9 replies to this topic

[68+86auto]

After having a GWP134EVO fail on a long trip, I am now needing to replace it. The seal failed but upon removal, I found cavitation. It had lasted about four years/50000km. I managed to locate a NOS pump in town but it was only a four fin version. It got me home but I am not convinced it has adequate capacity. I can't really find any reference to a four fin pump. The applicable models on the box only went to 77 so it could be around that old. The place I got it from closed about 20 years ago and it was old when they got it.

 

Ignore the missing impeller, I smashed it off to be able to use the pump to make a new gasket.

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I did get another pump sent to me while I was away but I'm not really happy with fitting a pump that the manufacturer couldn't even be bothered assembling correctly. The gap is way too large so the impeller needs to be pressed on further.

 

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Whatever I fit next will have no bypass.

 

 

Is there a certain pump I should be fitting to reduce the cavitation?

 

 

Stock 998 A+ HC in case anyone wonders.

Edited by 68+86auto, 18 September 2026 - 01:38 PM.

[Ethel]

That one ???

 

I am being a bit of a smart arse, but it could address one cause by reducing the flow speed.

 

There's plenty to consider: coolant temperature, pulley size, flow restriction before the pump...

 

I'm no fluid mechanics guru, but isn't where the cavitation ends & the bubbles collapse that the damage occurs? The piccy makes me wonder if that was on entering the impeller.

[Spider]

I'll get a chance in the new couple of months to check my own, but I fitted an EVO pump some years back now and been very happy - so far - with it. I've fitted them to many other cars too that I keep regular tabs on and these all do long high speed runs.

As Ethel touched on, there can be many causes for cavitation to occur, one in the original pump design being the location of the bypass, this was pointed out to me by a pump designer. Speed is a factor, most centrifugal pumps have an upper shaft speed of around 3600 - 3800 RPM, but making the impeller smaller does usually allow for a higher shaft speed.

 

Another thing is to be sure that all the air is removed from the cooling system, in most stock Minis, this is next to impossible as they don't have a separate header tank, this is something I've always fitted to them.

[PoolGuy]

Water wetter is another solution to cavitation.

[68+86auto]

I'll get a chance in the new couple of months to check my own, but I fitted an EVO pump some years back now and been very happy - so far - with it. I've fitted them to many other cars too that I keep regular tabs on and these all do long high speed runs.

As Ethel touched on, there can be many causes for cavitation to occur, one in the original pump design being the location of the bypass, this was pointed out to me by a pump designer. Speed is a factor, most centrifugal pumps have an upper shaft speed of around 3600 - 3800 RPM, but making the impeller smaller does usually allow for a higher shaft speed.

 

Another thing is to be sure that all the air is removed from the cooling system, in most stock Minis, this is next to impossible as they don't have a separate header tank, this is something I've always fitted to them.

 

I never understood the bypass purpose until you posted the picture of the old type of thermostat. This will be getting the bypass blocked off.

 

I have been wondering about getting a vacuum coolant filler.

 

I would fit an expansion tank of some kind but the engine bay is already packed full of aircon parts.

 

To make the air problem worse, this AC mini has a sandwich plate under the thermostat housing, making the top hose the highest point.

Edited by 68+86auto, 18 September 2026 - 10:54 PM.

[68+86auto]

That one ???

 

I am being a bit of a smart arse, but it could address one cause by reducing the flow speed.

 

There's plenty to consider: coolant temperature, pulley size, flow restriction before the pump...

 

I'm no fluid mechanics guru, but isn't where the cavitation ends & the bubbles collapse that the damage occurs? The piccy makes me wonder if that was on entering the impeller.

 

I did consider changing the pulley size but that becomes difficult on this engine. It has aircon which uses a wider belt than a standard mini therefore a pulley won't be available easily. I haven't checked what the belt width was on the factory systems but this early version is unusual. There is very little adjustment for the belt length too.

 

I actually wish I knew what happened first, there was a combination of problems which occurred and I don't know what happened first. After they started, I kept driving for about 500km. Lack of power on a large uphill section (although I've never been there before and never so loaded up). Then it began running hot around this time but I suspect a lot of this was the day getting warmer. Around the same time, the water pump started leaking. The float valve had failed around then too.

 

I am probably fine to just fit a new pump but I am wondering if there is some air trapped somewhere. I have been wondering for a long time if a vacuum coolant filler would be of any use.

[nicklouse]

Time for a Davis Craig?

[Spider]

I never understood the bypass purpose until you posted the picture of the old type of thermostat. This will be getting the bypass blocked off.

 

I have been wondering about getting a vacuum coolant filler.

 

I would fit an expansion tank of some kind but the engine bay is already packed full of aircon parts.

 

To make the air problem worse, this AC mini has a sandwich plate under the thermostat housing, making the top hose the highest point.

 

 

I've fitted them up in the wheel arch on cars with packed engine bays, I've also does a few down by clutch. . It doesn't necessarily need to be at the highest point, it does make it easier on the initial filling, but doesn't matter after that.
 

[68+86auto]

Time for a Davis Craig?

 

That is totally unnecessary.

 

All I need to do is make sure I have the most suitable type of pump.

[Midas Mk1]

If you’re concerned about trapped air, put an inline bleeder connection on the heater pipes at the bulkhead, ie highest point.
Worked a treat for me and makes bleeding easy. I too haven’t had much success with evo pumps in the past, had casings crack at new and after hard use, ended up getting a few nos uni part pumps.
I have the bypass blocked and have just drilled the thermostat in a few places, works great.