New impeller still overheating at idle..ughhhh

So I replaced my impeller on my 1998 Alpha one gen 2 3.0l mercruiser and it is overheating at idle speed goes up to 200-210 in the lake. But if I take the throttle to about 1500rpm it cools right back down to 150-160. I replaced the impeller, the base, gasket is correct, all hoses are free of debris, o-rings in kit where replaced. Could the water pump on the engine be suspect. Also the thermostat 140 is working good. This one has me stumped.... Please help.

Re: New impeller still overheating at idle..ughhhh

Howdy,


#1 determine if your raw water pump is pumping enough water to begin with.

"break" the hose from the transom to the t-stat housing and see if you have good water flow at idle.....if you rev it up it should shoot out pretty good.........

You must do this with the boat in the water. (doing it on "muffs" won't tell you much)

Re: New impeller still overheating at idle..ughhhh

Ok with the motor not running on the muffs there is no water coming out from the center prop area. But as soon as I start it it seems as though the full amount of water the hose is putting in is coming out the end of the prop.
So my next step is to remove the thermostat because I think maybe it isn't fully opening therefore not allowing enough water to bypass into the engine. At higher rpms in the lake it has enough flow to overcome the partially opened thermostat.

Is my logic sound or do i need medication?

Re: New impeller still overheating at idle..ughhhh

HT32BSX115 said:

Howdy,


#1 determine if your raw water pump is pumping enough water to begin with.

"break" the hose from the transom to the t-stat housing and see if you have good water flow at idle.....if you rev it up it should shoot out pretty good.........

You must do this with the boat in the water. (doing it on "muffs" won't tell you much)

Click to expand...

Well as I stated above there was what seemed like plenty of water flowing out of the end of the prop but absolutely no water from the hose at the t-stat when I rev it up to about 2000 rpm it shoots out.
Here is my question what would cause all the water to flow out of the prop if the pump isn't working correctly is there a bypass on the pump that allows it to flow to the exhaust side in the lower unit? I followed the manual to the t when installing the impeller I just cant understand what I did wrong. Would the little rubber seal at the top of the pump be the issue maybe I didn't get it all the way down? Thanks for your guys help.
Oh on a side note after realizing you said this has to be on a lake I submerged the entire drive up to the cavatation plate to see if that helped and the results were the exact same no flow at all until I throttle it.

Re: New impeller still overheating at idle..ughhhh

i'm also changing my impeller and ordered up a full kit. Ben reading a lot also.

http://forums.iboats.com/showthread.php?p=2752191

What i noticed is a plastic "hose to pump housing" part with two internal orings for the lower unit. However, the upper unit has the same type of fitting which IS NOT included in the water impeller kits. It's part of the upper unit seal kit here on iboats for like $35 or something.

If the upper units (top of the copper water hose) orings are bad or damaged upon replacement, then pump pressure at higher RPM's could result in the loss of raw water to the top end.

Sam

Re: New impeller still overheating at idle..ughhhh

Ok with the motor not running on the muffs there is no water coming out from the center prop area. But as soon as I start it it seems as though the full amount of water the hose is putting in is coming out the end of the prop.

Click to expand...

OK,


Well I'm not following you . How can you tell there's no water coming out of the center prop area if it's submerged in the water?

Also, there's relief ports at the bottom of the transom so you may or may not see a lot of water at the prop center even if the pump was working correctly.


You need to remove the hose from the transom, (that supplies raw water to the t-stat housing) start the engine (with the boat in the water) and see how much flow you have at idle.

That will tell you right away if there's enough (or any) flow at idle.

Re: New impeller still overheating at idle..ughhhh

HT32BSX115 said:

OK,


Well I'm not following you . How can you tell there's no water coming out of the center prop area if it's submerged in the water?

Also, there's relief ports at the bottom of the transom so you may or may not see a lot of water at the prop center even if the pump was working correctly.


You need to remove the hose from the transom, (that supplies raw water to the t-stat housing) start the engine (with the boat in the water) and see how much flow you have at idle.

That will tell you right away if there's enough (or any) flow at idle.

Click to expand...

I started this process by using the muffs and had alot of water flowing out of the prop area. But after your suggestion I used a large container and submerged the drive up the the cavatation plate well above the vents so there should be no issues with getting water then I started the boat with the hose detached off the tstat housing and I had no flow at 700rpm but at 1500 I had about 4 inches of water spraying out. So your saying that putting the drive in a tank of water up the the cavitation plate is not enough. Well I will try a lake trial tomorrow but I cant imagine that not being enough water. Thanks for your advice. I will let you know how it goes.

Re: New impeller still overheating at idle..ughhhh

From your description, it sounds like the water tube is mis-aligned. No water is being pumped up. At 1500 rpms the recirc pump can draw the water up the tube. I don't know if you can put the drive in full up trailer position and see clear enough with flashlight to see if connected. I'd give it a look see.

How did you do it? By removing the whole drive and then seperating? or just taking off the lower with the upper still attached? The latter can be more error prone. I do it with drive removed and then split. The lower is on a stand. When you set the upper onto the lower you can look through the front of the drive and make sure the water tube goes down in the plastic pipe with the o-rings.

If the tube is connected, then follow the water hose to the transom connection. Make sure it is not calapsed.

Re: New impeller still overheating at idle..ughhhh

imported_John_S said:

From your description, it sounds like the water tube is mis-aligned. No water is being pumped up. At 1500 rpms the recirc pump can draw the water up the tube. I don't know if you can put the drive in full up trailer position and see clear enough with flashlight to see if connected. I'd give it a look see.

How did you do it? By removing the whole drive and then seperating? or just taking off the lower with the upper still attached? The latter can be more error prone. I do it with drive removed and then split. The lower is on a stand. When you set the upper onto the lower you can look through the front of the drive and make sure the water tube goes down in the plastic pipe with the o-rings.

Click to expand...

The first time I had it apart I split it. The second time I just removed the lower unit. But that tube might explain why so much water is passing thru the prop area when on muffs. But what that doesnt explain is why when I unhook the tstat hose it still blows water but not until it hits 1500 rpm and the water pump isnt even hooked to that line so how could it suck it into the block...

Re: New impeller still overheating at idle..ughhhh

nitrosteve said:

The first time I had it apart I split it. The second time I just removed the lower unit. But that tube might explain why so much water is passing thru the prop area when on muffs.

Click to expand...

It sounded like the symptom would fit. The only other things to check when you are in there, the rubber dam in the lower and composite dam/seperater in upper.

I pulled my lower only, thinking I would save time, but quickly decided no way, and pulled the upper. It might be OK if you have a 2nd set of eyes to help guide the grunt holding the drive. For me, the eyes and the grunt are the same, and don't have the arm strength to hold it very long.

Re: New impeller still overheating at idle..ughhhh

Yeah,

If you disconnected the hose and had little or no flow at idle, you have a problem with the raw pump........Either it's failed/partially disintegrated or the plastic tube is broken/misaligned etc...


The raw pump is a positive displacement pump. It will even pump at 200 RPM.

I would pull the drive and split it. Here's a great description for doing it.


Cheers,



Rick.

Re: New impeller still overheating at idle..ughhhh

Problem solved... I went to take the lower off to take it to the local marina to see if I had done something wrong. I was careful not to disturb the tube on top of the pump when removing the drive. Well wouldn't you know it had twisted to the right and sitting next too the hole in the pump. I wonder if the neighbors think im weird dancing through the yard in excitement. So its back together and runs 135-145 all the time no matter what. Now on to the second problem but I will start a new thread. Thanks everyone for helping me. Come see the new thread it about fuel delivery issue at WOT.