overheated exhaust hoses

[jrwolfe]

I have a B28 with 260 hp Mercruiser inboards with closed cooling system. I had the impellor replace because of a failure with the present one. I have cleared the transmission cooler of broken impellor parts and have checked the heat exchanger for blockage. It was clean. The problem is at low rpm, 1000 rpm or less, the exhaust hoses overheat to the point you can't leave you hand on them. This first happened when we changed the impellor. We checked the water pump and found the new impellor had very flexible paddles or veins and replaced the new one with stiffer paddles. No help with the problem. At higher rpm, the hoses are properly cool to the touch. It seems the problem is lack of cooling water pressure at low rpm. Both exhaust hoses are hot so I think that should eliminate a possible clogged exhaust riser. If it were one side I would blame the riser. I run in fresh water so there is very low problem with rusted elbows. This is happening on the starboard engine. Any ideas on what to check next would be appreciated.

[Harry Babb]

When I had gas engines they were setup so that an engine oil cooler and a transmission oil cooler was installed inline on the SUCTION side of the raw water pump.

I started experiencing minor overheating problems........I found Grass (from the guys yard across the river) matted up on the water side of the oil cooler.......restricting water flow

Yours may not be setup this way but may be worth a thought.....any restriction is as bad as a blockage on the pressure side......I have even heard of hoses delaminating on the inside and restricting water flow.

On the same engine.......I found a rubber elbow that was collasping due to high suction......because a spiral spring on the inside (designed to prevent collasping) was rusted away and not supporting the hose properly.

My first thoughts when reading your post was that exhaust elbows were partically plugged.....but you addressed that

good luck

hb

[PaulJ]

My guess would be that the risers have some clogging at the exit side/port before you get to the hoses. Depending on how old your risers are, you might consider replacing them. To test this theory though, remove the exahaust hoses from the risers. Carefully, using a small thin flexible metal instrument (I use a hacksaw blade) gently insert the blade inbetween the inner exhaust tube and the outer, cast iron shell of the riser. Move the "blade" in and out (not so as to cut anything - do not use undo pressure if using a saw blade) and see if you can pull out any debries. Small pieces of the rubber impeller, coupled with rust can create blockages in the riser chamber right before the water/exhaust mix hits the hoses. If you have a small blockage, at low rpms (and thus low water pressure at the exit) you will develop hot spots on the hoses.... ask me how I know.

Paul J
1978 B28 - PALADIN
GM 350 V8 (260hp) FWC 4bbl Carb, Velvet Drive 1.52:1.00

[Carl]

Id look at a few things.

You checked to see that all pieces of the old impeller were located?



Risers can have restrictors cast into lower side to push water out the topside of riser at low speeds. If the restrictor rusts out water may only be flowing out the bottom. Is bottom of hose cool?

Top of riser is normally wide open...but rust/deposits can block flow.

Lack of flow can starve risers at low speeds.



Having two motors with no issues on one gives you a baseline for flow at each point. First, I usually feel for heat and pressure differences at each spot by touching a squeezing hoses. (Crunchy anywhere tells me a change is long overdue as are soft spots). If i cannot find a difference... I start disconnecting hoses at various locations and match flow and pressure till i see a difference for very short periods of time so as not to overheat anything. This usually narrows the problem down for me.

How old are risers? Fresh water may not have salt corrosion, but has mineral deposits that block flow.

As you worked on your pump...need to make sure you are not sucking air anyplace. O-ring, gasket sat correct, cover snug? Suction hose in good condition and tight?

Good luck....

[jrwolfe]

Thanks for all the responses. I do have the problem of hot spots on top of the hoses and cool bottoms. I thought that was just due to weak water flow. I will go back and check for any blockage and rust restriction. Thanks again.

[Rawleigh]

When an impeller goes bad you should fit all of the pieces back together to make sure that you got all of them.

[PaulJ]

What Rawleigh said!

FWIW... I made my own back-flush tool for my closed cooling system. It is made of a PVC ball valve connected to a female screw type hose fitting on one side and another fitting made to connect to the raw water exit of my heat exchanger. Disconnect the exit side of the raw water pump and I can back-wash the whole cooling system. I usually put the bitter end of the disconnected hose from the raw water pump in a bucket and see what comes out. Turn the ball valve on and off as I screen out what dumped in the bucket and keep going until clean. Usually get some small pieces of shell or some grass that made it through the sea strainer but cant find its way through the various holes in the oil coolers/heat exchangers. I usually do this once a season just to be on the safe side.

Paul J
1973 B28 - PALADIN

[Bob H.]

Jr, I had a similar problem and found shellfish growing in my oil cooler, sucked up mud from shallow slip and damn things grew, I would back flush the cooling system and see what you push out. BH

[bertram 20]

jr wolfe i to am having a problem on my b28 my port motor exhaust hoses are hot also, over the winter replaced old cast iron risers with the mercruiser stainless risers replaced all exhaust hoses and replaced steel y pipe and elbow with fiberglass ones.I also replaced my raw water pump, I did find out that i did not have enough water pressure at my house to really run engines so i filled a 55 gallon barrel up and the put hose in there and after a while barrell would go empty so i trailered boat down to boat ramp and backed boat into the water and ran engines starb motor no problem , port motor running and risers are cold so cold they were building moisture on top of them but exhaust hoses getting pretty hot. I thought i had exhaust hoses on perfect but when i took y pipe off and looked up at riser the hose was blocking water flow coming out of the riser so i re adjusted them and thought that was the problem but they still are running pretty hot i also can not seem to figure out what is going on i am leaning towards a riser issue even though they are stainless and clean as can be inside.

[jrwolfe]

I have people scratching their heads with this problem. I have checked the water flow from pump to trans cooler with a flow meter. also checked the flow from pump through the trans cooler to the heat exchanger with the flow meter and no change. The heat exchanger is clear from visual inspection. I have replaced the exhaust elbow with a used one I had and when we installed it, it worked perfect. The hoses were cool. Unfortunately, we had a leak between the elbow and the manifold. The next step was to remove the elbow and place a new gasket between the elbow and the manifold which we did. It was the proper gasket and once put into place it fixed our leak. Unfortunately, when we fired up the engine, after a few minutes we had our hot exhaust back. I can't imagine what we would have upset by just replacing the gasket. Maybe a new elbow will be the final answer. Its hard to understand why it worked and a change of the gasket put us back into a problem

[bertram 20]

did you check the exhaust hose were it connects to riser because what appeared to be on correctly on mine was not, when i looked up the exhaust hose with a flashlight i could see i was blocking water flow so i loosened them up and realigned them but it still did not help me but it may help you i even changed hose coming out of heat exchanger to the (T) and made it straight because i did not like the factory hose that bends down and runs along the valve cover because i thought there may be a flow issue there.still did not help me keep me posted thanks mike

[jrwolfe]

I have decided to replace both elbows. They have to be the problem. The question now is what to get. The prices for replacements range from the following brands: OSCO--$ 129, Barr-- $ 170, Mercruiser $ 250 & up. All are replacement with the proper parts number. Has anyone had experience with the aftermarket OSCO or BARR elbows of manifolds? I am tempted to go with the Barr products.

[Tony Meola]

I have not used them, but friends of mine have and they have held up well for them.

[Tooeez]

I have replaced the risers on my 28 several times over the last 20 years with the Barr risers. They are exact Mercruiser copies, and I get about 8 years out of a set before the salt water does them in. You can tell when it's time for new ones when they start to get hot.
That leads me to ask: is anything else on the motor heating up? If a restriction is keeping the water from exiting the riser at low speed, then it should be restricting the flow back all through the system--risers and heat exchanger, eventually causing the fresh water side to heat up. If that's not happening the water must be flowing through the system somehow. Have you compared the amount of water coming out of the exhaust pipes? If it's the same on both sides then it has to be that the flow is not being directed properly as it exits the riser.

[Rocket]

did you also change a gasket before the overheating problem first showed up? I had the experience of using the right gaskets , but one of the cooling holes in the bock to the head was either covered or uncovered (I can't remember which) and it caused my haustoria to overheat. fortunately I had saved the original failed part and we were able to overlay them to see what was different despite having "the right part".

[bertram 20]

I changed my risers today with a almost new set of mercruiser risers the 4inch hoses still get extremely hot on port engine, stab engine runs ok this is all going on in my driveway on the trailer I did find out that my water pressure from house is not enough to run them so I filled a 40 gallon barrel on deck and connected hose to raw water pump pick up and left water hose in bucket running also, my starb motor runs cool but port still runs hot is it possible I still don't have enough water to cool and if that's the case why wouldn't starb motor heat up like port engine any ideas ?

[jrwolfe]

With my problem, I wanted to check the raw water pump on the engine with the hot hoses to be sure it was pumping enough water. I re-plumbed the pump on the engine with the hot hoses to pump to the cool hoses to see if they remained cool which would tell me the pump was good. At the same time I re-plumbed the other pump to the hot hoses to see if there was any difference. As it turned out, the pumps were both good and it eliminated one variable. the other test I did was to by- pass the transmission cooler by running the hose from the water pump directly to the heat exchanger. If the hoses ran cool, it would tell you that the transmission cooler was partially blocked. Hope this helps.

[PaulJ]

Best tool I ever purchased to chase down cooling problems.... http://www.homedepot.com/p/Ryobi-Non-Co ... /100674438
I shoot temps on each part throughout the cooling system, including sending units, intake manifold, exhaust manifolds, risers, heat exchangers, hoses, etc. It will pin-point the temp differences. You can use your port/stbd engines as guides and mark down temp diffs between similar spots on each. Any area giving you more than 10 deg F diff between port/stbd is suspect.

My measurements at running temp (165 - 175 deg F) are (from memory)...
Heat Exchanger - 165-170
Intake-Head bolts - 180-185
Exhaust Manifolds - 175-180
Temp Sending Unit - 170-175
Therm Housing - 165-170
Risers - 98-109
Exhaust hoses at riser ends - 120-130

I use the IR gun to calibrate my electronic temp gauges. I've also installed mechanical guages for belt and suspenders.

Note... I bought the gun when I noticed hot spots on my exhaust hoses... temp was 325 deg. Solved by replacing clogged risers.

Paul J
1973 B28 - PALADIN

[jrwolfe]

Paul, Great idea. I am placing my order for the thermometer tonght. I agree it is important to get the base temps from all the components. Thanks for the idea. JW

[jrwolfe]

New elbows, problem fixed. I appreciate all the input and ideas. Thanks, JW