Exhaust Manifold too hot to touch

MI CC

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1998 Volvo 5.7L GS. Noticed one exhaust manifold too hot to touch although engine temp gauge never goes about 175. Went to replace impeller and it had many pieces missing and looked like it was melted in places as well. Removed impeller on other engine and same thing. Both were replaced 3 years ago and I never have seen any damage when replacing my impellers. I believe marina mechanic must have run them dry during storage last winter. Cannot find the missing impeller pieces on either engine. Replaced thermostat and housings since it was original with no change. Port engine has no issues at all as I'm able to touch both manifolds. Can the pieces get caught in the manifold? Should I remove the elbow and check that? Boat is in water and wondering what I can check next.
 

Scott Danforth

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start at the raw water pump and work your wait to the exhaust system until you find the impeller pieces.

usually from the raw water pump, the pieces go to the thermostat housing, where they may go to the exhaust branch.
 

MI CC

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I've checked everything up to the where the hose goes into the exhaust manifold. I installed brand new thermostat housings and flushed the hoses.
 

Bondo

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Should I remove the elbow and check that?

Ayuh,...... Welcome Aboard,....... Ya gotta find the bits, 'n that's the next logical place to look,.......
 

Lou C

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don't forget to check the little by pass opening in the OMC/Volvo thermostat housing...
at idle you should be barely warm (90-100) after coming off plane 125-130 or so. I have the exact same exhaust on my OMC and that's what I get consistently.

photo330576.jpg
 

Lou C

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On the OMC/Volvo exhaust manifold there is an opening at both ends, you can remove the drain plug on the back and and the water fitting on the front and run a coat hanger straight thru it to make sure there is no crud in it, only other thing you can do is drain water, pull off the riser, take off the manifold and dig out any deposits or trapped rubber in the cooling passages.
 

MI CC

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On the OMC/Volvo exhaust manifold there is an opening at both ends, you can remove the drain plug on the back and and the water fitting on the front and run a coat hanger straight thru it to make sure there is no crud in it, only other thing you can do is drain water, pull off the riser, take off the manifold and dig out any deposits or trapped rubber in the cooling passages.

Didn't realize that. Not going to be easy to get to the plug in the back but I will give it a try this weekend. Is there anything special I need to do to remove the riser if the boat is in the water?
 

Lou C

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that you have to be careful with, it depends on how high up the Y pipe the water comes with the boat at rest. It can be risky I'd probably pull the boat for that job. If not make up a cover for the top of the Y pipe to keep water from coming in if a wake rocks your boat. The water could be right at the top of the Y pipe or slightly lower. Y pipe diameter is 3.5" if you want to make something up to cover it. Make sure your bilge pump works and batteries are charged before doing this.....
 

MI CC

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Thanks for the heads up and info. I will try to take some measurements to see how high above the waterline the riser is before starting. Just replaced bilge pump switch and will be on shore power.
 

Lou C

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Also:
before removing the elbow, drain all the water out of the manifold, because once you break the seal of the gasket between the manifold and elbow, water sitting in the manifold can spill over into the center exhaust gas passage and run into a cylinder via an open exhaust valve. Make sure to have a couple of spare elbow/manifold gaskets and a manifold to cyl head gasket if you are removing the exhaust manifold too. (standard Chevrolet gasket will work for that, the other should be the OE Volvo gasket).
Keep in mind that the bolts might be difficult to remove. It might be easier if you warm up the engine before trying to loosen the bolts. You can just loosen them, let it cool, then drain the manifold (water might be pretty warm) then after it cools remove them the rest of the way.
I still think its better/safer to pull the boat for this. The most I would do in the water is change a thermostat or impeller on the Volvo because all you have to do is get the raw water intake line above the static water line and plug it with a wood plug of the right size. I've replaced thermostats a few times that way.
 

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MI CC

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Decided against removing riser in water. Water level appears to be extremely close to where it would be disconnected and of course it's the outside manifold/riser I'm having issues with which is extremely difficult to get to and work on. Inside is easily accessible.
Tried connecting clear tubing between thermostat and manifold but couldn't get it on manifold. The port on thermostat housing is 3/4" but on the manifold it's at least an inch. Had heat gun and couldn't get the clear tubing or a new replacement water line I had purchased on the manifold port. Any other tricks besides heating it up?
 

saf

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I am not an expert by any means but I had exact same problem (one riser overheating) last year. Turned out to be a faulty riser (ended up replacing it by VP warranty). Looked like the riser had some internal casting defects, bits of metal coming out of it. Once the riser was replaced the problem went away. Had no problem replacing it while in the water. You can start by loosening the rubber boot connection to the riser, if you see water seeping through, then you'd need to pull the boat; if not - you are good to go. Replacing risers was super easy. Just need to have a new metal gasket ready.
Connecting a clear hose is always a good idea. In my case it helped finding another problem - air being sucked in in the outdrive. You can use a 3/4'x1" connector to transition from 3/4'ID hose to 1"ID.
 

MI CC

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Took boat out last weekend and exhaust manifold reached 215 degrees within 5 minutes up on plane. Engine temp remained at 175. Decided to remove riser and exhaust manifold this past weekend and backflushed with power washer. Both looked decent and didn't see anything come out that could have been a problem. Reinstalled and temps still got up to 135 during idle. Do see some bubbles entering manifold (see video). The other manifold stays around 96-100 during idle. Didn't take it out to get up on plane. Anything else I can check?
 

Lou C

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OK in that vid do you have the clear hoses on both manifolds or just the one on the starboard side of that engine? If both have the clear hose do you see bubbles on both sides? Because bubbles in that hose can be an indication of exhaust gas getting in to the cooling water flow which usually means a blown or leaky head gasket. However, was the boat on land running on the water hose or in the water for that vid? Reason why I ask is because in the water if you see bubbles there, it points to the head gasket leaking exhaust gas into cooling water. On land this test can be inaccurate because when you run on the water muffs you can pull in a bit of air even when nothing is wrong. Any clear hose test is really only accurate with the boat in the water.
Beyond this, yours is a '98 correct? Volvo Penta used to use the same exact flappers that OMC used on their engines from '94-99. In '99 Volvo stopped using them because they can melt if you overheat or just the exhaust overheats and it can drop down and block the exhaust. When you pulled the elbow did you see a flapper at the top of the Y pipe? Was the little metal rod still there if there was no flapper? If so it could have dropped down and blocked the exhaust. Volvo put out a service bulletin that they felt these style of flappers were not effective at preventing water intrusion and should be removed. So that is something else to check.

Did you pull off the thermostat housing and check it for impeller bits? I would remove it test the thermostat (should open at 160) and make sure all the ports are open, they do start to neck down due to corrosion and any impeller bits if stuck in there will restrict flow. It would be ironic if all this trouble was caused by a bit of the impeller stuck right above the thermostat in front of the starboard side water nipple of the stat housing. This you would never see unless you pulled that rubber ring that holds the stat and removed it. Get a coat hanger wire and clean out the ports including the bypass in my diagram above. As far as pieces getting caught you are more likely to find them in the stat housing than the manifold, remember the flow from the impeller goes to the stat housing first, then to the manifolds (with a cold engine it flows through the little bypass port). If a piece of the old impeller got thru that, it could lodge right above the stat itself. Look at the diagram....
Ever try to run the engine briefly with both manifold hoses disconnected at the manifold and see if you get equal flow out of both?
 

MI CC

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Boat is in the water and I did not have a clear hose on other side. Thought I bought a long enough piece for both but had to keep it very long to avoid kinks so didn't have enough to put on other side. Boat is a 98 and I looked down Y pipe when elbow was removed and did not see anything. The first thing I did was replace the thermostat housing with brand new housing and thermostat so pieces couldn't be in the housing.
Over the winter, I had service replace the u-joints, gimball bearing and bellows on that drive. Could they have done something wrong that is restricting water flow coming into the impeller housing? I swapped impeller housings also which didn't make any difference.
 

saf

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This was exactly my situation last year, i.e. I saw air being sucked into the raw water circuit and one riser was getting significantly hotter than the other. In my case both risers were new and I heard all sorts of theories from the mechanic (the one who installed those risers for me) explaining the temp difference. Theories would range anywhere from "this is normal" to "air bubbles disrupt the flow and somehow direct more water to one side". The majority of other people I spoke with were saying "it has to be blocked flow on one side". Sure enough, they were correct. Turned out one of the new risers had internal defect (I guess). Once I swapped it with another riser (loaned to me by VP shop) temperatures got even. The air bubbles was a separate problem. In my case, I believe, the overheated exhaust resulting from overheated riser burnt the rubber gasket in the outdrive resulting in air leak. The easiest way to verify the blocked riser condition is to swap risers. Although you will need two sets of new gaskets for that. Alternatively, you can try connecting clear hoses to both manifolds and visually compare the water flow (chances are you will see slower flow on the "hot" side).
 

MI CC

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Id rather not go through the work of swapping manifolds if I don't have to. I raised up both outdrives some and I can visually see less exhaust flow coming out of the starboard engine. Was supposed to be going on a 40 minute trip this weekend. Will I be doing damage if the manifold is reaching the 215 temp for 30 minutes? Not sure how long this has been happening before I noticed it was getting hot.
 

Lou C

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Not sure if 215* sustained will cause a problem but keep in mind that you have the rubber hoses that come off the elbows to the 90* exhaust pipes and then to the Y pipe, you do not want to burn those because that will put water in the bilge!
I suggest that you try disconnecting both hoses to the manifolds on that engine have someone start it briefly while you observe flow. If flow on both sides coming out of the hoses is equal, then it must be a restriction on that side, either in the manifold or elbow cooling ports. I wonder if you were to compare the output of the impellers of each engine would be close to equal? Your thermostat housing is new so you should have no problem there.
You asked could the shop have done something to cause the issue with the servicing, the things I can think of are the plastic fitting in the pivot housing (possibly cracked) or if the hose from the pivot housing to the transom mount got kinked?
My Cobra cooling system is similar except for the fact that the impeller is the upper gear housing of the drive instead of engine mounted. The only problems I've had are marine growth in the water intakes (moored in salt water) and eventual corrosion and clogging of the cooling ports in the manifolds/elbows--same cause.
Now a couple of questions:
when you replaced the thermostat housing, how corroded was it inside?
how old are the manifolds and elbows? ever replaced? If so what brand was used? Because some of the aftermarket brands sometimes have problems with casting flash and slag left inside and certainly that could slow down the water volume flow through the manifold and elbow.
Fresh or salt water?
 

MI CC

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Boat has always been in fresh water and risers and manifolds are original. The thermostat housings weren't that bad but since they were original and easy to replace I just bought new ones for both engines. Manifold and riser didn't look bad at all. Risers don't get as hot as exhaust manifold so I don't believe the rubber hose connector will be an issue.
 

MI CC

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Finally swapped manifolds and risers on same engine. Heat stayed on same side but things changed. Whereas the manifold used to get extremely hot right were the water enters the manifold, it no longer does but eventually the riser got hot. The riser got up to 175-180 when on plane. Went down to 135 at idle. Manifold stayed cool. Other three risers staying around 120 on plane. At this point I'm thinking it's a water flow issue at the intake and will have to wait until the boat is pulled unless there are any other suggestions or tests I could try before end of season.
 
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