Bent push rods

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aj305

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HI,

Had trouble starting boat ( 1995 Maxum 2300sc Mercruiser 5.0LX Alpha leg) eventually fired up but with misfire and knock. Took rocker cover off found 3 well bent push rods (intake and exhaust cylinder 3, intake cylinder 5). Does anyone have any ideas on what caused this and how to fix?

thanks
aj
 
aj305 said:
HI,

Had trouble starting boat ( 1995 Maxum 2300sc Mercruiser 5.0LX Alpha leg) eventually fired up but with misfire and knock. Took rocker cover off found 3 well bent push rods (intake and exhaust cylinder 3, intake cylinder 5). Does anyone have any ideas on what caused this and how to fix?

thanks
aj
Click to expand...

I have a Volvo Penta 5.0 V8 in one of my boats which is effectively the same lump.
Is it raw water cooled? It is possible it is Hydrolock where parts of the risers/exhaust have corroded inside and seawater has got into the combustion chambers causing the piston to lock.
It's not the first symptom I would think of but it's possible.
 
causing the piston to lock.
Click to expand...
Not sure I see how a locked piston can lead to bent push rods !

I am tempted to suggest engine running cold or only run for short periods causes condensation under the rocker cover. That leads to rust on the valve stems, they fail to open freely and the consequence is bent push rods.
 
VicS said:
Not sure I see how a locked piston can lead to bent push rods !

I am tempted to suggest engine running cold or only run for short periods causes condensation under the rocker cover. That leads to rust on the valve stems, they fail to open freely and the consequence is bent push rods.
Click to expand...

Well was just throwing in some ideas.

The theory being that the starter motor turns the engine over, the piston is locked so the starter is putting pressure on the pushrods to open valves etc and can't because the piston is locked and the bore full of water.

As I say, just a thought.
 
Bent pushrods will be caused by piston to valve contact- 99 per cent of the time....
Your valve timing has probably changed- I'm not familiar with your type of engine, but if it has a rubber cam belt, thats the first point to look at.
If the cam is driven by chain and sprockets,the chain may have snapped, or the tensioner failed.
If its gear driven (or sprockets) the key may have sheared, either on the crankshaft or the camshaft, or the teeth on the gear stripped.

Thats the usual suspects covered, the 1% possibility is a foreign object in the cylinder, landed between the piston and the valve, any nuts missing near the air intake?

Whatever, timing cover and cylinder head off for a look see..
sorry!
 
Fire99 said:
The theory being that the starter motor turns the engine over, the piston is locked so the starter is putting pressure on the pushrods to open valves etc and can't because the piston is locked and the bore full of water.
Click to expand...
I think VicS is right, I cannot see this either.

What you are mistaking is a bent con-rod which are a totally different thing altogether and this certainly would be caused by the problems you describe of bores full of water whilst turning over the engine but not the fault that the OP describes.

As the OP stated that "when it did start there was a knock . . . " I suspect the cam belt could have jumped. If not, I would suspect rusty, corroded, seized rocker-shaft causing the problem of bent push-rods.

If the bores are full of water then the engine will not turn over at all as the water is non-compressible, therefore the camshaft would not turn, cam-followers and hence push-rods would not lift and therefore would not bend.
 
Take the rocker cover off and check if the rockers rock. Can you try and push the valves down with a screwdriver? Straighten the bent rods, reassemble and try turning the engine with a spanner on the pulley and look to see if everything moves as it should.
Something seized in the valvegear may bend the pushrod, otherwise the valves could have hit the pistons which can only happen if the cam has moved relative to the piston, broken chain would mean the cam wouldn't turn so valvegear won't move as engine turned.
 
lenseman said:
I think VicS is right, I cannot see this either.

What you are mistaking is a bent con-rod which are a totally different thing altogether and this certainly would be caused by the problems you describe of bores full of water whilst turning over the engine but not the fault that the OP describes.

As the OP stated that "when it did start there was a knock . . . " I suspect the cam belt could have jumped. If not, I would suspect rusty, corroded, seized rocker-shaft causing the problem of bent push-rods.

If the bores are full of water then the engine will not turn over at all as the water is non-compressible, therefore the camshaft would not turn, cam-followers and hence push-rods would not lift and therefore would not bend.
Click to expand...

Good thinking Batman. :)

I see your thinking.
 
Cam drive belt has been mentioned several times !!
Like to point out that the camshaft is chain driven (lack of research here ... at least look at an engine diagram folks )

It's a very short chain and I see no sign of a tensioner. Very unlikely I would think that the chain has jumped and put the camshaft timing out.

Two bolts and a dowel locate the sprocket on the camshaft so not likely that that has moved.

Cannot find out how the lower sprocket is fixed to the crankshaft ... Woodruff key I think so potential for that to have sheared.

Rocker shaft has also been mentioned.. There is no rocker shaft Rockers are individually mounted on studs.


Don't forget 2 cylinders have been affected, but only 2 out of 8. Foreign objects in two cylinders seems unlikely but presumably they are both on the same side being 3 & 5. If caused by valve timing being out then as the engine was running more than two cylinders would have been affected. If it had been out by that much probably would not have run anyway.
 
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VicS said:
Cam drive belt has been mentioned several times !!
Like to point out that the camshaft is chain driven (lack of research here ... at least look at an engine diagram folks )

It's a very short chain and I see no sign of a tensioner. Very unlikely I would think that the chain has jumped and put the camshaft timing out.

Two bolts and a dowel locate the sprocket on the camshaft so not likely that that has moved.

Cannot find out how the lower sprocket is fixed to the crankshaft ... Woodruff key I think so potential for that to have sheared.

Rocker shaft has also been mentioned.. There is no rocker shaft Rockers are individually mounted on studs.


Don't forget 2 cylinders have been affected, but only 2 out of 8. Foreign objects in two cylinders seems unlikely but presumably they are both on the same side being 3 & 5. If caused by valve timing being out then as the engine was running more than two cylinders would have been affected. If it had been out by that much probably would not have run anyway.
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Your problem Vic is that you are applying logic..... Not a common trait on the forum generally and those who apply it are often lost in a fog of "nutty professor" answers or guesses by people with little experience of the problem. Having stripped several of these in the past I can say with great authority that I don't have a clue what caused the problem from the information given but your logic is impeccable so I wont repeat it.
As far as what to do about it it's pretty obvious.

1. Remove timing chain and inspect chain, sprocket, key etc. My suspicion is, probably not the cause but check anyway. The fact that you have more than one cylinder affected is interesting but not conclusive.
2. Remove head and check for obvious damage. My guess is you will probably find bent valves but not necessarily.
3. Regardless of what you find remove all the valves and check that the stems are straight and not coked up to the extent that they can jam open. This is a POSSIBLE cause of your problem as when an engine stops some of the valves are indeed stopped in an open position and if the engine is heavily coked or there is any water at all run back from the exhaust that can corrode valve stems they can jam open and either hit the piston or cause the rods to jump off their seat when it starts. This is a more likely cause if the engine has not been run for some time.
4. Check the piston crowns for damage and turn the engine over by hand ensuring that it moves freely and the pistons reach the same height in all cylinders (to see if you have a bent con rod)

There is no doubt that when you do this you will find the cause yourself and rectify.
Don't forget to check the anti-syphon valve if you find even the slightest indication of water in there (well check it anyway!)
 
VicS said:
Rockers are individually mounted on studs.


Don't forget 2 cylinders have been affected, but only 2 out of 8. Foreign objects in two cylinders seems unlikely but presumably they are both on the same side being 3 & 5. If caused by valve timing being out then as the engine was running more than two cylinders would have been affected. If it had been out by that much probably would not have run anyway.
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Saw this once on a Volvo.... two out of four push rods were not just bent but due to material composition had actually broken. There was no evidence of miss timing and byt the time I saw the engine everything seemd free thought the tappet clearances were not quite correct on the two intact units.

My only thought was as Vic said that the engine had been run cold or not run at all for a while and that the valves had been sticking open. this had either allowed the pushrods to dislocate from the ball cups at one end or the other, or allowed the valve tappets to move out of position so virtually extending the length of the push rod by a few MM . This then would have caused the piston to clobber the valve head.

Strangely enough the valves wern't bent...

As far as I know this particular engine never repeated this saga and ran fine afterwards with out any timing adjustments.
 
YOU CANT HAVE PUSHRODS

AND

A

TIMING CHAIN

OR

A

CAMBELT.


MUTUALLY EXCLUSIVE FEATURES.

Pushrods lift the valves on an Overhead Valve engine, where the cam is located off the crank.

Overhead Camshaft Engine does not have pushrods, it has cam followers where the camshaft(s) is mounted in the head and usually driven by an externally mounted chain or belt driven off the front crankshaft pulley and tensioned through idler pulleys.

Pushrods can be bent purely by engine overspeed as the spring rate recovery speed is exceeded and the pushrods cannot function as the cam is lifting but the valve gear is still accelerating downward. Result:misery.
Also can be caused by overlapping valve timing where it is possible to have the vlave touch the head if cam timing is disturbed. Not normal on low to medium output engines, especially older V8s, although very prevalent on modern automotive applications.
 
YOU CANT HAVE PUSHRODS AND A TIMING CHAIN OR A CAMBELT.
MUTUALLY EXCLUSIVE FEATURES
Click to expand...
Maybe that does not read as you intend it to.
You have a camshaft, be it an overhead one or not and something must drive it. An overhead one can be chain driven or belt driven.
An OHV camshaft is frequently chain driven but can be gear driven. Cannot think of a belt driven one but there is no reason technically why it should not be.

Push rods mean it's OHV not an OHC.

I think as suggested you have to check the camshaft timing but I doubt if you'll find it wrong.
Pretty certain that the problem will have been caused by sticking valves.
Whether or not they have been struck by a piston is another matter. It may or may not be possible. Always is possible with a diesel engine but not necessarily so in a gasoline engine. Depends on engine design.

Also perhaps be checking for broken valve springs.

A Boatmike says its a strip down job until the likely cause is found.
.
 
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FullCircle said:
YOU CANT HAVE PUSHRODS

AND

A

TIMING CHAIN

OR

A

CAMBELT.


MUTUALLY EXCLUSIVE FEATURES.

Pushrods lift the valves on an Overhead Valve engine, where the cam is located off the crank.

Overhead Camshaft Engine does not have pushrods, it has cam followers where the camshaft(s) is mounted in the head and usually driven by an externally mounted chain or belt driven off the front crankshaft pulley and tensioned through idler pulleys.

Pushrods can be bent purely by engine overspeed as the spring rate recovery speed is exceeded and the pushrods cannot function as the cam is lifting but the valve gear is still accelerating downward. Result:misery.
Also can be caused by overlapping valve timing where it is possible to have the vlave touch the head if cam timing is disturbed. Not normal on low to medium output engines, especially older V8s, although very prevalent on modern automotive applications.
Click to expand...
Yes you can.

You're forgetting what actually pushes the pushrods - cams on a camshaft which is itself located in the block and chain driven.

These engines aren't Overhead Cam units at all.
 
andrewbarker said:
Yes you can.

You're forgetting what actually pushes the pushrods - cams on a camshaft which is itself located in the block and chain driven.

These engines aren't Overhead Cam units at all.
Click to expand...


I will believe just this once that you misunderstood me. Suggest reading a little more closely.

Thats why I called it an overhead valve engine. The camshafts on those are *usually* driven by gear off the crankshaft.

Happily we haven't introduced Desmodromic valve gear intot his conversation, nor yet hydraulic tappets.
 
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