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I have a 1999 P38 4.6HSE, with a Thor Engine and a gas conversion. When I bought it there was an intermittent overheating problem, as I managed a garage at the time I got onr of the mechanics to look at it and we replaced the cheaper parts first, thermostat, water pump and viscous fan, the car would run fine for hours then suddenly overheat, the guage just shot up.

We did a gas test and that showed the head gasket had gone, however when we flushed the system it appeared that st some point a sealant of some kind had been put in and there was a lot of gunge in the system, we had a hosepipe runnning water through it for hours clening all of this out.

We did the head gasket and everything seemed fine but after about 20 miles the overheating re-occurred as winter was coming on I just got some k-seal and put it in and this cured the problem.

A few weeks later I leant the RR to a friend and when it cam back it was running very badly but not overheating, I had it diagnosed only to discover that a valve had burnt out.

The RR was then left for a few months until I started doing some work for a guy who owned a garage and he said he would get one of his mechanics to look at it as long as I wasnt in a hurry, so over a period of a few months the engine was stripped the valves done and another set of head gaskets put on. Once it was rebuilt it ran perfectly but although the heater was warm it never got hot.

Anyway after about 600 miles one day it just suddenly overheated no warning just straight into the red. I stopped waited for the engine to cool down and got the vehicle home. I checked everything and the top hose was highly pressurised but I wasn't losing water. I have checked it daily and had no more problems until last night.

Because I had to travel about 50 miles and the weather was bad I jumped into the RR, I got about 3 miles and it overheated I was on the motorway so waited until it cooled down then carried on after a mile or so it did the same, I had to continue this course of action starting and stopping until I got to a garage forecourt where I let it cool completely before topping up on coolant. Once I did the vehicle was fine but I am now wary of going anywhere in it, can anyone suggest a solution, could the radiator be clogged? someone suggested a porous block but I really have no clue as Im not a mechanic.

Thanks
Jon
 
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Yes, its no surprise.... and yes the 4.6 blocks have porousity issues.

From such sudden/rapid symptoms I would suspect that the k-seal has given up (-it does not work/last forever-) but another gas test will quickly confirm this.

It may not be relevant but how long ago were the head gaskets replaced.... just 600 miles ? Were checks made to remove airlocks ? Were any other 'sealants' used which may have blocked up the heater ?
 

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Did they service the heads when it was last apart?

I had similar overheating issues. I had the heads cleaned, valves pressure tested, the face milled, new head bolts, and of course new gaskets. Haven't had a problem since.
 

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I had heat issues also. I ended up putting in a new radiator and it fixed it. The new radiator cooled the engine much better. I am betting my old one was at 25% efficency.
 
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