In another post I told of my buddies 606A loosing coolant and having pressure in hoses after cool down. So we did a full head job. He also replaced the radiator.
With head job done drives great however pressure still present in upper rad hose next morning after cool down.
What I discovered:
I opened the coolant reservoir cap and took a sniff. Strong smell of diesel fuel in coolant and some floating on top! This is Clean fresh diesel fuel floating not black oily residue. My diagnosis is that the Fuel Preheater has internally broken some how allowing fuel into the cooling system.
There is no other place on the engine where the coolant comes in direct proximity of the fuel.
Suggestions? Am I missing something?
He will be replacing the preheater.
Mark

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91 300GD SWB with transplanted 87 OM603 turbo motor

Some years and Models have a Fuel Heater. However, I don't know the details.

If the Fuel is heated by the Coolant a leak there could be the issue.

Something else that could hold the pressure in would be if the Coolant Pressure Cap is not working.

I also believe someone mentioned that there is some sort of test strips or sniffer to tell if combustion gasses are getting into the Coolant. But, I don't know if that would still work if Fuel was previously in the Coolant.

I have always wondered if there is any way to attach a Ballon to the Coolant System and let the Engine idle to see if it fills with Gasses.

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84 300D, 82 Volvo 244Gl Diesel

The fuel heater is indeed the first thing I'd check.

Thanks guys for the confirmation and suggestions.

The NAPA tester tests for gasses and would not be accurate as the present diesel fuel would give a false reading.