2002 5,7 Mercruiser exhaust manifold keeps cracking - 5 times this summer

[kenny nunez]

A good time to look at the through hull intakes. Does your boat have one for each engine?

 

[mart284]

A good time to look at the through hull intakes. Does your boat have one for each engine?

Yes, one for each engine, extra one for genny on the bottom of the boat. We did check the flow while boat was in the water and there was water coming freely, but will physically check them anyway. Both filters were clean.

 

[Lou C]

Converting your readings from kPa to psi you have about 14 psi on the starboard engine and 20 on the port engine, not sure if that can cause a problem, did you get a trouble code for low water pressure? I know on my brother's boat he did and it limited the RPM due to the low water flow, he had only about 180*F on the gauge though. Your temps on both engines are pretty equal the read out is about 150-154, although the gauges read higher like 170...
You are also losing about 350 or so rpm on the starboard side.
Still the evidence doesn't add up. I'd think if you're cracking manifolds due to low water flow the starboard side engine would be running a good bit hotter.
Probably one of iboat's top ten inboard engine mysteries.

 

[mart284]

Converting your readings from kPa to psi you have about 14 psi on the starboard engine and 20 on the port engine, not sure if that can cause a problem, did you get a trouble code for low water pressure? I know on my brother's boat he did and it limited the RPM due to the low water flow, he had only about 180*F on the gauge though. Your temps on both engines are pretty equal the read out is about 150-154, although the gauges read higher like 170...
You are also losing about 350 or so rpm on the starboard side.
Still the evidence doesn't add up. I'd think if you're cracking manifolds due to low water flow the starboard side engine would be running a good bit hotter.
Probably one of iboat's top ten inboard engine mysteries.

Issue is, that NO ALARMS, NO BEEPS - nothing. Just cracks on the bloody manifold. Temps are fine, water and oil pressure is fine. Old gauges are not too reliable, they have been showing funny numbers all the time I owned the boat. One engine has been from the day one slower to speed up..
After quadruple checking everything, we still have no clue what is causing it. clear hoses show waterflow, heat gun did not show any differences'.

After seventh beer the great idea is that we have issue with water flow after having some longer 3800RPM stint and slowing down causes lack of water, exhaust is still hot and moment later cold water arrives. But why only one side, this probably requires more stronger drinks

Thanks for ideas and support - everything is appreciated (except - sell the boat)

 

[Lou C]

Well if you're on the seventh beer you're working on a 12 pack eh, I rate jobs by how many beers are required to prevent tools from being thrown, bad language used, in fact I had to switch to cussing in my mother's tongue (Italian) so the neighbors don't understand what I'm saying lol.
Getting back to the problem, cracking manifolds, if it isn't water flow, then I'm thinking it must be related to expansion when the engine heats up, I'm still wondering if the cyl head mating surface isn't even and when the manifold is torqued down but then heated, it is flexing unevenly and cracking.
Now I don't remember but have all the cracked ones been aftermarket, and not Mercruiser? If that's true I'd go at it again, use either a Merc or Barr manifold, measure the flatness of the mating surface, make sure that those bolts thread ALL the way in, for even clamping force, and keep your fingers crossed or as the Brits like to say "touch wood" lol!
It's a beautiful boat I'd hate to see you sell it.
I can tell you that when my engine overheated, the manifolds were at 375*F, and did NOT crack, OEM OMC one piece batwings, I tested them with acetone after that and used them 3 more seasons.

 

[mart284]

..
Getting back to the problem, cracking manifolds, if it isn't water flow, then I'm thinking it must be related to expansion when the engine heats up, I'm still wondering if the cyl head mating surface isn't even and when the manifold is torqued down but then heated, it is flexing unevenly and cracking.
Now I don't remember but have all the cracked ones been aftermarket, and not Mercruiser? If that's true I'd go at it again, use either a Merc or Barr manifold, measure the flatness of the mating surface, make sure that those bolts thread ALL the way in, for even clamping force, and keep your fingers crossed or as the Brits like to say "touch wood" lol!

All cracked manifolds are aftermarket ones and from two different brands - hard to get other in Estonia. We checked the surface and it was straight.

We will go first for water flow -pipe by pipe... and hope to find something there. If nothing at all, then for next cracked one in 2025 I will bite the bullet and order original one with customs and shippings and $$$$$. I hope it will not go that far, but I had similar hopes in last few times too

 

[cyclops222]

Has anybody cut open the manifolds to check if the CASTING is DEFECTIVE ? Compared to other cylinders where cooling water enters & exits ? Same area points to defective castings.
And or racing WOT for many minutes and then a sudden slow down. ........Cut open all the cracked pipe sections !! Also cut open none cracked sections. You have to see something different from the none cracked pipes.

 

[mart284]

Sad to state that nothing has been done with the boat - will start dismantling process next month... Sadly standing alone in the winterspot and waiting for a better summer...