exhaust manifolds rust!!

[bingshmo]

1996 Mercruiser 4.3 lxh gen+

How much rust is acceptable in side my exhaust manifolds to be reusable? Took em off today and they are flaking off pretty large flakes of the cast iron. Is there anything I should do to them before reinstalling to make em last longer? Lastly, should I be worried about rust flakes causing blockage further down in the exhaust system? Thanks.

 

[HT32BSX115]

Re: exhaust manifolds rust!!

Howdy,


Well, the amount of rust does have an upper limit. When it starts to block total water flow, you'll start having over heat problems.

Remember, the rust is the metal that used to be part of your manifolds. As it forms and flakes off, the thickness of the walls separating the water from the exhaust, gets thinner and thinner until it's "rusts thru". They usually clog before they rust thru (you hope)

You cannot "make them last longer" unless you install closed cooling (after you replace your engine and manifolds)

They WILL last longer if only operated in fresh water.....in salt, aside from using "Salt-Away", The "rust-rate" is largely fixed.

The rust flakes won't really hurt anything since they're flushed overboard through the risers...unless they're plugged too.


When they get plugged with rust, they should be replaced. (period)



Regards,


Rick

 

[dg512055]

Re: exhaust manifolds rust!!

I took mine off over the winter and saw large flakes also. I assume you have the one-piece manifolds. I filled mine with muratic acid to dissolve the rust out, followed by a blast with the pressure washer into the water inlet holes. Then I filled them with Jasco Prep & Prime (phosphoric acid) to passivate the iron. This left them nice and clean with a black oxide coating on the inside. I don't know how long they would take to rust through given what they weigh, but it should take a long time. Mine are original from 1997 (I think), and have always been in fresh water. I also stripped and repainted them while they were off. I also re-torqued them 3 or 4 times after I ran the boat.

 

[bingshmo]

Re: exhaust manifolds rust!!

Sweet answers guys. I think I will do exactly what you did dg512055! Also glad to hear its quite normal.

While were on the subject.. When I took off the coolant hoses I could see they were nasty as well. So I'm sure the whole motor's nasty too. Whats the best way to clean out (flush) and preserve the entire coolant system on these mercruiser open cooling systems?

 

[HT32BSX115]

Re: exhaust manifolds rust!!

I filled mine with muratic acid to dissolve the rust out,

That is VERY risky. as I said before. The rust IS the metal in the manifold. When they rust, and clog, it's time to replace them before they destroy your engine.

If you clean them out, youwon't really have any warning at all when they start to rust through or crack.....


If it happens during your winter "layup" or while it's sitting for a few weeks etc, you'll be replacing the engine.

Manifolds and risers are somewhat cheaper than an engine.......


Good luck on that....

 

[jerryjerry05]

Re: exhaust manifolds rust!!

Air is the worst thing for your manifolds.When you winterize leave them full of antifreeze.
Air always accelerates the rusting process.
Fresh water manifold should last 15-20 years.Salt less.
Your at 14 years and probably can get a couple more.
The more you run a moter will have an effect too.
The thing is if your worried about roaching your motor change the manifolds. Get a fresh water cooling system.Then the only thing you have to worry about is the risers after another 15 years.
Keep the impeller changed yearly.Fresh oil and keep an eye on things.J

 

[Brentathon]

Re: exhaust manifolds rust!!

Here we go again..........I can't believe how many people try to skimp and save on exhaust manifolds and elbows...
Just replace them at standard intervals (i.e. 5-6 years for manifolds, 3-4 years on elbows)
The most common engine issue on this forum is overheating, and manifolds/elbows are such a common culprit.

 

[Lou C]

Re: exhaust manifolds rust!!

Right, replace em...remove at 5-6 year intervals if in salt....and when winterizing....drain...and then fill with -100 AF with corrosion inhibitors...get the air out....which reduces OXIDATION (RUST)....air doesn't freeze but it RUSTS....

On my '88 OMC 4.3 I replaced them after I got it they were then 16 years old and still good, this set has about 6 seasons on them....so this fall I'll check them for rust flakes...

 

[bingshmo]

Re: exhaust manifolds rust!!

Wheres the best place to buy manifolds for best price? Any upgraded or aftermarket ones?

 

[HT32BSX115]

Re: exhaust manifolds rust!!

Wheres the best place to buy manifolds for best price? Any upgraded or aftermarket ones?

Here's some examples.

http://www.iboats.com/Exhaust-Manif...0468091--**********.120339262--view_id.222204

http://www.wholesalemarine.com/pc/S...rs/Exhaust+Manifold+Mercruiser+4.3+Litre.html

http://bpi.ebasicpower.com/mm5/merchant.mvc?Screen=PROD&Store_Code=eBasicPower&Product_Code=OSC9745

Some of the after market ones are imported. GLM comes to mind. Be careful with them though as they frequently leak. Seems the machining is not all that great.

In a fresh water boat they only need to get replaced every 15+ years or so....etc.......it doesn't make a lot of sense to try to save $100 on the really cheap ones......

 

[bingshmo]

Re: exhaust manifolds rust!!

Thanks for the info.. By the time I replace my 1 piece Merc manifolds with new manifolds, risers, and elbows I will have spent over $800. Now I can see why people consider ways to try and make the ones they have last.

 

[dsmmace]

Re: exhaust manifolds rust!!

I am thinking of replacing my one piece manifolds with two piece aluminum ones from HGE. Any experiences with them?

 

[Brentathon]

Re: exhaust manifolds rust!!

Wheres the best place to buy manifolds for best price? Any upgraded or aftermarket ones?

I've had good luck with aftermarket ones.......in fact, that's all I've used...never had a problem.
Aside from possible casting issues or machining issues.....cast iron is cast iron....it's not the most complex/intricate part of the powertrain.

 

[buzzm19]

Re: exhaust manifolds rust!!

I love this forum and learned a lot and also try to help out when I can. I do see some help here, marine exhaust manifolds are painted black for a reason as black dissipates the heat best and on those thick castings the more heat you get rid of the better. Buz

 

[c.fish]

Re: exhaust manifolds rust!!

Is there thing afterthe risers that can also get blocked- I have exhaust coming out front of riser. Have not removed them yet.