As a preventive measure while I am taking care of tensioner issues, I have been planning to replace my water pump. I thought all the new/rebuilt ones had a plastic impeller, but this one in metal.

In searching the archives, I have found opinions that the metal might be better, and replacing the water pump as a "while-you-are-at-it" measure might not be as necessary as thought.

Any thoughts on this?

TIA,

Neil

 

I've been through 2 water pump replacements over the 10 years I've had my 928 and all of them have been metal. Personally I wouldn't want a plastic impeller.


Ken

Upraor 85 928S
Vlocity 89 C-4 37,000 miles

 

I have yet to hear a definative response on why metal is actually better. On my rebuilt one, from 928 spec. the impeller was plastic. On my engine block where the water pump is situated, there are 2 gouges. I assume this is from a failed metal impeller.

P.S. always replace the water pump while in there. You have to pull so much off on the 928, you'll be kicking yourself if the old waterpump fails in a year. I take the opportunity to replace cam, oil and crank seals as well.

 

If a plastic impeller fails, it has a much lower chance of damaging the block.

 

There has been much discussion regarding the pros and cons of prophylactic water pump replacement. The popular notion is that it should be changed while you are in there, but there is the possibility that the rebuilt pump might fail prematurely... this has been the experience of more than one Rennlister.

 

Including me....the bearing on mine went south, causing a coolant leak of about a gallon a day. Forced me to do a timing belt job nearly two years earlier than scheduled. I'm just glad it didn't seize up.

 

The rebuilt one I got from 928 Specialist is also plastic. Can’t recall where but I recall reading about metal impeller has higher probability of damaging the block if it fails. Fact or opinion? Not sure.

I must agree with everyone to replace it. Mine seized up, and I lost several nights of sleep before finding out nothing else were damaged. This is definitely not where you want to save a few (or a few thousands) pennies.

 

What are new pumps coming with??

 

Plastic/cast iron can be debated forever - plastic having a lesser mass, easier on the belt: plastic can freewheel on its shaft...... and so on.
Last summer's bearing cage failure in an '88 allowed the shaft to tilt, causing the pulley to eat away at the (perfectly good) TB - and the plastic impeller to touch the block recess and/or the pump housing: this broke the bond between the impeller and shaft = freewheeling - so hi temp & boil over accompanied the tension alarm.
The replacement is cast iron - so "it all depends" on the failure mode, which who knows how to predict? Would could assume that seal failures vastly out weigh bearing failures: only the rebuilders know for sure. Any water pump autopsy experience there?

 

New pumps have plastic impellors . There is a small knurled metal ring which is bonded to the plastic the metal ring is what contacts the shaft when the impellor is pressed on . What has happened to several pumps is the metal ring gets loose from the plastic and then not spin all the time . The metal impellor is pressed directly on the shaft . In a few very rare instances the bearings in the pump may have failed with out killing the seal ? )which allowed the cast iron impellor to machine away some of the aluminum off the engine block . I also think that is is more likely that in those cases the metal impellor was NOT PRESSED down far enough on the shaft and the "damage " occurs on start up only to be discovered years later on a subsequent water pump change . Personally I prefer the metal impellor but I am an old fashion kind of guy !

 

everyone knows my story. first pump, metal, and it leaked throught the bearing. put on the plastic one , and one year of racing later, (with strange water cooling flow issues) the shaft spun inside the impeller, (the knurling is JUST NOT DEEP ENOUGH@@@!!!) so, the moral of the story is. if its not broke, dont fix it!!!! a waterpump only , is not a big job. plastic pumps SUCK because the knurling is too shallow for the injected molded plastic (terrible design flaw!!) the metal one is fine, just more weight (what a couple of ounces!) usually, the mode of failure is leaky bearing. As Jim says, metal impleller coming off and ruining block is rare. the plastic is probaby fine for gentle driving applications. racing with the rpms and heat, probably is too much for a plastic impeller water pump unless it is perfect from the start!

MK

 

Thanks Jim & Mark......

Next question......what do the *recent* rebuilt pumps come with......plastic or metal?, or does it depend on what you turn in as a core?

The last one I received some years back was metal.........just curious if metal is still available or will we have to suck down the plastic ones in the
future?

 

Mine from 928 spec. has a plastic impeller. I got it about 6 months ago. My core had metal.

 

I have not turned in my core yet (still on the engine), but I got the pump a couple of weeks ago from 928intl.

 

The official reason that Porsche changed over to a plastic impeller was to save weight !
That's what's stated in the technical service manual for '90