably Ok....
Under 100 psi is a Core motor...

Any motor shouldn't have more than 10%- 15% difference between the cylinders...
 
achris

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: Cylinder Compression Test Values

nigels said:
For a 1999 4.3 the manual says 100psi on any one cylinder is about as low as it should be before a overhaul is due but the manual does not say where the starting point is. In other words if you did a compression test on brand new engine what psi would you get?
Click to expand...

On a brand new engine? Dunno, never done it.
On a run in engine? About 170psi (depends on the gauge and the length of hose) at cranking speed with the throttle wide open and the engine warm.

HTH,

Chris....
 
N

nigels

Seaman
Joined
Oct 2, 2011
Messages
54
Re: Cylinder Compression Test Values

The test was done by an independant mechanic I hired when I bought the boat, I was there when he did it but cannot say if the throttle was open or not. The engine had been run but left to cool for at least 2 hours. No squirts of oil. His hose was short only just made the cockpit floor. The results were as follows
#1 200
#2 196
#3 195
#4 200
#5 194
#6 196

I was impressed by the consistancy but thought the readings were too high, my thoughts were atmospheric 15 x +-10:1 150 so where is the extra coming from?
 
achris

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: Cylinder Compression Test Values

nigels said:
... so where is the extra coming from?
Click to expand...

Gauge calibration? Doesn't take much to knock those gauges around, and most of them just live in the bottom of the mechanic's toolbox...

Buy your own gauge, do your own test....
 
paultjohnson

paultjohnson

Lieutenant Commander
Joined
Jul 29, 2010
Messages
1,560
Re: Cylinder Compression Test Values

The owners manual for my 86 4.3 states...At a compression ratio of 9.3:1, 180 PSI Compression Pressure...
 
N

nigels

Seaman
Joined
Oct 2, 2011
Messages
54
Re: Cylinder Compression Test Values

paultjohnson said:
The owners manual for my 86 4.3 states...At a compression ratio of 9.3:1, 180 PSI Compression Pressure...
Click to expand...

That's great, my +- 200psi figures can't be right I need to redo the test, I will get my own tester and do it in the spring, (just winterised the boat).

I wonder though what the scientific explanation is; Why atmospheric pressure +- 15psi x 9.3 = 180psi when in theory it should be 140psi ??? is it because of heat or perhaps back pressure in the exhaust.

It would be interesting to learn, any one know?
 
Don S

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: Cylinder Compression Test Values

Why atmospheric pressure +- 15psi x 9.3 = 180psi when in theory it should be 140psi ???
Click to expand...

It's normal for the newer engine (99 is in that range) to have 160 to 175 compression, but it being down to 140 -150 range is about normal for that year engine with normal minimal maintenance that boats get.
 
You must log in or register to reply here.
Top