Mercruiser V6 problem - anyone help?

[Infamous]

this is going to be a bit of a long post so i apologise in advance, but there are alot of symptoms and i feel they all might be relevent so il list them all ..........

i have a recently rebuilt Mercruiser 4.3 V6 engine that ive had alot of problems with since i put it back in the boat (an 18ft cuddy). the main problem is the engine will run ok at 2000rpm which is the revs i use to get from my mooring out to sea, but as soon as i throttle up to get up on the plane the revs get to about 3800rpm then the engine seems to overfuel and caugh/splutter/almost stall. when i back off the throttle the engine is running on 5 cylinders and the smell of fuel is very strong from the exhaust. so far i have replaced the rotor arm, coil, all 6 leads + king lead, and 3 sets of spark plugs. I've also changed some parts of the carb, and opened up the air filter/flash guard to make sure the engine is sucking in enough air. all of the above has been done 1 thing at a time but nothing has helped. have been playing with the fuel mixture while doing the above and today i know ive got that right - 5 of the plugs have turned a lovely golden brown but number 1 plug is black and sooty. every time ive found it running on 5 cylinders its always been number 1, or number 1 and 1 other cylinder.

speaking to 2 different people today and ive had suggested that it could be 2 problems, but which 1? or is neither solution right and is there something ive overlooked?

cause 1 - small failure of the head gasket causing a VERY small ingress of water into the bore causing the mixture to weakin in number 1 bore and not fire properly. this is possible as when the engine was rebuilt earlier this year the head gaskets from the old engine were re-used, however the old engine had never really been under load (run for 2 hours max) and suffered big end bearing failure in number 1 piston.

cause 2 - the valves not adjusted properly. when the engine was rebuilt the valves were set but i was told that after a 'running in' period the hydraolic valves will need re-adjusting to set them properly. would 1 sticking valve cause the syptoms listed above? and surely if all 12 valves were adjusted at the same time and to the same tolerances then there would be more faults than just with number 1 piston?

also could a small fault with the dizzy cap cause number 1 plug not to fire properly?

please somebody help, this engine is driving me insane with this fault and the season is slowly slipping by and ive got a boat i cant use!!

thanks for reading and hope someone has the answer

Mark

 

[tinkicker0]

also could a small fault with the dizzy cap cause number 1 plug not to fire properly?



Mark

Absolutely. A small crack or track to earth in that particular tower of the cap may allow the plug to fire under low load conditions but as soon as you open her up, peak voltage in the system climbs higher than the cap's ability to insulate, so the spark tracks to ground.

 

[Tidnock]

YOU CANNOT REUSE HEAD GASKITS

its obviously something that manifests when you place the engine under load or when it gets hot, if I understand correctly once the splutter has started and you drop onto 5, it does not cut back to 6 again when you drop back to 2000rpm or below. have you done a compression check on number 1 when it is hot, check them all and make sure they are close to each other, this will test the valves and head gasket you mention.

 

[Tidnock]

In fact if you have reused you head gaskets, change them straight away, you have a problem waiting to happen

 

[Infamous]

YOU CANNOT REUSE HEAD GASKITS

its obviously something that manifests when you place the engine under load or when it gets hot, if I understand correctly once the splutter has started and you drop onto 5, it does not cut back to 6 again when you drop back to 2000rpm or below. have you done a compression check on number 1 when it is hot, check them all and make sure they are close to each other, this will test the valves and head gasket you mention.

yes once it has dropped to 5 it doesnt come back to 6 untill the engine has cooled right down and plug has been cleaned or replaced. done a compression test (results were about 150 average i think) and all cylinders are close enough (within about 15psi). i did question re-using the head gaskets but the person who helped me rebuild the engine (a marine engineer apparently) assured me that they were 'as new' and hadnt ever been under load and would be perfectly ok to use again. i did give them a good look over and have to admit that i couldnt see any compression marks or nicks or any damage on them at all, although i think its probably worth replacing them as you say - if they aint damaged yet, they very soon will be.

also going to add a new dizzy cap to my parts list, thanks Tinkicker0. i had quick look at the cap today but couldnt see it well enough to comment on its condition, when in doubt - replace is my motto on boat engine parts, just aint worth the hassle not to!!

 

[Tidnock]

yes once it has dropped to 5 it doesnt come back to 6 untill the engine has cooled right down and plug has been cleaned or replaced. done a compression test (results were about 150 average i think) and all cylinders are close enough (within about 15psi). i did question re-using the head gaskets but the person who helped me rebuild the engine (a marine engineer apparently) assured me that they were 'as new' and hadnt ever been under load and would be perfectly ok to use again. i did give them a good look over and have to admit that i couldnt see any compression marks or nicks or any damage on them at all, although i think its probably worth replacing them as you say - if they aint damaged yet, they very soon will be.

also going to add a new dizzy cap to my parts list, thanks Tinkicker0. i had quick look at the cap today but couldnt see it well enough to comment on its condition, when in doubt - replace is my motto on boat engine parts, just aint worth the hassle not to!!

once you have torqued/pulled the head down and as you do (in sequence) that gasket is compressed and forms a joint/seal, once that has happened then the gasket is used whether or not you start the engine, I do not thing you should spend any more time or money until you have replaced them, one last point have you interfered with the valve timing

 

[ExcaliburII]

Yep, here's another vote for the head gaskets.

As suggested above, run the unit until it's warm and under enough load to make the misfire appear, then take all the plugs out asap and do a compression test on all cylinders for comparison. If one is down compared to the rest, look carefully at the head gasket around that one when you get the head off and you may be able to see where it was blowing, which will put your mind at rest on that score at least.

I think the normal mode of failure of a head gasket under load is for the combustion gases to leak out during the firing stroke (usually into the nearest waterway), rather than the water forcing its way into a cylinder. There won't be enough pressure difference between the water jacket and the cylinder on the induction stroke for anything like long enough to get a significant amount of water in while it's running IMHO. Of course that's not to say that water won't leak in when the engine is stopped once a passage has been blasted across the gasket, so I would be fairly keen to find the problem asap.

 

[Infamous]

once you have torqued/pulled the head down and as you do (in sequence) that gasket is compressed and forms a joint/seal, once that has happened then the gasket is used whether or not you start the engine, I do not thing you should spend any more time or money until you have replaced them, one last point have you interfered with the valve timing

il pick up a pair of gaskets tomorrow and replace them, to set the valves i have to strip the top of the engine right down so makes sense to change them now. i dont think you can set the valve timing as such - there's no timing belt or chain to adjust as the valves are hydraulic and run off the crank (which is what im used to working on car engines). when the engine was rebuilt the block and both heads were stripped to their component parts and inspected/cleaned. the valves were then adjusted as per the manual - no1 piston TDC, tighten inlet valve till stem can just be turned by fingers, and so on through cylinders and inlet/exhaust valves in order. i cant remember the exact order but it was checked and double checked as the correct way before i started. i was told that after a few hours use the valves need to be checked and usually re-tightened as they can get a bit noisy when they bed in, and there certainly starting to get noisier.

 

[ExcaliburII]

there's no timing belt or chain to adjust as the valves are hydraulic and run off the crank (which is what im used to working on car engines).

This isn't right. On a 4 stroke engine the valvegear must operate on every second revolution of the crankshaft. Hence you have a camshaft which ALWAYS runs at exactly half the crank speed. Sometimes this is achieved by gearing the camshaft directly to the crank (in which case the camshaft gear is twice the size of the one on the cranskshaft), sometimes it is driven with a chain or, more usually these days, with a toothed belt. Whichever method is used, the actual relationship of the camshaft to the crankshaft is critical, which is why there are always timing maks which have to be lined up when the engine is assembled.

 

[Tidnock]

This isn't right. On a 4 stroke engine the valvegear must operate on every second revolution of the crankshaft. Hence you have a camshaft which ALWAYS runs at exactly half the crank speed. Sometimes this is achieved by gearing the camshaft directly to the crank (in which case the camshaft gear is twice the size of the one on the cranskshaft), sometimes it is driven with a chain or, more usually these days, with a toothed belt. Whichever method is used, the actual relationship of the camshaft to the crankshaft is critical, which is why there are always timing maks which have to be lined up when the engine is assembled.

The reason I asked is: we do not know how far you have had the engine stripped down, it is possible for a petrol engine to run with the valve timing a tooth out, but do not complicate things, just replace the head gaskets and go from there, if you have had the cam drive gear off it may be worth checking, one can to this at TDC and ensure the valves are at balancing/rocking point both closed back at forth just either side of TDC

 

[Infamous]

This isn't right. On a 4 stroke engine the valvegear must operate on every second revolution of the crankshaft. Hence you have a camshaft which ALWAYS runs at exactly half the crank speed. Sometimes this is achieved by gearing the camshaft directly to the crank (in which case the camshaft gear is twice the size of the one on the cranskshaft), sometimes it is driven with a chain or, more usually these days, with a toothed belt. Whichever method is used, the actual relationship of the camshaft to the crankshaft is critical, which is why there are always timing maks which have to be lined up when the engine is assembled.

sorry i may be getting confused - the valves on my engine are pushrod operated with a rocker (i think its called) from the pushrod to the valve, and the manual i origionally had was for hydraulic valves which is why i had to check and re-check how my valves were adjusted (it confused me more at the time - modern cars are what im used to and this engine is 80s!!). the only timing marks on the engine are on the pulley, which indicate TDC and (i think) 16deg before TDC. there are no camshafts in the heads, definately no chains, and the only belts run the alternator/PS pump/water pump. again this is something i'd never come across before and had to ask where my cambelt was (oops).

 

[Tidnock]

sorry i may be getting confused - the valves on my engine are pushrod operated with a rocker (i think its called) from the pushrod to the valve, and the manual i origionally had was for hydraulic valves which is why i had to check and re-check how my valves were adjusted (it confused me more at the time - modern cars are what im used to and this engine is 80s!!). the only timing marks on the engine are on the pulley, which indicate TDC and (i think) 16deg before TDC. there are no camshafts in the heads, definately no chains, and the only belts run the alternator/PS pump/water pump. again this is something i'd never come across before and had to ask where my cambelt was (oops).

all four stroke engines, petrol and diesel have at least one cam shaft, they can be over head cam (OHC) or push rod. there are various methods for driving that camshaft, those being, belt, chain or gears.

Your engine is a push rod engine not (OHC), it has one cam shaft in the middle of the "V" under the inlet manifold. So indeed, the cam shaft has to be timed to open/close the valves at the correct moment in the 4 stroke cycle, if you have not disturbed it in the rebuild dont worry if you have it needs checking, change the head gaskets

 

[pistonbroke]

here you go