Guidance or Suggestions Appreciated
 

So I have a 2003 Boxster 3.2 with 79,000 on the clock that I bought as a project.

Having noticed the build up of brown little bits in the coolant, I flushed the system a few times and got it back to what I thought was clear. Since however, I have found the same buildup as pictured in the attached.

Having seen the chocolate intermix in most cases I was hoping it might be the leftovers of the cooling system flush, but completed a combustion leak test in the coolant reservoir just to have a little more confidence moving forward with the project.
The results, again pictured in the attached came back positive. The fluid remained blue for a few minutes but then began to change. I completed the test again with the engine at operating temperature and the fluid remained blue throughout the process. Later research explained that this could be due to the engine heat and expansion closing any cracks.

From this I have removed all of the spark plugs and performed a leak down test on the cylinders from cold. All six plugs look consistent with carbon deposits and no signs of coolant contamination along with the carbon buildup within the cylinders. The results of the leak down test were consistent across all cylinders with minimal pressure lost other than that coming through the oil cap and dip stick from the expected blow by. No bubbling coolant, no exhaust or intake escape and no hissing from the other cylinders.

Because the car was a project that was supposed to be mainly bodywork from a small frontal I can't comment on coolant loss prior because it has never been on the road. The only coolant loss I had put down to the system bleeding itself and the coolant pressure test I completed seemed to be fine.

During the tests the engine fans kick in and the engine doesn't seem to overheat. The temperature builds steadily and it takes some time and consistent revs to persuade the fans to kick in.

Would a leaking heat exchanger possibly result in a positive with the combustion leak test

If there are any suggestions out there as to a direction to explore or highlight any steps I may have missed before resorting to having to drop the engine for confirmation it would be much appreciated.

Many thanks.

no pictures? perhaps a harder look at what the 'little brown bits' are?

You still need to send the photos.

I wouldn’t worry too much. I flushed my system 3x using a specialised cleaner after noticing deposits (scaling) in the expansion tank when the car was new to me
You can never completely empty the system, so there will always be some residue left. I would trust the overpressure test you performed which I also did @ 1,1barg. The system kept the pressure for at least 10min.

Do you notice a loss of coolant, because this could be caused by "something". But you should rule out your cap which number should end with 04.

The 'little brown bits' could be your water pump impellor disintegrating..