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still overheating @ 3000 rpm +/ waterpump cavitating? sucking exhaust gas?

K

Kevin Conley

New member
water out of supply hose (into bucket) from inner transom looks good up to 3000rpm, after that flow lessens, and is "airated", a mechanic friend said it's back pressure from exhaust getting by the waterpump plate and can be fixed by sealing the plate w/ silicone. Transom is completly under water. '87 Merc. 260, alpha 1 o/d. Tried all the usual stuff - new waterpump, thermostat, clear manifolds & riser, new Y pipe w/ flappers. ( needed due to leak at bottom ) I've replaced lots of o/d water pumps but never had a problem like this before. Does this explanation sound reasonable? has anyone else experienced symptoms like this?
 
The transom hose kinked when it enters the transom, or a clog on the outside of the fitting that goes thru the transom, or tube not insreted correctly from the water pump to the upper tube.

You didn't miss a o-ring, or tore the o-ring, i use a dab of silicone.

You replaced the housing ? if not could it be worn where the impeller touch's the inside, not perfectly smooth. I always change the housing just not worth the time and money, when i help my friends i tell them buy the housing or i ain't coming over.

See i have a stringer, and when i do mine i always put in a new housing also.

That exhaust bull don't make sense to me.
 
New w/p base impeller & housing, new outer transom hose - not kinked - can run all day w/ fully loaded boat at < 3000rpm - if kinked or clogged that wouldn't be possible. Didn't miss or tear an o ring - all o rings assembled w/ a little greese. Like I said, I've done lots of these and never had airated water comming straight in from the transom - air has to be comming from somewhere - seems like no other logical answer but from exhaust. As stated transom plate is completely submerged.
 
Don't want to bother you anymore, may i ask one more question ?

You use a oem outer transom hose ? See the aftermarket ones come a little to long ?

But what the f--k do i know i'm a rank amature i'm not even house broke yet and i'm still trying to figure out what a screwdriver does.

Good luck your gonna need it.
 
All sierra parts, have never had a problem w/ sierra parts as yet. Guess there is always a first time. Pulling boat out tonight to take a look, will pull & split drive, probably replace the top water pocket housing as a "just in case". will look at outer supply hose for signs of kinking, & take w/p housing off to inspect internals, inspect copper tube ect. What else is there to look at? Gotta take care of the airating problem before looking at downline restriction type problems ( which have already been looked at - I kind of started the process backwards because I don't own a trailer and the boat lives in a slip, I was trying once again to find an easy way out without pulling boat and drive). My bad.
 
I split the drive again last night to inspect, didn't see anything abnormal until I started reassembly. Just as a sanity check, I held my finger on the impeller as I turned the shaft - guess what I found - yep you guessed it - the brand new impeller just installed spun it's hub! Now before anyone starts to berate me for not installing properly, yes I greesed the cup, no I didn't overtighten the nuts, yes I turned the shaft as I pushed down on the housing, ect. ect. ect. I guess I should have taken a clue from my previous post "I never had a problem with Sierra parts before, I guess there's always a first time". So this is a first for me. I'll let you all know what happens next.
 
Brass hub, popped right out with little effort, nice bright polish on outer surface. Haven't had a chance to get back to it yet, wife's jeep's electric fan exploded - puncturing radiator, so priorities have shifted for the time being. Should have time to work on boat this weekend.
 
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