3.0 at idle overheats

[Big Del]

Hello iboaters I have an 88 3.0 mercruiser inboard/outboard. It is a fresh water only boat I'm second owner. I did the Gimbal and all new bellows with impeller last year with help from searching your post. Now this year I have had a great season of boating until this weekends trip. I was at a new lake in here in Oregon doing a lot of idling and I noticed my temp was at 200. It has never been above 150 so that through up a flag. I had been in some weedy type area I thought I had stayed out of the weeds. I accelerated my boat to possibly knock off any debris temp came right down instantly. I thought it was just something blocking my water inlet. After cruising at idle for a couple minutes I noticed the temp was slowly climbing again. I did this all day anytime I idled for more than a couple minutes the temp would come up. I pulled the inlet hose to Tstat it didn't appear to be clogged any ideas.

 

[HT32BSX115]

Re: 3.0 at idle overheats

Hello iboaters I have an 88 3.0 mercruiser inboard/outboard. It is a fresh water only boat I'm second owner. I did the Gimbal and all new bellows with impeller last year with help from searching your post. Now this year I have had a great season of boating until this weekends trip. I was at a new lake in here in Oregon doing a lot of idling and I noticed my temp was at 200. It has never been above 150 so that through up a flag. I had been in some weedy type area I thought I had stayed out of the weeds. I accelerated my boat to possibly knock off any debris temp came right down instantly. I thought it was just something blocking my water inlet. After cruising at idle for a couple minutes I noticed the temp was slowly climbing again. I did this all day anytime I idled for more than a couple minutes the temp would come up. I pulled the inlet hose to Tstat it didn't appear to be clogged any ideas.

Howdy,

Overheats at idle are usually the raw pump not getting the water up to the engine in sufficient amounts.

Pull the water line that comes from the transom to the T-stat (water inlet) when the engine is idling and see if you get a good stream of water. That's a VANE type pump. even at idle it should pump a fair amount of water. If you have a LOT of water you may have blockage somewhere else.....It's also possible that the impeller has failed again (or lost a few vanes and they're blocking down stream somewhere...)


Regards,


Rick

 

[Big Del]

Re: 3.0 at idle overheats

OK so I searched idle overheating and didn't find anything about a raw pump? Is that diffrent than the impellar? If I have alot of water flowing through the transom inlet what would be plugged start at t stat and work through the exchangers? Thanks for your help, I would assume you can only test this in a tank or in the water.

 

[HT32BSX115]

Re: 3.0 at idle overheats

Yeah. you have to have "muffs" on the back or have it in the water. By raw pump I mean the impeller.

If you have an oil and /or power steering cooler (and it's between the raw water pump and the t-stat you may have pump remnants clogging the heat exchanger. pulling that hose to the t-stat from the heat exchanger while you're running at idle will reveal blockage if the flow is weak etc....

Most stern drives have the raw water from the transom go to an oil cooler etc before going to the engine.

I am assuming you tested the T-stat and checked the drive water pickup holes etc....

Do you know the condition of the exhaust manifold and exhaust elbow? Sometimes if the exhaust manifold/elbow are severely rusted they can block the flow of cooling water when it exits the manifold. that reduces the overall flow of water thru the engine and can cause overheats. The T-stat can be fully open but because the water cannot exit
fast enough you can't get enough cool water into the engine either....

 

[Big Del]

Re: 3.0 at idle overheats

Yes I have checked the drive inlet holes, they appear to be free of debris, the impellar I changed looked fine it had all the fins on it still so I don't believe it is causing any blockage. I don't know much about the exchangers I haven't had them off, but I do know the PO and he said it had not seen saltwater and it hasn't since I owned it. This is a bare bones blue water no power steering or oil cooler. When I was out on the water I pulled the hose that went to the tstat not knowing until I read on here that is the hose coming from the raw pump and it was empty. I didn't know it was ok to run it with this hose off I wish I would of that would of been the time to test it. My question about running it with muffs was thinking that the pressure from the garden hose might not show an idle overheat. It sounds like I need to pull the outdrive off again and inspect/replace the impellar.

 

[Big Del]

Re: 3.0 at idle overheats

It sounds like I need to pull the outdrive off again and inspect/replace the impellar. Is there any thing I could of done wrong the first time to get less than a year estimated 20hrs of run time on a new merc impellar.

 

[HT32BSX115]

Re: 3.0 at idle overheats

If you took that hose from the transom off when the engine was running (it's ok to run with it off briefly....briefly means 30 sec or so) and little or no water came out, You have either a blockage or a failed raw water pump impeller.

With the hose off you should have gotten pretty good water flow even at idle.

 

[Big Del]

Re: 3.0 at idle overheats

Ok, so I did some testing tonite. Removed the lower water inlet screen plastic ribbed thing and shined a light up the outdrive, no blockage. With the muffs on the engine, it did not overheat at idle. I think the water pressure is enough to help it cool. Pulled the water inlet hose from Tstat and flushed it with the garden hose backwards and good clean water came out outdrive no apparent blockage. Pulled hose from Tstat to exchanger and flushed good clean water came out outdrive port, used light and looked into exchanger it looked farely good shape rusty color but no flaking I scraped at it with a flat head srewdriver a little and it seemed very solid. The only thing I didn't do was pull Tstat itself but at this point it is unlikely a Tstat. So I'm thinking that I only got a year out of this (merc bought) impellar pump. What would cause the pump to go out so soon. Should I look at a housing possibly.

 

[HT32BSX115]

Re: 3.0 at idle overheats

They have been known to fail in a year.....it's usually for a reason though. I just changed a GEN II impeller that was 10 years old that actually could have been run longer....

This is probably not a good comparison since the GEN II impeller is vastly different from an older type Alpha impeller....

if yours failed only after a year you could have had a defective one or you have something else causing a blocking problem.

When the drive is in the water the pump is completely submerged so it's not a priming problem....(my bravo pump will suck a tub full of water dry using muffs with a hose in a container of water)

If it doesn't over heat on muffs your t-stat is probably opening.....you can check it on muffs though thermostats rarely fail......... you could take your hose and put it in a large tub of water (after priming everything) and see if the raw pump will pull water out of the tub thru the hose via the muffs.

It sure sounds like the impeller has lost a few vanes.....

 

[achris]

Re: 3.0 at idle overheats

.... Pulled the water inlet hose from Tstat and flushed it with the garden hose backwards and good clean water came out outdrive no apparent blockage.....

This shouldn't happen... The design of the impeller/pump is such that the water should have 'dead-headed'. No water should have come out of the drive! You have a problem with the pump or impeller....

Chris........

 

[Big Del]

Re: 3.0 at idle overheats

That is intresting another finger pointed at the impellar thanks guys.

 

[HT32BSX115]

Re: 3.0 at idle overheats

I think you're right. everything is pointing at the impeller.

Did you say you changed it the last time or did someone else?

 

[Big Del]

Re: 3.0 at idle overheats

My buddy and I did the gimbal, bellows and impellar last year. It was a full day of work for us.

To just replace the impellar do I need to pull the whole outdrive off or can I just split it on the boat? Also last time I used genuine Merc impellar is there a diffrent brand I should use? I read something about the housing wearing I didn't see any excessive wear on my housing is there something I look at.

 

[achris]

Re: 3.0 at idle overheats

Yes, you can just remove the lower unit leaving the upper on the bell housing... I wouldn't use anything but genuine (impeller). Check the housing closely, look for any signs of distortion or even a slight melt that allows the cup to move in the housing... Check the base for any signs of melting, especially close to the driveshaft...

Post photos if you're unsure...

Chris.........

 

[Big Del]

Re: 3.0 at idle overheats

I will post pics of the housing, it is just a little suspect to only last year. It has minimal hrs but I realize on some stuff that is worse than using daily. It will be a couple weeks before I get a chance to tear it apart. Thanks for the help, and any added advice will be apreciated.

 

[HT32BSX115]

Re: 3.0 at idle overheats

What he said!!

Chris,

I just noticed you're in Perth! (I'm a little slow!)

I just LOVE that place!

I spent a few days there back in 1981. Flew in there on a USAF C-141 to pick up a couple of (deceased) Navy seamen who were there on a shore leave. Seems they'd had a little much to drink and were roaring around out in the country and hit a "roo"..... must have been a big one....


They weren't ready for us to take them the first day so we got a couple of additional days there. WHAT A NEAT PLACE! in later years I made many trips back to and including RAAF Learmonth, Alice Springs and Richmond Base.


Cheers,


Rick

 

[achris]

Re: 3.0 at idle overheats

Rick,

It's changed a bit since then... .. Big roos are killers, literally....

You're welcome here any time you like... I have a guest bedroom made up and ready to use. I'll even take you out fishing

Chris........

 

[HT32BSX115]

Re: 3.0 at idle overheats

Thanks!

That would sure be fun,

I'll bet the fishing is just incredible around there!

 

[Big Del]

Re: 3.0 at idle overheats

OK a couple weeks turned into 6 months, it's been cold and the boat was tucked away for the winter.

I took the lower unit apart today and found the water pump to be in good shape, all the fins were in place a couple of them were twisted the wrong direction. I did notice a slight melt on the housing by the shaft. I'm going to replace the impellar and housing, I was just wondering if the housing melt could have something to do with my overheating issue.

I was hoping to take it apart and find a missing fin on the impellar, is there any way to flush the water jackets in the outdrive to make sure I don't have some sort of blockage. Or is there something else I could look at for the overheating?

 

[Big Del]

Re: 3.0 at idle overheats

I'm heading to the parts house in a couple hrs I was wondering if there was anything else I should pick up that could be suspect in the overheating?