SBC V8 Water Pump Failure, 3 Styles of Pumps Tested BYPASS THERMOSTAT UPDATE Options

[nick mironov]

My Renegade water pump failed yesterday. The pump impeller is stamped steel and the blades all just folded over or sheared off. I understand that the cast impellers are much stronger, but that not all manufacturer's cast models will fit into the Renegade housing. Anybody have experience with cast impeller pumps that will fit into the Renegade housing?

Since there is no bypass in the Renegade system, Renegade instructions are to limit the motor RPM to 3,000, or less, untill the thermostat opens. I have observed this, but the pump still failed after 5,500 miles. Renegade now makes a bypass kit, which I going to try, but I would still like to get a more durable pump.

Nick

[LvSteveH]

This is Scott using Steve's 914 World account for now...

First of all...I have seen the failures that are discussed in this thread. There are several reasons why the failures have increased over the years including lower quality parts used to build the impellers, higher RPM's due to better aftermarket engine technology, cheaper thermostats that don't flow enough volume when open, more debris and blockages in the ageing engine blocks, and just the fact that there are so many SBC 914's and 911's that have finally found the road...and not just a dark corner of the garage covered in dirty laundry and kids toys!

My little orange 914 from years back that some of you named "Peal", went through a few water pumps in it's day. I was stranded on a cold windy night on the long State Line grade with just such a failure...so I completely understand the frustration. BUT...it was the cast impeller/water pump that failed that time at the shaft seals and not the typical carnage with the impeller failures. That engine was frequently spun to 6800 RPM and was never driven easily I can assure you.

Where we saw the most consistent failures was with customers that revved the engine past about 2500 or 3000 RPM before the thermostat was open. Drilling two or three small holes in the T-stat was the easy way to relieve the pressure a little so the water pump would have a fighting chance. Often the pump would sustain damage after just such a high RPM rap of the throttle when cold, but it would still move some water. BUT, just like the paper clip, eventually the blades would give up and curl themselves around the shaft.

In an attempt to help our little water pumps to live longer, we developed an easy fix with a bi-pass T-stat design. Water temps have been consistent, water pump failures seem to be less, hose pressures have been more stable, no need to drill holes in the T-stat, heater core temps have increased on cold days, and it's easy to install.

In summary, keeping the RPM's down when your engine is cold, is a good idea in general and may help your water pump last. Installing the cast water pump was a solution I have tried, but they are not completely immune from failure. Different impeller designs will change flow rates that will effect your ability to cool. Using a good quality, high flow T-stat, with two or three small holes drilled, will work very well for this application. Flushing your block and cooling system completely will also increase flow and thermal transfer allowing the engine to cool more efficiently. Installing the new bi-pass system seems to be the next logical step if you are still having issues.

On our end, we have looked into impeller suppliers and we think we are using the lesser of all the evils. There is some really bad junk being made out there, but the ones we have are lasting when reasonable care is exercised.

If you have any further questions, you are welcome to call Steve or myself here at the shop. Thanks for all the good input here on 914 World without the drama that finds it's way into these boards from time to time. Keep up the good work!

Sincerely,
Scott >>> Renegade Hybrids
www.renegadehybrids.com
(866) 498-2421 toll free

[nick mironov]

I purchased Scott's / Renegade's bypass thermostat solution. Nice, neat solution. It installs fairly easily. The only problem was that the kit's Y-fitting for the bypass connection at the pump suction did not fit because of my particular setup (I have steel tubing under the car rather than hoses), so I made a T-fitting (pictures later).

It is now installed and the cooling system is "burped". The coolant seems to come up to temperature twice as fast as before, which is very nice (and unexpected). I will report on the results after some varied driving conditions.

Even though I strictily followed Renegade's 3,000 RPM limit when cold and had the holes drilled into the T'stat, the pump still failed, and while driving only 35 mph. The pressure bounce (see results above) of the stamped impeller test suggests that there is some significant cyclic pressure effect for that impeller style. I suspect that it is this cyclic pressure variation that ultimately makes the blades fail from bending fatigue (just like bending and breaking a paper clip). The cast pumps did not exhibit this condition, so it must be something about the geometry of the stamped impeller. My guess is that if you open up your pump housing your blades will already be bent, and it is only a matter of time before one fails and breaks off. Note that the test of the broken pump (one blade missing altogether and 5 remaining bent blades still moved water pretty well. One would not suspect a problem until it was too late.

In any case, a bypass is the way to go, and the Renegade kit is very simple. I am further hedging my bets and using Slits's cast impeller with the Renegade bypass kit.

Nick