Removing Risers...

Sea Daze

Sea Daze

New Member
Apr 27, 2010
32
Miami Fl
Boat Info
1993 240 Sea ray. 7.4L V8 mercruiser. Garmin 540S GPS/chartplotter/Fishfinder
Engines
454 7.4L V8 Mercruiser
Hello and good morning to all..

My boat has been over heating.. I wanted to remove and check the risers.. Anything I should look for? I don't know what to expect once I remove but what are some things I should be looking for once I do? Its a 454 I/O. Im assuming that all i can check for is some type of clog or debri.. If I dont find anything then im thinking problem is with manifolds.. Please help :smt100
 
I had the engine fully serviced about 7 months ago. Replaced impellers, starter, all belts, wires, and rebuilt kit for carb. the boat started to run very strong but now its over heating. My mechanic told me that the risers, and manifolds are the original ones that came with the boat from factory.. Boat is from 1993.. Im thinking the same that im sure im going to have to replace manifolds and risers.. But was wondering if there is anything maybe in the risers that I can look for that will improve the over heating. I also noticed not to long ago that there is no thermostat either.. Previous owner reomved it. Is replacing the manifolds a DIY job?
 
Generally, you're looking at the "meat" of the metal in the passageways - primarily between the mani and riser. It'll be obvious if it's bad, or not. But, do a search on this forum - you'll come up with other owners who have posted pics, to give you a reference. You could also post your pics.

But, before you do that, maybe we can save you the effort of removing those heaving things. When's the last time the impeller and t-stat where changed? Have you been running in shallow water (sand/mud)? Also, what do you mean by "overheating"? Can you quantify that?
 
If that's all original on a 93' boat, I wouldn't waste time pulling them to check them, just replace them. also the T-stat, and all hoses.
 
See what I get for being a slow typer!

Sounds like the previous owner knew there was a problem and removed the t-stat as a band-aid. You can remove the mani's and take a look, but like mentioned above, you're likely due for a set (get the Merc's - too many people have expressed problems with others).

If you're comfortable turning a wrench, it is certainly a DIY - although the space that you're working in certainly not to your advantage.
 
When removing risers and manifolds be careful not to snap off the bolts. Going through that process now. Not pretty.
 
The impeller was changed 7 months ago, and there is no thermostat currently installed. Before the service that boat was not running well at all. Top speed was 18 knots at 3.5K RPM engine was nto reving higher then that. There was no overheating at all// Motor actually stayed very cool. Obviously once the carbs were rebuilt the motor started working much stronger and RPM now top at 4.5 to 5K. At first the engines ran ok and did nto over heat, but the first time I took it out for a longer trip temp gauge climbed to 170 which is 3/4 of the way before topping out. This past weekend it past the 170 mark and hit over 200F.. Now if I bring the RPM back down to about 2K RPM the engines immediatly cool off. But I have to run the boat at around 6 knots.. way to slow for my taste.. Could this be a simple issue since I do not have a thermostat on it? I do go to allot of sand bars and have kicked up sand time to time.. How can I check if the impeller is good without having to remove?
 
another quick question.. I want to take it out this weekend.. Am I harming the engine by taking it out if maybe the manifolds do need to be changed? If I keep it at low RPM I can keep the temp low on the engine.. I would have to maintain a slow boring speed but rather that then nothing..
 
The only sure way to tell is to remove it. It may only be 7-months old, but sand wreck havoc on an impeller awful quick. It's about a one-hour job.
 
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Sea Daze said:
another quick question.. I want to take it out this weekend.. Am I harming the engine by taking it out if maybe the manifolds do need to be changed? If I keep it at low RPM I can keep the temp low on the engine.. I would have to maintain a slow boring speed but rather that then nothing..
Click to expand...

If it is the manifolds, yes, you could do serious damage... like needing a new engine type of damage. I hate to paint a gloomy picture, but it's pretty hard to give a solid diagnosis via a forum.
 
My comments are if the shoe fits?..something to consider and look at, but first I would say for sure you need risers, manifolds, t-stat, etc

1. But do you have water input strainer baskets? yes?no
2. Next pull one of you hoses going from the sea cock or strainer output, (are there any obstructions from you raw water through hull input to the sea cock or to strainer) is it open or partially clogged? Is this from sea cock/stainer hose very flexible, old hoses will compress, Think of it as like when you pinch off a ordinary water hose, it is closing, while the engine is running under higher rpms at times,restricting water flow.
3. Do you have a heat exchanger, you may want to pull the ends and check the tubes for blockage
 
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The risers and manifolds should have been changed about 13 years ago, and then again about 7 years ago, and then again last year. Borrowed time.
 
Thanks guys.,. Looks like I got another project in my hands... but whats new.. This forum kicks major A$$.. and so do its members..
 
If it's a fresh water cooled system, the manifolds might be fine, but the risers are certainly due for a change.
 
as boykster says, risers are on borrowed time, manifolds might be OK if closed cooling. When the mechanic changed the impeller did the old one come out with all the vanes? If not, did he locate the missing parts in the system and remove them? If closed cooling be sure you take
off and inspect the end of the exchanger that has the zinc hub. You might find a large graveyard of zinc ends plugging the exhanger
 
I had typed a longer reply and my browser ate it -

Happy Hour IV hits most of what I had typed; if closed cooling, the manifolds should last as long as the block, as long as there aren't any cooling issues or gasket leaks, but the risers should definitely be replaced. I just replaced the risers on my 22 year old 454 for the 3rd time, and I inspected the manifolds (closed cooled) when I had everything apart and they look great.

As for your overheating issues, I'd be wary of broken impeller blades being left over from a prior change blocking water flow, or some sort of sediment/sand blocking flow.

If the cooling is all raw water - then by all means replace the manifolds ASAP!!
 
If your engine is fresh water cooled ( closed sys ) check as said by Pseudomind the heat exchanger. I had a 5.7 that would do the same thing at high R.P.M . and as soon as I backed off temp would drop back to normal.What it turned out to be was the impeller had failed and was replaced but all the rubber chunks that came off went through cooling system and got stuck on one side of heat exchanger. As R.P.M would go up water flow would rise and parts of old impeller would block off cooling tubes in exchanger, as R.P.M dropped off peaces would fall back off tubes and let water flow and cool engine. When I took end cap off exchanger and removed old impeller peaces and some that were in some of the tubes, engine temp was back to normal. I had no strainer in cooling system but put one in and have not had temp problem. I do run a t-stat. It could still be manifolds as they are really old and chunks of rust and scale could be blocking water flow. If they are as you say that old I would replace them. I know they are very pricey but they cost less then a new engine. If they are bad at the riser they are not to good in the manifold itself. If the manifold fails it will let water get into engine . Bad news then. Good luck.
 
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