Merc Alpha Gen 1 overheats at idle

[Bay2755]

I changed the impellers this spring and used the boat for 6 hours at low rpm's about 1000. The engines ran great and the temperature was normal. The second time I went to take the boat out I started it and after a ten minute warm up, (wife was talking to another boater) the over heat alarm came on. I shut the engine off but turned it back on right away and increased the rpm's to 1500, the temperature slowly came down to normal. We went out for a short cruise and when ever I dropped down to 1000 rpm after a few minutes the temperature goes up to 200, increase the rpm's and it drops down to normal. The other engine is fine no problems at all.

Any idea's what I could be causing this.

 

[Bay2755]

Re: Merc Alpha Gen 1 overheats at idle

I've been reading every over heating thread on this site and one suggestion is the adjustment inside the hoses on the block. I will be at the boat tomorrow and pull the hoses off and have a look at the thermostat could switch it with the other engine or test it in hot water.

Any other ideas.

 

[HT32BSX115]

Re: Merc Alpha Gen 1 overheats at idle

one suggestion is the adjustment inside the hoses

That's a new one. What adjustment?

Overheating at idle is usually a bad impeller. When you rev up the engine even a "bad" impeller will move a little more water.

When you changed your "bad" impellers were they devoid of vanes? If they were, did you find all the pieces?

Also, there may be pieces of previous impeller failures crammed in your oil/PS cooler.

 

[Bay2755]

Re: Merc Alpha Gen 1 overheats at idle

The impeller was not bad. I bought the boat in late August and when it was pulled for the winter changing them was on my list as I did not know how old they were. They were deformed from use but not broken or missing any vanes.

Edit to add

The check balls at the thermostat housing are adjustable?????? I read this in one thread and it was also mentioned to me.

 

[HT32BSX115]

Re: Merc Alpha Gen 1 overheats at idle

Hi Ken,

When I installed the replacement 454/Bravo in my boat the previous owner told me it had a "recent" impeller.

It wasn't in all that bad of shape actually......when he didn't tell me that he didn't bother to locate the broken vanes from a previous failure.

Every one of those vanes were crammed in the oil cooler. It would pass water but not a lot.......

 

[Bay2755]

Re: Merc Alpha Gen 1 overheats at idle

Hi Ken,

When I installed the replacement 454/Bravo in my boat the previous owner told me it had a "recent" impeller.

It wasn't in all that bad of shape actually......when he didn't tell me that he didn't bother to locate the broken vanes from a previous failure.

Every one of those vanes were crammed in the oil cooler. It would pass water but not a lot.......

Thanks for the reply

Can I check this by not pulling the boat out of the water. I have to get a hauler in each time and it's $150.00 each way.

 

[HT32BSX115]

Re: Merc Alpha Gen 1 overheats at idle

You probably can.

If the remnants found their way thru the hose from the gimbal to the oil or power steering cooler.

Another way to check water flow is to disconnect the hose from the transom (which is your raw water supply ).

Connect another piece of hose to it so you can point it overboard,

Have someone start the engine and run it briefly....just long enough to observe how well the water flows out of the hose. Even at idle it should be a fairly good stream. Don't run it more than a minute or so. You will have NO water flowing thru your manifolds, risers or the rest of your exhaust system. It will all get very HOT very quickly.

You oil cooler and/or power steering cooler is usually between the transom and the thermostat housing so that the raw water flows thru them before it gets to the thermostat housing.

Edit to add

The check balls at the thermostat housing are adjustable?????? I read this in one thread and it was also mentioned to me.

There appears to be 3 different thermostat housings for your year engine.

If you do not have housings that look like the one below, you probably don't have the balls...uh....check balls!

The image below is from Doug Russell parts for an 88 5.7. They show 3 different t-stat housings. click that link and scroll down to "Standard Cooling System Design" I II III etc. It will show the different systems. Yours should match one.

If you do have the one like below, and yours is a raw water system, and you're using it in salt, it's probaby corroded enough for them to stick. You also may need new housings. (the price will give you a heart attack!!)


Boats ain't cheep!

 

[Bay2755]

Re: Merc Alpha Gen 1 overheats at idle

I printed all 3 out and am off to the boat shortly. The boat has only been used in fresh water since new Lake Simcoe and Georgian Bay I am the 3rd owner. The boat is raw water cooled, I will pull the hose at the oil cooler and see what I find there first, and check the flow as you suggested.
If the boat over heated right away after changing the impeller then I would think bad impeller but the 6 hour cruise at 1000 rpm with no over heating is what is puzzles me. I'll let you know what I find maybe post a picture or two.

 

[Bay2755]

Re: Merc Alpha Gen 1 overheats at idle

It looks like I have solved my over heating problem but the temperature is still a little higher than the other engine.
1- I removed a hose and started the boat, lots and lots of water.
2- I tightened the rod that has the 2 nylon check balls.
3- Start engine, no difference.
4- I remove the nylon check balls and the problem is worse.
5- The temperature at the thermostat housing is 175.
6- I remove the thermostat housing and replace the thermostat.
7- I start the engine and it climbs to 200 then it comes down to 165 and stays there at 700 rpm.........

Problem solved bad thermostat.