1998 Mercruiser 5.7 Header hot to touch

[WarRod61]

I have a 1998 Bayliner with 5.7 Ltr. carb Mercruiser I/O. The starboard exhaust manifold riser is too hot to leave your hand on it. The port riser is warm but not too hot and you can leave your hand on it. I replaced both exhaust manifolds in the past but did not replace the risers but I did check them for obstructions. Not long after, I found the raw water pump impeller was deterioated and replaced the impeller and removed all of the impeller pieces from the thermostat housing. I also replaced the thermostat. When I ran the boat in the water. The water temp. is 175 degrees Farenheight and the engine is running great but again the starboard exhaust manifold riser is too hot to leave your hand on it and the port is warm but you can leave your hand on it. During fresh water flush out at home using the water hose and engine at normal operating temperature the water coming out the port side is cool to the touch but the starboaard side water is very warm but not too hot that it would burn you. Question: Do I have a problem or am I worrying for nothing?

 

[Bondo]

Re: 1998 Mercruiser 5.7 Header hot to touch

Question: Do I have a problem

Nope,... Not that I can see....

 

[jtybt]

Re: 1998 Mercruiser 5.7 Header hot to touch

Pull the hot riser and check for rust scale build-up in the water passages.

 

[WarRod61]

Re: 1998 Mercruiser 5.7 Header hot to touch

I will pull the hot riser and elbow to check the passages. If my memory serves me the elbow looked pretty rusted up (internally), I used screwdrivers and steel rods to attempt cleaning out the passages but that has been last year. Since I have already replaced both manifolds and both risers are clear and the only problem that I am having is the starboard side, do you think I should just go ahead and replace the elbow? WarRod61 Thanks for you info.

 

[WarRod61]

Re: 1998 Mercruiser 5.7 Header hot to touch

I have been reading some of the other postings about 5.7 Mercruisers with exhaust manifolds hotter on one side. I did have an impeller that was completely dissintergrated and to the best of my knowledge I retreived most all of the pieces. What I have been reading indicates that I may have some impeller pieces that may have been pushed into the manifold or exhaust elbow after the new impeller was replaced. I believe the best approach is to pull the riser first then the exhaust manifold and examine for debris and if I do not find any debris then replace the starboard elbow. Please advise!

 

[bruceb58]

Re: 1998 Mercruiser 5.7 Header hot to touch

I have always found that risers need to be replaced more often than manifolds...usually 2 risers to every manifold. Any reason why you didn't change the riser when you changed the manifold? Personally, I would just replace both risers and be done with it.

 

[RoyR]

Re: 1998 Mercruiser 5.7 Header hot to touch

I have exactly the same problem on my 5.0l V8....the starboard side is hot enough to cause the water to steam, even though the boat temp is reading just below 175F.

Removed the risers earlier this year and everything looked good. Had the impeller replaced last week as preventative maintenance.

Went out this weekend and noticed steam? I think I noticed the starboard side being hotter before, but there wasn't such a big temperature difference. I lowered the stern drive, while on the trailer, into the water and both sides was flowing equally strong???

Going to try and figure out what is going on this weekend.

PS. Wish they made these hoses transparent....

 

[bruceb58]

Re: 1998 Mercruiser 5.7 Header hot to touch

You also want to pull the thermostat housing and check for rust buildup in there. Have found the outlets that go to the manifolds/risers blocked up and prevented full water flow to one side.

 

[NHGuy]

Re: 1998 Mercruiser 5.7 Header hot to touch

Correct me if I'm wrong but if they are same you might try swapping and rechecking temps after that.
You might also check for stuck or broken shutter valves too.

 

[RoyR]

Re: 1998 Mercruiser 5.7 Header hot to touch

Correct me if I'm wrong but if they are same you might try swapping and rechecking temps after that.
You might also check for stuck or broken shutter valves too.

Swapping what?

 

[Maclin]

Re: 1998 Mercruiser 5.7 Header hot to touch

I think you need to aim an infrared temp gun at it and get a real temperature. There may not end up being much difference between the temp your hand can stand and too hot to touch.

 

[WarRod61]

Re: 1998 Mercruiser 5.7 Header hot to touch

Thanks to all for the good inputs but one in particular BruceB58, when you say replace risers, do you mean elbows because the riser are only three inches high and I can visually see that they are clear with no obstructions but the elbows are difficult to see through and are pretty rusted? One other thing is my shutter valves are missing could they be down in the wye duct just before they port out through the transom? Do I really need the shutter valves because it has been working fine without them? WarRod61

 

[bruceb58]

Re: 1998 Mercruiser 5.7 Header hot to touch

Risers = Elbows

I guess those spacer things are called spacers!

If you don't have the shutters, you definitely need to look to see if they have fallen down.

 

[WarRod61]

Re: 1998 Mercruiser 5.7 Header hot to touch

Thanks BruceB58 and everyone else. I have a lot of work to do but I do feel I have a problem. I don't believe it is too serious but I don't want to take a chance and damage my engine or get stranded out on the water. I will get back in about a week and let you know what I find. Again thanks for the inputs to help me resolve this problem. I am going to look for shutter valve parts. I know the thermostat is new and the thermostat housing is clear. the hose going to the starboard exhaust manifold is clear. I will disassemble to starboard riser and exhaust elbow and inspect for obstructions. Last resort, I will replace the risers and exhaust elbows. WarRod61

 

[Dshow]

Re: 1998 Mercruiser 5.7 Header hot to touch

I have a 2001 5.7L that I have posted regarding steam out the starboard side thru-hull exhaust. I have had this for 3 seasons now and (knock on wood) it hasn't caused me a problem. I had the thermostat housing off last winter and it was clean. Temp gage always reads ~170F. One thing I've notice is the starboard hose going to the exhaust manifold is warmer than the port side, so it start before the exhaust manifold. Since both the starboard and port hose are fed from a single point (thermostat) more hot water must be diverted to the starboard side.

I like the switch sides test. I'd like to find some "U" shaped tubes that I could connect to the thermostat housing to route the starboard output to the port side exhaust manifold and vice versa.

 

[Maclin]

Re: 1998 Mercruiser 5.7 Header hot to touch

During fresh water flush out at home using the water hose and engine at normal operating temperature the water coming out the port side is cool to the touch but the starboaard side water is very warm but not too hot that it would burn you.

How can you differentiate between the port and starboard water exiting? Do you have thru hulls?

I would get an IR gun on it first just to be sure the temp is out of bounds.

 

[RoyR]

Re: 1998 Mercruiser 5.7 Header hot to touch

I will do the IR gun this weekend and report back!

 

[RoyR]

Re: 1998 Mercruiser 5.7 Header hot to touch

Met a new mobile boat mechanic that is reasonable and seems to know what he's doing. Invited him to come over to have a look, dropped the stern into a bucket of water and ran the motor for 10 minutes with no sign of over-heating....WTF? Next time I go out, I will take the IR temps, just so that we have them for reference.

 

[dudetek]

Re: 1998 Mercruiser 5.7 Header hot to touch

I think you are all wrong. I have a 2001 Checkmate with 5.7 EFI 260 HP. It always steamed out of both the pipes since I bought it in 2008. I recently had the impeller replaced. The impeller never broke, but it had "set" to it. Yesterday I was out on Lake Michigan running hard. Stopped the boat and noticed steam only coming out of one pipe. I called Mercury, and the tech told me it is by design that the Starboard manifold is 30 - 40 degrees hotter than the Port. That makes Starboard warm to the touch while the Port is cool. My temp never went over the midpoint on my gauge. It is just normal. Sometimes it just pays to make it simple and call those that design the thing.

 

[bruceb58]

Re: 1998 Mercruiser 5.7 Header hot to touch

Dudetek,

The OP hasn't been here since last September when this thread was active.