470 the death of me

[2fishy4u]

So, to no surprise to me the engine is acting up again. mercruiser 3.7lx 470 alpha one gen one. After a long repair on the transom assembly boat finally gets back in water. And to no surprise the heating issue is back from 6 months ago. Last time around I replaced the exhaust manifold, riser, thoroughly cleaned the heat exchanger and flushed the cooling system. It will idle fine and run fine up to 3200rpm. Anything over 3200 and it slowly heats up, like 5 degrees every 5 minutes. Impeller is brand new, and there is plenty of water flow coming out of the riser when I remove the hose. I am completely at a loss as to where to look. And I am sick of the only mechanic in my marina that charges 100 per hour and takes literally 3x the recommended time for a job. He even charges full price to diagnose. I understand the cooling system, but I dont see where the issue could be at this point. Any suggestions will be greatly appreciated.

 

[gm280]

I have absolutely no experience with your type engine. However, have you actually tested the heat sensor to verify it is working properly? It can be measuring, but still be off as it gets warmer. Maybe a good idea is to remove it and verify the raw temperatures it sends to the gauge.

Also verify the gauge while at it. I would do that before changing anything else. Once you know those things are reading correct, then proceed to other possibilities. You could fight this problem forever until you verify you are actually reading the correct readings. JMHO!

 

[2fishy4u]

Thanks Gm, unfortunately I have a infrared thermometer, and the gauge is reading correct temps for where the sensor is at. The port engine blew when I bought this boat because the gauge was reading wrong and it overheating badly. So I replaced all gauges and sending units. And I watch them like a hawk. And check them every week for accuracy.

 

[Bt Doctur]

Ok. sounds familiar. You will need to drop the lower unit and put the muffs on with full water pressure. You will most likely find water leaking from under the water pump housing. The base is on the suction side of the pump,at planing speed the lower housing has exaust gas and no water present.so instead of sucking water its sucking hot exaust

 

[2fishy4u]

Thanks BT, A year and a half ago I had yet another heating issue but on the starboard side. Turned out there was no water coming from the impeller. I did not want to pull the boat so the mechanic put in a high volume electric pump sucking in water through a through hole and pumping through the heat exchanger and out the exhaust. It worked great, and got me through the season. So, I pulled the hose from the impeller and let it pour out the back. And I hooked up the pump instead. It did better than the impeller, I was able to get to 3600 rpms before it would start to gradually heat up. So unfortunately I dont think it is an impeller issue, and I dont think its exhaust as it was just pouring the impeller water out the back.

 

[Scott Danforth]

in addition to what BT Doc mentions about the raw water pump, did you pull the ends of the HX off to clean out the passages? you could have fouling.

also, do you have the larger 4" diameter HX? or the original 3"?

 

[2fishy4u]

Thanks Scott, I do have the 4" heat exchanger. And cleaning it was how I solved it last time i had this issue. But its hard to believe its fouled again 6 months later. The starboard has gone 2 years without cooling issues. But anything is possible. I will pull apart the cooling system, flush the engine side, and swap out the Heat exchanger with the spare.
One last question, could a partial bad head gasket allow exhaust into the coolant without letting any oil or water to mix? AKA no oil in coolant, no coolant in oil? Just exhaust heating the coolant?

 

[Scott Danforth]

Durring deep watter horizon spill, HX's were getting fouled on 5 minute intervals on the service boats. If you run in crud-filled water, you need to clean often. Its HX maintenance 101

 

[2fishy4u]

Ok, so I pulled all hoses and made sure there is no blockage, bends or kinks. I flushed the engine side, pulled the heat exchanger and replaced with the spare. Put everything back together and blah wasted half a day. So, it will idle at 170, its normal temp is 160. going over 1500 rpms gets the temp back down to 160, but at 3200 rpms the gradual increase starts. I shut down once it hits 180. So, should I assume somehow exhaust gas is getting into the coolant? And if so, what tool do I need to purchase to figure out if exhaust gas is present in the coolant? And if it is I am looking at a cracked block or Head gasket? It ran perfect before going into the yard, So I was hoping for a kinked hose or something from when we put the engine back in. But of course its never that simple for this beast.

 

[2fishy4u]

Exhaust manifold leaking exhaust into coolant, at least I'm starting to figure things out on my own. Thanks everyone for the advice as always. Time to get back on the ocean and enjoying this awesome hobby! And this crappy boat lol