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Old 03-22-2002, 01:33 PM
pmizell's Avatar
Benz Zealot
Join Date: Jun 2001
Location: Houston, Texas
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Angry Coolant overflow probs after head gasket change

Hey guys, need some help here please --

Recently replaced my head gasket and I'm now experiencing very hot running temps (110 to 120). Aux fans work fine. Apparently I have a leak in the coolant system somewhere, because I get coolant overflow when high temperatures are reached, even when driving. Expansion tank cap just replaced along with thermostat. All hoses are tight. Any other ideas where the leak/problem is coming from?

Does the overflow tank itself pressurize and possibly a bad one could be the culprit? How about the expansion tank?

I'm stumped. The radiator is only about 6 months old.

Any help appreciated.



'91 300E, 208k miles
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Old 03-22-2002, 02:37 PM
Fimum Fit
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I fear that you will have to go back and redo the gasket job

Your headgasket may not be sealing compression pressure, or the head (or block) may have a crack which was not detected when it was tested during the gasket job. It is slightly possible that the cause lies somewhere else (such as the old-time Ford problem of a bad water pump seal allowing the pump to suck air into the system when the motor is running) but it is extremely unlikely, under these circumstances.
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Old 03-22-2002, 03:05 PM
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Not exactly what I wanted to hear...

First off, thanks for the reply Fimium, however bleak....

How does this explain, though, the fact that this overflow was not occuring prior to head job?

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Old 03-22-2002, 06:35 PM
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Does the overflow tank itself pressurize

Yes it does. I had to replace one on an M103 car last year, but I'd suspect your problem is head gasket and not expansion tank, especially if this didn't occur until you did the head work.
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Old 03-22-2002, 07:51 PM
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Gasket was in perfect condition (Elring), used new headbolts, 2 perfect 90 degree torques after 70nm tighten, head was resurfaced etc....... I just don't want to believe that this is the cause. Maybe another 90 deg torque down could solve it, who knows. Anyhow, it's at the shop being diagnosed. I'll report back when I find out more.

Thanks for the responses..


'91 300E, 208k miles
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Old 03-25-2002, 11:48 AM
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Diagnosis from shop:

Bad/leaking monovalve. They recommend replacing the hose too, as it's bulging at connections due to oil exposure.

This sound correct? Does the monovalve switch on and off to seal in pressure? Also, I understand that there's a monovalve repair "kit" available that's alot cheaper than replacing the whole valve -- however, I only see the monovalve listed under fastlane for $80. Anyone have a p/n for the kit?

Thanks for any insight as always!


'91 300E, 208k miles

Last edited by pmizell; 03-25-2002 at 12:28 PM.
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Old 03-25-2002, 05:27 PM
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Did you bleed the cooling system? There's a bleed port on the head above the #1 intake runner. You're supposed to open the bleed port while you're filling with coolant so you don't trap air.

How do you associate coolant overflow with a leak?

I doubt the monovalve has any bearing on your overheating problem. All it does is regulate coolant flow to the heater core. Maybe W124 monovalves do more than W123 and W126 monovalves do.

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Last edited by sixto; 03-25-2002 at 06:25 PM.
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Old 03-25-2002, 06:19 PM
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You mentioned you replaced the thermostat at the same time you did the head gasket. Is it possible you have a bad or faulty thermostat. If you still have the old one, maybe put it back in and hope for the best.

The monovalve thing sounds odd. Don't know how that could creat your run hot problem.

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Old 03-25-2002, 07:18 PM
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I would start by the most obvious.
1) check for coolant leak and condition of hoses (colapsed?
2) have your rad. cap tested for pressure.
3) Make sure the system is completely purged of air
4) Check thermostat
5) have the coolant analysed for presence of combustion gases.
If all of the above is perfect, start praying...... and thnk about
water pump??
blocked radiator???
fault head gasket, cracked head/cylinder block (very unlikely if combustions gase test is negative)
Good luck. let us know what you found
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Old 03-26-2002, 09:26 AM
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The tech got ahead of himself when he told me the monovalve was bad. The shop foreman cleared the diagnosis up and told me that the hose to the monovalve needed replacement because it was "collapsing" as JackD put it, due to oil exposure -- causing too much pressure to build up at the connections and ballooning.

New hose (special order) will be put on and the diagnosis will continue after that, if not fixed.

Cap is new. Thermostat was replaced about 6 months ago, so this could be the culprit. To test, can it simply be removed from water pump housing and see if problem is resolved?

Thanks again for the responses..... gotta love this forum!


'91 300E, 208k miles
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Old 03-26-2002, 10:19 AM
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Is the expansion tank cap on securely?

I know this sounds like a dumb question, but when I flushed my coolant for the first time, I had closed the cap to where is clicked on the detents on the expansion tank spout. I had high temp problems and coolant overflow, collapsed hose from the coolant pump, etc., for about a week until it dawned on me that the cap would rattle when the coolant I twisted the cap some more and presto! Problems disappeared!

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