PRRT - better but not perfect (FIXED.ish)

[Smithy77]

Hi all,

My new K series R400 engine (with DVA mods running 230bhp) used to suffer (badly) from overcooling when it had a standard cooling set-up (standard rad, standard bypass, no heater, 82c stat with jiggle valve). Cruising with ambient temps of 10-15c saw the coolant settle as low as 60c

I have replaced the rad with a Coolex item built to standard spec as the old one was weeping. I also decided to try a PRRT acquired from Mankee (  ) which was tested and seemed to be functioning correctly, age is unknown. So current plumbing has the standard 16-20mm bypass deleted with a "standard Rover" 32mm bypass between top hose and PRRT. The PRRT is fitted in the bottom hose between rad and submarine.

For further reading and an understanding of how it works see here

This mod has completely eradicated any over cooling and temps remain rock solid at 80c under all driving conditions. However, it doesn't seem to work quite right when at idle. If I leave it to come up to temp, all the hoses warm up with the exception of the bottom hose, as you would expect. The problem is, the water temp then continues to rise beyond 92c (switching point of fan) and will settle at about 95c. At this point, the bottom hose (before the PRRT) and lower O/S part of the rad is still cool while the top of the rad is too hot too touch and the fan has not cut in, suggesting that there is little to no flow through the rad. If I then rev the engine slightly, temps almost instantly start to fall and the fan kicks in and cuts out again at about 86c, so the thermo switch appears OK. 

I am hoping that some of the more scientific members could offer their thoughts on what is happening. I get the fundamental principles of how the PRRT works (from reading the above link) but perhaps there is something which someone else can see to explain this behavior.

The PRRT relies (somehow  ) on pressure to operate correctly and comes in a small variety of temp and pressure spring ratings (see here). I believe mine is the 82c stat with soft spring PEL500110, but I haven't verified this myself. I'm not sure if the "pressure" is required to help open the rad circuit or help keep the bypass circuit open. I read somewhere that the soft spring must be used on our engines as the pressure at idle isn't enough to operate the PRRT correctly with the harder spring versions, but I am not sure on the theory myself.

My first thoughts to resolve this are:

1. Try a brand new PRRT PEL500110 with current plumbing

2. Try reducing the bypass from 32mm to 25mm to help direct more coolant through the rad

Hope someone can help. Thanks in advance!

[Superwhite R283]

Initial thoughts are it might be something to do with the water pump, as this is engine (rev) driven, and the problems is only on tick over?

[Titanium7]

Have previously fitted two of these systems in the past and both have worked perfectly and held the temperature at a steady 80C no matter what. The first car always was a s*d with air blocks and the PRRT sorted this issue. The second car, a R500 had the same over cooling issues as yours and it cured it immediately. 

[Ivaan]

Daft question... you have removed the old stat?

[Mankee]

Yep, we definitely did that before re-installing the engine.

[Smithy77]

Darren - it had a new metal impeller water pump fitted for the previous rebuild in early 2015, so maybe 3000 miles on it. The pumps can leak or have nackered bearings, but I think I'm right in saying that there isn't really much else to go wrong with them which would effect flow rate.

Titanium7 - I can see a couple of subtle difference between our set-ups. I see you still retain the standard bypass from coolant rail to OEM stat housing - this, in theory, should increase overall "bypass" flow and reduce rad flow. You also appear to have an R500 triple pass rad fitted? I think I'm right in saying these rads are higher resistance than standard rads, so again, this would, in theory, reduce rad flow. So, even though these two differences would, in theory, worsen my problem of lack of rad flow, you report no such issues. This would suggest, therefore, that my bypass diameter is not the problem and would point more towards the PRRT being at fault.

One other potential factor here, Titanium - what did you have your idle set at? Mine is about 900-950rpm I think. It could be as simple as a low idle speed causing the problem. I found that raising the revs by only 200rpm or so was enough to get the coolant flowing through the rad....

Ivaan - perfectly valid question, and as Mankee kindly pointed out, we did indeed fit a gutted stat in the OEM stat housing

[Titanium7]

Not triple pass but Radtec standard previous car at 200bhp had the Radtec Extremme Tickover at 900rpm is a tad low, liked to keep it around 1000rpm always with a little blip to start with to get the charging going into the battery.

[Smithy77]

Thanks, will see what effect raising the idle to 1000rpm has

[garybee]

" it had a new metal impeller water pump fitted for the previous rebuild in early 2015, so maybe 3000 miles on it. The pumps can leak or have knackered bearings, but I think I'm right in saying that there isn't really much else to go wrong with them which would effect flow rate."

 

A common failure mode of water pumps is for the pulley or impeller to work loose from the shaft (failure of interference fit).  This often means that the impeller is only turning part of the time (heat/rpm related).  I've seen many fail this way.

[Smithy77]

Interesting Garybee, I stand corrected - you have a good point Darren!  

I've ordered a new PRRT so will see if that sorts it first. If not, perhaps the pump could indeed be worth checking.

[Smithy77]

Just to update this thread, new PRRT behaved exactly the same. Increased the idle to 1050rpm and bled a bit of air from the top of the rad and it now warms up and controls temps at idle just as you would expect.

[Superwhite R283]