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Old 11-20-2003, 11:32 PM
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Cooling system loosing pressure?

Here's one for you guys...

My expansion tank needs topping up once a week, though very little (like 250ml to 500ml maybe).

I suspected that coolant was being lost thru the expansion tank overflow hose, so I tied a small piece of cloth at the end and sure enough, later that day, the cloth was wet and the coolant level was a little lower.

Do I have a crappy cap or what???

I am certain that the coolant is lost thru' the overflow hose.

Btw, my car's runs at around 82 deg C, abt 89 deg C in stop-go traffic.Ambient temps where I live are around 32 deg C.

The only time I ever hit 100 deg C was when I blew an aux fan fuse while having a shot electromagnetic clutch in the car at the same time.

Your thoughts, guys...


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Old 11-21-2003, 09:48 AM
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Sure sounds like a bad cap to me that's not holding the proper pressure.
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Old 11-21-2003, 11:47 PM
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Yeah, I'm also thinking that's probably it and it's a cheap shot anyways...

Definitely not a head gasket problem I think coz I am losing water from the overflow hose and not overheating coz the water is lost in small amounts.

Also, no oil in water and vice versa.

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Old 11-22-2003, 08:01 AM
sunil190e-1.8's Avatar
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replace the cap and let us know if you cure the fault
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Old 11-22-2003, 10:16 AM
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Replace the cap and watch the water level. This is the first thing to do. More than likely that is your problem. The tank cap is designed to hold a certain pressure in the tank. Anything in excess of that pressure and it alows it to leak out through the overflow tube. It also has a gasket that can be bad.
Now what is causing the "overpressure". the caps do get old and allow the antifreeze out at 15 Lbs. of pressure instead of the 23 or 25 they are designed for. Of you cap would be the problem in this case.
The bad senario would be a blown or leaking head gasket. My sons Mustang had the heads off 3 times because of a leaking headgasket. The symptoms, pissing out antifreeze and bubble in the radiator. A couple of tests can let you know. A CO test can determine if hydrocarbons are getting into the antifreeze. The use a dye and draw some of the antifreeze into the tester. If it changes color they you probably have headgasket problems.

Now don't get upset. The majority of the times its the expansion tank cap. I would say 95% of the time. Especially if you have never replaced it. The gasket on the cap goes bad and ruins the seal.
So replace the cap. And watch the water level. If its still goin down ask a independent shop to do the CO dye test.

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Old 11-22-2003, 11:41 PM
Mike Murrell's Avatar
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It's not uncommon for the 103 overflow tanks to leak. Been there.
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
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Old 11-23-2003, 08:12 PM
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I'll swap the cap and let you guys know my findings.

Thanks, y'all.

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Old 11-24-2003, 12:50 PM
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Also, the base of the cap, on the tank, may be leaking. Not the cap itself. In this case you need a new tank @ $40 (or used one)
good luck
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Old 11-24-2003, 05:01 PM
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Location: Leicester UK
Posts: 39
Funily enough my 1.8 190 is doing similar things.
I flushed out & refilled the whole cooling system about 2 months ago.
Driving home the other day the low coolant lamp came on & I had to top up with about 3/4 of a litre.
I put it down to air in the system when I refilled it, but my cap has seen better days. I now realise that there is a spring in the cap which "lets of steam" at a certain pressure, even if the rubber seal is still doing its job.

Hopefuly if this a common problem it will be solved with a new cap.
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Old 11-28-2003, 08:45 PM
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Thumbs up Solved! New cap did it, of course.

Swapped out the cap on Tuesday for $6.50.

Checked the coolant level today and it is now consistent!

Interesting side note - on Monday night, just 3 minutes into a 30km drive, the coolant light came on.

Stopped the car, turned engine off and opened cap to check the coolant level but it was good (coz it was just checked and topped off in the evening).

Started car and coolant light went off and never came on again.

I put this down to dirt/crap on the low coolant sensor and will take a look at it when I have the time.

I believe it needs a good cleaning.


Also, the base of the cap, on the tank, may be leaking. Not the cap itself. In this case you need a new tank @ $40 (or used one)
Kind of a strange coincidence but my kid brother's 190E and needs a new cap/tank coz he is losing coolant from both the overflow hose as well as the base of the cap.

Thanks guys.

This was an easy one.


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