The problem: The car runs on the hot side, near 100 C. The operating temps actually vacillate between ~92 C and 100 C, during normal driving (no heavy loads, AC off, ambient temps around 75 F). I know that this is within acceptable range and is considered normal by some, but in comparison to my 85 300TD it is 10-15 C hotter. I bought the car a couple months ago, but haven't driven it more than 500 mi yet (I've been working on it). It was filled with green coolant and had this problem when I bought it.

Previous work:
1) I degreased and citric acid flushed the system. It is currently filled with 45% Zerex G-05:distilled water. There was ALOT of scale build up prior to the acid flush. All flushes were done with the thermostat out, heating the car up to operating temp for 15-45 min while running the defroster, pulling the radiator plug - block plug - temp sender and upper radiator hose and flushing with a garden hose.
2) The thermostat was replaced with a new 80 C Behr unit, installed correctly. I have tested it (in the thermostat housing, out of the car) with boiling water and it extends fully to block off the radiator bypass, but there is no way to tell if it opens "fully".
3) I have tested the temp sender/dash gauge by grounding the temp sensor and immersing it in 90 C water next to a laboratory thermometer. It reads true within a couple degrees.
4) I removed the radiator, filled it with boiling citric acid and let it sit over night. I power washed it at the local car wash from both sides to clean any dirt and debris. I am fairly confident that it should have good coolant and airflow. There are very few bent fins.
5) I have swapped the coolant tank cap on both cars, the system does pressurize when hot and hold pressure overnight.

The questions:
1) How is the fan clutch supposed to work? When the car is cold and I shut off the engine the fan stops immediately. When it is hot the fans spins a few extra revolutions. This is true of both cars. In direct comparisons the fan clutch on my 85 300TD (which runs cooler) has more resistance when spun by hand. Not a huge difference, but noticeable.
2) I have seen posts stating that the water pump impeller blades had worn down. How is this possible?
3) Other than swapping parts between the two cars, what other diagnostics can I do?

EDITED on 07/24/07: For the record it was the thermostat, but please read all the great suggestions.

The water pump blades wear down due to erosion. The metal is harder than water, but water resistance "gets harder" the faster something tries to move through it. It sounds like a combo thing to me, weak water pump and weak fan clutch. Also, make sure the fan shroud is in place. HAve you tested the temp sender? Sometimes they can send a too high or too llow reading to the guage. The gauge an also read wrong. Have you used a non contact thermometer?

What do you mean by "holds Pressure" overnight? Is that from running the engine or from you manually pressurizing the system? It is a bad sign pf pressure stays overnight after running the engine.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

99 W210 E300 Turbo Diesel, chipped, DPF/Converter Delete. Still needs EGR Delete, 232K

90 Dodge D250 5.9 Cummins/5 speed. 400K

Gone and still missed...1982 w123 300D, 1991 w124 300D

yup. what he said.
holding pressure overnight that YOU pumped up with a pump is a good thing.
holding pressure overnight from the engine running is a bad thing that indicates a cracked head or blown gasket...
pressure should build up due to heat of expansion and dissipate when the engine cools.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

rrgrassi,

I am certain that the dash gauge is reading true (or darn close to it). See previous post 2nd paragraph #3.

I also did the unscientific (but free) test today of holding the upper vs lower radiator hoses after a 20 minute highway drive. Upper hose was almost too hot to hold, lower hose was only warm. Despite the qualitative nature of this test, it clearly indicates that the radiator more than sufficiently dissipates heat and therefore eliminates concerns about the radiator and fan clutch.

Oh bugger.

So what's the next diagnostic step? compression test?

not sure, but you could start with releasing all pressure in the cooling system, and put a gauge on the cooling system, start the car and watch the gauge. how long does it take to build up pressure? how much pressure does it build? instant pressure spike, blown gasket. pressure jumps over 20 psi in 5 minutes? cracked head... sorry.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I think so anyway (fingers crossed).

I must have confused myself about when the system was pressurized. The last two mornings (and after work today), I removed the expansion tank cap without any discernible pressure release. Maybe, MAYBE, a slight gurgle from the upper radiator "overflow" hose.

I hooked up the mityvac as a pressure gauge to the expansion tank (via a #6 one-hole rubber stopper) and tested that it held pressure up to about 8 psi (for a couple of minutes at least). I released the pressure, then started the cold engine and watched the pressure gauge for about 8 or 9 minutes. There was at most 1-1.5 psi.

I see no evidence of coolant in the oil, oil in the coolant (though I have not had either analyzed), coolant loss or white smoke/sweet smelling exhaust. So, as of now I think I can put horrible thoughts of a leaking head (gasket) out of my mind.

More info: It appears as though the car will only run hot while being driven (locally or on the highway). If I start it and let it idle it will run at around 85 C. If I park it while it is around 100 C and leave it idling, it will eventually return to ~ 85 C.

Next up I will pull the water pump and inspect it. Gaskets and time are cheap.

Now on to the part of every repair job, where I break something:

While I had the mityvac on the system, I decided to try to pull a slight vacuum. I was kind of thinking that I might be able to pull any trapped air out of the system (OK, so it was not exactly clear thinking). I could hear the system sucking air from somewhere in the front and bubbles in the radiator. I tried to pump pressure back in to the system and now there is a small leak somewhere in the radiator. The fan sprayed the coolant around. Doh!

IIUC the water pump fails when you start to weep water out of the built in hole, can't fathom how you can wear down impeller. I've snapped off a hold down bolt on water pump, be careful! You found a hole in rad, correct?

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83 SD

84 CD

That occurs when the shaft seal fails, which is totally different than the impeller failing.

Fan clutch, fan clutch, fan clutch! swap clutches to verify. Years ago we would test a clutch by blowing compressed air against normal rotation thereby stopping the clutch with engine running. Once engaged, the air could no longer hold it. Check temperature on dash when you can no longer hold the fan with air.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

I also have a very hard time imagining how the water pump impeller could be worn down. But with the amount of pitting and scale I found on the thermostat housing (around the hoses), I can't rule it out. But the t-stat housing is aluminum and the water pump is iron, it still seems unlikely. The only way I'll for sure is to pull it.

Besides, at this point, insufficient coolant flow seems to be the most likely explanation to me.

I think I actually created a hole in the radiator, or unplugged one to be exact. I'm certain that this hole in the radiator has nothing to do with the car running hot.

Agreed, pulling it is the only definite way to determine whether or not there is a problem with the impellers. I would go with Jeremy...check t'stat, again. I imagine you'll replace belts, if necessary, were you to decide to check water pump. Will still need to deal with radiator, keep us posted.

__________________
83 SD

84 CD

Doug, I assume when you did the test of the temp sender in a cup of boiling water the engine was not running. If so, the readings you have may be different (lower) than if the engine was running. Afaik the temp gauge runs at battery voltage and is not regulated. The voltage when the alternator is charging is typically 13.9 to 14.5 v. , with engine off battery voltage is around 12.2 to 12.7 v. The difference can affect the gauge reading.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Your assumption is correct that the car was off when the test was performed.

But, the dash gauge does not read any differently when the engine is running normally vs when it is stopped (obviously with the ignition keyed "on").

It did occur to me that when I did the test the engine was cold, and that if the wiring from the sender to the gauge were faulty (internally corroded) it could generate excessive resistance as the engine heats up, giving false high readings. So, I bypassed that wire with some 10 ga wire from the sender to pin #2 on the instrument cluster. The readings were identical.

A replacement radiator (Nissens) is on the way along with the waterpump housing gaskets. I'll probably pull the waterpump this weekend for inspection.

I pulled the water pump this morning, its fine. Also the inside of the waterpump housing is squeaky clean.

FWIW, the entire removal process was very, very easy. No rounded off/broken bolts. No problems at all.

I still think its inadequate coolant flow, due to restricted passages (t-stat, radiator or engine (god I hope not).

So, I cleaned up the water pump and reinstalled it (had to wait for the water pump to housing gasket), degreased the fan clutch (it was dirty), installed a new Nissens radiator, and refilled it with recovered (filtered) coolant.

It still ran at just under 100 C.

I finally broke down and swapped the thermostat out from my 85 300TD (which has always run a rock steady 87-90 C).

Sure enough, now the 84 300TD runs at 87-90 C and the 85 300TD runs at just under 100 C!

Both thermostats are Behr, stamped 80 C, made in Germany. They look identical but the one from the 85 300TD has a very small MB star stamped on the "foot" plate. It was in the car when I bought it 5 years ago.

What I would really like to do is test them side by side by immersing them in hot water and watching 1) at what temps do they begin to open, 2) at what temps are they fully open, and 3) if they open by the same amount.

If I do get around to collecting this data, I will certainly share it.



Thanks to everyone for all the suggestions.