I am trying to diagnose a cooling problem with my M104 CIS motor. I've installed a new water pump, thermostat with ball valve is pointed between 9 and 10 o'clock position as pictured in the 124 manuals, bled the water system, but I still get gradual overheating problems on the freeway. The needle goes over the 100C mark cruising at 70mph and backs down if I slow down.

I notice my upper radiator hose has a lot of pressure behind it and I cannot squeeze it after a drive. The hose is not stiff/caked with stuff lined on the inside, I can squeeze it just fine if I release the pressure via coolant tank cap. I am also hearing sounds of quiet/slow pressure leaking from the coolant tank cap. Is this sign of overpressure? Do I have combustion pressure leaking into my cooling system? Is there a way I can confirm/test this to see if I really need to pull the head for a head gasket replacement?

I am not seeing an oil in my coolant.

Excessive pressure can be due to a head gasket leak, it can also be due to overheating. There are kits you can use to test for the presence of combustion products in the coolant, but they may or may not be reliable.

Does the upper radiator hose still have pressure on it in the morning when the engine is cold? Assuming the cooling system is tight and not leaking, that's usually a pretty definitive test for combustion products in the cooling system.

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Do a cylinder leak down test with the rad cap off and look for coolant to rise / bubbles as a sign of a combustion chamber to coolant leak. This is the only way to reliably check for a combustion to coolant leak.

Could be a clogged radiator internally and/or externally. If you have an infrared thermometer check for cold spots in the radiator when it is running warm.
Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Years ago I had a W126 300SD that had the same symptoms. It would gradually rise in temp at freeway speeds. Otherwise ran fine.

Luckily, I also had at the time a mint W123 300D turbo, which coincidentally, used the same radiator.

Swapped out the radiator from the no problem W123 into the W126 300SD, and the problem went away. Ordered a new radiator for the W123 300D and all was right with the world again.

Jim

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They make a tester to do that , warm it up, sample some coolant, install a chemical and if it turns yellow if you have combustion gases in cooling system if its stays blue your ok

my car never has pressure in hose,but I use 100% poly gylcol anti freeze,and a 72c thermostat

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

Why 72C? M104.980 is, I understand, designed to run at around 87C.

Any reason?

Mine always seems to be around 90C so I think all's well.

RayH

Engine wear is minimized around 80-85F. Maybe a bit higher with synthetics. I've seen people running cool thermostats, and it's always a mistake. I'm not even sure where you would buy such a thing.

Pure glycol is also a mistake, but it's one virtue is that you'll see much lower system pressures.

Cooling systems are pretty simple, but it can be hard to pinpoint a problem. Overheating at speed usually indicates a clogged radiator, the question is whether the air or the the water side is clogged. I'd start with a flush and a new thermostat (stock temperature). Also peel off the aux fans from the front of the radiator and clean out the inevitable collection of dead bugs.

You can test for combustion gas in the coolant with a chemical test kit if there's other reason to suspect a headgasket. Other reasons would be things like sludge in the coolant or sump, evidence of oil contamination in the header tank, or disappearing coolant.

I had a Ford truck that was running hot at speed, ended up being a bad transmission. I swapped everything in the coolant system, then the trans died. Is the transmission in the car acting up at all?

engine temp runs 78c during winter,no hotter than 82c summer,with a stock radiator.Evans PG is different,put no pressure on hoses,and gaskets,no water to ruin system.M104 are known to eat head gaskets,9 years I have not had to replace one are trans coolers are known to rot and ruin the trans.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

I'm leaning more towards combustion gas in cooling system at this point. I popped open the reservoir tank after letting the car sit for about 2 hours in the garage and I immediately smelled exhaust gasses from the opening.

I'll do a leakdown test this weekend to confirm this, but I already went ahead and put in orders for new parts.

104 010 20 20 - Headgasket kit - 1x
104 050 01 58 - Valve stem seal kit - 1x
104 203 00 08 - Metal hose to thermostat - 1x
103 203 00 82 - Rubber connector hose for thermostat - 1x
104 990 00 12 - Cylinder head bolts - 14x
102 016 03 76 - Cylinder head washers - 14x

Additional notes/information:
- I've been seeing random puddles of coolant on the floor everywhere I park. I haven't determined exactly where it is coming from since I've seen the puddle form on both sides of the engine.
- This motor was originally from another car (1991 300CE @ 200k miles) and I swapped it into my 190E that was running just fine. I did a basic compression test prior to installing the motor and the numbers looked good so I did not tackle a headgasket job.
- With above in mind, my radiator, expansion tank/cap were all relatively new (5 years old) and working properly with the original M103 motor. No clogged radiator or leaves stuck between condenser and radiator. I also did flush out the block with a garden hose attached to the water pump inlet with the drain plug loosened until clear water came out.
- Driven around 70 miles since motor install (this last Saturday). Issues started developing roughly after 40 miles. (Monday morning trying to commute to work).


@MCallahan: Issue is not transmission. I have a freshly rebuilt (0 miles as of last weekend) 717.411 transmission attached to the motor. It feels and drives normal so I am ruling that one out, but I do understand your viewpoint with extra engine load.

Cool, just one that caught me totally off guard. Was driving me insane trying to figure why it kept overheating. Trans wasn't slipping noticeably even at highway speeds (no tach, though), but it was generating enough heat that the cooling system couldn't deal with it.

John5788, inspect the water pump weep hole as the source of the coolant leak.

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Fred Hoelzle

I'll take a look when the head and intake manifolds are off, but it is a brand new Graf water pump. Before I put the motor in my car, I replaced the water pump. Is it possible that the extra pressure from combustion gasses can destroy a new water pump?

I've replaced almost everything on the motor (including resealing the oil pan and rear main seal+cover) except the cylinder head, head gasket, and upper/lower timing covers. So I'm hoping that this last area I didn't go over is my failure point.