I recently had my 72 280SEL radiator recored due to leakage. The source was a small crack in the top portion of the radiator. Other leaks developed during the cleaning process and thus the decision to recore the whole thing. The car normally runs around 180 degrees.


The problem is that I now get dripping from the radiator overflow tube after the engine is turned off - something that didn't occur before. My mechanic recommended replacing the stock MB cap with a 16 lb cap from the local auto supply store. He says the coolant pressure rises with the engine off (it doesn't seem to leak with the engine running). I replaced the cap, but it still leaks from the overflow tube. Is it safe to go to a higher pressure cap (maybe 18 lbs) or is there some other problem I should be aware of?

Thanks for your help,
Mark W.

If you are positive that you are getting an accurate temperature reading, I would take a look at the radiator neck to ensure that the cap has a smooth, even surface to seal against.

I don't really have an answer as to why it does that. The fitting on the neck it's tube were meant to send overflow to the ground or it wouldn't be there in the first place is my theory. My 4.5 did the same thing occasionally. You might consider rigging up an after market plastic coolant recovery tank. Any auto parts store has them. Mounts nicely on the drivers side in the engine bay. No more mess on the ground. I guess those tanks were mandated by the EPA or DOT or both at some point to stop the pollution as all later cars have them as part of the cooloing system.

Of course, consider I have no idea what I'm talking about! ))

STS

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The motor will rise in temperature after turning off, because the water pump stops moving the coolant.

A long shot, but, you aren't topping the radiator to full everytime this happens are you? You are not meant to fill a radiator to the top, but leave a few inches for expansion.

(In laymans terms)

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Thank you for the suggestion. I was periodically filling the coolant level to very near the filler neck. I'll let the fluid level drop and see if it stops leaking on its own.

Mark W.

And no, I've been told that it's NOT ok to use the higher pressure cap. You'll have to ask REAL mechanic as to why. I'm sure I could find a section 14 in some manual that simply says "You MUST use a 14lb cap" with no supporting reason.

-CTH

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Bear in mind that the cap acts as a pressure release valve in an overpressure situation. If the pressure rating is upped too high, the pressure will find another point of escape, such as through your wallet.

I agree with PaulC... a higher pressure cap only puts more overall pressure on your coolant hoses and at some point, a fitting or hose failure is more disastrous than the dribble from a cap.

Too high a coolant liquid in the rad and it will overflow when hot. The correct level, when the engine is cold, is when the liquid reaches the lower part of the roundich level indicator you can see in the back of the rad when looking through the neck.

I believe your original cap was marked 100, but this pressure (14.5psi) may be hard to locate. The 100 refers to the percentage of a bar of pressure (14.5psi) used as a rating. Most caps I've seen recently are 120 or 140, i.e 120% or 140% of a bar, and this can be too high on a car with an old heater core. Do some shopping and get the 100 cap. In a pinch you can simply use a common old Stant cap with a 14.5-15psi rating.

Regarding your fill procedure, there should be a pronounced flat area visible in the OEM tank top somewhat below but near the neck. This is the fill line. Above this mark leads to the piddle drips through the overflow tube. A recovery system is helpful to purge air and raise efficiency of the whole system. Any parts house will have something that will fit. Take the new 100 cap and add the sealing gasket from the recovery kit so that there is a positive seal at the cap. Fill the radiator to the top, add coolant to the recovery tank and run through several drive cycles allowing it to cool overnight. At the end of the week you should have a purged system and a slightly lower level on the recovery bottle. Refill it to the cold line and you are good to go.