Low Compression in two cylinders and blowby

[Jclarke81]

Hello, Been searching the forums and haven't run across this one specifically. I have 1998 Mercruiser 5.7L Alpha 1 Gen 2 outdrive. Was running great all summer now won't go past 1200 RPMS. I pulled the plugs and had several fouled plugs as well as a lot of carbon buildup in the carb. I replaced the carb with a new one from guaranteed carbs and all the spark plugs. Put everything back together and fires up on land. Take it to the boat ramp to tune the carb using a vacuum gauge, highest vacuum was 18, just into the green range on the gauge. Go to water test and still will not get up past 1200 rpms sounds like it's stumbling etc. Take it to a trusted marina they run a compression test all cylinders in spec(140-150psi) accept 4(50psi) and 6(110psi). There is a ton of blowby at start up. Could this be a head gasket or is it def the rings and would require a complete teardown?

 

[jimmbo]

4 and 6 are next to each other, and since they both have low Numbers, a Bad Head Gasket is strong a possibility

 

[Jclarke81]

Would I still have blow by with a head gasket?

 

[jimmbo]

All engines have Blowby, even brand new ones. Yours is 24 yrs old, and unless you bought it new, you don't know it's History.
You say it has a ton of Blowby at Startup. Is the Spark/Flame Arrestor covered in Oil?
If it is pushing a bunch of smoke out the Exhaust at startup, that is not Blowby,

 

[Jclarke81]

The carb and flame arrestor had a ton of what I thought was carbon buildup but could of been oil on it, which is why I changed the carb. There's a lot of blue smoke when running, more at startup. And the pcv hose has a ton of smoke coming out of that side of the engine in comparison to the other.

 

[Jclarke81]

On second thought I can add oil to the cylinder, recheck compression. If it goes up that's supposed to mean rings right? If I had both spark plugs removed and it went down that would lead to believe leaking into the next cylinder and head gasket? Would that mean I'd have to get the whole head machined? Block would be ok?

 

[Bondo]

On second thought I can add oil to the cylinder, recheck compression. If it goes up that's supposed to mean rings right? If I had both spark plugs removed and it went down that would lead to believe leaking into the next cylinder and head gasket? Would that mean I'd have to get the whole head machined? Block would be ok?

Ayuh,..... Welcome Aboard,..... It'll be cheaper, easier, 'n quicker to just buy a Long Block crate motor,.....
Either new, or reman,....

 

[Jclarke81]

Ayuh,..... Welcome Aboard,..... It'll be cheaper, easier, 'n quicker to just buy a Long Block crate motor,.....
Either new, or reman,....

That's what I was afraid of. I'm not skilled enough to be replacing a whole motor.

 

[jimmbo]

The carb and flame arrestor had a ton of what I thought was carbon buildup but could of been oil on it, which is why I changed the carb. There's a lot of blue smoke when running, more at startup. And the pcv hose has a ton of smoke coming out of that side of the engine in comparison to the other.

Sounds like it is plain worn out.

 

[tank1949]

Hello, Been searching the forums and haven't run across this one specifically. I have 1998 Mercruiser 5.7L Alpha 1 Gen 2 outdrive. Was running great all summer now won't go past 1200 RPMS. I pulled the plugs and had several fouled plugs as well as a lot of carbon buildup in the carb. I replaced the carb with a new one from guaranteed carbs and all the spark plugs. Put everything back together and fires up on land. Take it to the boat ramp to tune the carb using a vacuum gauge, highest vacuum was 18, just into the green range on the gauge. Go to water test and still will not get up past 1200 rpms sounds like it's stumbling etc. Take it to a trusted marina they run a compression test all cylinders in spec(140-150psi) accept 4(50psi) and 6(110psi). There is a ton of blowby at start up. Could this be a head gasket or is it def the rings and would require a complete teardown?

if you have put plug wires back in correct order, expect to pull head.

 

[Scott Danforth]

do a leakdown test. determine if its head gasket, rings or valves

could also be the factory (cheap) GM valve seals leaking oil down the valve steams

Know what needs to be replaced prior to jumping to pulling a motor

if its just the heads, have them redone.
if its the rings, motor is coming out.

pulling a motor is easy. just need to understand righty-tighty, lefty-loosy. and the order of operations. drive comes off first. then motor comes out.

 

[NHGuy]

Yup, heads can be repaired or replaced if the rest of the engine is ok. If you need to check the rings on 4 and 6 you could pressure test the port head on the starboard bank. BTW that's a known spot where head gaskets fail, often from high pressure like water in the cylinders or too much timing. I'd just inspect the gasket and if the fail is visible, and the piston tops are ok, I'd expect the rings to be good.
You also have to find out why the problem happened. This can have happened from exhaust water intrusion into the cylinders. Test the manifolds and risers. Or maybe just plan to get fresh exhaust when you go back together.
If it's head failure put it back together with good MLS (multi layer stainless steel) or composite gaskets. Don't use thin metal head gaskets. They do not seal well. Composite and MLS gaskets seal well, MLS are reusable. Only thing is they can be a long time to order.
Cometic MLS gaskets can take 2 months to come in.
Summit Racing equipment is a good place to buy this stuff. Sometimes you can get part numbers at Summit and find the parts from Rock Auto where the pricing is great.
Both of those sources are good for parts because they do not lie about inventory. Many other parts places will take your money, then tell you the part has a long wait.

 

[Lou C]

Agreed if it’s just the heads & head gaskets it’s not a bad job & can be done with the engine in the boat in many cases. You’ll need a machine shop to evaluate the heads and re-do them if they’re salvageable or if not (cracks & corrosion) you can easily find reman heads just get the same casting number. Other things is you must use good gaskets (I like Fel/Pro) and a thread chaser to really clean out the cyl head bolt holes in the block is a must. If the head bolts are rusty replace with new (ARP). And measure the block for flatness with a straight edge & feeler gauges (less than .003”). Chevy small block is very simple & easy to repair. One advantage of inboards & I/Os.

 

[QBhoy]

I’d say you’ve either had and overheat or a manifold failure at some point in the past…it’s ruptured the head seal between the two cylinders…

 

[Lou C]

HGs can blow in different ways, on mine I had a water leak into 2 cyls, not enough to hydrolock but enough to cause rough running due to water + corroded valve stems but the compression test results were close to normal. The important thing is to act on it quickly, get the water out and change the oil as many times as it takes.