Overheat at Idle Only...Replacing Impeller...Parts and Procedure Advice?

IslanderVT

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Mar 13, 2013
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Hi Folks,

'88 Mercruiser 260 with Alpha 1 drive...all factory original. Raw water cooling in fresh water only its whole life.

Problem is getting increasing temperature at idle that quickly restores to normal over ~1000 RPM. No other issues....runs like a champ.

Has original Mercruiser temp gauge (3-tone orange lettering at the bottom) w/o numbers, but it has always run at the first mark on the gauge in normal mid-season water (slightly lower in colder early-season water). I think the temp gauge is working correctly because the behavior is repeatable. I'll try to verify with a thermocouple at the temp sender.

This boat sat in my garage for 6 years. It needed a gas tank, stringers, bulkheads, deck, etc....which I did this spring.

I suspect the impeller took a set and is now pumping at lower volume. I should have changed it initially, but coming off all that other work I mostly wanted to get the boat running and shake out any other problems....thankfully there are few.

I'm planning a full replacement of the impeller and housing. I've done impellers in the past but not in many years, and not with any advice from experts like you...so a few questions.

1) Parts:

- Should I go with OEM or Sierra? Any difference worth picking one over the other? I'm inclined to go OEM unless there are other strong opinions.

- From another post, it appears that I need these 2 kits for a full make over...this is a '88 vanilla Alpha 1:
Quicksilver 46-96148A 8 (Water Pump Upper Housing Repair Kit)
Quicksilver 46-44292A 3 (Lower Base Kit for an Alpha One)
Also fresh gear oil, and a pair of new drain plugs for the lower unit.

Sound right? Any other bits and pieces I might need? In general, fasteners are in good shape without much corrosion, but I haven't split the lower unit in a while. I'll check hoses, but I replaced the upper water hose between the transom assembly and hull a few years back, so that should still be good.

2) Procedure:

I greased the splines (with actual spline grease!), u-joints, shifter, etc when I hung the drive this spring, so I don't plan on removing the whole drive, just the lower unit. Is this OK?

Is there a good thread or video anyone can recommend for this repair? I've done these before, but over time I've probably lost some of the key points for doing a perfect job...which is my goal.

Anyone have any key tips or tricks they could share?

Do people normally torque fasteners? Should any gaskets or o-rings be coated with grease prior to install?

3) Other things while I'm in there...Anything else I should attend to while I have it all apart?

Thanks!
 

stonyloam

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Re: Overheat at Idle Only...Replacing Impeller...Parts and Procedure Advice?

1) OEM
2) http://forums.iboats.com/mercruiser...placing-merc-alpha-style-impeller-387864.html Easier with the whole outdrive on the bench but doable. Make sure you place your control in forward and the gearhousing stays in forward (locked CCW and held with bungee) and you turn the prop CCW only to line the driveshaft splines. Torque everything that you can. Make sure the ss washer is in place on the lower shift shaft.
3) if there is any heat damage (melted stuff) take a look at the water pocket cover to make sure it is good shape (do not try to remove it lust look LOL).
 

Bondo

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70,470
Re: Overheat at Idle Only...Replacing Impeller...Parts and Procedure Advice?

Ayuh,.... On topic, but off subject,... Check the Check-balls in the T-stat housin',....

Over-heat at idle, 'n only at idle is a common trait of the check-balls leakin'/ not seated,...
 

IslanderVT

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Messages
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Re: Overheat at Idle Only...Replacing Impeller...Parts and Procedure Advice?

Thanks stonyloam and Bond-o.

Getting OEM parts locally.

That manual is very helpful, as was the video on the sticky. Great tip on the bungee...will try that. I'm a believer in torquing outdrive parts, especially with plastic involved.

I don't expect any heat damage...this is a thru hull so never any exhaust in the drive.

I'll also check the balls...I remove those hoses to winterize the manifolds and risers, and I've cleaned crud there, so worth a look. I always compare the volume and temp of the water from each exhaust tip while on muffs, and they appeared to be the same.

Speaking of that, could I check the impeller by measuring combined discharge from the exhaust tips? Should be the same there as it is coming in the intake? At a minimum, I'll do a before and after video to confirm the new impeller's better.

Any other service I should do while this is apart?
 

stonyloam

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Re: Overheat at Idle Only...Replacing Impeller...Parts and Procedure Advice?

Speaking of that, could I check the impeller by measuring combined discharge from the exhaust tips? Should be the same there as it is coming in the intake?

I am not sure it would tell you much, because the water coming from the muffs is under pressure and so it is not being moved entirely by the impeller.
 

IslanderVT

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Joined
Mar 13, 2013
Messages
48
Re: Overheat at Idle Only...Replacing Impeller...Parts and Procedure Advice?

New impeller and pump rebuild kit solved my temperature problem.

I changed both the top and bottom 1/2 of the pump so its all new with OEM parts. Water testing yesterday showed lower, and perfectly stable engine temp over a range of conditions.

I tried a before and after comparison of the water flow rate as measured at the exhaust tips using muffs. We did the 15sec sample and multiplied by 4 to get GPM.

At idle, before pump change 1 GPM...after pump change 2 GPM.

At 1000 RPM's, before pump change 2 GPM...after pump change 4 GPM.

So the new pump about doubled the water flow.

Despite doing this test with muffs, the "after" numbers came out exactly at the spec for the factory flow rate check that is supposed to be done with the drive submerged and measured at the inlet at the engine.

Not saying this method (using muffs and measuring at exhaust tips) is a substitute for the factory procedure, but it worked for me and will now be a simple quantitative yearly check for our impeller health....which sure beats my previous practice which was to compare flow rate and temperature with my hand...and didn't catch the low flow rate this year.
 
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