4.3l mercruiser alpha one over heat after an hour at idle

[Southpaw sportfishing]

Hello if anyone has any ideas I would love a fresh perspective. 4.3l mercruiser alpha one will over heat after an hour at idle with all of a sudden no water flow from out drive. New impeller, new exaust manifolds and risers, new water pocket and tube, new motor, new circulating water pump, new tested t stat 140, Clean y pipe. No kinked inlet and transom hose. Was running amazing about 150- 160 with plenty of water flow then jumped up to 180 with no water flow coming out of exaust then 190 so shut it off. Same prob both times, one on muffs, one at the dock. I'm lost on what I'm missing, any help would be very appreciated

 

[QBhoy]

Sounds like a problem...but firstly I would maybe say that you shouldn’t run your boat at idle for an hour..and especially not on muffs.
Sorry to sound condescending, but it’s true.
All the best. Sounds like the impeller is at fault.

 

[GA_Boater]

An hour is a long time to idle. What happens if the RPMs are raised to 1500-2000? Be careful doing that on muffs, the hose may not supply enough water.

Was it overheating before all the work was done?

Maybe a chunk of old impeller is blocking water flow?

Welcome aboard.

 

[Rick Stephens]

I'd be thinking impeller, new or not. You might pull the hose coming from the drive/transom up the port side of your engine, off the thermostat housing. Then turn on the water to the muffs, see if any flows through. (should not flow until engine is started if the impeller is intact) Then fire up the engine and see what kind of flow you get out of it. You can even measure the gallons per minute by dumping into a bucket. I don't recall how much it is on an Alpha1, but someone here will know.

 

[achris]

... I don't recall how much it is on an Alpha1, but someone here will know.

Depends on the drive ratio... Alpha Ones behind 4.3s were usually 1.84:1... They should pump 12.4 litres/minute. Now, that is at EXACTLY 1000rpm (use a shop or calibrated tacho, not the dash tacho) and it can only be done on the water. On muffs is no good... Run the engine only for 15 seconds with the drive hose disconnected (3.1 litres in 15 seconds for 1.84:1)...

Chris......

Service bulletin #82-4...(MC1, R, MR and Alpha One all use the same pump)

 

[QBhoy]

Fine and expert advice above !

 

[Walt T]

I can honestly say all my years of working on boats I have never ever done that. Usually pretty easy to tell if a sea pump is going bad. By the way I wouldn't replace it in fall, I put those off until spring so the new impeller is less likely to take a "set"
Of course, lots of impellers go all winter and are fine in the spring, It's just how I do it. It also gets the customer back in and maybe decides to have me do something else.

 

[QBhoy]

Depends on the drive ratio... Alpha Ones behind 4.3s were usually 1.84:1... They should pump 12.4 litres/minute. Now, that is at EXACTLY 1000rpm (use a shop or calibrated tacho, not the dash tacho) and it can only be done on the water. On muffs is no good... Run the engine only for 15 seconds with the drive hose disconnected (3.1 litres in 15 seconds for 1.84:1)...

Chris......

Service bulletin #82-4...(MC1, R, MR and Alpha One all use the same pump)

Chris

any idea what my Alpha 1.47 should see ? My boat eats impellers running as high as she does on occasion.
Ta

 

[achris]

any idea what my Alpha 1.47 should see ? My boat eats impellers running as high as she does on occasion.
Ta

Same as the 1.5:1.... The actual difference is 2%, and you're very unlikely to be able to measure that in water volume. It equates to 76ml.... In a 9 litre bucket....

 

[QBhoy]

Same as the 1.5:1.... The actual difference is 2%, and you're very unlikely to be able to measure that in water volume. It equates to 76ml.... In a 9 litre bucket....

Thanks Chris.
I’ll probably just continue to replace every year anyway. Im prettt certain the visible volume of water coming out the exhausts reduces at the end of the season, compared to the start. Hard to tell exactly. It comes out mainly through the transom but also from the corsa marine side pipes too. Seems to vary in quantity on occasion. Likely the old path of least resistance effect with different trim and conditions.
It does take a hammering but not sure I could warrant or even find a bottom end with a low water pick up for it over here in the uk. Didn’t know such a thing even existed until someone on here told me a while back.

 

[achris]

Why not change to an engine driven pump and either through-hull or transom mounted pickup?



Chris........

 

[aimlow]

First question..WHY idle at a dock for an hour???

 

[QBhoy]

Why not change to an engine driven pump and either through-hull or transom mounted pickup?



Chris........

Never thought of that.

 

[achris]

First question..WHY idle at a dock for an hour???

Probably because he has a 1MW getto-blaster annoying the living.... out of everyone in the same zipcode... He needs the engine running to, a) keep the power up to the noise maker, and b) if he doesn't, there's not enough charge in the batteries to start it.... :facepalm:

JMHO.

Chris........