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  #31  
Old 07-09-2012, 09:20 AM
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Quote:
Originally Posted by Brian Carlton View Post
It cannot blow it's cap and lose coolant unless the coolant boils
Again I will tell you that the cap releases at a pressure point, not a temp.

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I had this same problem and I spent months trying to solve it. In my case it was a head gasket. I don't know if your problem is a head gasket, but you should test for it. The whole painful thread is here : http://www.peachparts.com/shopforum/diesel-discussion/292852-cooling-system-riddle-1991-350-sdl.html

I never detected combustion gases in the coolant reservoir. This lack of gases in the coolant convinced me that it was not a head gasket. What did convince me to change my head gasket was pressure testing the coolant system while driving.

Here is what I did:

I installed a "t" fitting for the small radiator vent hose that goes to the coolant reservoir. Actually, I just removed one end of the hose, installed the "t" on the end and another short piece of hose to the radiator fitting.

I installed a pressure gauge on a piece of hose long enough to reach the bottom of the windshield in the driver's view and attached that to the "t". Use a plenty long hose so you can position the gauge where you can read it.

Went for a drive. I could watch the pressure build when the engine was under load, but not overheating, and remain stable when the engine was loafing along. I could see when the cap would release. I could see the pressure in the system the next day.
______________________________________________________________

The other thing to look at is your radiator. If it is no longer viable, it will not be able to remove the extra heat that putting the engine under load produces.

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Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO
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  #32  
Old 07-09-2012, 09:44 AM
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Quote:
Originally Posted by GregMN View Post
The other thing to look at is your radiator. If it is no longer viable, it will not be able to remove the extra heat that putting the engine under load produces.
Quote:
Originally Posted by Hooverfull View Post
I have replaced:

Thermostat
Radiator Cap
Radiator
Coolant 50/50 mix
Fan Clutch
Fan
Shrouding
Water pump

I have not replaced the temp sender, but it seems to work since it seems to know when the car is going uphill. I do think the needle on the guage is a little jumpy. Also, the car has actually overheated, blown out of the cap when the temp was high.

Other than the overheating issue, the car runs very well.

People have mentioned the head, and I am starting to believe them.

Any ideas?
He replaced the radiator as mentioned in post #25.

DID YOU SHOOT THE ENGINE WITH THE IR GUN WHEN YOU SAW 110-120°C ON THE CARS TEMPERATURE GAUGE?

If so what temperature did it read?

My educated guess, not to be confused with a WAG, is that you have a cracked head. You have replaced everything except the temperature sender. The symptoms you describe match my symptoms prior to replacing my #14 head with a #22 head. The head on that engine does not like 120°C. Perhaps you will get lucky like Greg and only need a new head gasket. So are you feeling lucky?

I had my coolant tested for combustion gasses and none were present yet the head was cracked and warped when it was sent out for testing.
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  #33  
Old 07-09-2012, 06:46 PM
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Quote:
Originally Posted by Hooverfull View Post
Well, situation has changed. It used to not register above the 110 mark or so, but one day, it actually did on a modest grade. The data from the IR gun was before the coolant boiled over. Now the coolant does boil over on hills.

I can drive on flat land forever. Car runs great. Once there is a hill though, the temp shoots straight up.
Can you perform one test, please?

Get it good and hot and then bring it home and park it.

The next morning, squeeze the upper radiator hose. Does it feel soft and easy to squeeze or is it quite hard, as though high pressure is in the system?

Now, carefully open the expansion tank cap and observe whether gas forcefully escapes from the cap when you do it. The system might be under a slight vacuum and air might rush into the reservoir. You need to pay careful attention and make a clear determination whether air is rushing out or rushing in.

Report back with result.
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  #34  
Old 07-09-2012, 06:48 PM
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Quote:
Originally Posted by GregMN View Post
Again I will tell you that the cap releases at a pressure point, not a temp.
What happens to the pressure in the system when the coolant boils?
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  #35  
Old 07-09-2012, 08:09 PM
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Quote:
Originally Posted by Brian Carlton View Post
What happens to the pressure in the system when the coolant boils?
When the pressure in the system is greater then then vent pressure of the expansion tank cap, the cap vents it. We both know that.

My point is that the cap vents based on the pressure in the system, not the temp. of the coolant in the system. (The cap is rated at a pressure. A t-stat is rated at a temp.) Boiling coolant is a source of pressure. But, not all pressure is from boiling coolant.

My experience was that my cooling system was building up enough pressure to vent coolant out the cap without overheating. The pressure in my system was combustion gases due to a failed head gasket.

I do not know what is causing the OP's coolant venting problem. But he seems to have replaced everything that could cause boiling coolant.

Quote:
Originally Posted by Hooverfull View Post
I have replaced:

Thermostat
Radiator Cap
Radiator
Coolant 50/50 mix
Fan Clutch
Fan
Shrouding
Water pump
So, if he is not boiling coolant to create pressure, it must be something else.
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Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO
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  #36  
Old 07-09-2012, 09:29 PM
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Location: Central Florida area
Posts: 186
This may be a little off the topic - all MB's in that era came with belly encapsulation panels. With the encapsulation panels on, the main air exit in the engine compartment is through the big air dam in the panel right below the cooling fan. Which way the engine compartment cools better - with the encapsulation panels on or without?

Tan

'91 300D 2.5t
'87 300TDT
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  #37  
Old 07-11-2012, 01:44 PM
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Posts: 12
I'm also having the same issue. My system does hold pressure overnight, however there is no water in the oil or exhaust in the water, no white smoke during startup or missing coolant. I took it to my mechanic and he tested for the presence of hydrocarbons in the coolant and there was none.

Last edited by dawgz83948; 07-12-2012 at 10:37 AM.
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  #38  
Old 07-11-2012, 02:21 PM
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Quote:
Originally Posted by GregMN View Post
When the pressure in the system is greater then then vent pressure of the expansion tank cap, the cap vents it. We both know that. My point is that the cap vents based on the pressure in the system, not the temp. of the coolant in the system.
Since we both know that the cap vents excessive pressure in the cooling system and we both know what happens to coolant when it boils, you statement is fundamentally obvious.
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  #39  
Old 07-12-2012, 10:37 AM
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Hey, OP have you bypassed the EGR valve?
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  #40  
Old 07-12-2012, 10:53 AM
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Quote:
Originally Posted by Brian Carlton View Post
Since we both know that the cap vents excessive pressure in the cooling system and we both know what happens to coolant when it boils, you statement is fundamentally obvious.
What does a 50/50 colant water mix boil at? My guess is somewhere between 275 to 350F. The engine has to get damaging hot to boil the coolant. Combustion gas into the head could boil the coolant locally where the 2 meet, pressurizing it and cause the cap to vent.
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  #41  
Old 07-12-2012, 12:19 PM
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Quote:
Originally Posted by funola View Post
Combustion gas into the head could boil the coolant locally where the 2 meet, pressurizing it and cause the cap to vent.
If you have combustion gas entering the cooling system, that alone can pressurize the cooling system enough to cause the cap to vent, you don't need to boil any coolant.
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2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO
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  #42  
Old 07-12-2012, 12:26 PM
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Quote:
Originally Posted by funola View Post
What does a 50/50 colant water mix boil at? My guess is somewhere between 275 to 350F. The engine has to get damaging hot to boil the coolant. Combustion gas into the head could boil the coolant locally where the 2 meet, pressurizing it and cause the cap to vent.
A 50/50 mix of Zerex G-05 will boil at 139°C. with a 140 cap.

Note, however, that you cannot really approach this figure without some bubbles developing.

I would agree that the engine cannot overheat at 120°C. with this setup.
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  #43  
Old 07-12-2012, 01:33 PM
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Quote:
Originally Posted by Brian Carlton View Post
Can you perform one test, please?

Get it good and hot and then bring it home and park it.

The next morning, squeeze the upper radiator hose. Does it feel soft and easy to squeeze or is it quite hard, as though high pressure is in the system?

Now, carefully open the expansion tank cap and observe whether gas forcefully escapes from the cap when you do it. The system might be under a slight vacuum and air might rush into the reservoir. You need to pay careful attention and make a clear determination whether air is rushing out or rushing in.

Report back with result.
I ran the car yesterday for about 2 miles; squeezed the radiator hose today and it held pressure. Opened up the cap. System is holding pressure and blowing out. I was careful to note it was not at a negative. I started the car up and noticed some smoke/steam? coming up out of the overfill cap hole immediately after starting. Could it be exhaust gas?
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  #44  
Old 07-12-2012, 01:42 PM
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If you had steam soon after starting it that's not a good sign. The coolant isn't going to be that hot that soon.

I still vote for a cracked head or if you are lucky just a head gasket.
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Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss
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  #45  
Old 07-12-2012, 01:47 PM
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Quote:
Originally Posted by Hooverfull View Post
I ran the car yesterday for about 2 miles; squeezed the radiator hose today and it held pressure. Opened up the cap. System is holding pressure and blowing out. I was careful to note it was not at a negative. I started the car up and noticed some smoke/steam? coming up out of the overfill cap hole immediately after starting. Could it be exhaust gas?
Sadly, Bio is correct.

The pressure in the cooling system overnight coupled with immediate combustion products bubbling out of the cooling system confirms that you've got to pull the head and see what you've got.

Sometimes you get lucky............sometimes you don't.


Sorry.

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