brand new 350 mag mpi getting hot at idle

rockslut

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Jul 19, 2009
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Ok guys i will try to explain from the beginning. Lost the 5.0 last Sept 30 mile off of Fort Bragg tuna fishing. manifolds decided to rust through. Always fan very well for a 2000 hour saltwater engine
In April we bought a new bobtail 350 mag mpi..I installed it myself, ,also went with a full closed water cooling system. everything went smooth. first shake down run took the boat to a local reservoir. (Oh ya the boat, its the 2001 Bayliner trophy 23' lives on a trailer very clean boat ) So engine seemed to run real good for first 15 min wile checking for leeks ect.. i asked it to get up on plain. Plenty of power Jumped up out of the hole couldn't of slapped the smile off of my face. temps climbed quick with in 2 min alarm was singing, So i backed it down to a idle and temp came down quickly to about 175deg (dash gauges) i forgot the one tool that i really needed the temp gun. I did that about 5 times not letting it get to 190-195 before i would let it cool, hoping maybe a air bubble needed to be worked out. Very discouraged i put it back on the trailer.
So I was running the original alpha 1 gen 2 with a brand new impeller kit in it. that out drive never had any problems cooling the old over worked 5.0. One thing i noticed when installing the new impeller. there was some erosion under the impeller plate, there is a small aluminum wall that separates the water from the exhaust over the 20 years in service salt water has made a gape to were the impeller lower plate would not seal the air out. hard to explain I felt that i was getting air sucked into the cooling system at higher rpm and that was not allowing to cool at speed. So being a 95% offshore boat i felt i didn't want to jimmy rig it with jb weld or epoxy. So i bought a brand new mercury out drive. The new out drive i ordered with the recommended gear ratio for the new engine of 1.47 the old one was 1.62 So i was expecting posable prop change.
This morning set out to another near by lake to test. Engine fires up like it should, I let it warm up a bit wile idling away from the dock. then i hammer down to see what happens. First thing i notice was the lack of power getting up on plain, i kinda figured with the higher gears it wouldn't be as peppy. So heading across the lake at 33mph and boat temp was a consistent 175deg. ran it for 10 -15 min. Im thinking fixed, not so fast. Backed it down and was planning to do some trolling. The wife wanted to know how slow we were going to troll since the Kokanee like it slow. The temp quickly ran up to 185deg 195 and the little alarm started singing again. So we ran around the lake for another 1/2 hour temp was steady 175 when running. But quickly rose when under 1500rpm. I did bring the temp gun this time and everything seemed normal. Manifolds were never over about 110deg. Checked every hose and all seemed to just verify what my dash gauges were. telling me. When I got home i ran it on the ears and with a high flow unregulated hose spicket The temp again rose to 190 untill i bumped up the rpm to about 1500 to cool it back down.

So thats my story, sorry for the long read !

I have checked and double checked the routing of the cooling system. When the outdrive was off i had my Daughter put a hose in the gimble housing water inlet and checked flow buy taking hoses off. Nothing was clogged, unrestricted flow throughout. When running the new outdrive in the driveway there was plenty of water flow exiting out.

I feel like i have thrown a lot of good parts at this boat and it keeps kicking me in the butt.
Any suggestions would be much appreciated.

For you prop guys.... im running a 14.5 -17 four blade its getting me about 3800rpm at WOT 33mph. I need about 1200 more rpm to get to 5000 were mercury suggests. Top speed is not as important to me 30-35mph is fine. would rather have the mid range power for easy hole shots when weighted down with big fat guys and lots of ice plus better control in rough sea..
 

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tpenfield

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I can't see anything wrong from the picture you posted :D

You say it is a FULL closed cooling system (right)? What brand of system did you get? Usually they come as a kit for a specific engine.

Have you checked the coolant level?

Did you have to add any coolant?

Can you post pictures of the heat exchanger (HE) and all the hose routings?

Did a new thermostat come with the HE kit?

Usually an Alpha 1 water pump does not have enough 'power' to effectively cool a closed cooling system. You might have to convert to an engine mounted sea water pump (what the Bravo outdrives have). But let's rule out the other stuff first
 

rockslut

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Jul 19, 2009
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Thanks for the reply.
Yes coolant is full and overflow tank is at half
yes came with thermostat and i have already pulled it and verified that it is working properly.
the kit is a monitor full fresh water cooling kit. I called them about the Alpha 1 cooling capacity. The tech insured me that the kit was designed to work with the apha 1 pump. I had read the same and asked before i ordered it.
 

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rockslut

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Its hard to get my head wrapped around the fact that all i did was change the out drive and the symptoms changed from high speed over heating to low speed now.
Would u think that trolling speed heating maybe coming from wrong prop/gearing. Comparing to a car with standard tranny idling around in 3rd gear. But then again it gets warm just sitting in neutral idling.
 

Rick Stephens

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I'm not sure how you prop down far enough from a 17 inch to gain 1200 RPM. Michigan Wheel makes a 13". You might be headed there.

I've been a kokanee addict for years. Tend to fish at 1.2 to 1.6 MPH. Once last year the fish seems to want to bite at a race car 1.7 MPH. I tried trolling plates. I tried gas powered kickers - which will do the job and I wouldn't be without if I went on the big blue. But gas motors are noisy and have to run 100 foot plus lines to really accommodate the fish moving away from the sound. I just installed an electric bow mount motor with GPS to make it easier to control. I couldn't have ever done kokanee fishing successfully with just the sterndrive.

I've not ever run closed cooling.... so I'm not a source for help there. From what I have read though, you may need to run a second fresh water pump.
 

Searay205

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May 27, 2018
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2000 hour why go freshwater cooling? You had exceptional success in running what you had. Remove the freshwater cooling system and see what happens.
 

Searay205

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At idle (600 rpm) my seawater pump puts out about 1.6 to 2 psig at 4000 rpm about 13 psig. Alpha 1 gen 2 saltwater cooled. My guess hopefully i am wrong is not enough water pressure at idle to overcome line losses.
 

Lou C

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Did you bleed all the air out of the antifreeze side of the system? There are a lot of places air can wind up given that you have a feed and return line for each manifold. I would pick up a Lisle radiator bleeder funnel and run it on the water hose till all the air is out of the H/E. For example when filling the manifolds you'd want to fill from the bottom up to push out all the air.
 

rockslut

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Thanks Lou that could help i really could have some air in there still.

Sesray 205 i was just thinking the same thing today. I was just hoping to protect this brand new engine. But if i can get another 2000 hours out of this one i dont care whats running through it.
 

Searay205

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I have to agree with lou C. you may still have a ton of air in there. Is there a fitting on the intake manifold? temperature sensor? When i used to fill car engines and one was available i would remove and it would help tremendously in getting the air out. I assume you have an overflow tank? get it hot and carefully burp into overflow tank. rinse and repeat. even my polaris highlifter took 3 or 4 times to burp the air out and it held about 3 quarts of coolant. finally got air out and no more overheating.
 

rockslut

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Im going to grab one of those funnels tonight on the way home. Last night i checked water level and added about 24oz. So there was some air in there. Kind of hoping there is more this would be an easy fix. thanks again for the reply's. sometimes i over look the simple things. fingers crossed !!
 

Searay205

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You get an air bubble and circulating pump on engine can't move water forward you are overheating. I was watching diesel brothers the other night. they were 100 miles into trip started to overheat. guy pops radiator cap a fountain of water blew out and then a huge amount of air. filled with water never overheated again. obviously you don't want to do that with antifreeze but i was shocked at the amount of air that blew out. 3 seconds of steam after the water shot 15 feet in air, lol.

Curious did you have 2000 hours on original manifolds?
 

rockslut

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mandmj, i may invest it one of those. trying to decide between that and the big yellow funnel kit

searay205, no i bought the boat with around 1000 hours and it had brand new manifolds. so i did get about 1000 hours out of those. We use saltaway after every single outing. thats funny about the diesel brothers episode. i just watched it last night, makes since. I just picked up a vesselview device so i will be able to see more accuracy my engines data temp,RPM ect...
 

Searay205

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I ran across a guy the worked for Mercruiser, He said the key with manifolds and salt water is 140F. Under 140F the salt won't crystallize and attach to iron. My old 3.0 liter had a 140F thermostat. I guess due to fuel efficiency they changed thermostat to 160F which is above the recommended temperature. I thought I would change my thermostat to 140 on my current engine but it would throw a low temp alarm according the diagnostic manual. 1000 hrs on manifold holy heck!!! I alway thought saltaway was bs, maybe not......
 

QBhoy

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Few things in my mind about this one. Apart from the cost of doing all that...but that’s another story.
my main thought just now is that I am pretty sure the temp a fresh water cooled mpi runs at and thermostat set up is different from a raw water. The raw water runs much different temp than closed.
I take it the ecu was changed for the new engine too ?
 

rockslut

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QBhoy old engine was carbureted no ecu. fresh water kit came with its own thermostat, I think it was a 160deg or165 and i just went and checked the one that came in the engine is a 160
 

Searay205

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I pulled my file at idle (600rpm) my water pressure is 3.2 pig not 2.2psig as I said earlier. At 2500rpm i get 7.8 psig. You have plenty of water pressure at idle to get through your exchanger. Get the tallest funnel you can, i have one must be 18" and hold it tight start engine and let level come up as high as it wants. I bet you see some good burps
 

rockslut

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Ok Searay i picked up one of those big funnels made for burping cooling systems. I also picked up a Vesselview so now i have accurate gauges on my phone. First the funnel worked great a ton of air bubbles came through, engine probably ate 32oz more coolant. The new vesselview was easy to set up and proved to me that my 20 year old gauges were not accurate. The temp was off by quite a bit. Almost 12deg once warmed up. Engine idled at around 160-165 for 10 min.
Taking it out sat morning with a new prop and hoping everthing go;s well i need to kill some fish !!
 

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Searay205

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I love it!!!!!!!!!!!!!!!! You may still have air but hopefully normal cool hot cycles will burp out through expansion tank. Funny about your gauges. My gauges led me to believe mine was borderline hot. Then I hooked up Rinda and determined they were borderline low (February) . You would think they would calibrate the gauges so operating temp is middle of gauge but what the heck. shoot that looks like a standard size gauge probably get a replacement for $15. I had an oil gauge like that showed 15 psig when it 38 psig. I knew gauge was wrong but annoyed me so I replaced. Didn't help anything but made me feel better.
 
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