My Compression Numbers?

[dg512055]

I have done a compression test on the old 4.3 LX gen1 today. Here are the numbers:

200, 205, 200, 205, 200, 205.

I am using a gauge with the schrader valve in the spark plug hole fitting, and running through 5 commpressions for each cylinder. The manual says 180 psi. Do these numbers seems strange to anyone?

DG

 

[Don S]

Re: My Compression Numbers?

IF the spark plugs look normal, I would guess your gauge might be reading a little high. Could also be the pressure you have on your engine. The 180 in the manual is a "Typical" reading on a good engine.

 

[Mischief Managed]

Re: My Compression Numbers?

If you've been running high octane fuel in it for awhile, the high compression number might be due combustion deposits built up on the pistons and heads, but I'd test the tester to make sure it's accurate before worrying about that.

 

[picklenjim]

Re: My Compression Numbers?

Could be that the guage is off but I wouldn't worry about it beings their all consistent. That's what's important.

 

[TilliamWe]

Re: My Compression Numbers?

The manual says 180 psi. Do these numbers seems strange to anyone?

Okay, Don or anyone else, would you like to educate me/others on exactly what other factors, besides compression ratio, affect the PSI on cylinders? I am sure my thinking could be faulty here, but I thought if two engines had the same compression ratio, then their PSI #s would be the same? Or am I forgetting about the total "volume" of the cylinder? And if I am, the cylinder volume of a 4.3 and a 5.7 is exactly the same, so why wouldn't a 4.3 and a 5.7, with similiar compression ratios have similiar PSI readings?

What I am getting at is that a 5.7, with around a 9.8 to 1 compression ratio, will produce a lot more than 180psi, and so should a 4.3.

But regardless of all that, dg, those number look fine, especially since they are so close to each other. You don't appear to have any "dead" or "sick" cylinders. They all seem pretty equal to me, and that's good.

 

[Bondo]

Re: My Compression Numbers?

Ayuh,... Cam timing,+ overlap have alot to do with it....

 

[Gary H NC]

Re: My Compression Numbers?

I think they are fine also....I have 2 different compression gauges and they both read different.

 

[Don S]

Re: My Compression Numbers?

The OP's boat is a 97, it's very possible that at some point in it's life it was repaired and possibly the heads and/or block trued up due to warpage. That would increase compression equally on all cylinders.

 

[mkast]

Re: My Compression Numbers?

How was the compression test performed?
Engine cold or at operating temperature?
Carburetor or injected?
Throttle opened or closed?

 

[dg512055]

Re: My Compression Numbers?

Engine cold, all plugs out, carbed engine, throttle closed. Ran the starter long enough to run the tested cylinder through 5 compression cycles. At this point the gauge was not increasing any more. The plug hole fitting does have a valve in it. I do not think the engine has ever been apart. I replaced the thermostat a few weeks ago and it was original. No other evidence of engine disassembly.

DG

 

[Don S]

Re: My Compression Numbers?

What do the spark plugs look like?
Sounds to me like you have a good worry going on for no reason at all.

Read this and try again.
http://www.boatfix.com/merc/Bullet/97/97_25.pdf

You also said the engine was cold. Had it been sitting a long time, maybe have fogging oil in it.
If it was winterized, then run it for a while, let it cool then take a compression test and follow the service bulletin link above.