Cooling System Riddle 1991 350 SDL - Page 3 - PeachParts Mercedes-Benz Forum

barry123400 · #31 01-26-2011, 08:32 PM

No detectable exhaust gasses is too big a concern to pull the head yet. You might try a some water wetter. You are the first case to my knowledge to report a possible hot spot in one of these engines.

Otherwise I am at a total loss about what is causing this problem.

Bob Albrecht · #32 01-26-2011, 08:52 PM

It could be two problems.

If the radiator is bad and the temperature is getting too high and the thermocouple for the coolant temperature is bad you could be overheating and not know it.

Try running the heater full blast when you are driving at freeway speeds to see if it happens then.

babymog · #33 01-26-2011, 09:04 PM

Possibly a big air bubble in the system?

GregMN · #34 01-26-2011, 09:40 PM

Again, thanks for the responses and trying to keep me from pulling the head.

Keep in mind that I have 2 temp gauges in opposite ends of the cooling system and they both show normal cooling system temps.

The car has been driven almost 1000 miles since this engine was installed.

I cannot image there being an air bubble in the head after this many miles and never any indication of overheating.

In one last attempt to detect combustion gas, I will install the modified cap to run the system with no pressure. Then I can drive hard at full temp and safely open the cap to sniff for gasses. I will report what I find.

GregMN · #35 01-27-2011, 12:22 AM

I installed a modified cap so the system would not pressurize. I drove the car a few miles. Under a heavy load, WOT 0 to 60 several times, the car's temp gauge went from 80 to 100 in a very short time. When I cruised, at no load, it quickly dropped back to 80 as fast as it rose. When I stopped, the coolant level was where it was when I started. I tested for combustion gases in the coolant tank and found none.

When pressurized, the coolant temp never got above 80.

So:
Still no gases detected.
The temp spikes under load when not under pressure.

When the system was under pressure, under a load the pressure would spike but the temp would remain steady.

josha37 · #36 01-27-2011, 12:26 AM

without pressure the temp spike was most likely from boiling coolant in the head. So without building pressure you did not have coolant transfer? put a real cap on it and do not fill your radiator all the way up, just leave a air pocket and with a real cap on it try again.

dude99 · #37 01-27-2011, 01:44 AM

I had an old Dodge Royal Monaco (land yacht) and it started doing something very similar to this. No over heating but as soon as we stopped it would start pushing it's coolant into the expansion tank. Solution was a new thermostat. Can't remember why. Your original post says you have a good thermostat, but I'd try replacing it anyways...

winmutt · #38 01-27-2011, 08:01 AM

Quote:

Originally Posted by GregMN

Thanks for your response but...

Not overheating, just over pressuring.

Thermostat. How did you check it?

GregMN · #39 01-27-2011, 01:41 PM

Re-filled the system with the coolant and installed the new cap so the system would pressurize as normal.

I removed the small bleeder hose from the reservoir and attached a hose to the reservoir. I pressurized the reservoir until coolant filled the bleeder hose and them blocked the end of the bleeder hose. At this point the radiator is full to the top. I then sucked the rest of the coolant from the overflow bottle into the reservoir. The reservoir is now full to the "max" level.

I drove the same route I did last night. When I got home I found the following:
The overflow bottle had about 4" of coolant in it. (it started empty)
The system was pressurized enough to be pushing air and/or coolant into the overflow bottle (very little, very slow gurgling) for about 5 to 10 minutes after I turned off the motor.
The reservoir was still full up to the "max" level.
There were some very slight gurgling sounds that seemed to be coming from the intake side of the head.

1/2 hour later:
The reservoir was still full up to the "max" level.
The pressure, as felt by squeezing the upper hose, had dropped, but not to zero.

I will check the reservoir level after it has cooled a lot more.

GregMN · #40 01-27-2011, 01:49 PM

Quote:

Originally Posted by josha37

without building pressure you did not have coolant transfer?

Yes, it did push out about twice as much under no pressure as it did under pressure. The coolant level in the reservoir was at "max" when I first got home. This morning the reservoir was down about 1/2 way which required a re-setting of the system to "full" before this morning's drive.

winmutt · #41 01-27-2011, 02:40 PM

How did you test the thermostat?

GregMN · #42 01-27-2011, 03:40 PM

Does anyone know the normal pressure range for the cooling system?

I installed a pressure gauge in the bleeder hose that I could see while driving. At WOT I could see the pressure increase instantly. Once the throttle was released, the pressure would stabilize at whatever pressure it had built to. I was able to push the pressure to 25psi, above the 20psi release level for the cap, but as soon as I released the throttle it would bleed down to the 20psi level and stay there.

So:
The pressure builds instantly in response to WOT under load.
The volume of the pressure produced at WOT is more than the cap can bleed off. I was able to get it up to 25psi during the short WOT runs.
At no time during this, or any other test drive, did the temp gauge indicate any increase in temp.

One hour after shut down and the pressure is still at 16psi.

It has to be combustion gasses, yet no gasses are detected in the coolant system.

funola · #43 01-27-2011, 03:58 PM

Pressure in a cooling sytem is due to expansion of the coolant from heat. Did you try releasing the pressure, then drive it? You should see minimal pressure rise (from heating) since the coolant is near full hot (cooled off a little from idling while you release the pressure). If it goes up to 25 psi immediately, then it has to be from combustion gas.

GregMN · #44 01-27-2011, 04:12 PM

Quote:

Originally Posted by funola

Pressure in a cooling sytem is due to expansion of the coolant from heat. .............. If it goes up to 25 psi immediately, then it has to be from combustion gas.

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.

winmutt · #45 01-27-2011, 04:37 PM

Funny. What you describe fits busted Tstat. Assuming the engine was already warm. I'd ask the question again but you probably would just continue to ignore it. Let me restate that. Its either busted head gasket or a busted tstat.

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