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  #1  
Old 08-09-2004, 12:01 PM
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Overheating, with no leaks and no thermostat

I have read a number of threads lately, mainly about thermostats and overheating but couldnít find my answer. I own an 84 300SD that has been overheating. A few days ago when my wife was driving, it ran low on water and overheated. I replaced the water and inspected all hoses, none were leaking. When I noticed it was getting hot again I removed the thermostat to begin diagnosing the problem. I test drove it for about 5 min at 2000 rpm then about 10 min at 3000 rpm. After the 10 min piece, the temp began rising again and I inspected the system. The water in the plastic reservoir was bubbling (boiling) and was being forced out the overflow tube. I checked all hoses and they were all hot (both those attached to the reservoir and those on both sides of the thermostat connection). My first thought was the top hose from the reservoir to the radiator was restricted but I removed one side and blew air through. It passed through freely. I visually inspected the radiator from the front and the back and did not see any soil (dead bugs, dirt) sticking out. My next guess is maybe the water pump has quit turning but havenít checked it out. Has anyone else experienced (and resolved) this or do these symptoms sound familiar?

Thanks,

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  #2  
Old 08-09-2004, 12:28 PM
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You may want to begin with a new T-stat, with fresh 40/60 or 50/50 mix of MB-Coolant, and see where that leads you. If you are still running hot then check the radiator temperature in the center (with warm/hot engine). You may also want to check with a local repair facility and see if they can help you determine if there is a head gasket problem.
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Old 08-09-2004, 01:21 PM
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Did you read the posts about air pockets? I've actually gone to the extreme of parking a vehicle in the most nose-up angle I could find in order to help air find its way to the front and out. It's amazing how much air can be trapped sometimes.

Also, maybe there is a reason it was low to begin with? Maybe the rad is clogged and it blow the water out?
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  #4  
Old 08-09-2004, 09:13 PM
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On Mercedes, the thermostat is also responsible for closing the radiator bypass and directing all the coolant through the radiator. Reinstall the correct thermostat and see if there is a difference.
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Old 08-09-2004, 09:20 PM
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Normally a bad water pump simply leaks. Normal flow, just leaks on the ground. An extreme water pump failure could be a seizure, broke impeller, or rotted impeller, but Iíve yet to see one.

My guess is the rad is clogged, has air pockets, or maybe both now that water was lost. A Ĺ decent rad test is to feel it with your hand from the front while itís running and warmed up. If you feel cold spots they are likely clogged. Remember water cools as it goes across, so one side will normally be a bit cooler, but youíre looking for a significant drop and it will normally be cold all the way across. Some rad flush might do the trick for a quick fix.
If there are air pockets then there will often be surges of flow. With the rad cap off, gushes of water out the fill hole are typical signs, even after you shut the motor off. If you can run it with the cap off and it doesnít surge, itís probably air free.
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Old 08-09-2004, 10:04 PM
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I appreciate everyone's input. I will do some of the quick tests tonight. Problem is I get home late tonight and tomorrow. For the quick tests, I'll report back tomorrow for the longer ones it may take a few days. I'll keep everyone informed.

Thanks everyone.
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Old 08-09-2004, 10:11 PM
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Dumb question; You're sure the belt is there for the water pump?

Gilly
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  #8  
Old 08-09-2004, 11:00 PM
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start it up, see if the upper hose gets pressurized before the engine gets up to operating temp. That would indicate a blown head gasket, or block/head cracked. when the block expands, compression goes out the crack. when the engine cools the crack shrinks, so no water flows into the cyinder. could be block or head.

has it overheated before??
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  #9  
Old 08-11-2004, 09:31 PM
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Refill with proper mix coolant (not water) and replace the thermostat and get a new radiator (overflow tank) cap -- the cap has failed and isn't holding pressure. This allows the coolant to boil, overheating the engine.

Do not run without a thermostat in a Benz engine, the coolant will not circulate through the radiator without the bypass passage closed by the flap on the back.

Use only Whaler or Behr OEM thermostats from a good indpendent mechanic or the dealer, do not buy on at the local parts place -- the "clones" usually don't seal (and in fact often will not physically fit in place!).

If you still have overheating problems, look for a leak at the water pump at the bottom weep hole -- might only be a trickle. Also check the AC drain off the transmission for coolant from a leaking heater core. Last, make sure the "cold" side of the radiator gets hot when the engine is warm -- if not, the core is plugged and it's new radiator time.

Peter
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  #10  
Old 08-13-2004, 08:53 PM
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Some updates,

Did the majority of the checks recommended. Warmed the car up and checked the radiator, it was hot from one side to the other and from the top to the bottom. Still have both belts to the water pump and both are tight. No oil in the coolant, no coolant in the oil (bubbles). Unsure as to whether I have air pockets or not, still could be some. (Since then I have read threads on this and will make sure to "burp" when I replace the fluid). Tried another diagnostic test, pulled the top hose (small) from the resevoir to the radiator and let it drain in a jug while the engine idled. What came out was mostly a frothy liquid, it sputtered and sometimes nothing came out for 10 seconds. I expected a small but consistent stream of coolant. Remebered that when I removed the old thermostat no fluid came out of the housing (or connected hose). Seemed odd, like maybe an air pocket?

Then the suprise showed up this morning. Coolant on the ground under the engine. The water pump is now leaking! I would have thought the leak would have been noticable when the overheating started.

I ordered a new water pump and thermostat. It will take a few days to arrive. After I replace will update.

My explanation now is that maybe the thermostat was sticking and then the water pump stopped pumping properly (not circulating the coolant fast enough to prevent overheating).

Anyway, will keep everyone informed and thanks again.

PMD

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