What PSI is a Cyl considered dead

[Tail Chaser 661]

I believe I read 90 psi. A very long time ago.:huh:
I have one still firing at 90 psi now.
I got a little water in the oil.
A pinhole in a riser.
And a new engine waiting to install.
But still need to pick up another 7k in parts to finish the repower.

I'm tempted to. Just buy new risers.
Change the oil again, Getting rid of all signs of water intrusion.
Then try to finish out this season with this old engine.:huh:
Any opinions on this?
The worst thing that could happen is.......
I break down, Get capsized and Get eaten by a shark waiting for help to come :smt043

 

[midway]

Did the engine hydrolock, if it did then you could have a bent rod, if not then you have a weak cyl. Bent rod could be bad, it could break at any time but if not then I would think if your engine is still running good, not knocking or running extremely rough, change the oil a couple of times to clear out the water, put the new manifolds and risers on and see what happens. If you have 1 cyl at 90 psi and the rest at 150 then you definitely have a weak cyl, if they are all in the 100 to 90 psi range then you have a weak engine. As you say what is the worst that could happen and I think you pretty well covered it. :grin:

 

[JediJD]

The rule of thumb is not a single cylinder pressure amount, it is a difference of more than 20% between cylinders. On a carbureated engine...even a pressure of 15-20# might be enough to get you some type of combustion going and be technically not dead, but engines are about balance. (On a port fuel engine, the full fuel amount of each cycle would probably sit in the plenum and make the mixture too rich to light off...a carbureated engine is only going to pull as much of the air fuel mixture from the intake manifold as the negative pressure differential permits...so the mixture is still probably going to be volatile)

In any event, throw one cylinder out...make the piston a comparative dead weight being thrown around a crankshaft...stresses and problems tend to multiply.

 

[Tail Chaser 661]

Thanks for the info,
I got bad internet info two years ago. I had water in the valve covers for the first time. Since a guy with race boats said, This is only condensation and normal. I took his word for it. After showing a oil sample to a mechanic. :smt013

Been needing riser gasket replacements every other year.
Ran the boat at higher cruise speeds last time out.(3500rpm)
That brang out even more issues. Failing risers.
First time I did not change out and upgrade a potential problem part. NOW its bit me :smt013

I have 5cyls at 125psi, 1 at 90psi, 1 at 130psi, and 1 at 150psi.
3 yrs ago I had 7cyls at 150psi and 1 at 130 psi.
Funny, That one low cyl is still at 125 psi.

 

[sbw1]

Is that the original engine without a rebuild? How hours did it run?

 

[JediJD]

Your frustration level may be at the point where you have no further interest in looking into this, but you may find a wet reading to be informative...

Squirt a little 30w oil in the spark plug hole and do the compression test again. It will give you a better idea of ring wear vs. valves. Big improvement in compression=bad rings. Not so much=bad valves.

Valves are a relatively easy short term fix...much easier than rings. If it is valves, you could tear the top off the engine in a long afternoon, have the heads turned around in a day or so, and put it back together on the third with new gaskets. Not a fun job, but not rocket science either.

 

[Tail Chaser 661]

Your frustration level may be at the point where you have no further interest in looking into this, but you may find a wet reading to be informative...

Squirt a little 30w oil in the spark plug hole and do the compression test again. It will give you a better idea of ring wear vs. valves. Big improvement in compression=bad rings. Not so much=bad valves.

Valves are a relatively easy short term fix...much easier than rings. If it is valves, you could tear the top off the engine in a long afternoon, have the heads turned around in a day or so, and put it back together on the third with new gaskets. Not a fun job, but not rocket science either.

Its probably the orig engine with 840 hrs now.
I bought the boat 5 yrs ago at a consignment lot. Never got to talk to the Orig owner.
Live and learn. I was always looking in the wrong area for damaged risers. (Water jackets) The failure is where the water hose goes into the backside of the riser. Corroded a small hole straight through into the exhaust outlet.
Just enough to contaminate the oil. May have caused the engine to stall off idle at the ramp also. Cured with a tune up. The sad part not knowing the history of the boat.
I went through just about everything, and changed out most parts. Getting bad info on the water ingestion. I did not change out the manifold/risers to be safe. :smt021
Being a raw water cooled salt water boat. I think pulling the heads etc, Would open up a whole other can of worms.
I would probably install a upgraded Vortec intake manifold while I was at it also.
All that work would be a easy and comforting job.
Having saltwater run through all the bearings twice now.
Kind of creeps me out for doing anything with this engine.


Not having a service manual yet for my 8.1 EFI engine.
And not wanting to deal with the shops of what kind of EFI Smartcraft junk I need to make the 8.1 run. Different hydraulic connectors, etc Is making me put off the install with the 8.1 :smt009

If I would have bought a Carb engine. 502ci Carb :grin:
One look, and damn lets get this thing installed.
I did not find any complete, New turn key big block carb engines out there.

 

[Alex D]

Even though even pressure is important, absolute is too. Normal excellent values are anywhere from 160 to 180 psi in a compresssion test. Once you go below 120 you have more serious engine wear. Most often being the valves.

 

[Tail Chaser 661]

Thanks for the good info everyone :thumbsup:
I'm done with the grief. The Merc 8.1 Mag is going in.

 

[BarryCF]

The Merc 8.1 Mag is going in.

That boat is going to be a Hot Rod...

 

[sbw1]

If that is the original engine it probably has more than the hours you show. The hour meter on that boat was easily defeated and there was no electronic control module to verify what you show on the Hobbs. If the 8.1 fits and the stringer system and transom will support the torque, the boat will be a good runner. Some of the old 260s could be had with twin small blocks and were fast. They also sat very low in the water at the stern. How much does an 8.1 weigh vs. the twin small blocks? Count yourself lucky you did not find the 502. They used a quart of oil every 3-5 hours even when new.

 

[Tail Chaser 661]

The 8.1 is like 200 lbs heavier then a single 5.7. And I am no where near stern heavy now. A little more ballast may be a good thing.
Another CSR member with three other friends all had early 80"s 26' DA
They had single and twin 5.7's, a 454 HO, and a 502ci. The 502ci outperformed them all.
He also mentioned the 8.1 Mag has more midrange tq then his 502ci.
He has 8.1's in a bigger Sea Ray

 

[stevea661]

Its probably the orig engine with 840 hrs now.
I bought the boat 5 yrs ago at a consignment lot. Never got to talk to the Orig owner.
Live and learn. I was always looking in the wrong area for damaged risers. (Water jackets) The failure is where the water hose goes into the backside of the riser. Corroded a small hole straight through into the exhaust outlet.
Just enough to contaminate the oil. May have caused the engine to stall off idle at the ramp also. Cured with a tune up. The sad part not knowing the history of the boat.
I went through just about everything, and changed out most parts. Getting bad info on the water ingestion. I did not change out the manifold/risers to be safe. :smt021
Being a raw water cooled salt water boat. I think pulling the heads etc, Would open up a whole other can of worms.
I would probably install a upgraded Vortec intake manifold while I was at it also.
All that work would be a easy and comforting job.
Having saltwater run through all the bearings twice now.
Kind of creeps me out for doing anything with this engine.


Not having a service manual yet for my 8.1 EFI engine.
And not wanting to deal with the shops of what kind of EFI Smartcraft junk I need to make the 8.1 run. Different hydraulic connectors, etc Is making me put off the install with the 8.1 :smt009

If I would have bought a Carb engine. 502ci Carb :grin:
One look, and damn lets get this thing installed.
I did not find any complete, New turn key big block carb engines out there.

I haven't changed out the risers or manifolds out. I did blow them out and visually inspect what I could see and it looked ok. The port ones where the problem originated between the 5 & 7 cyl looked new, the starboard ones were pretty clean.
Now you've got me thinking :wow::wow:

 

[Tail Chaser 661]

That boat is going to be a Hot Rod...

There is some 25' Parkers and Stripers in my waters that I may be tempted to leave in my wake :lol:
Nice part is......I will probably still be turning a low rpm :grin:

 

[Tail Chaser 661]

I haven't changed out the risers or manifolds out. I did blow them out and visually inspect what I could see and it looked ok. The port ones where the problem originated between the 5 & 7 cyl looked new, the starboard ones were pretty clean.
Now you've got me thinking :wow::wow:

Tough spot your in Steve,
Your ready for a shakedown run, Need a break from wrench turning. And not sure of your manifolds.:wow:
Glad I'm not in your shoes :lol:

Not knowing better. There was a little high spot. Looked similar to when rust forms under painted metal. Had a 1/8" lifted pitted look inside the exhaust port. I thought it was a slight carbon build up that was not scraping off very easily. Noticed it in Pre 07 boating season. So it took a little while to fail.

 

[fc3]

I think the easiest and the best way to inspect the exhaust system components is with a bore scope. Until recently bore scopes were pretty darned expensive. I found a few that are inexpensive enough and good enough that everyone can afford to have one.

The top of Mercruiser exhaust elbows have a 1/2" brass plug that connects to the exhaust gas passage. These are used on some export engines for O2 sensors. See the photo below for an example of elbows with said plug. Simply remove the plug and insert the probe of the bore scope. If you find evidence of water, it's time to repair. Otherwise, you're good to go after reinserting the plug. The process is so easy that inspections can be done every couple of weeks rather than just once a year.

Best regards,
Frank