Pressure of Radiator Cap 83 500 SEC
 

I am having a heck of a time ( I searched) finding out the correct pressure rating for my radiator cap on my 83 500 SEC 120 or 140?

The cap I have is in German & has 120 on it
I think I have seen 140 caps listed in catalogs
Do I have the right cap?

I have a Behr radiator
126 505 0155

Thanks

Most of those cars came with a 1.0 bar cap...replacements are 1.2 bar.

No need for a higher pressure unless the engine has issues!

Thanks
It has a 1.2

Since my engine is still running hot 95 / 100C
though less than the 115/ 120 before the changing of
thermostat
senosr
radiator

All I see for the car is 140 in MB parts

I don't know if the rad cap could be a fault for the hot running temp
but its cheap enough if it could make a difference

Just don't know whether I have to stay w 120 (harder to find)
or the 140 would be good?

A little higher pressure cap will make it cool better if the cooling system is slightly below the required capacity to cool the engine. If all is fine, it will not make any difference, I think.

A slightly higher cap is worth a try, they don't cost much.

Tom W

I would be much less worried about the manufacture of the cap than I would be about the pressure rating of the cap.

Since after all the replacments & the engine is still running in the 96- 100C
range would a 140 cap be better?
Is there any danger from the extra pressure going from 120 to 140 on the system?

Thanks

not really and danger, just watch the temp gauge. All the change in the cap pressure will do is cause the cooling system to allow the system to cycle the fluid in the reservoir sooner or later that it would. I think the higher the bar on the cap the longer the coing cycle, but dont quote me. I would say with your new radiator and thermostat, the opening time of the seal on the radiator cap to the fliud reservoir is irrelivent as someone else mentioned. THe higher cap only seems to help when the system is already working at its max and yours is not going to be with a new radiator regardless of the higher operating temp you are experiencing

Not sure what you mean by working at max
just want to get the best one for my system.

Theoretically the highter the pressure the higher the boiling point of water.
So a higher rad cap would likely keep it a little longer to attain the higher pressure. Guess the only good thing would be if she redlines with a 140 the boiling point would be higher than w a 120 thereby halping the coolant not to boil but what does the boiling point have to do with it?
Its the temperature itself that can hurt the heads & gasket
so it shouldn't matter if its boiling or not accept at a boil there is a liquid & gas state...now don't know if thats a factor as a gas is air & air will lessen the capacity of the cooling system which is already stresed.

Seems like it really doesn't matter??????

:freak:

It really doesnt matter I would say.
You have a new radiator and thermostat, you are not losing any fluid from a bad head gasket or any leaks, there is not much more you can do to get the engine to run any cooler. I guess you should have the mech make sure the aux fans are coming on and off as they are supposed to and that the temp sensors are all working as they should. But after that it the car is getting air and the rad and therm are new, it is likely your car is running at its Max cooling capasity right now. It is not the end of the world running ar 90-100 as long as that is where it likes to stay. The big thing would be to see if it cools down while on a highway trip in moderate weather, say 85-90 max hi temp outside. If the car cools down to a average of 90 c then it is going to be fine. All cars like to heat up when not moving and the outside air is already hot. I have had my small 190E engine temp at 100 to 110 easy sitting and idling in an enclosed garage ( no, I vent the exhast outside) and it was winter time outside.
I would buy a new radiator cap, get which ever one you want and use it. I would say that if the 140 is the one for sale on line everywhere it is likely because at some point MB determined that they wanted to use the 140 and they now recommend the 140. I have seen that on several diff MB models. Mine has a sticker right on the reservoir that says use only a cap with 120 marked on it, but the dealer will sell you a 140 every time because that is the new part number that the old part was changed from in my car's case. I would bet yours was changed too in the parts lisst at some point. the online sales shops are not making anything up, they simply go by what the parts list at now, I would say that the 140 is now listed at the correct cap. Have the dealer run the part number or look up what cap they would order, it is probably a 140, eitehr way, it really wont hurt a thing.

Dealer didn't know 120 from 140
so I opted to hold off on that until I got more info.

I checked the expansion tank tonight
it was cool but still under pressure
may have been a tad warm...

A slight coolant seeped out oupon opening.

Coolant looked higher than I had filled it
It took 24 ounces last tiime I checked it after I ran it after they iinstalled the new radiator
I can only assume it had air or the shop didn't bleed it right.

Today it was to the neck
higher as I guess it was a bit warm

I am a bit concerned over some scum floating on top
sort of a oily scale
It didn't have that with the old coolant before the radiator change so I am wondering what that is hoping its not a gasket leak

Obviously cap is holding pressure
just don't know if it is as rated.

On way home today may have ran a bit cooler
did see some 90 C

My oil was clean & I remember the coolant looked fine before the rnew adiator so maybe it had some residue in it?

Alwasys something...

They did go from 120 to 140 but not sure if the older systems run better on the 120.

Thanks so much!

:P

I would try the higher pressure cap. I would start by finding out the pressure rating in pounds per sf and then go get one from any auto parts store that is rated higher and try it and see if it helps.

I think if the level in the expansion tank rose, maybe that means the cap is leaking when the engine gets hot.

It is a pretty inexpensive experiment.

Tom W

Thanks
Maybe the cap is off
although a warmer expansion tank will be higher than a cold one

Still I can't imagine after 3 or 4 hours it was too warm...


MB cap are $16
If you think its worth a shot.
I still have to pick up lic plate brackets & touch up paint


Might just order the 140 MB cao

Thanks......

have you checked the fan clutch on the engine to make sure that it is locking up?

I'll have to look further into how to test fan clutch
I'd assume a reported good mechanic might look at it as he did have the car in twice trying to address the hot engine...

Thanks

Don't know about water pump either
but it must be working least somewhat

I had wondered if the mech had bothered to check the aux fans...
You should be able to test at least part of the aux fan operation by starting the car and then turning on the A/C. The fan should run with the AC on. It might take a few seconds, but the fan should turn itself on with the a/c running. That would at least give you a place to start the diagnosis, you will know if the fan will run at all. OH, if the charge in the a/c system is low, freon level, the a/c compressor will not kick on, and in turn the fan might not turn on...so, if the ac is in the on position and the ac air inside does not get cold, and the fan doe snot come on, it is likely because the ac freon level is low, not a aux fan problem