mercruiser and volvo penta exhaust manifold problems

This page discusses the common problems found with marine

EXHAUST MANIFOLDS !!

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If you arrived here from the main page, did you guess right??,
I'm sure you did if you have 'been there before' however most folks never give a thought to the manifolds when they start to get a misfire or lack of power, they go straight for the plugs/leads or carb, and any of these may well be the problem, however around 50-60% of my weekly conversations with people start with them discussing a misfire or lack of performance and ending up with them buying a set of manifolds.
If you are very unlucky and didn't spot the symptoms in time you will usually end up with a sump full of emulsified oil and lots of thick creamy goo in the rocker cover.

Why it happens is this,

Your manifolds are cast iron and as such are porous to a degree, they are also running in a very harsh environment getting hot then cold, Expanding and contracting lots and lots over the years, and all the time they are having cold water pushed through them, (mainly salt water).
From the minute the engine is first started the corrosion starts, and after a few years (About 10) the internal walls that divide the exhaust gas from the cooling water start to break down and allows the water to get into areas it definitely shouldn't.
If you are in luck you will catch the problem as an early misfire. If you are not so lucky the engine will continue to 'breathe in' the water through the exhaust ports and will start to run rough as the plugs get wet, the water then finds its way into the sump and quickly mixes with the oil and emulsifies into a creamy liquid (which thickens when cold) and the condensate in the rocker cover mixes with the oily mist in there to form a different creamy goo but slightly thicker than the stuff in the sump.

If at this point you are still running the engine, when you switch off there is good potential for the water remaining in the manifolds to run straight into the exhaust ports (no exhaust pressure now) and fill the cylinders.
Be very carefull trying to re-start the engine from now on as it can hydraulically lock as the piston comes up, and cannot compress the water as it does with the air, this causes the starter motor to fail in most instances, especially in engines from the early 1990's on which have the small PMGR Starters fitted.

IF AN ENGINE IS LEFT IN THIS CONDITION FOR MORE THAN A SHORT LENGTH OF TIME CORROSION SETS IN RAPIDLY ON ALL INTERNAL WORKING SURFACES AND A TOTAL REBUILD WILL BE NECESSARY.

ENGINES LEFT FOR MORE THAN A FEW DAYS WILL START TO GET RUST GROWING ON THE VALVE STEMS AND EVEN IF THE ENGINE TURNS OVER THE VALVES MAY NOT RETRACT PROPERLY AND COULD COME INTO CONTACT WITH THE PISTONS.

BE VERY CAREFUL WHEN ATTEMPTING TO TURN OVER AN ENGINE WHICH HAS HAD WATER INGRESS AND BEEN LEFT FOR A WHILE, THIS SHOULD BE DONE BY TURNING THE CRANK WITH A WRENCH AND WATCHING THE VALVES.

DO NOT USE THE STARTER UNTIL IT HAS BEEN ASSESSED THAT EVERYTHING IS ROTATING AND CLEARING OK

If You have an engine full of emulsified oil please call for advice, do not attempt to just drain and refill it !!

If the boat has been used in salt water from new and your Manifolds and Risers are 10-12 years old or older you should consider replacing them ASAP, and if they are older still than that then you are on borrowed time, we have seen manifolds that have reached 15-18 years but it may be that the boat has spent some of its life in fresh water.

Fresh water is nowhere near as corrosive as salt water and Manifolds can last for years and years in fresh water.

Now that you're all scared to death, you may simply have a misfire caused by a plug or lead or dirty fuel, but if you suspect water ingress pull out all the plugs and look at them, if there is any debris or deposits on any of them it is probably salt and would be salty to the taste, however we cannot recommend licking spark plugs but that is up to you, ( I quite like 'em)