We have made a bit of a breakthrough here!

In doing a v6 conversion we have always fitted brand new water pumps as part of the job, but on some v6 swaps a few people have reported that if the engine is left to idle for long periods of time, especially after a bit of a thrash, they would start to very slowly overheat.A quick blip of the throttle and the temp would settle to normal again.

Now it occurred to us that this was no normal fault, if the thermostat was faulty it would overheat all the time,or stay stone cold... if the radiator was partially blocked it too would give a constant problem.You also have to factor in the fact that not everyone has seen this little oddity with the coolant temp.Indeed all systems are checked for fan operation, rad switches checked and systems fully bled.

So what was causing the odd V6 mr2 to heat soak around the engine if left to idle but not others?

Well we put in a lot of time into this one, mainly because if we are fitting brand new water pumps,thermostats and doing everything by the book then what else can we do as mechanics?

We have been getting our water pumps from our local parts supplier that i have been using for the last ten years, he only supplies good quality parts, none of the cheap rubbish you can buy off ebay or from china.Normally it is a QH (Quinten Hazel) water pump we would use, who are a reputable make of spurious parts, and it was only last month when we happened to have a QH pump on the bench, a genuine Toyota one and a Blueprint pump all side by side that we stumbled on something VERY important.

It appears that different makers of spurious pumps have DIFFERENT designs of pump impellor blades!

Take a look at the pic below....


Notice anything odd? Both of these pumps are brand new 3vz-fe pumps, the one on the right is a QH pump and the one on the left is a Blueprint pump.You can see the impellor design is completely different between the two..... we compared them both to the genuine toyota pump which shared the same design as the Blueprint pump on the left.

So, we then fitted the new Blueprint pump to a conversion we did that was having the mystery heatsoak issue and after a quick bleed up of the system it was 100% cured.

What is basically happening here is that the QH pump impellor is not moving enough coolant at engine idle, probably fine for a camry with its short run to the radiator but on an Mr2 it was just lacking enough push unless it was given a little blip of the throttle now and then.

We have spoken to QH directly about this, who were initially shocked that their pump design was not the same as toyotas, and they have promised to change the design.Wether they do or not remains to be seen!

So this is an important find, you presume by fitting a brand new water pump that you are doing everything in your power to do a good job, sometimes you have to look into it a little deeper.

Bottom line is if you do buy a QH pump make sure it has the same impellor design as the pump on the left.Otherwise buy a Blueprint one that shares the toyota design.

To date we have no idea which conversions we have personally done have which design of pump, it may or may not have been one design or the other, who knows! All i do know we have been fitting new pumps to them all.Ironically if we left the old knackered toyota pumps on (presuming of course nobody ever changed it during its life on the camry) the issue would never have been spotted.

So a bit of an important cooling system breakthrough this one, up to now it only applies to the 3vz, 1mz pumps appear to be all the same.

Hopefully this new information helps you lads out.