Cooling System Riddle 1991 350 SDL - Page 4 - PeachParts Mercedes-Benz Forum

dude99 · #46 01-27-2011, 04:48 PM

As mentioned above, I second the T-stat.

winmutt · #47 01-27-2011, 04:50 PM

If the engine is warm and you nail it on the hwy and its not open you are going to get a heck of alot of pressure built up instantly. On a gasser or non turbo diesel it could be slow as you stated. On a turbo it will be hella quick.

GregMN · #48 01-27-2011, 05:42 PM

Quote:

Originally Posted by winmutt

If the engine is warm and you nail it on the hwy and its not open you are going to get a heck of alot of pressure built up instantly. On a gasser or non turbo diesel it could be slow as you stated. On a turbo it will be hella quick.

OK, I'll bite.

How would the pressure build instantly and then stay at it's new higher level?

I could understand the water pump creating momentary pressure on one side of a blockage (defective stat), but wouldn't the pressure then even out in the cooling system after the WOT condition was removed?

3 hours after shut down the pressure is 15psi.

Thanks,
Greg

funola · #49 01-27-2011, 05:51 PM

Quote:

Originally Posted by GregMN

Expansion from heating of the coolant would not be instant. That is why I but the word "instantly"in bold. Put foot to the floor, look over at gauge, watch gauge move instantly.

It has to be combustion gas, but no gas is detected.

Yeah, expansion of coolant could not be instant, that's why I said to release the pressure when it is full hot, then go for a drive and nail it. Since the coolant has already expanded, it cannot expand much more, so any pressure that you see instantly is from combustion gas. Does that make sense??? Did you release the pressure first then go for a drive and nail it? You say no gas is detected. Maybe you let the combustion gas escape before it was captured? I don't know what your tester looks like but if you can put the probe in a balloon then the ballon over the expansion tank cap, then you can be more certain of capturing some combustion gas.

winmutt · #50 01-28-2011, 09:22 AM

Quote:

Originally Posted by GregMN

OK, I'll bite.

How would the pressure build instantly and then stay at it's new higher level?

I could understand the water pump creating momentary pressure on one side of a blockage (defective stat), but wouldn't the pressure then even out in the cooling system after the WOT condition was removed?

3 hours after shut down the pressure is 15psi.

Thanks,
Greg

I think you should review the thermostat and how it works in these cars. If it was closed, pump speed irregardless, there would not be enough cooling capacity of the system to keep the system cool. As such the system will be alot more sensitive to heat. Turbo charger is essentially a heat pump at any level. With a closed tstat the system is going to take much longer to cool than normally. Its a cheap relatively easy fix. You said you tested it, how did you test it?

GregMN · #51 01-28-2011, 11:01 AM

20 hours later and the pressure is still at 15psi.

I will remove the thermostat and test before I order a head gasket. I still think it is a pressure problem not a heat problem. Any pressure caused by heat would have gone away when the engine cooled. By my observation, expansion due to heat accounted for no more than 5psi of the 20+ that developed on the last run.

I have a recipe for crow. I would love nothing more than to be having that for lunch today. To go, of course. Eating it while driving down the highway at 75 with all the coolant staying in it's proper containers.

josha37 · #52 01-28-2011, 11:20 AM

easy way to keep coolant in proper containers is to stop filling it all the way to the brim and leave a air pocket under the rad cap ... fluids dont compress, i read the way you are filling it and it is a bit over kill. Something to think about

winmutt · #53 01-28-2011, 11:25 AM

Quote:

Originally Posted by GregMN

Any pressure caused by heat would have gone away when the engine cooled.

Ive popped the cap after an overnight cooling and it was still pressurized. The rad cap is set to leak at 16psi so it sounds to me like you just have a good tight system

GregMN · #54 01-28-2011, 12:01 PM

I put the car's thermostat and a new thermostat in pot of water with a thermometer. The car's stat started to open at 170, the new one started to open at 172. Both fully opened at about 180.

GregMN · #55 01-28-2011, 01:29 PM

Installed the new thermostat when I reassembled. I ran the engine until the thermostat opened and then topped off the coolant. So the coolant was up to temp and the pressure was a zero to start this trip.

I drove to town and the pressure got up to 10psi. and stayed there until I got to the hwy. As soon as I accelerated up to 70 the pressure climbed to 22psi and stay there. When I took a slower route home it dropped to 20psi (the caps release pressure) and stayed there.

GregMN · #56 02-04-2011, 03:46 PM

The cooling system pressurizes to 20 psi (the release pressure of the cap) and stays there. The pressure drops to 15psi once the engine is stone cold and will hold that pressure for days.

New Radiator
New Cap
New Thermostat

The cooling system never goes above it's normal operating temp as indicated by 2 temp gauges located in completely different areas of the cooling system.

I still cannot detect any combustion gasses in the cooling system. The gas detector has been tested and it works.

There is no oil in the coolant or coolant in the oil.

I still cannot think of any other explanation for the pressure source.

The new head gasket has arrived.

I do not want to R & R the head only to find that was not the problem. If no one else can explain this, then I see no choice.

Please, somebody stop me.

josha37 · #57 02-04-2011, 03:55 PM

while a head gasket it required to do the job make SURE you have a machine shop check the head for cracks and straightness.. Im sorry at this point i cannot stop you.

winmutt · #58 02-04-2011, 04:16 PM

Are you aboslutely sure all the hoses are routed correctly?

If it makes you feel better my 87 was parked for 2 days this weekend and I put some water in on Sunday. When I opened the overflow tank there was pressure in it. Not that this really helps your "boilover" problem...

Fulcrum525 · #59 02-04-2011, 04:20 PM

This is very similar to what I went through last year.

Pressure on the coolant system.....

A forum member was nice enough to bring his tester over and it tested positive for exhaust gas in the coolant so I did the head gasket.

What sort of tester are you using to detect gasses?