compression on a 93' merc 200?

[bigfish21]

What should the compression be on a 1993 mercury 200 fuel injected motor? I have found a boat and i am going to check it out tomorow and plan on doing a compression check on it. I am kinda figuring it should be around 120 to 125 but not sure. Any help would be greatly appreciated. Thanks

 

[Oldman]

u need to have leak down test done all so.

 

[wrechin2]

Compression SHOULD be around around 120-130 on that engine with stock heads. BUT a compression test does not tell you the health of the engine. I have built many that have had PERFECT compression and have high leakage due to mechanical reasons. A leak down test performed at 1/2 ATDC will tell you EXACTLY how healthy a engine is. The only time I rely on a compression test is when I build a modified engine to ensure I and using the correct octane of fuel.</p>

Examples....I had a 87 150XP come in today that is having issues and the gentleman just recently bought and both times he had it out had problems with it running. I ran a compression test and all were between 86-93 PSI. This is normal for a XP/GT 150 as they are low compression engines. I then done a leak down test on it and found #1 had 73%!!! None of the cylinders passed!!!! A healthy engine will have 10% or less leakage. This engine has mechanical issues But had PERFECT compression!!! We pulled the cover off the side and looked at the side of the piston. It does not have a lot of carbon and the rings are still free. It shows to have fatigued rings that have lost their tensile (spring) or possible egg shaped cylinders. </p>

Thursday I performed a compression leak down test on a 84 GT150 that had 84-89 PSI on compression and found #1-52%, #3-63%, #5-51% and then #2-4%, #4-4%, #6-5%. Because all the star had high leakage, we pulled the head and found it was carbon build up from a failed thermostat that caused that bank to ran cold and coked up due to it never got hot enough to burn off any carbon. </p>

What is funny is in the OMC factory manual it tells you that you can have good compression and can still have mechanical issues. I have yet to find it in any of my mercury factory manuals. But will still apply. </p>

These are just my opinions and personal experience. </p>

Here is a 150 mercury that the OWNER started the thread and was standing beside me the whole time and actually done most of the leak down test. 
</p>

http://www.chattanoogafishingforum....5&highlight=150&highlightmode=1&action=search

 

[bigfish21]

Thanks very much old man and James. I will for sure have that done also. That is something i had never thought or heard of before. i have alway thought if the compression was good then the motor was good.

 

[Oldman]

me too until i listened to the wrechin 2 report.he is the guru.

 

[wrechin2]

The sad part is there are a lot of places that solely rely on compression. I have built several engines with issues that "OTHER" places said was not a mechanical issue because it has good compression.

I worked at a Dodge dealership. One day I had a Dodge truck come in with a misfire on cylinder #7 only at a idle. It went away above 1300 RPM. On those we could monitor the misfire with the scan tool. I swapped plugs to see if it followed the plug. It stayed on #7. I performed a compression test and they were all 175-180 PSI. So I switched wires with #5 and the missed stayed on #7. I checked the injector driver and it was ok. I then swapped injectors. I was scratching my head and the shop foreman told me to do a leak down. I told him, "but the compression is good!???" I performed the test and found it to have 80% leakage. With the tool on it it not only showed it had had leakage but you could also hear the air running out of the intake. I pulled the head and found the intake valve was just starting to burn. I took that over onto my boat engines and it has worked out great. A lot of outboard racers use them to determine if it is time to re-ring. This will tell you the "TRUE" health of a engine.

I always do mine at about 1/2" to 3/4" ATDC because carbon can build up at TDC and give a false reading. On most that fail you can do it at TDC and they will pass. If the cylinder is good it will have under 10% leakage all the way down to the idle relief or the port. Which are generally at least 1.200" ATDC. Prime example is Ditchdoc666 engine( link I installed) . It passed at TDC. But failed at 3/4" with 70%. we pulled apart and found the 2nd ring completely coked in.

Just my experiences.

 

[wrechin2]

ALSO, a high leakage engine will seem to run fine on top end due to the duration of time that it can "BLEED" off pressure. Dodge truck example. The issue went away at 1300 RPM. Most high leakage outboard engines will only exhibit low RPM issues like poor hole shots or poor idle. JME...
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