If you are reading this and asking yourself how do you know when your manifolds and risers are bad?, Then chances are they may already need replacing.
For a lot of petrol engine owners exhaust manifolds and risers can be one of those items that only get looked at once an issue arises.
Manifolds and risers are one of those engine components that although they are a major component, depending on use could almost be considered a consumable item such as filters and oils.
It all comes down to how often you use your boat? And how often are you flushing your engine?
Manifolds can also be a sticky subject due to the cost of replacement and sometimes the minimal hours of use.
In this article I’ll be covering a range of topics such as.
- Manifold anatomy
- How they work
- Types of manifolds
- How to identify failing manifolds and risers
- Common manifold issues
- What action you can take to increase the longevity of your manifolds and risers
Manifold anatomy
Exhaust manifolds are made up of an exhaust tube for every cylinder your engine has and around all of these are water jackets allowing cool sea water to circulate around the hot exhaust tubes, controlling the temperature.
You will notice along the very bottom of some manifolds there is a long pipe, this is a kind of balance chamber for the circulating water to be shared between all cylinders.
How do manifolds and risers work
This is a brief overview of how manifolds work and their role in keeping your engine cool.
Firstly manifolds, or more accurately exhaust manifolds and risers are essentially chambers that funnel your exhaust gas from your engine cylinders out through your exhaust pipes .
Both your manifolds and risers have water jackets in them that allow seawater to circulate around them.
The heat building up in the manifolds from both hot combustion gas and heat from the hot engine block gets transferred into the water and circulated around your manifold then exits the exhaust riser on its way out through the exhaust.
This heat exchange in the manifolds and risers help to keep the engine in its optimal temperature range.
A blockage anywhere in this system can and will cause engine temp to rise and even overheat.
Types of manifolds
- Cast iron
- Alloy
- Stainless
Cast iron manifolds
Let’s go back a few years to the good old cast iron manifolds these were seen on most petrol engines of various brands and to be honest i really liked them, yes they corrode inside over time as one would expect in a marine environment but they are heavy, tough and with regular flushing and periodic inspection would give you a decent working life.
The cast iron manifold, while in theory not a great idea for salt water use, has proven itself a robust component when looked after properly by flushing regularly and running as much as possible to keep internal corrosion to a minimum.
Alloy
Now is the age of the direct injected petrol engines with alloy manifolds and catalytic converters for emissions depending on model.
Alloy manifolds are far more compact, have exceptional heat dispersion properties and are a lot lighter than just about any other manifold out there.
Also corrosion resistance is great considering how much lighter and thinner the material is.
Stainless
Stainless manifolds can be a bit of a curly one, on one hand the corrosion resistance of stainless is exceptional but on the other hand stainless is much more brittle than alloy and iron so much more susceptible to cracking.
Stainless manifolds have been looked at as a performance upgrade.
How to identify failing manifolds
weeping manifold ad riser
Visual checks
Visual checks are simply having a good look around all the water hoses and sealing surfaces of the manifolds and looking for anything unusual like rusty run marks coming from where the riser joins the manifold or maybe a hose that looks frosty around where it fits onto the manifold these are the first signs that your manifolds and risers may need some attention.
Watch your temps
One of the first signs of a manifold failing is going to be the engine running hotter than normal.
If your engine is creeping up into the 90 – 100 degrees celsius range you are reaching overheating temps.
Pull out bungs for inspection
manifold full of shale
If you are finding your engine is getting hotter than usual and you suspect manifolds to blame, I would pull out any easily accessible hose or bung to inspect for sand, silt and shale, starting at the lowest point first.
You will notice along the very bottom of a set of manifolds there is a long pipe, this is a kind of balance chamber for the circulating water to be shared between all cylinders.
This pipe is usually the first part of the manifolds that block up due to being at the bottom, and sand, silt and shale sinks down and settles there slowly building up from the ends until the whole tube gets blocked.
There will be a bung at the bottom of this pipe usually in the middle, where you can undo this and see what comes out.
You want clean free flowing water coming out. So if nothing comes out or you have lots of sand, silt, shale or other debris coming out, the manifolds will need removing and flushing.
One thing you can do before committing to manifold removal is push a small piece of wire up the hole first just in case the hole has only a tiny amount of something blocking it.
Usually once these bungs have been removed they can have a thin skin or crust sitting in the hole this is usually nothing to worry about and after piercing water should flow unrestricted.
Separate risers from manifolds and inspect
You can easily remove an exhaust riser from an older style cast iron manifold if you suspect they may be starting to corrode heavily inside.
Make sure to take note of any bracketry and nut / bolt positions prior to removal and
remember to clean up the mating surfaces and install a new gasket prior to reassembly.
blocked manifold water jackets
Remove manifolds and inspect
The best way to inspect manifolds and risers is obviously going to be removing both the risers and manifolds and having a good look inside.
At this point you can flush the water jackets with a garden hose also to clear everything out.
Make sure to take note of any bracketry and nut / bolt positions prior to removal and
remember to clean up the mating surfaces and install a new gasket prior to reassembly.
water has entered exhaust tubes
Pull out spark plugs and check for water
wet spark plug, water has entered cylinders
Another sign of failing manifolds is an engine running rough or not running on all cylinders.
This is more common with alloy manifolds but does still happen to the cast iron and stainless manifolds as well.
An easy way to diagnose this condition is to pull out the spark plugs,
any water or rust on the spark plugs tip will indicate water has gotten into your cylinders.
How this happens is water leaks through from the cooling jackets into the manifold’s exhaust/ dry side and into the top of the exhaust ports in the engine block, which allows entry into your cylinders.
If you find yourself with water in your cylinders it is very important to turn your engine over with spark plugs removed to expel any water from cylinders and spray some lubricant like WD40 or INOX into your cylinders to prevent corrosion around pistons.
Remember to always isolate your spark / ignition and fuel delivery systems before turning an engine over with spark plugs removed and if you are unsure of what you are doing here please see your local marine mechanic for assistance, stray spark and fuel is an explosion waiting to happen.
Common manifold issues
Cast iron issues
- weeping between the risers / manifold and manifold / engine block.
- Corrosion between risers / manifold and manifold / engine block.
- Gasket failure
- Loose nuts / bolts
- Internal cracking or pin holes
Alloy issues
- weeping between the risers / manifold and manifold / engine block.
- Corrosion between risers / manifold and manifold / engine block.
- Gasket failure
- Loose nuts / bolts
- Internal cracking or pin holes
- Blockages from manufacturing swarf
Stainless issues
- weeping between the risers / manifold and manifold / engine block.
- Corrosion between risers / manifold and manifold / engine block.
- Gasket failure
- Loose nuts / bolts
- Internal cracking or pin holes
- Pin holes and cracks in weld areas
What action you can take to increase the longevity of your manifolds and risers
- Regular running of engines
- Freshwater flushing
- Acid flushing
- Periodic inspections
Regular running of engines
The best thing you can do for your manifolds / risers and your whole engine for that matter is run your engines as much as possible.
By running your engines regularly you are circulating water around your raw water system essentially flushing your engines.
This regular running reduces the build up of salt and corrosion in the system by not allowing the salt and corrosion to take hold and grow in the first place.
By the way, salt and corrosions best friend is an engine that sits for long periods without running, allowing salt crystals and corrosion to grow.
Freshwater flushing
Depending on your set up, some boats can be flushed from the engine bay if they are equipped with a freshwater flush kit.
But others can be flushed by placing “earmuffs” (flushing device) on to the outdrive leg water pick up.
Flushing an engine after every use is your best defence from salt and corrosion build up.
There are also anti salt and corrosion products on the market that you can incorporate with your regular flushing to increase protection.
Acid flushing
Can i Flush my manifolds with acid to clean them in situ?
I hear this a lot and the reality of this situation is if you are asking the question then it’s probably too late for an acid flush.
Yes you can definitely acid flush your manifolds and yes it can be beneficial to do so as preventive maintenance but most people in my experience dont know this is an option until they have an issue and start researching how to fix it.
And acid flush will only break up small amounts of salt build up.
If you are having temps starting to rise then inside the manifold jackets will likely be looking quite bad and they should be removed and inspected at a minimum.
Periodic inspections
Water weeping from manifolds and riser mating surfaces can be a number of things such as loose mounting bolts / nuts (these can become loose from years of heating up and cooling down), gaskets that have failed for whatever reason, corrosion building up between the joins and warping of flat surfaces from overheating.
Infrared Temperature guns can be a handy investment when trying to diagnose failing manifolds; you can simply point the gun anywhere over the manifolds and risers to determine if there are any hot spots.or if the boat temp gauge is reading the same temp as what the engine actually is.
When doing any work with the raw water side of your engine in the water always make sure to shut off any main water valves you may have and or plug any hose that has the potential to siphon water into your boat.
As an example when removing your exhaust hoses off the manifolds make sure you cable tie them upwards and above the boat’s waterline.
When replacing, manifolds always replace as pairs. If one needs replacing chances are the other will be close behind
Conclusion
How do you know when your manifolds and risers are bad? . . .
Firstly by understanding how a set of manifolds and risers work and being able to identify what type you have on your engine, you can easily start to look for the first signs of manifold and riser troubles, assess whether you need to monitor or rectify an issue before it becomes a problem.
So as you can see your manifolds and risers are an extremely critical part of your cooling and exhaust system and by being proactive in looking after them you are ensuring a reasonable service life is achieved.
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