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wofjsr

Monsson 320 Exhaust Manifold getting really hot

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wofjsr

Launch boat for this season and everything seemed fine, after I got up to about 30 MPH for about 1 minute I shut it down to open motor box. I noticed that exhaust manifold was smoking. On each side the center exhaust port had burn't away the grey paint form exhaust manifold.

So I started taking off the impeller and replaced it, it had some damage. I unhooked the lines going into Oil cooler (I think) and it was cloged with weeds. Assume I had sucked up lots of junk when I launched it. Cleaned out everything I could and it still seems to be getting hot.

Any clues? or additional things I can do to diagnose?

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NorCaliBu

Since your tranny cooler was clogged with weeds you obviously sucked up a bunch. Check the whole intake line starting at the external water intake scoop on the bottom of the hull. Weeds could get hung up at the intake shutoff ball valve (if you have one) and also on the water pump itself. I would also change out the thermostat on the engine block because they don't like to be overheated. No.gif Good luck, keep us posted.

oh and Welcome.gif

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wofjsr
Since your tranny cooler was clogged with weeds you obviously sucked up a bunch. Check the whole intake line starting at the external water intake scoop on the bottom of the hull. Weeds could get hung up at the intake shutoff ball valve (if you have one) and also on the water pump itself. I would also change out the thermostat on the engine block because they don't like to be overheated. No.gif Good luck, keep us posted.

oh and Welcome.gif

Thanks for the clues.

Today I took out thermostat put it in cup of boiling water and it opened up about 3/8 inch. Yesterday I cleaned out the intake scoop and I am sucking water pretty good through new impellar. No intake shutoff valve. When I get a chance to get to the store to get a new thermostat I will change it out.

Does anyone know if can just take the thermostat out for a quick run just to see if the exhaust stays cool?

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NorCaliBu
Does anyone know if can just take the thermostat out for a quick run just to see if the exhaust stays cool?

You can but the engine won't get up to proper operating temp so it may not run very well. It would tell you if that was your problem though.

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mrothwell
Does anyone know if can just take the thermostat out for a quick run just to see if the exhaust stays cool?

You can but the engine won't get up to proper operating temp so it may not run very well. It would tell you if that was your problem though.

I thought that the manifolds had a bypass that did not go thru the t-stat, I dont recall for sure.

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vette-ski
I thought that the manifolds had a bypass that did not go thru the t-stat, I dont recall for sure.

Correct. Water from the raw pump goes up to the stat housing and then down into the engine circulating pump. At cold start when the stat is still closed, the circulating pump really isn't doing anything so all of the water bypasses to the exhaust manifold and out the exhaust. When the engine warms up and the stat opens, the circulating pump starts pushing water through the motor and out the t-stat where it then goes to the manifolds like the original bypass water did.

Short answer you can take the stat out for a quick cruise without hurting anything, but I don't think that is your problem because you should have water through the manifolds and out the exhaust even with it closed. It's either your impeller (which you replaced) or a clog somewhere.

Did your engine temp go up too, or just hot manifolds?

Edited by vette-ski

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wofjsr

My temp gauge never gets above 140 degrees before I start seeing the paint burn off my manifolds. I don't think I even get the engine up to 160 to open the thermostat.

After doing some thinking I have settled on the following diagnosis. Comments or suggestions would be welcomed!

1. I think my Temperature sensor back to the ECM is bad.

The ECM thinks the engine is cold since the sensor is bad and keeps running the mixture rich to try and warm it up. This extra fuel causes the cylinders to get hot and the water coming through the bypass is not enough to cool the manifolds. This also may explain why the engine is not running great when it gets warmed up.

As to why it's only the center manifolds? I think two cylinders per side exhaust into the center manifold port and the outer most manifold ports only exhaust 1 cylinder. Two cylinders = twice the heat and build up at a faster rate. If I had the guts to run it longer I am sure the outer ports would burn the paint off, they were already starting to get discolored.

I took the sensor out and measured resistance, it was open. I warmed it up with hot water and also flame and still open.

So seeing how the sensor is an indmar specific part, not a generic automotive part I had to have my delear which is 2 hours away order one and was not in stock. Based I prior experience, I am sure the first one they order will be the wrong one.

If things go well I should be skiing by June 7th which is basically 1 month later than I would have liked to.

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jgouveia3

did you try pulling the risers to see if there is any blockage in the passages? one of my manifolds (starboard) was hotter than the other for a bit, and a couple days later blew a hole through the riser. when i replaced it, the riser was almost completely blocked (i am in salt, so i expected it - this season i did new risers and manifolds (although now that I have it all apart, the manifolds where actually still in good shape)).

Edited by jgouveia3

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vette-ski

I personally can't see it being the sensor that is the problem. It could be broke, but the system is designed to operate correctly when it IS cold. That's what happens every time you start up your motor when everything is at ambient. So if the ECM thought the motor were cold and making adjustments, it would just be running like a cold start situation, which shouldn't be enough of a difference to overheat manifolds. Basic assumptions here are that you just didn't re-paint the manifolds or buy new ones and that when this is happening they really do feel hot to the touch. Paint burning off is pretty common on the bottom sides right where they bolt to the motor (around the spark plugs). I doubt you will find any manifolds out there that have had any use at all that are still fully painted on the bottom.

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electricjohn

Are you sure you removed the right water temperature sensor. The one for the ECM has three wires plugged into it. The ECM would also give you a check engine light or audable signal if the sensor failed like you described. If it is the three wire sensor, you can get a replacement at an auto parts store. Remember, it is just a marine-ized car engine. Even the ECM is made by the same peoplpe who make the ECMs for GM cars. If the sensor did indeed fail, the ECM would default the sensor reading to 32* F which would make your engine run rich, but a rich burn is actually cooler than a normal or lean burn. I would still look for the root cause of the hot risers. See if you can run water thru them with a garden hose or something. One more thing to ponder, clogged fuel injectors or fuel filters could cause a lean burn situation which creates high exhaust gas temperatures.

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wofjsr

jgouveia3

Thanks I didn't try that yet because I didn't have access to new gaskets. Will buy some and take apart.

I did run a garden hose through bypass hoses and it didn't back up and drained through transom exhaust.

vette-ski

Manifolds are stock original since I bought my boat 12 years ago. I thought for a moment that they just started to corrode and the paint broke down, but the center ports do get very hot to touch after a while. The paint has burnded off all the way up to the gasket between the riser and manifold.

electricjohn

The 96 monsoon with Multi port has two single post sensors, one for the guage in the intake manifold and one smaller 3/8" single pole in the thermostat housing. Checked wiring on one in intake and it ran back to temp guage. Assumed the other smaller one in thermostat housing ran back to ECM.

Also I am not aware of a check engine light on the 96 malibus with Monsoon multiport injection. Agree it would be a good idea, maybe the output driver is already in the ECM without a wire and light. Good idea on fuel filter, injectors I hope not.

Thanks again and I agree the temp sensor doesn't sound like the root cause.

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footnlongline
My temp gauge never gets above 140 degrees before I start seeing the paint burn off my manifolds. I don't think I even get the engine up to 160 to open the thermostat.

After doing some thinking I have settled on the following diagnosis. Comments or suggestions would be welcomed!

1. I think my Temperature sensor back to the ECM is bad.

The ECM thinks the engine is cold since the sensor is bad and keeps running the mixture rich to try and warm it up. This extra fuel causes the cylinders to get hot and the water coming through the bypass is not enough to cool the manifolds. This also may explain why the engine is not running great when it gets warmed up.

As to why it's only the center manifolds? I think two cylinders per side exhaust into the center manifold port and the outer most manifold ports only exhaust 1 cylinder. Two cylinders = twice the heat and build up at a faster rate. If I had the guts to run it longer I am sure the outer ports would burn the paint off, they were already starting to get discolored.

I took the sensor out and measured resistance, it was open. I warmed it up with hot water and also flame and still open.

So seeing how the sensor is an indmar specific part, not a generic automotive part I had to have my delear which is 2 hours away order one and was not in stock. Based I prior experience, I am sure the first one they order will be the wrong one.

If things go well I should be skiing by June 7th which is basically 1 month later than I would have liked to.

I don't think its ECM related. Use a fake a lake and run boat on trailer to check water flow from exhaust. BTW Running lean produces more heat than running rich. I bet you have some cooling passage blockage. idg. edg adg. odg.

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