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350 mag exhaust issue

Old 12-29-2015, 07:43 PM
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Default 350 mag exhaust issue

I've got a "02" 28' formula w/ twin 350 mags, I keep blowing the flapper valve in the y pipe if that's the proper name its the rubber butterfly that stops the water from coming back in i already lost one motor from it, repowered that side it happened again, then lost the flapper on the other motor. My temps are running fine at 180 and can't seem to figure out what the issue is, if anyone has any info on this please help me out merc. has not been very help full thanks!!

Last edited by bigpao; 12-29-2015 at 07:45 PM.
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Old 12-29-2015, 08:34 PM
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you should be running under 170 degrees. Scan the engine to see what the water temp reports while running also look at your water pressure readings from the scan, they are important. You should have ECM 555 - this is the stock Merc ECM that started in model year 2001.

Sounds like you have a water flow restriction to that side of the exhaust inwhich makes it run hot melting the rubber off the flapper valve aka water shutter. Also the flapper should move open/ close very freely. I would check that as well.

Also your exhaust hose/ tube possible not mounted correctly off the exhaust riser causing a water flow restriction blocking some of the water ports. (exhaust riser )

Gen VII water pump housing not putting out enough volume of water flow. Probably internally scared. Junk pump anyways.

Do not rule out water circ pump either for cooling - yours is 14 years old and they need to be changed depending on hours and water conditions ran in. Running one over 10 years is only asking for trouble plus if they never get drained for winterization worst yet.

Last edited by BUP; 12-29-2015 at 08:39 PM.
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Old 12-29-2015, 08:51 PM
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I know your problem but everything i said in the post above is valid as well to come into play.

I will need to see pics of your exhaust and then the question is has the riser ever been off and has new gaskets been installed ???

Or do you have Mercruiser Dry joint exhaust manifolds and risers or any aftermarkets dry joint exhaust manifolds / riser installed ??? I am 99 % sure I know your problem if so or I have already listed it in the post above.
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Old 12-29-2015, 10:44 PM
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Originally Posted by BUP
I know your problem but everything i said in the post above is valid as well to come into play.

I will need to see pics of your exhaust and then the question is has the riser ever been off and has new gaskets been installed ???

Or do you have Mercruiser Dry joint exhaust manifolds and risers or any aftermarkets dry joint exhaust manifolds / riser installed ??? I am 99 % sure I know your problem if so or I have already listed it in the post above.


That was a huge help thank you, the manifolds are about a year old on booth motors replaced them when we repowered last year got them from merc. Saying that they're guenuine merc. Parts but say "made in China" on the inside used gaskets from merc. Had questions on the gaskets on which way to put the bigger holes got two different answers from two different mechanics not much help there, and the circulator is 2 years old, thanks again
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Old 12-30-2015, 09:59 AM
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That was my point as well - what style of manifold and riser is installed - Dry Joint style or the older style ?

Then my other question which style riser gasket was installed ??

Next question which position was the riser gasket installed ??? - they have to be positioned correctly - if not will cause a cooling problem.

Next question do you have riser extension blocks installed inbetween your manifold and risers ???? If the correct gaskets need to be installed there as well.

All of this needs 100 % to be known. There is more to it than just replacing the exhaust system and thinking you did correctly plus have the correct parts installed. Just saying, trying to make all of this clear to help fix the problem.

Pics of what you have and the riser gasket plus which way it was installed would help most likely fix your problem.

Post # 2 has all good info in there as well on top of what I am asking in this post. I think you might have the riser gasket installed in the wrong position causing your problem especially if you have dry joint manifolds and risers installed.

Last edited by BUP; 12-30-2015 at 10:09 AM.
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Old 12-31-2015, 04:31 PM
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Originally Posted by BUP
That was my point as well - what style of manifold and riser is installed - Dry Joint style or the older style ?

Then my other question which style riser gasket was installed ??

Next question which position was the riser gasket installed ??? - they have to be positioned correctly - if not will cause a cooling problem.

Next question do you have riser extension blocks installed inbetween your manifold and risers ???? If the correct gaskets need to be installed there as well.

All of this needs 100 % to be known. There is more to it than just replacing the exhaust system and thinking you did correctly plus have the correct parts installed. Just saying, trying to make all of this clear to help fix the problem.

Pics of what you have and the riser gasket plus which way it was installed would help most likely fix your problem.

Post # 2 has all good info in there as well on top of what I am asking in this post. I think you might have the riser gasket installed in the wrong position causing your problem especially if you have dry joint manifolds and risers installed.
Okay thanks again it's winter up my way I'm going to get into the project in the couple weeks can I contact you when I get into it
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Old 01-01-2016, 08:33 AM
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I think the best way for people to help you on here is first take pics of the motor with the exhaust installed. Take pics of everything. Including all hoses and thermostat housing. Probably wise to pull the thermostat housing and see what thermostat you have. 180 seems High. My motors are run hard in south Florida in the summer and I never see above 165 or so. Also how long are you flushing the motors. Most dont like prolonged running on the hose. Even if the exhaust riser gaskets are put in backwards, that tends to make the exhaust hot which can burn flappers, but not make the motor run hot. I do not have the motor expertise that BUP has. I do have a lot experience in exhaust manifolds especially in DRY JOINT MANIFOLD retro fits. Much more too it than just bolting them on. Does your Thermostat housing have the T on the top with 2 extra hoses going to the risers?

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Old 01-01-2016, 10:35 AM
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Stock OEM engine serial number raw water cooled OM300000 thru OM599999 inwhich should fall under an 2002 350 mag Mercruiser. This should not have any check balls in the T stat housing nor the T.

Again this is the stock set up. One of the reasons I never brought that up. Just saying. But of course with todays boats and many have been passed around especially on the repo market that do not have the correct nor stock set ups that they once had to begin with. I still doubt your has the check balls in the T for the T stat housing.

IMO your problem is something I posted or possible could be a T Stat issue or water passage blockage of some sort especially in the water ports of the exhaust manifolds / riser areas.

IMO again I think if in fact you have Dry joint exhaust manifolds and risers - the riser gasket is not positioned correctly., Seen this about a 500 times no BS thru out the years.
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Old 01-01-2016, 10:46 AM
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Some added info for whomever

http://www.offshoreonly.com/forums/g...e-options.html
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Old 01-01-2016, 11:43 AM
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The other thing that comes to mind is check valve(s) problems and or they are somewhat restricted - these are installed to help prevent heated water draining back down to the cool fuel assembly after engine shut down that can cause vapor lock ( fuel boiling away in the cool fuel from engine heat soaked water ) - They are installed in the cooling portion (hoses) (location ) depending on which water drain system you have - multi drain system or easy drain system set up.

Basically everything is cover here for the possible why the melting of flapper valve aka water shutter and possible engine temps on the high side if in fact your temp gauge readings are spot on. I would scan the the engine to see true engine water temps as well so you can rule whatever out or a real problem. 180 is on the high side for your app if this a correct temp reading.

Last edited by BUP; 01-01-2016 at 11:48 AM.
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