Exhaust Gas Leak, 2005 Mercruiser Mag 496 HO

JamesTT

JamesTT

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I noticed an unusual ticking noise from the engine after a run yesterday. There was gas escaping at the joint on the exhaust system between the manifold and elbow. I presume the gasket has failed, can anyone familiar with the engine advise if this is this a big problem and if its safe to run the engine?
Thanks
 
Sorry to hear of your troubles.
Are you sure it's exhaust gas escaping from there?
Because if it's vapour, there's a water reversion problem - which might be more dangerous.
Either ways, I definitely wouldn't run the engine before checking what happened.
If it's just a failed gasket, you might wish to take the opportunity to replace it on both sides, removing also the turbulator while you are at that.
It's the steel plate #15 in the drawing below. Originally designed to catch moisture, but widely recognised as useless at anything but restricting the exhaust flow (hence the engine performance) at WOT.

I'd be more worried about the ticking noise, though.
Can you tell us more about it? Is it there just at idle, or also at higher rpm? With silent choice open or closed (or both)?
And how did it develop? When you say "after a run", do you mean that it wasn't there before, you ran the boat normally and both the noise and the exhaust escape appeared? :eek:

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Hi Mapis, thanks for your help.

I have started the boat in the last couple of months but not had chance to run her out to sea, not sure if its relevant but it has been very cold here recently.

A very slight ticking noise was evident on start up, but the serpentine belt was also quite noisy and squeaky as the engine was damp and it was very cold. I let the engine get up to temperature, checked all the guages, all seemed fine, i tried to investigate the ticking noise but couldnt be sure where it was coming from/ if it was there at all.

I ran the boat out to sea for 15 to 20 minutes with the exhaust open and closed, the ticking was now more evident both ways, it was much louder upon return to the marina at idle, the engine other than that, had run fine at WOT and at idle.

Upon investigating back at the marina after the run I could hear and feel the gas escaping from the joint and pulsating against my hand. I didnt notice any moisture on my hand at the time.

Please let me know what you think now with the further information?
 
James,

Any exhaust leak on a boat is a worry; apart from the chance of water ingress (into the boat or engine), the chance of carbon monoxide ingress is a real concern (no catalytic convertor on your engine). I'd first check the bolts are tight, they do come loose. After that, replacement gaskets and tightening the bolts with a torque wrench.

Graham
 
JamesTT said:
not sure if its relevant but it has been very cold here recently.
Click to expand...
Well, I hope you winterised the engine, otherwise I'm afraid it could be very much relevant... :(
I mean, even assuming that the closed cooling circuit is properly filled with antifreeze, there's still plenty of raw water around (heat exchanger, exhaust, etc.) which can make some serious damage when freezing.
The 496 has a manual air pump designed to empty the raw water circuit. Did you use it as explained in the manual?
Actually, I still prefer the more traditional winterisation, filling the raw water circuit with antifreeze rather than leaving it empty, but the (much quicker) pump method is MUCH better than nothing.
Btw, was the exhaust leaking from port or stbd side (or both)?

Re. the ticking noise, it's hard to tell where it comes from without listening at the real thing.
The fact that you ran it also at WOT, and as I understand it ran fine (usual RPM/speed, correct levels on all gauges ?), is good news. In fact, valve(s) damaged from water reversion should affect the engine performance, to some extent.
Hopefully, the noise could just come from some of the belt-driven accessories.
Anyhow, I think it's worth have the boat checked by someone with a specific experience on these engines. Which btw not all Mercury mechanics are! If there is any Mercury Racing shop in your area, I'd definitely ask them rather than any "normal" Mercury mechanic.
In fact, even if the engine we're talking about in theory doesn't belong to the racing line, in my experience there's much more experience about it among mechanics used to deal with performance boats and blue engines.

Good luck!
 
A leaking exhaust will very often cause a ticking noise ( common on car engines, when the head to manifold gasket has started to fail). From the drawing, it looks that the gaskets between the manifold and riser are dry joint ones; no water passing through to the riser through the gaskets, so there'd be no chance of water ingress via the gasket (unlike the VP ones, where the sealing surface between the water and exhaust is very narrow!). It's worth checking the tightness of the bolts (plus the manifold to head bolts) in case they have become loose, but if the gasket has been leaking for a while then it may well have become damaged due to blow by. Certainly on VP engines changing the manifold and riser gaskets is straightforward, I can't see the Mercruiser ones being much different in that respect.

Graham
 
Thanks Graham and MapisM,

I'm 100% sure now that the ticking noise is the exhaust gas escaping from the joint, as you say Graham, i have heard it on car engines before. The belts were squeaking as they were damp, i only mentioned this as it was initially masking the ticking that was much more evident upon my return to the marina, when the belts were dry and quiet.

There is new and good antifreeze in the closed cooling system and i had used the eazi drain air pump to drain the raw water side prior to the frosts this year.

It appears to be starboard side only and all guages/ smartcraft readouts were normal, the boat ran really well and fast, but that may of been due to the fact i havent been out for a couple of months and had missed the buzz.

MapisM, I note and agree with your concern re dealing with a mechanic familiar with the engine, I will check the bolts this weekend and re asses the situation.

Thanks again
 
Doh! I didn't even consider the leak as the reason for the noise...
Actually, I've never heard of that happening on these engines, but it might well be as simple as that. I wish you it is, of course!
 
GrahamHR said:
it looks that the gaskets between the manifold and riser are dry joint ones; no water passing through to the riser through the gaskets, so there'd be no chance of water ingress via the gasket
Click to expand...
Just for the records, yep, a dry joint it is.
I mentioned water reversion just because it's a known problem in some installations of 496, particularly in the HO version and particularly with switchable exhaust. But if that happens, it's indeed reversion (all the way back from the point where water is mixed with gas), not water ingress from the gaskets.
 
Are you sure its the manifold to riser joint, where is the leak exactly, if its on the inside of the manifold towards the cylinder head it could be the manifold to cylinder head gasket leaking, I have had a few of these blow a section out and make a ticking sound. If the leak is front / rear or on the outside of the riser joint then its the joint you mentioned.
 
As an update, the riser has now been split from the manifold. I am advised (although i havent seen it yet) that the sealing face of the aluminium manifold is corroded and will not hold a seal with new gasketts, options are to replace both manifolds, circa £400 each or attempt to skim back down flat. Anyone had aluminium manifolds skimmed?
 
What year is your block? IIRC, Mercury used alu manifolds only in the first years of production of the 496, swapping them for iron manifolds afterwards.
Btw, I have a couple of iron manifolds (2006 vintage) laying somewhere in my garage, leftover from the installation of the CMI exhaust.
They're not new, and should be thouroughly cleaned before installing, but they worked for just 150 hours or so, therefore they would definitely be fit for purpose, if you're interested.
 
Whoops, I just noticed that the year of your engine was in the subject of your OP.
Tough luck, it must be one of the last blocks with aluminum manifolds.
Which were widely recognised as not good enough for salt water use, hence the new iron ones Mercury came up with.
If yours lasted till now, they already performed much better than most.
 
Hi James

sorry to hear about your risers but it is about par for the course for salt water boats.
I saw some stailness mild performance ones on a US forum that were uk products I will try and dig it out.
Not a lot more expnsive than tin but will last longer and will breath better.
 
Kipper said:
or go the whole hogg
http://www.cpperformance.com/p-14692-cmi-small-block-chevy-sport-tube-exhaust-system.aspx
Click to expand...
Actually the exhaust in that link is for small blocks. The 496 version of CMI system is this one.
That's what I have in my boat. They look and sound fantastic, and they do improve the engine performance a bit.
But based on my experience, I wouldn't recommend JTT (or anyone else with the same engine) to go that route.
They're simply not worth the cost, compared to other exhausts which can bring comparable improvements for a much lower cost, as those built by Dana Marine.
Besides, they do require some attentions when using the boat, to avoid the risk of reversion in the last cylinders.
 
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