coolant system pressure - PeachParts Mercedes-Benz Forum

its_snowing · #1 12-09-2006, 07:38 PM

question:

my 1982 300sd. i drov eit up to vermont from boston. after i had it parked for some time (several hours), i came out to unplug the block heater, which i had had on for a short while. i noticed that that coolant reservoir was almost overflowing, though i had defintely not overfilled, and it seemed to still be up to full pressure. i opened up the cap and it popped a little, and some coolant squirted out. however, the collant level remained high - all the way up to the cap. so, i put the cap back on and left it. when i next went out to drive, my car was running hot, up at around 100 degrees. i normally run around 80-85, maybe up to around 90 on long climbs. whats going on here? im wondering why the system would take so long to depressurize, and why it would now be overheating. when i would squeeze the hose coming from the radiator, it was soft, and i could hear bubbling in the reservoir.

on the drive up, it did not start running hot. this only happened after i put the cap back on. could i have put the cap on too tight or something? i ve got to drive back to boston tomorrow afternoon, and am a little worried about overheating. any ideas?

#2 12-09-2006, 07:49 PM

Sounds like an air bubble. Did you change the coolant recently?

its_snowing · #3 12-09-2006, 08:05 PM

no, but i did have the system pressure tested for what i thought was a leak. might that have done it?

t walgamuth · #4 12-09-2006, 10:03 PM

i suppose it might have, though i cannot see a reason for it to do so. brian recommends taking off the upper hose and filling it from there to avoid having an air pocket.

good luck

tom w

its_snowing · #5 12-10-2006, 11:02 AM

update: after sitting all night, the coolant in the reservoir is still pushed way up high, though not all the way to the top. the small hose leading from the reservoir to the radiator is very hard to compress, though the larger hoses still feel spongy enough to press in. if it is a water bubble, which hose do i disconnect to fill with water, i wonder?

#6 12-10-2006, 02:19 PM

Run the engine without the radiator cap and see if you can spot bubbles or the fluid "pulsing".

its_snowing · #7 12-10-2006, 03:21 PM

will do. what might that tell me, if there are bubbles?

carnut · #8 12-10-2006, 03:39 PM

I'll bet its a weak pressure cap. Lifting early to transfer coolant to the tank. most people never pressure test the cap itself, just the cooling system. Being filled to the top in the radiator is correct and I would think rule out air trapped. You cannot install the cap "too tight?" If the cap is over 3 years old, change it on principle.

t walgamuth · #9 12-10-2006, 04:59 PM

yeah, i was thinking bad cap too.

tom w

maddogg20 · #10 12-10-2006, 10:02 PM

My car is doing the same thing. ill come out after it has sat to cool and there will be a puddle on the ground under the overflow tank tube. I replaced my cap but I still dont think that its the right kind. It is a 13lb cap. Is that the right kind?

#11 12-10-2006, 10:05 PM

Quote:

Originally Posted by maddogg20

I replaced my cap but I still dont think that its the right kind. It is a 13lb cap. Is that the right kind?

You should use the cap that has 120 or 140millibars stamped on it. (17.5, or 20psi)

its_snowing · #12 12-11-2006, 07:53 AM

update:

i drove the car from vermont to boston with no major problems. every now and again, the temp would hit 100, but for the most part it ran in the 85-90 degree range. i will pick up a new cap tonight.

on a related note, im having new engine mounts put in in a couple of weeks. is there anything else i should have done while the engine is out? i know theres a hard-to-reach freeze plug around the back of the block, but im wondering if theres anything else that might cure my cooling-sytem ails that i could get done while its up on the hoist.

t walgamuth · #13 12-11-2006, 08:47 AM

not that i can think of.

i hope they arent taking the engine out for mounts.

tom w

its_snowing · #14 12-11-2006, 09:05 AM

maybe i misinterpreted what they were saying about lifting the engine "just a little bit." perhaps theyre only going to get it up enough to fit the moutns in. its sounded like an expensive repair - they were talking in the high 300s. in any event, its shaking like theres not tomorrow.

carnut · #15 12-11-2006, 01:11 PM

The engine is jacked up maybe 2 to 3 inches! simply to slide out the old and slide in the new ones which are taller than the worn out collapsed ones. Maybe two hours labor for front mounts. Whats their hourly rate? Are they replacing trans mount and engine shocks as well? Also you should have this question on a new post so in the future someone can do a "search" more easily to locate it.