Overheating problem

[Kidnotorious]

I built this engine from my old truck engine and got most of my part from a marine scrap yard. I have a 5.7 liter 350 vortec 1997. Alpha one foot. New impeller and impeller housing, the whole unit. 4" diameter straith headers out the back for exhaust. According to the mercruiser book my cooling system is hooked up perfectly the way it's supposed to be, i checked it many times, it all looks good. Now my exhaust manifolds are fairly hot all the time and my headers are scorching all the time, but my problem is that, when starting out theres no problems and i can drive for over an hour with no problems, but if i stop and let the engine sits for a while even though it cools down some as soon as i start out again it starts overheating big time. Tried again without a thermostat and started out fine from dead cold, drove for a while and no problem, stopped and let it sit a while about 45 minutes and when started to drive again, overheating big time. The only way to cool it back down is to rev the engine in neutral at about 3500rpm, temp goes down and then i can go. I think i'm getting a steam pocket in the block somewhere and the high revs push it out when theres no thermostat but why it's getting this steam pocket, if it's that, i don't know. Might be because the exhaust is heating up so much, but why. And i don't really like driving without a thermostat cause this way it cools down way to fast when i rev it.
If anyone has any ideas, it would be much appreciated.

[tuantom]

I know Ford small blocks have some variation where the steam holes are machined in the blocks and heads - so you have to pay attention when you start mixing and matching components. Sometimes you have to drill your own steam holes in the either the block, head and/or gasket - so it seems possible that you may have a similar issue by putting newer heads on an older block.

If it is the case; you're not the first one to come across it - and someone on a small block chevy or hot-rod forum is bound to know.

[Kidnotorious]

No, i didn't mismatch the engine, the heads belong with that block, both are 1997. Just some of the marine components are from a scrap yard, all mercruiser and all rebuilt. Do you think this still applies cause everyone says that the exhaust system is supposed to be cold to the touch. Plus i'm not sure that it's getting a steam pocket, it's just my best guess according to the way it's behaving.
Thank you.

[tuantom]

I'm a little confused by what you're calling the headers and the manifold. Is it stock Mercruiser? The exhaust shouldn't be hot if it is. To clarify; do you mean the manifold (cast iron and bolts to the engine) is scorching and the riser (bolts to the manifold and to the 4" hose) is warm?
When I hear header, I think of insanely expensive, high performance stainless steel tube header.

If it is a stock Mercruiser exhaust system, the gasket between the manifold and riser has to face the right way to get proper water flow through the manifold. Could the gaskets be backwards?

Is it spitting water out the exhaust when it starts to overheat?

Have you checked the flow into the manifolds?

[Kidnotorious]

The exhaust manifolds are mercruiser cast iron and do bolt to the engine and the headers are stainless steel 4" diameter, but i made them myself and they go straith out the back, the're the ones that are scorching and the manifolds are pertty hot. There is no riser in between them, but the gasket has a slot on either side a square pocket in the middle and a 3/16" hole in the front and back of the pocket. When driving normaly, when it's not overheating, water is coming out the back of the headers and it is very hot. Even when it overheats water is coming out.
The manifold are very clean inside no rust blockages anywhere.
A marine mechanic told me to put clear hoses temporarely to see if there is air in the system and to see if the water is flowing in the right durection, so i'll be trying that tomorrow.
If you have any other ideas please let me know.
Thank you

[tuantom]

OK - you made your own risers - aka collector pipes. Headers = head pipes (attached to the head).
Do you have any pictures?

Still ... If both your engine and exhaust manifolds are getting hot - it sounds like a water supply problem. When the thermostat is closed, or if a water passage in the engine is blocked, the water will bypass the block and be routed through the exhaust manifold - which it would be cool. If Your exhaust system is the bottle neck, then both engine and exhaust will heat up - as it would if your supply is low. Make sure that your supply from the water pump is healthy and bubble free while you're at it.

[Frstbyte77]

try tilting the boats bow abnormally high in the air while on the trailer and run it for a few mins. this might help work any air to and out of the front of the motor...

[Kidnotorious]

Well i'm gonna run it with clear hoses to see the flow and then i might be able to see if there are any restrictions or flow problems and see if there is air bubbles in the system. Will be doing this tonight if the weather clears up a bit.
Here's some pic of the exhaust, the water and exhaust stay seperate till it passes the bend.

[Frstbyte77]

The exhaust looks nice... wish i had the tools to make somethinglike that

[Kidnotorious]

Well thanks for your help guys. I found the problem, it was a blockage, but it was my stupidity. When i put my headers on, the manifolds were a bit rusted and there wasn't much of a surface left to seal so i put a good amount of high temp gasket glue to ensure it would'nt leak, but it was too much and it plugged to water ports completely. Yes i know, i'm a tool. Plus i checked my compression and i blew the head gaskets from overheating so it will be a week or so till it's back in the water. Thanks alot guys.

[TollyWally]

Very nice work on that motor.

[powerabout]

It might be working but
it looks like you have a different version manifold versus the thermostat housing and are recirculating the water in centre rise manifolds which were designed for total loss ( unless heat exchanged)and had spring loaded pressure relief valves in the thermo outlet.
the old log style manifolds had block off plates under under the risers with just a pin hole to bleed the air off you will need to copy that but your inventing a cooling system now.
Saying that I cant see why it wouldnt work as your plumbing has followed the routes to suit the thermo housing you are using.

I had a few customers who changed the old style log manifolds to aluminium log types ( ski boat stuff) and the lower water volume in the manifolds meant there wasn't enough water in them to prevent them boiling the recirculated water so thay had to be plumbed the same as the center rise manifolds where the water once entering the manifold leaves the engine via the manifold to riser joint.