raw water input in cylinder - mercruiser 470 (3.7L)

[mcwinfrey8]

Hi, i've recently purchased and rebuilt my mercruiser 470 and got it back in the boat this weekend. It started lovely to my amazement and holds it's internal fresh water up to temperature like a new one. The problem however seems to be when i run it connected to a hose through earmuffs on the leg (sea trials next week), as left idling for a few minutes sounds sweet, then a quick rev and it's down on three cylinders (always seems to be number 3 that dies). Turning off the hose input(THOUGH NOT IDEAL TO DO WHILE RUNNING I KNOW!) seems to bring it back onto four cylinders again in the space of about 10 seconds! Is this my exhaust manifold posing a problem due to internal corrosion or simply too much water pressure up the leg for the rpm of the engine? Will running the hose without the engine running fill the cylinders? many thanks, Ben

 

[crazy charlie]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

I have heard of instances where that is exactly what happened.The motors in question were the ones designed with valve overlap I believe.

 

[tashasdaddy]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

it sounds like a bad ex. man.

 

[Haut Medoc]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

I would not completely shut off the water input while the engine is running.....You will damage the impeller.....Running the hose (engine off)should not put water in the cylinders, if it does something else is up.....While running, turn the water pressure down, not off....The worst that should happen is that it will run warm.....JK

 

[mcwinfrey8]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

Hmmmmm. Do you know if the ex man is seperated inside, one for gas flow and the other for the cooling water preventing any water entering the exhaust ports? Is having the input water running when not running the engine a no no for this reason? I'm tempted to try it in the sea and give it the benefit of the doubt!

 

[mcwinfrey8]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

Thanks Haut Medoc, i feared that! As if i shut the motor off then restart with the hose still running, the engine runs on three and when i remove #3 plug it is wet with water! i guess it MUST be the exhaust manifold

 

[Haut Medoc]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

I'm not wholly familiar with your specific set-up.....If your carb is on the manifold....I suggest you update the topic header & put 470 in it...They are a little rare , but there are some members in the forum who own them & would be able to shed more light than me....<br />Since a 470 is a pretty old rig, how old is the manifold? It might be wise to invest in a new one anyway....<br />Not suggesting that you start throwing parts at it, but that's one part that can't hurt....JK

 

[mcwinfrey8]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

Carb is on the inlet manifold on the other side of the engine, fresh water coolant filler tank is on top of the exhaust manifold. Boat has been left for a year out of salt water without being flushed. The water entering the engine is definitely coming from the input at the leg. Is there anything else but the exhaust manifold (assuming this IS raw water cooled in the first place) that i could be getting the water from? will change the topic header. Thanks JK

 

[Haut Medoc]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

The riser or elbow is your problem or the gaskets at your separater plate underneath it....I had a chance to look up your system.....The only way in is through the elbow or gaskets....The only other thing I can think of is that a piece of crud has lodged in the outle hole in the elbow & is diverting a small bit of water the wrong way....Pull the elbow & have a look...If the mating edge is deteriorated, replace it....JK

 

[MrBill]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

Your impeller is now shot...it will continue to run at good temp as long as you're providing pressurized raw water, but the sea-trial will tell the story. I cooked an impeller 4 years ago with four 60 second starts without water supply (by accident), but I hope you luck out. Problems is you've put wear on it and it may be ok for a while and then fail.<br /><br />Your water intrusion problem sounds like an issue with the exhaust manifold and/or riser, raw water cools the riser. If you pull the riser, chances are you'll find corrosion and it's letting water down into the exhaust manifold. Pull the riser and evaluate.

 

[MrBill]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

Your impeller is now shot...it will continue to run at good temp as long as you're providing pressurized raw water, but the sea-trial will tell the story. I cooked an impeller 4 years ago with four 60 second starts without water supply (by accident), but I hope you luck out. Problems is you've put wear on it and it may be ok for a while and then fail.<br /><br />Your water intrusion problem sounds like an issue with the exhaust manifold and/or riser, raw water cools the riser. If you pull the riser, chances are you'll find corrosion and it's letting water down into the exhaust manifold. Pull the riser and evaluate.

 

[mcwinfrey8]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

Cool thanks guys. new break through this evening however......! when i pull off the return from the heat exchanger into the exhaust it picks ups again after 15 or so secs then replacing it drops it back on three after about the same time. Also when i remove the pipe after it starts to miss fire, i get a gush of water from the inlet pipe, suggesting a blockage somewhere in the exhaust outlet through the leg? Also, the centre of the impeller emits a small spray of water, whilst the two outlets on either side of the upper part of the leg are under great pressure. Can i point my finger at a blockage?? Ben

 

[Haut Medoc]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

Yep.....Pull the drive & have a look....<br />Grease the u-joint while you are at it..<br />They probably need it anyway....<br />Could be a broken exhaust shutter lodged in there...JK

 

[mcwinfrey8]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

You're very efficient JK! Many thanks. When you say "pull the drive" do you mean unbolt it from the transom and free from the boat altogether? Will dig out the grease gun!! Thanks again.

 

[Haut Medoc]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

Yes!

 

[mcwinfrey8]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

I fear many more man hours to come on this craft!! There goes my weekend! Thanks all

 

[mcwinfrey8]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

Update on progress. . . . pulled leg as suggested and ran a hose in to exhaust outlet of the mated face and water flowed out of prop centre with no problems. Guess the trouble lies within the upper half, all that's needed now is to find where..........!!!

 

[mcwinfrey8]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

Problem solved, the exhaust riser has a hole directly above no 3 cylinder, so as the water circulates before being dumped out the exhaust, part of it is spilling into the cylinder! Going to make a brass insert and give it a go. Thanks for all your help. Ben

 

[Haut Medoc]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

Replace it now!....JK

 

[mcwinfrey8]

Re: raw water input in cylinder - mercruiser 470 (3.7L)

JK, do you know the difference between the old style exhaust manifold and the new one on the 470? Only mine has the riser with built in filler cap,whereas one i went to look at today is completely different to look at but is the same block! Can i use the whole manifold etc from the older engine? What was the reason to change to the new type? Ben