High compression on 350 magnum mpi

[Kkoski]

Hi all, first time poster here. I attempted to go out for a family cruise today and could not get the boat to plane out or go over 3300 rpm. Brought the boat over to a guy that knows a few things about marine engines and through a series of checks he found that 6 of 8 cylinders have compression around 260psi. I have a bunch of bent push rods now which he says is why I could not get more power out of the engine. He is confused as to what would cause the extremely high compression and is not sure what to do next. Any ideas or suggestions would be appreciated. The boat is 2005 chaparral ssi. Only about 200 hrs on it.

 

[achris]

9 year old engine. When were the exhaust elbows last replaced? Sounds very much like they have corroded through and are leaking water into the cylinders, hence the high compressions...

Chris....

 

[alldodge]

Hi all, first time poster here. I attempted to go out for a family cruise today and could not get the boat to plane out or go over 3300 rpm. Brought the boat over to a guy that knows a few things about marine engines and through a series of checks he found that 6 of 8 cylinders have compression around 260psi. I have a bunch of bent push rods now which he says is why I could not get more power out of the engine. He is confused as to what would cause the extremely high compression and is not sure what to do next. Any ideas or suggestions would be appreciated. The boat is 2005 chaparral ssi. Only about 200 hrs on it.

:welcome: to iboats

I don't see 260psi from an engine with bent or straight push rods. With that high a number my first guess would be your mechanic needs another gauge. Either way with bent push rods the heads need to come off at the minimum. Then you can check if the rest of the engine needs to com out and also check out the exhaust

 

[Kkoski]

Gents, Thanks for the responses. I will try a second gauge before getting into anything else. Regarding the exhaust concerns if they were leaking water into the heads wouldn't the oil look milky?

 

[alldodge]

Gents, Thanks for the responses. I will try a second gauge before getting into anything else. Regarding the exhaust concerns if they were leaking water into the heads wouldn't the oil look milky?

It should, but this is your first posting and we don't know everything about what lead up to the issue. Looking from the outside, water can get into the engine, but the issue IMO to figure out is how the push rods got bent. If water filled the head completely, and if the engine didn't hydo-lock, and the seal was perfect, I still don't see how water could bend the push rod. Something stopped the valve from coming open, and my first guess is the timing chain stripped or slipped. So getting another gauge is not going to tell you why you have bent rods.

 

[Kkoski]

AllDodge, Thanks for responding again. There was nothing of any note that happened before this issue. The boat always ran fine. It was even idling in the slip for about 1/2 hour prior to heading out to the lake and it sounded fine. The guy that is looking at the boat wants to start with taking the heads off and to see if he can find anything. He is a well known local guy and a good friend of my father. I even went and talked with the local mercruiser mechanic and he is confused and not sure where to start. Could it be that my fuel rail just decided to pump too much fuel into the cylinders and the excess pressure caused the same thing that you think the water cold have. I will check the viscosity of the oil to see if it is full of gas.

 

[alldodge]

Lets start with , "Why do you think or know you have bent push rods"??????

 

[Bondo]

Ayuh,.... Welcome Aboard,..... Either yer Mechanics are not Diagnosin', or yer not ginin' Us the whole story,...

Has anybody mentioned findin' water in the cylinders durin' the compression test,..??
Yer Gas idea,... Was Gas found,..??
On the ole spark plugs,..??
Damaged plugs,..??

I'd have thought, Water was found,..... 'n if so, the exhaust manifolds are the 1st thing to come Off, for inspectin', 'n testin',....

How'd the bent push get found, without openin' up the motor,..??

 

[Kkoski]

Gents, The only reason I know the push rods are bent is because they (my father and his friend) pulled the valve covers off to start searching and found them. They did not mention any water or gas found. That is the entire story as I know it. Not sure where to go from here.

 

[achris]

Gents, The only reason I know the push rods are bent is because they (my father and his friend) pulled the valve covers off to start searching and found them. They did not mention any water or gas found. That is the entire story as I know it. Not sure where to go from here.

Exhaust elbows? Last changed?

 

[Kkoski]

Achris, I have had the boat 3 seasons and I haven't changed them. I didn't even know that this was something that needs to be changed. As far as I know the boat has always been a fresh water boat in the Great Lakes Area.

 

[achris]

Pull the elbows and check them. 10 to 1 on, they (one or both) have corroded through. That's where you'll be getting water in the engine...

 

[Kkoski]

I will give that a shot and report back in day or so

 

[tpenfield]

Been there, done that on hydro-locking. Here are my thoughts . . .

Typically, you would bend the connecting rods, but I suppose push rods could bend too.

Tell your mechanic friend that liquid in the cylinders would result in a very high compression.

I did not see in the posts so far, what if any kind of liquid was actually found in the cylinders, if so what was it?

6 of 8 cylinders having liquid/high compression might be from a source that is common to all cylinders, like the intake manifold.

Would you post the actual compression numbers that were found?

Have the spark plugs been removed and the engine spun freely?

Got any pics of the valve train/cylinder heads?

 

[Kkoski]

Gents, I discussed all the feedback with my father. There was no water in any of the cylinders and he said everything looked quite clean. There are only 2 bent push rods. The intake on #2 and out on #4. What prompted them to pull the valve covers was originally low compression on those 2 cylinders which makes sense that they are that low ( 100 psi) while every other cylinder was between 220 and 240 psi. Another thing I neglected to tell you all is that the boat sat in the water for 2 months and was not started or moved. The boat stays in the Great Lakes and there are these lovely little zebra mussels that love to attach to anything that stays in the water and had totally covered the entire hull of the boat and out drive (bravo III). Could these little things have blocked the exhaust which is causing the pressure to be too high? This will probably be the last post until I get time to pull the exhaust elbows off to see if there is any signs of them in there. When they are off I plan on checking compression again to see if it is back in the normal range which I assume is 170 psi. Any opinions would be appreciated before I head down this path. Thanks again

 

[achris]

Have his compression gauge checked against a known good one. If there was no fluid in the cylinders, and the cylinders with the bent push rods was still coming up to 100, then that gauge sounds like it's reading very high. I suspect the better question now is why you got 2 bent rods. Before pulling heads (I would still go ahead and pull the elbows as they need to be checked anyway), find a different compression gauge....

A 'normal' compression on that engine would be 100 to 150, depending on the gauge, with the lowest not less than 70% of the highest... That's the book spec.

Chris.....

 

[alldodge]

Gents, I discussed all the feedback with my father. There was no water in any of the cylinders and he said everything looked quite clean. There are only 2 bent push rods. The intake on #2 and out on #4. What prompted them to pull the valve covers was originally low compression on those 2 cylinders which makes sense that they are that low ( 100 psi) while every other cylinder was between 220 and 240 psi. Another thing I neglected to tell you all is that the boat sat in the water for 2 months and was not started or moved. The boat stays in the Great Lakes and there are these lovely little zebra mussels that love to attach to anything that stays in the water and had totally covered the entire hull of the boat and out drive (bravo III). Could these little things have blocked the exhaust which is causing the pressure to be too high? This will probably be the last post until I get time to pull the exhaust elbows off to see if there is any signs of them in there. When they are off I plan on checking compression again to see if it is back in the normal range which I assume is 170 psi. Any opinions would be appreciated before I head down this path. Thanks again

A partially plugged exhaust would reduces performance of a running engine but would not increase pressure during a test, engine is not turning over fast enough. Water in the cylinders could as tpenfield mentioned, but you stated there was none. Only way to get a bent push rod is something has to stop a valve from opening all the way. Valve comes in contact with a piston, rocker arm seizes up, valve stem seizes against guide or the like. I agree with achris and get a known good gauge and retest with all spark plugs out of the cylinders

 

[achris]

Something just occurred to me.... Exactly how was the test done? I've heard of people removing one plug at a time and actually having the engine running to test each cylinder... :facepalm:...

 

[Scott Danforth]

I have come across bent push rods when the valve stem gets rusty from sitting a long time with the valves open. basically the cam moves the lifter, however the rocker arm cant move the valve so the push rod bends. I see this often trying to resurrect motors that have been sitting for years in both old trucks and boats.

I agree with Ted, usually hydro locking results in the piston becoming closer to the crank with the rod bending.

 

[Kkoski]

Gents. I have no idea how the test was done. I decided today that the motor will come out in a couple of weeks and get worked on during the winter. The boat will be put in storage for the winter and in the spring we will put them back together and see how she goes. I will post back when we find the problem(s)!