Cooling System Riddle 1991 350 SDL
 

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?

... put a new radiator cap on it

As noted in the post, it has one.

Unless you are blowing radiator hoses up it is either not sealing or is not strong enough. Me and the master tech next to me say... put a new cap on it. if you really need something else to check have a assistant put it in gear and put a load on it while you check for exhaust gas in the rad.

Do you have before photos to ensure that the hose routing is correct?

Thanks for your responses.

Caps (3) have been swapped from systems with no issues.
A new cap has been installed.
A replacement reservoir from a system with no issues has been installed even though the original one tested tight.

But...with your encouragement, I will have the brand new cap tested.

Thanks

it could be as simple as your using the wrong one for that rad so the sealing surfaces dont meet. Does it ever hold pressure?

X3.

We have seen this condition on race cars. Start to puke after x minutes, new parts list like yours. Often we try yet another 'new' cap and it works. In some cases we go so far as to use a heavier cap. (Not that it physically weighs more, but the pressure rating is higher).

have you tested the cooling system pressure (with the "new" cap installed) and note at what point it starts to push fluid? Perhaps the pressure is hitting 15psi, pops the cap and then pushes fluid out. The indicated pressure should then be even lower as the system evacuates.


this may sounds crazy, but are the water pumps any different between the two motors? rads different? Hoses different?

What direction does the water pump turn on each motor?

With a real cooling system issue i would expect him to complain of over heating (hoses water pump direction). My vote stays cap.

Just had these exact symptoms on a buddy's diesel Jetta -- it, too, was the radiator cap. The o-ring was missing. A new cap (and o-ring) fixed the matter completely. Maybe you just happened to get three bad caps!?!?

I'm thinking radiator cap also. You need to get a reading on actual pressures in the system when this is happening.

Thanks for all your responses.

The cap held 30 psi in the tester. Seems to be quite a bit for a cap rated at 20 psi.

I will order a new cap anyway. It will be here tomorrow morning.

Coolant is not going to displace itself with air. Either there is a local hot spot in the engine, or air is being introduced to the suction side of the system, which could then lead to a hot spot.

but at higher engine speeds, the cooling system should self bleed.

Am I wrong?


But clearly, if the system is NOT self bleeding, I would try to find out why.

quick question.. have you pressure tested the system? a leak on a suction side would be plausible of you fuel heater circuit is on the suction side of the heater core.. i still think cap.