MerCruiser 5.0 Carb Engine Compression Test, Two Piston came back with Zero

[Robertse]

Hi, This MerCruiser 5.0 Carb Model Engine is on Bayliner. The Engine has not been started for a few years, I tried to start it it almost kicked over, so I did a compression test with the following results. Piston 1 - 85, 4,=90, 6=0, 8=60, 7-70, 5=0, 3= 130, 1=100. I know overall the majority of the total pistons reported low compression. The strange issue is that Piston 6=0, and 5=0 both reported Zero Compression, however they are on opposite heads, so the issue might not be head gaskets, do you think it is Value issue on these two postions. Warm Regards,

 

[racerone]

Remove valve covers.-----Likely a good idea to see if valves are all moving.

 

[Scott Danforth]

head gaskets are almost never an issue - water in the cylinders from failing riser gaskets usually is

is this a fresh water boat, or a salt water boat?

with numbers this low, at a minimum, pull the exhaust, pull the intake, then pull the heads and investigate what needs to be repaired. I would pull the motor and do a teardown myself.

 

[Scott06]

Hi, This MerCruiser 5.0 Carb Model Engine is on Bayliner. The Engine has not been started for a few years, I tried to start it it almost kicked over, so I did a compression test with the following results. Piston 1 - 85, 4,=90, 6=0, 8=60, 7-70, 5=0, 3= 130, 1=100. I know overall the majority of the total pistons reported low compression. The strange issue is that Piston 6=0, and 5=0 both reported Zero Compression, however they are on opposite heads, so the issue might not be head gaskets, do you think it is Value issue on these two postions. Warm Regards,

How many is a few years ? Yes I would assume the valves are hung up or the seats are corroded. You can probably pop a valve cover, take the rockers off and smack the valves with a dead blow. would also put some marvel mystery oil in the cylinders let it soak .

Typically when engines have sat I take the plugs out and spin it on the starter until oil gets up to the rockers. longest I had an engine sit was 8 years but it was in a dry garage. If near humidity and it wasnt layed up correctly and fogged or oiled may be an issue. Defeinetley change the oil use something like rotella T4 change it after you run it a bit. no way there isnt sludge in there

 

[muc]

I'm more of a diagnose it before disassemble it kind of guy.
Leakdown test.
Bring #6 to TDC (on the compression stroke) and put some compressed air thru the sparkplug hole. Listen at carb, oil fill and exhaust. Hear where the air is escaping, repeat for #5
If it's coming out of the oil fill, no need to start taking things apart in the boat. Motor is coming out.

 

[kenny nunez]

Whether it is a Ford or Chevy engine it probably has stuck valves, if so then either there are some bent push rods or just some of the rocker arm studs have been pulled up. No big deal hammering them back but you have to free up the stuck valves. If you can do that then squirt some Marvel Mystery oil in all the cylinders then spin nit over and then check the compression.
I had some stuck valves in a 351 from a salvage yard, some bent push rods and pulled studs. I freed up the valves hammered the studs and replaced the push rods. Turned out to have even compression afterward. I got lucky, for a change.

 

[Lou C]

I think Muc's way is the most efficient to find out what needs doing. The fact that it's #s 5&6 suggests the manifold to elbow gasket, or leaky elbow and/or manifold, but it is worth doing more investigation to get a better diagnosis, and the leak down test will do that for sure.