1991 350 SDL with an engine out of a 1987 300D

I drove the engine donor vehicle 400 miles home with no overheating or coolant loss problems.

After it was transplanted into the SDL, the cooling system is pushing coolant out of the reservoir into the overflow container in the fender well. It does not overheat unless it pushes enough coolant out to be low on coolant. It does this at freeway speeds above 65. Below 65 it does not seem to do it. It can take 75 miles of freeway speed before it pushes out enough for the low coolant warning light to come on.

All coolant missing from the cooling system can be accounted for by the coolant in the overflow container. There are no external leaks.

It is pressurizing the cooling system enough to push coolant out the 20lb cap, but not overheating to do it. It must be combustion gases. I have tested for combustion gases in the cooling system and found none. I used a Uview 560000 Combustion Leak Tester. I have used this tester after every trip since I got it and it has never indicated combustion gases. I was so sure it was combustion gases that I tested the tester by sniffing the exhaust at the tail pipe and found that the tester worked as it was suppose to.

There is no steam, no white smoke, and no anti-freeze odor in the exhaust.

I have tried running an unpressurized cooling system by removing the return valve on the bottom of an old reservoir cap so all expansion gases can pass out to the overflow tank. It will still eventually push out enough coolant for the low coolant warning light to come on.

It has a new radiator
new reservoir cap
swapped out the coolant reservoir from an SDL with no cooling problems
fixed all coolant seepage, there are no leaks
the thermostat operates as it should
the fan and fan clutch work as they should
the heating system works and there is no anti-freeze odor

I have 2 temp gauges, the stock one and one with a sensor at the return end of the coolant loop that heats the VO fuel line and the filter in the trunk. Both gauges confirm that the cooling system does not get above 180 -190 range while this is happening. When enough coolant is pushed out, the VO loop gauge will fall as the loop goes dry and the stock gauge will start to rise as the system becomes low on coolant. I have never let it overheat.

If it is not overheating
If no combustion gases are detected in the coolant system
What could pressurize the coolant system enough to do this?

__________________
Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO