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MG MGF Technical - PRT and Heater in a MGF is this right?

After 6 months, my MGF is nearly back on the road

It got a little hot one day early Jan and shot a mixture of oil/water from the head gasket onto my exhaust wrap which caught fire!

Check out for the pics. Perhaps I should write something for FTF?

Anyway, as one of the mods I fitted to improve reliability during the rebuild, I fitted a PRT.

I Purchased a new PRT from rimmers (grey 82degree) and got the hoses from a breaking mgtf.

After much swearing, I managed to get the whole lot back wired in.

Anyway car back up working now, and dying to go in for its MOT.

Bleed the system fully ((loads of times now up ramps and back down again) btw mike saturs bleed nipple screw for the front rad is great for bleeding air.!)

Ive noticed a strange thing since all of this

The heater (interior) seams only to warm up luke warm (Airlock I say to myself)

But when I drive the heater warms up!

When I stop for a while it cools down.

Im very lucky to have an emerald on my car because of all the engine work I have done on it over the years, so I can see everything thats going on in the system, and all looks ok (to me!).

The rad fan kicks on and off, and the coolant sits at no higher that
101 degrees when idle then back down to 96

When driving the cars heating circuit sticks around the 83 - 88 degree mark.

Any ideas?

I think (well it was some time ago!) that I swapped around the way the bypass / normal circuit connected to the heater pipes (due to lack of rubber pipe length)

Could this be the prob?

Also I have quite a good water pump with large fins on (motobuild race
job) could this be the culprit (although I never had the problem

I dont want to get another HGF after 6 months of a full rebuild!


Chris Glen

hmmm, it's possible that the wider bypass means that there isn't enough pressure to feed the heater unless the engine is reving
Will Munns

When you were bleeding the system, were you getting good fluid pressure at the bleed point on the heater circuit? Did it vary with revs, as Will has suggested?

How interesting, i would never have thought to bleed the heater system with the engine running.

Physics are not my strong point!

Anyway this is what I did (with a topped up expansion).

I got the car up on the ramps (front) and opened up the heater valve. Nothing came out! Opened the radiator bleed vavle and that was ok.

Started the car, and left it warming up. Still nothing coming out.

Whilst the engine was running, I sealed the heater valve back up and pushed it down the ramps. Opened up the heater valve and some bubbles came out, removed it and after a hiss, coolant flowed. Screwed the valve back in.

I then reveresed the car up the ramps, and opened the valve again, a little more air came out but not much. Now there was a nice flow of coolant coming out and very hot.

Asking my mum to hold the engine at 2000 revs I watched what happend. The coolant did not come out any quicker or slower.

Is this correct? I don't know as Ive never tried it before.

The pump is working good guns as the expansion tank is constantly being topped up, and the rad fan is coming on and off etc.

I have realised now though that the heater air is always look warm.

If the worse thing thats happened is the larger heating circuit is stopping the car heater from working 100% i can handle that.

Or could it be something to do with the ducting from the heater matrix to the body? I did have to replace my heater resisitor before the car went bang. Could this cause luke warm air?

Thanks for the advice so far....
Chris Glen

This thread was discussed between 16/05/2008 and 17/05/2008

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