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Compression test on inboard motor


rick99

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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 by Bozeman Bob
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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 by JAYRFRED
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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.

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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.

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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 .

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