Bubbles In OM617's Coolant Reservoir - A Head Gasket Leak For Sure? - PeachParts Mercedes-Benz Forum

Richard Howard · #1 08-06-2013, 04:52 PM

While checking transmission fluid level with the OM617 running in the 84 300SD after a long drive, I noticed considerable bubbles rising constantly in the coolant reservoir. Experience with a few leaking head gaskets in other diesels in past years produced almost a panic response, remembering the chain drive to the overhead camshaft in our MB diesels and the work necessary to replace a head gasket.

So I drained the coolant and refilled the system with distilled water to get ready for a possible use of a good quality block and gasket sealer carried in the coolant. While letting the engine idle for 15 minutes at normal running temperature, with the thermostat open and not on bypass, I saw no bubbles in the coolant at any time.

Is there any other reason for bubbles being seen in the coolant in a running OM617? I think not. With the 21+:1 compression in these engines, I suspect that a compression leak can be the only cause. Yet, why no bubbles when idling at normal operating temperature with water only?

The engine runs well, without roughness, and power is good at 235,000 miles. No compression check has so far been made as it hasn't seemed necessary.

Any opinions about what is going on will be much appreciated.

jeffr0000 · #2 08-06-2013, 06:12 PM

Before pouring goop in the system, get yourself a head gasket tester. It's a bottle of blue juice that'll turn yellow in the presence of exhaust gases (or maybe yellow to blue, I forget). If you buy a kit, it'll should come with a tube you pour it into. It has a pointed end with a one-way filter and a bulb at the top to draw the gases into the blue fluid. You shove the pointed end of the thing in the overflow tank filler with the overflow hose still stuck in there bubbling like before and pump the bulb to draw gases into it.

Here's a video of one in action. Testing for leaky head gasket - YouTube

If it turns out it's blown, sodium silicate is the snake oil you need. Most the seal stuff in the autoparts stores will list it. In my experience sodium silicate works to seal up minor leaks, although how long it'll work is questionable and unknowable.

SD Blue · #3 08-06-2013, 06:35 PM

Personally, I wouldn't put any kind of "miracle, gooey, sealing magic solution" in the cooling system. It may have been alright on cars back in the 50's but I've personally seen it cause more problems than it temporarily fixed. Then when you go back in to repair it correctly, it's a nightmare to get the "magic solution" removed to prevent further problems.

From what I've seen in the FSM, a magic solution will cause problems with the thermostat venting and probably reek hovoc on the monovalve, at least. There is a lot of information here on this forum to help you through doing it correctly, once.

I'm not sure, but the thermostat venting may be causing the bubbling. Also, a head gasket problem usually results in a loss of coolant. Are you seeing coolant levels drop?

Richard Howard · #4 08-06-2013, 07:09 PM

SD Blue / Sam: Thanks for your thoughts on the use of any type of sealer in the cooling system........ I tend to agree that it's not the right way to solve a problem in these engines. The coolant valve for cabin heat that reacts to voltage changes between 12.6 and zero from the climate controller would surely get gummed up.

Regarding the thermostat venting, I recall having read comments about pulling a vacuum on the OM617 cooling system before adding coolant via a device intended for the work, in order to eliminate all air from the highest points in the coolant passages. I wondered about the effectiveness of this and have instead always used a floor jack under the driver side lift pad when draining the system, and under the passenger side pad when filling the system...... to angle the car to hopefully get as much coolant out as possible, always draining when the coolant and thermostat are hot, and to fill the system as completely as possible.

Is there, in fact, a "best" way to fill the W126 coolant system with an OM617?

SD Blue · #5 08-06-2013, 07:37 PM

When I refilled mine, I put the nose up on ramps and filled through the top radiator hose. Several folks, here on the forum, recommended this method and it worked like a champ. If I recall correctly, I only had to add a small amount, later on, to the overflow reservoir.

Richard Howard · #6 08-08-2013, 08:04 PM

For what it's worth, I'll report that I now believe the bubbles were from trapped air in the path from the head back through the heater core, past the voltage-controlled water valve, and through the pipe near the exhaust back to the water pump, finally being purged.

I still can't understand how air could remain in this heater-core-related area for so long with routine driving. Is the heater core higher than any part of the water passage in the OM617 cylinder head? I don't think the MB engineers would have designed such a situation.

The car now has fresh antifreeze, distilled water, a new thermostat, and the system purged of air with repetitive blips of the throttle to bring revs up to about 1800. The water pump undoubtedly sent pressure pulses into the head that then exited through the fitting and hose behind the oil filter, moving the air along until only water remained in the system.

I still cannot believe the above but there seems no other explanation since the engine runs smoothly and power is good....... so apparently no leaking head gasket exists.

funola · #7 08-08-2013, 08:58 PM

The heater core is lower than the cyl head. Did you have the heat on? It has to be on to get flow through the heater core. High RPM driving will purge air better than idling or blipping the throttle at idle.