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  #1  
Old 12-18-2001, 12:52 PM
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Unhappy Strange cooling system problem - exhaust gas entering system??

Hi all,

My 1987 300D developed a strange problem suddenly. The cooling system remains under high pressure even when stone cold (left overnight). Yes, I have replaced the radiator cap. The car has never done this in the previous 4+ years I've owned it. The only way I can think of abnormal pressure in the cooling system would be from exhaust gas entering. How do I test if this is happening? The coolant looks clean and smells fine (to me.) There is no oil in the coolant or vice-versa. I suspect either the head gasket or a crack in the head (shudder.) I'm at a loss here... Help!


Dave M.
Sacramento, CA
1987 300D - 228kmi (weird cooling system problem)
1987 300D - 234kmi
1984 300D - 205kmi
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  #2  
Old 12-18-2001, 01:45 PM
woody
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exhaust gas in radiator test

Hi GSXR: Before I bought my 500 sel, I was leary of owning an aluminum engine, cracked heads, etc. So I convinced the owner to let me take the car to a smog shop in San Diego and they put the probe that they normally check emissions at the opening to the radiator cap and it will sniff any exhaust you may be getting from a cracked head, etc. Hope this helps, woody
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  #3  
Old 12-18-2001, 04:21 PM
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The way you discribe,things it dosnt sound as if your car has a problem.It sounds as if the radiator cap you replaced is just doing its job well.
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  #4  
Old 12-18-2001, 04:32 PM
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My '87 300D holds radiator air pressure a long time.

If you are not losing any coolant, I can't imagine you have much of a problem. I think your cooling system is just doing a good job now at holding pressure.

Ken300D
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  #5  
Old 12-18-2001, 04:52 PM
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Hi Ken,

The problem is that about 12-24 hours after the engine is shut off, the pressure forces coolant out somewhere near the water pump or thermostat area - I can't tell exactly. (I'm guessing the water pump seals.) When the engine is hot it doesn't leak. I'm speculating that when the metal w/p body cools, the seals shrink a tad and allow seepage. After all, the system is designed to hold 1.4 bar at full operating temp, not at ambient. If I find it is coming from the w/p I'll probably replace it anyway.

However, I don't feel the real problem is the leak (although that's not good), I think it's the fact that there is high pressure when cold - which is NOT normal! Usually, when the engine cools off the system is under slight vacuum. The upper radiator hose may even be "compressed" a little bit - but there is NEVER any pressure present when cold.

Almost the entire cooling system was replaced ~3 years ago - new radiator, water pump, fan clutch, thermostat, all 5 hoses in the engine compartment, new plastic reservoir/tank, radiator cap, and factory MB coolant installed w/RedLine Water Wetter. It's been doing great up until a few weeks ago.


[Side note: This proved the dealer to be totally wrong in their claim that "if it's not in the red, it's normal operation." What a load of crap. The car used to run at 105-115C in the summer, and after replacing all the above it now refuses to ever exceed 90-95 except under the most brutal conditions (i.e., 110F ambient, climbing a hill with the A/C on). The problem in my case was a bad radiator AND a bad fan clutch. I changed the other stuff as preventive maintenance. My experience with several other 1987 300D's is the same - they should never get over 90-95 most of the time, and 105+ indicates a serious problem.]


Dave M.
Sacramento, CA
1987 300D - 228kmi
1987 300D - 234kmi
1984 300D - 205kmi
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  #6  
Old 12-18-2001, 05:51 PM
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I have had cars over the years that when cold still had pressure.
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  #7  
Old 12-18-2001, 06:14 PM
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Unhappy

I know, I've heard that from other people too. My point is that this car NEVER, ever did this until recently, and my others don't either. I'm not talking about a little pressure, I'm talking about a LOT of pressure; 10-15psi or so. The cap almost shoots off the top of the reservoir when I loosen it. That's not normal.


-DM
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  #8  
Old 12-19-2001, 10:10 PM
Mark Elrod
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Head is cracked

No doubt about it, crack in a combustion chamber. Extreme pressure in that engine forces gasses into the cooling system, the crak has not gotten large enough to allow coolant to be forced back into the cylinder, not yet.

All cooling systems should not only drop pressure from hot to cold but will actually develope vacuum when cold.

sorry for the bad news.
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Old 12-19-2001, 10:56 PM
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Mark, that's what I was afraid of. I'm hoping it is the head gasket and not the head itself. I'm probably going to try K&W Block Seal before tearing into things too far - it worked last time (to stop an oil dribble into the coolant, 3+ years ago) so who knows, it could work again! If not I'm in for a $1-$3k repair. Ouch... too bad the rest of the car is pristine, and I'll have to actually fix it... [;-)
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  #10  
Old 12-20-2001, 05:02 AM
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I think people are confusing what happens with a conventional US car (unpressurized) coolant overflow tank vs. the Mercedes pressurized tank. In the US system, the car burps fluid out under pressure into the overflow tank, then pulls it back when cooling off. The Mercedes system is simply a radiator with part of it (the plastic tank) being "see through". In the US system, there is not supposed to be much air in the pressurized area. In the Mercedes system, there is significant pressurized air inside the system.

Since people that have this engine are telling you that this is normal, and you also indicate the radiator cap is now new, maybe what you need is a new water pump and not a new head?

I know the dealer will not be as happy........

So, we should get more people in here that actually have a 603 or 602 engine to tell you their experience. Maybe we both have cracked heads (in the engine).

Ken300D
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  #11  
Old 12-20-2001, 08:04 AM
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On my 300D that coolant reservoir seems to stay pressurized for at least 24 hours after shutting the car down. In all honesty I have never seen anything like it and have pretty much come to the conclusion that it is normal.
I do not see any indication that I have an engine problem. Car starts and runs good and maintains level in the reservoir and I do not see any oil residue that would indicate a problem.
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  #12  
Old 12-20-2001, 11:54 AM
C Amos
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What an intresting post. Sure am glad it is not my car though!

Unfortunately, I agree with Mark Elrod.

Starting with a cold system, no pressure, and run it as hot as you
like, once it cools it will return to the same volume.

As Ken rightly states, the MB system keeps all of the coolant under pressure, and accounts for the natural expansion by
the elasticity of the plastic resivoir in the loop.

The only way the pressure could increase to something greater then what was in the system on startup is if there was a leak adding pressure.

Craig Amos
1987 300D turbo
270,000 miles
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  #13  
Old 12-20-2001, 12:24 PM
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Question

Thanks, Craig. That's what I think too. I'm really puzzled by the number of folks who say their systems hold pressure when cold - I posted to the MBZ.org diesel email list and got similar responses over there. I'm not sure if they're confusing the "rushing air" noise when opening the cap to be pressure instead of vacuum, or if there are a bunch of other 602/603 engines that are all going to need head gaskets soon as well!

Anyway, follow my logic here if you will:


The cooling system (602/603 engine specifically, but should apply to almost any engine) is a sealed system, with the radiator cap only acting as a pressure relief valve at 1.4bar (~20psi). When it's cold (ambient temp, 50F at my house) if you open the cap, there will be zero pressure in the system. Close the cap - same thing, zero pressure. Drive the car - the coolant gets hot, and expands, which causes pressure in the system. Shut down the engine, it cools, the coolant contracts, it returns to ZERO. If it doesn't there is something entering the system. If there were a leak, it would either lose coolant, and/or not hold pressure in the first place!

To explain how vacuum can exist, picture the pressure relief valve (rad cap) opening when the engine is hot, and releasing some pressure into the overflow tank. When the system cools, there will be less coolant in the system, which will result in slight vacuum. As I said, this is how my car acted for years prior to this month.

I'm going to try the K&W Block Seal route before tearing into the head job. I'm almost afraid to ask the dealer for a quote on the job. Any comments on things to replace while it's apart? I was going to have the timing chain & valve lifters changed, wasn't sure if there's something else I should add to the list...



Thanks again,

Dave M.
Sacramento, CA
1987 300D - 228kmi
1987 300D - 235kmi
1984 300D - 206kmi
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  #14  
Old 12-20-2001, 01:33 PM
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An additional source of air pressure is a certain amount of liquid water boiling off into water vapor and that water vapor is contained in the relatively large air pocket in the pressurized recovery tank.

When the system cools off that water vapor does not fully condense back into water - therefore some residual pressure.

No matter what creates the pressure, whether water vapor, heat, or leaking exhaust, it won't get any higher than the release value of the radiator cap. So, you will always have the same pressure in the cooling system at engine shut down (given a reasonable period of operation in which the system heats up fully).

So my theory is that the reduced volume of the coolant when it does cool down is not sufficient to "draw down" the large pocket of pressurized air in the recovery tank.

So, with a non-leaking Mercedes coolant system you get residual pressure. It is a unique feature of the design that keeps the "overflow" tank pressurized. Being used to the unpressurized American/Japanese overflow tanks, I was a little startled too when I took the Mercedes radiator cap off the "recovery" tank the first time.

If you had a small exhaust leak that was pressurizing the coolant system, it still would not pressurize the system beyond the radiator cap rating. And we know a non-leaking system holds that pressure even when cold. I say its just water vapor, and drive on.

--------------
OK. It's not like my theories don't get shot down sometimes.
--------------

The whole reason I got the '87 300D so cheaply is because the Mercedes dealer had the owner primed for a head replacement when all it needed was one glow plug - so he dumped it at a "quick sell" price. I guess the dealers just see dollar signs when these engines come in with any issue - because they know its an easy $2-3K. It's an easy shot, "Oh yeah, ALUMINUM head, yeah that needs replacing......."

Ken300D
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  #15  
Old 12-20-2001, 02:08 PM
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Easy test. With the engine cold remove the cap, then start the engine and let it run. Once warm give her a little throttle, not much just a little. If coolant starts running over through the neck on the tank, chances are good your need a head gasket (or head). If nothing happens hmm...nothings wrong. I don't think the water pump will circulate hard enough to flow out the expansion tank on this system.
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