Replaced t-stat, same results. She actually drives a couple miles holding temps just fine. Then, is seems, after all of the coolant is warmed up it starts to climb slowly. This is from a overnight cool down.

Figured I had the part, worth a shot.

Even though I am certain my old viscus fan is bad, I am going to reinstall. Upon closer inspection, I dont feel there is enough resistance in the fan. Is there a good way to test it? I did notice that if I hold high RPM in park the fan quiets down. Not sure how long, maybe 30 seconds? It actually sound like the cars RPM is dropping (but clearly its not) - if that makes sense. But I can move it by hand when cool - tighter than the old one. But when hot, there is no noticeable change in resistance.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Well new viscus fan was not change. So I removed the temp bell housing, lower and upper hoses and took the shop vac to it. Sucked whatever could have been in there, if anything, out. Made the the vac did not sounded restricted. With the t-stat it was, I could tell. Pulling the lower hose before it and it flowed fine. Basically a reverse flush I would love to actually flush her, but that may be a last effort. I got her up to temp to where the t-stat opened with expansion cap off and now I am letting it cool back down (still with cap off).

If this did not help, I will be pulling the water pump next weekend. I dont have another on hand right now and if I am going to pull it, I may as well replace it anyway.

On a good note, all the water is coming out crystal clear.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Well hope and dreams crushed again, can only think maybe water pump now for whatever reason.

So guess I will be driving it running hot until I replace the pump or she dies. Guess its a good reason to finish the 750il and finally put the Benz down.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Sorry it's not working. Thanks for the updates. Good luck and let us know what it finally turns out to be.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Are all your belts there and are they tight?

Yup. In fact, with the water pump there are two belts. Both also assist in power steering. So if they went out or slipped then power steering would be affected.

Today I cleaned the condenser and radiator. Tested 2 t-stats. Purged it twice more, once from dead cold to warm up.

At this time, I can only lean towards water pump. T-stat(s) certainly open. Viscus fan clutch is new, even old viscus clutch yields same results.

So thinking water pump. Its probably original anyway. So if it does not solve the issue I dont feel I am wasting money.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

There should be a thermostat to stop heated water from flowing through the washer tank.

My SL does not get driven in winter so I bypassed the heated washer and heated base of windshield.

Given you are running straight water, dump in a bit of antifreeze to color the water. It gets difficult to find leaks with straight water. Straight water has the best heat transfer rate but needs elevated pressure to prevent boiling.

I'he been looking at maybe getting an extra overflow cap and rigging up a vac line to it. Supposedly pulling a vacuum will get any air out as well as act as a leak detector. If it can't hold vacuum you have a leak.

I thought there was suppose to be one. Guess mine is not working, never has as long as I owned the car. Not a top item on the list right now though! But I will look into it.

I dumped some stuff that shows up under black light. Pretty sure there is no leak, the upper hose remains "inflated" for a long time when the car is off. Under pressure, I should be good to 125 degrees celsius. Without pressure, no more than 100c. Antifreeze at 50/50 mix only adds a few degrees to boiling point.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Pretty sure no leak. If there is one, it would have to be pretty tiny and thus not significant. I can certainly try the vacuum test.
When off, the car maintains pressure in the cooling system for a long time. At least an hour if not more. Never really checked longer than that.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Forgot to add. I just pressurized the cooling system with a garden hose and tap water. I really, really got sick of buying up the local inventory of distilled water - even if its cheap.

I pulled temp sensors and the plug on the back of the intake. Also pulled the air vent hose on top the radiator.

I did this for two reason. One, I descaled the system with corrosive acid that needed to be flushed out. And two, this way I could almost guarantee no air in the system. I wont go through details, but yes I made sure no air was pre t-stat too. In short, engine running and water coming out everywhere. Good times. Still runs hot...

Water pump ordered.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Have you actually used an infrared thermometer and checked the engine's temps in various places (top of radiator...water pump housing....upper hose....intake area...sensor area) to see if its actually running at those temps?

The engine fan should be ROARING above about 85C.......the one on my 420 fully engages over about 84-85C and moves a TON of air, my front aux fan also comes on low around 85C With the A/C on.....

I just did the intake job on my 420 a few weeks ago and did not burp anything, I filled the coolant just by filling the expansion tank and running the engine...0 issues, runs at 82-85C, even sitting idling with A/C on a hot day it never even hits 90C. I am running about a 45% mb coolant to 55% water mix in mine right now.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

No. But to some degree, I trust the gauge. Only because the aux fans kick in right around 110, and the dash gauge reads that. So two sensors agree on temps at 110 at least.

My new viscus clutch grabs much better, so I can hear the fan now really well. The fan is roaring when I start the car. After a bit, it disengages, as it should. Its a short jump to the highway for me where the car will now reach operating temps. And yes, at 85-ish I can hear the viscus fan spool up. I can verify with a rev (especially if I drop into neutral). But temps still slowly climb. The only thing that seems to save her form overheating is the aux fans.

And I believe that, hence my frustration! I have replaced the t-stat before and other operations that resulted in major water drainage during work. Always filled it back up, squeezed the upper hose and never had an issue.

So like I said, thinking water pump. May as well, its ordered anyway. So once I know all the components are new (viscus clutch, t-stat, pump) I will be forced to rule those out and look into other possibilities. Blockage, bad radiator (technically blockage) or even some sorta blown head or exhaust getting into coolant system. I doubt a head or exhaust issue since the radiator cap never blows over. And I see no oil or other signs related to those issues.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

I've seen fins come off stamped steel water pumps before when the cooling system is run low. Water , air , vibration and the fins come off.

This car has a cross flow rad correct? I've seen cross flow rads fill with silt on the bottom and block water flow. ( had this occur on a Ford Aerostar, the rad inlet tank was tapered at the bottom making the perfect place to silt to pack. )

It is becoming time to do a leak down test. This involves removing spark plugs, turning engine to top with valves closed. Pump in compressed air ( 100 ish PSI is fine ). This tests integrity of the combustion chamber.

In your case you are looking for air bubbles in the water. If so, there is a cracked head / bad head gasket.

They are top to bottom flow radiators on the V8 W126's.

It could be a bad t-stat....they can come right out of the box bad sometimes. Maybe get one from the dealer and swap it again? I've seen one on am OM617 fail right away and had to replace it again.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)