Compression test on inboard motor

[rick99]

I’m looking to buy boat with in board motor. I’ve run compression test on out board. What about in boards? Do you run compression tests on those?

 

 

Sent from my iPhone using Lake Ontario United mobile app

[rolmops]

It is probably the most crucial test to run,because low compression often translates into having to rebuild the engine, which on a big inboard can easily cost thousands of dollars.

 

[rick99]

Does anyone know what the acceptable compression range is for a v6 Chevy around year 2000


Sent from my iPhone using Lake Ontario United

[Bozeman Bob]

Anywhere from 140 to 220 would be acceptable numbers. My engine rebuilder tells me 150 is the magic number. The big thing is the numbers should be within 75-90% of each other, [ depending on who is giving you that % info.]  So for 75% if one is reading 150 the lowest one should not be under 110.  All plugs should be out and coil wire/ignition disarmed. Wouldn't hurt if the engine was warmed up first if that's possible at this time of the year, but not a deal breaker. All gauges read different so a Harbor Freight might read 145 and a Snap On may read 150, the numbers will even vary if made by the same company for that matter.I would only be worried if all the numbers were below 115-120. If you have reading of 130's the engine may be good for another 5-10 years depending on oil changes and abusing the throttle from a standing start. A rebuilt 4.3 is 2,000 tops, probably get a new short block  for that amount of money,if not a lot less. 4.3s are a dime a dozen in the bone yards for that matter.

Edited February 17, 2018 by Bozeman Bob

[JAYRFRED]

A leak down test would actually be the most accurate. That test would not only identify leakage, but you could actually identify the location of the leakage.

Examples being:

1) intake vales - hissing in the intake

2) exhaust valves - hissing in the exhaust

3) piston rings - hissing in the crankcase breather

4) head gaskets - hissing in a plug hole other than the one you are pressurizing

 

good luck

Jay

Edited February 22, 2018 by JAYRFRED

[Bozeman Bob]

The leak down test is not going to give you compression numbers [ which are the most important ] and is more time consuming and the last time I checked will requires a compressor or a air tank at the very least. IF you had a low number in one of the cylinders then ,yes, go to a leak down test, to possibly verify where your losing the compression , but if your numbers are good there is really no reason for it.

[JAYRFRED]

3 hours ago, Bozeman Bob said:

The leak down test is not going to give you compression numbers [ which are the most important ] and is more time consuming and the last time I checked will requires a compressor or a air tank at the very least. IF you had a low number in one of the cylinders then ,yes, go to a leak down test, to possibly verify where your losing the compression , but if your numbers are good there is really no reason for it.

 

I obviously have no knowledge of what type of leak down tester you have used, but the one I use clearly indicates to me percentage of leakage.

All I am saying is that a leak down tester is much more indicative of a cylinders current state of sealing compression; if you know how to use it an intrepet the information obtained by the test.

[Bozeman Bob]

Basically a compression test will tell you there is or isn't a problem. The leak down will help pinpoint where the problem is originating from. If you have good compression there is no need for a leak down test. 

[jimski2]

Test the compression first dry, then squirt some oil in the cylinder and test again. A major change would indicate ring or cylinder damage. 

[chugbug]

I agree with bob do a good compression test warm engine if possible , your not going to be able to check everything in the backyard with someone else's boat .but a basic compression test will give you a idea if you should buy it or not and like stated look to see if the numbers are close write them down as you go per cylinder if you get a crazy figure on one go back at the end and double check it .

[devoknevo]

The actual number on a compression test is irrelevant, as long as all cylinders  are within 10% of each other you are OK.

[Bozeman Bob]

4 hours ago, devoknevo said:

The actual number on a compression test is irrelevant, as long as all cylinders  are within 10% of each other you are OK.

 Not exactly, if you have numbers under  85-90, in most if not all cases, the engine is in need of a rebuild.

[lost a lure]

13 hours ago, devoknevo said:

The actual number on a compression test is irrelevant, as long as all cylinders  are within 10% of each other you are OK.

So if they are all 0 its's all good?

[rick99]

Thanks for all the great input, this is very helpful.


Sent from my iPhone using Lake Ontario United

[devoknevo]

On February 23, 2018 at 12:29 PM, Bozeman Bob said:

 Not exactly, if you have numbers under  85-90, in most if not all cases, the engine is in need of a rebuild.

  

Correct!