98 Mercruiser 5.7 manifolds different temps?

Hello again everyone, I've looked through threads and couldn't find an answer for my issue(other than what I've already done). I replaced my risers and manifolds on my 98 Mercruiser 5.7 with Alpha I Gen II drive. I've never noticed this before but after replacing the manifolds and risers the starboard manifold gets quite a bit hotter than the port side. I thought maybe it was just because it was on the hose but I took it to the lake and let it get up to temp(170 for my boat is normal) and the port side was about 98 while the starboard side was about 128. I've read that the two sides can vary(many reasons for why they vary but either way I've heard its normal) but should they be that much different? The boat runs great, but why such a big difference. I wish I would have known about this early so I could have checked the flappers(the flaps that keep water from back flowing into the exhaust) while I had the risers off but I never checked them since the boat has always ran great. Any suggestions on why this is happening or is this normal? I am thinking about taking the risers off again to check the flaps, but I want to see if there is another solution before I have to replaced the brand new riser gaskets after taking them back off. I've already took the hoses going to the manifolds off and they are clear. Didn't see much rust/debris in the thermostat housing. Thanks in advance. I just want to be clear of any issues when summer comes up and its used every weekend.

Your year and design show

160* thermostat so a running temp of 170 is correct.

A thermostat of 140 which is the other choice for OTHER YEARS and designs would give a running temp of 150-160*

The water hose routing shows only one hose to bottom of each exhaust manifold which supply the cold water to each side. NO separate hose to Elbow (Other designs).


So, with your design you have less ability to control/supply fresh cold water to exhaust system.

Are you using in Fresh or Salt water and if salt water, how long has it been used in this environment?



Below is the image of your thermostat housing. The parts and order in which they are installed make a difference how the water flows.


The thermostat housing top casting you have 4 connections.

One, is cold water in (oil filter side of casting)

two, are cold water out, Smaller diameter and feed exhaust manifolds. These have cold water all the time before thermostat opens.

one, large hose which connect to on engine circulation water pump.

Operation-

Engine cold, Thermostat closed, Cold water comes in thru one hose on thermostat housing, it is sent out two smaller hoses directly to exhaust. It also flows into engine thru pump.

When engine block water temps reach ~160* the thermostat opens and this 160* +/- water now mixes with the fresh cold water and gets circulated thru engine and to exhaust manifold. Because of this you have no control over the water temps going to the exhaust manifolds.

So there are only a few possibilities.

1. Low water flow from Outdrive impeller assembly (Impeller, Missing slinger, dam between water pump assembly and thru prop exhaust not present or eroded away or not installed at all or properly) This should be explored to confirm this is functioning as designed.

2. Clogged exhaust elbows- You said you replaced.

3. Clogged small hose output of thermostat housing, Do both outputs look exactly the same or is it possible one has corroded and is not allowing enough water flow?

4. Exhaust manifolds have corrosion issues, Less likely but possible.


Images of thermostat and parts layout and orientation. Also water circulation image.

Ayuh,..... Manifolds rarely to never run the exact same temps,.....

I've had it for about 5 years and it has always been in fresh water. I don't know about the use before I got it. I read some people say they run about 20 degrees different, but then some people say they had this issue and found a flapper broken and replacing that fixed the issue. Should they run 30 degrees different though? That seems a bit far off to run that much different.

The flappers assist in keeping water from back flushing into exhaust elbows/manifolds, say when you come to a quick "stop" and water rushes back at boat and also reversion to a degree that occurs as a result of the exhaust.

If a Flapper breaks and falls down into exhaust it can hamper exhaust flow if it is in the exact right position but one would expect the entire engine to get hotter not just one side. Both exhausts flow/combine into ONE and the go thru transom. If a flapper falls it will make it to that point where both sides combine before it may cause an issue. The "rams horns" down tubes have NO restrictions and are round and smooth until they combine into ONE at the transom.So the flapper will not get stuck in the down tube.

To answer your question, Yes there is typically a difference in temps between the two sides. That difference varies and 20* is typical.

What are you using to measure the temps? A infa-red gun?

Try this, while in neutral, run the engine at 2000 rpm and measure the temps again and let us know what the difference is.


You really dont give much history of maintenance on the engine or drive. The list I posted would be part of that history. 1 and 3 are the most important in my opinion.

If you cannot answer that all the things listed are known good especially 1 and 3, then you are asking for an answer that is only a guess about why or if what you see as temp differences are acceptable.

If you brought your boat to a reputable shop they would confirm all that I mentioned in previous post before saying the temp differences are acceptable.

Is this after 20 minutes or six hours? A car engine and a marine engine have very different thermal loading/dissipation characteristics. Run it for an hour or two up on plane and then check it. Prolly less difference by then, but even if not it's still within expected range if it's as you describe.
Which brand of manifolds and risers? Genuine? Osco? Other?

I used an infrared temp gun to measure the temp. I have changed the oil, impeller, impeller housing, lower end fluid, oil and fuel filters regularly(I'd say about every 8 months, I only put about 20 hours on it a year and I don't run it hard) other than that I haven't done much to it. It was owned by an older couple that lived on a freshwater lake and seem to have taken very good care of it while they had it. I don't have maintenance records of what was done though. I replaced the impeller, oil, oil filter, fuel filter and lower end fluid about 6 months ago. I just put new Mercruiser OEM manifolds and risers with new gaskets on it 4 days ago. I took all hoses that comes from the thermostat housing and checked them, they were all clear. This could have been going on since I got it and never noticed, I'm not even sure that there is an issue. This weekend I'll run it around for an while and see if it changes anything.

It sounds like everything is normal including your maintenance.


I have included a link to the parts breakdown from this site below of your lower unit showing the impeller and associated parts.

The two items you did NOT mention and I DID is of concern, It these parts are not functioning as they should they possibly will allow exhaust gasses into fresh water supply witch could cause temp issues.

I am not saying you have any issues but next time you split the cases to work on impeller these should be examined and the one item #2 should be in perfect shape or replaced.

Item 25 is referred to as a slinger, It slides over shaft and sits on top of impeller housing and keeps exhaust from entering the impeller housing


https://www.marineengine.com/parts/...-up/gear-housing-driveshaft-standard-rotation

A little update on the issue. I looked at the old gasket for the riser and found that I put the wrong gasket on. There was an oval shaped spot that I thought was the gasket hole for the water to flow through, it turns out it was just the outline of the manifold and there just just a small hole in the riser gasket for the water to flow through. Looks like the website I looked on was wrong about which gasket goes on this riser. My fault for not double checking. New gaskets are on order and I will check the flappers while I have it apart. Also have a new thermostat kit on the way. I don't see how having more water flow would cause the temp difference but we will see. I'll post an update after I run it with the new gaskets and thermostat.

When you have single water flow to exhaust manifold and none to the elbows it is critical to have the correct gasket that allows full water flow thru manifold to elbow

Link below shows the correct gasket and orientation. Item 5. Large open holes for full water flow all the time

https://www.marineengine.com/parts/...u-0m599999/exhaust-manifold-and-exhaust-elbow

I'm a little confused. Mine looks like the one on the left side of the picture with small holes in the gasket, not the open ones on all 4 sides. Both types of gaskets are #5. When I look at the elbow it has open holes on all 4 sides but the gasket has 2 oval shaped holes and 2 small pin holes on the gasket. So which one would be correct? Did they have the wrong gasket on before(the one with 2 oval holes and 2 small pon holes)?

The first picture is with the old gasket on the old riser, the second picture is just the bottom of the old riser with no gasket on it. Was the gasket that was on it the wrong one? Why would the riser have 4 big oval holes and the gasket just have 2 small holes and 2 oval holes? This is where I'm getting confused.

The one on the left is the correct one.

The image on the right ( item 5 for riser kits) is for a SPACER/riser kit used when the elbow height has be higher for certain boat applications such as thru hull exhaust. The gasket used between manifold and spacer has all four holes which allows full water flow to top gasket between spacer and elbow which is the same as the gasket used for your application (manifold to elbow). If this was not the case and you used the same gaskets with small holes the spacer would get much hotter than it should.
Think of it like this, When a spacer is used and the gasket has all four open holes between the manifold and spacer, It is as the spacer is a part of the manifold. It is just extended higher.

The correct one has two large holes and two small holes. The small holes control flow so the water does not exit manifolds to fast without taking the heat with it.

If all four where wide open the manifolds would get much hotter. The small holes allows some water flow so the hot water itself doesnt build up pressure and blow out the gasket



Below is a copy from Manual.

Just make sure gasket is oriented the way it shows in the image below.........open holes in gasket on the left and right and small holes front to back

Ahhhh, I understand now. I'm still not sure why they are running different temps even though the gaskets are the wrong ones, both have the same gasket in them. I will install the new gaskets and thermostat and see what happens. I am definitely going to check the flappers this time. I'm new to working on marine engines so this is definitely a learning process for me. Thanks for your help! It makes much more sense now. I will give an update once the new gaskets are installed.

Just a little update, I put tye new gaskets in and replaced the thermostat and now the engine temp runs just over 170(normally it runs right at 170) I'd say it runs about 175. Didn't overheat or anything but it runs slightly hotter with the new thermostat. As far as the manifold temps they are still running the same temps. I checked the flappers and they are good. Not sure what to do now, I've checked everything I can think of. I'm guessing it is normal and I've just never noticed it. If anyone has any other suggestions it would be greatly appreciated because I don't know what else to do.

Put old thermostat back in if you want otherwise enjoy you boat.

Remember, as you have only one cooling hose to each manifold and no second one to the elbows and all downstream parts are working coreectly, not much else you can do.

Your systems elbows will run hotter and it is what it is..