Firstly, I want to thank everyone here profusely! This is my first post but this forum has already saved me $2,500 in labor costs since buying my 83 300d back on New Years Eve...on a whim

My brother and I bought the car together for $1,200 with the intent of slowly "restoring" it while I use it as an occasional driver. It supposedly has 179,000 miles which I have no reason to doubt at this point. It runs well and all the electrical gadgets work the way they are supposed to. Unfortunately, the PO installed a WVO system, and from what I can figure out from reading his documentation, it is a "proper" system which I will be removing as part of the restoration. From what I can tell from the records the alternative fuel was only used sporadically for the last 5,000 miles so hopefully whatever damage has been done can be reversed. Anyway, sorry to bore you all...on with my problem.

My brother and I immediately replaced a few things: Oil cooler, Oil cooler lines, engine mounts, engine shocks, radiator, thermostat, power steering pump and all the belts. After putting it all back together, the car still starts, runs and drives well...what an accomplishment. However, the temp gauge stays right around 85C around town, which is good, but when you get on the highway and go faster than 75 mph, the temp starts creeping up in relation to the increase in speed. At 80mph the temp gauge hovers just a hair above the 110C mark, which is hotter than I am comfortable with. When you slow back down to speeds under 70mph the temp returns to a normal reading of 85C and stays there.

The coolant level reads full in the coolant tank, the oil is full, radiator is new and so is the thermostat.

I thank you all in advance for any advice you may have for me.

Cory

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Former owner, always looking.

1984 300D Silver/Blue (deceased 1997)
1983 300D White/Palimino, Miami, FL 230,000mi (deceased 2013)

Water pump, auxilary water pump? It sounds like coolant distributution can't keep up with the high RPMs...

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1989 300E 144K

Could it be a bad thermostat?

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-Typos courtesy of my mobile phone.

I was thinkning that, too.

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1989 300E 144K

Did you blow out the radiator fins making sure your getting good air flow? Your sure the temp is really that high, check using a hand held "temp sensor gun" My 85 240D temp gauge reads high after hard freeway driving, a good "smack' on the dash drops the temp, verified by the sensor gun.

Did you guys adjust the valves?

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

Sounds rudimentary, but make sure your cap is on good and tight to the second "notch"

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

The thermostat should be installed "arrow up". You might want to go to the trouble to verify the accuracy of your sending unit. I had to test three in my 300 before I found one that was reasonably accurate.

I agree, verify the temperature of the radiator and the engine block/head with a non-contact thermometer before taking any action.
Did you take any steps to 'burp' the system when you replaced the radiator and thermostat? These cars are notorious for getting an air bubble in the back of the engine/heater core once they have been emptied. They should be filled thru the top radiator hose with the nose of the car pointed uphill to avoid getting an air bubble.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I know this sounds silly, but when you filled the cooling system, did you mix the coolant 50/50 with water? Straight ethylene glycol doesn't cool nearly as well as a water/glycol mix and can cause symptoms like you're describing. I've seen it many times.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

Yes, my T-stats have been working fine for a long time so I forget that I went through several (all OEM, no junk.) before I entered this period of "OK tempts" That arrow can be hard to see even when you know its spoz to be there.

So don't rule out "outa the box, NG"

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

Do you have green coolant in there? I received my car with the green crap and it ran hotter than it should. When Pierre flushed and replaced the coolant, it ran like it should.

This is a hot topic (punn intended). I will try to reply to everyone's suggestions so far. I am pretty sure the Aux water pump is working...I can hear it humming when the CCU is on with the ignition in the on position and the engine not running. I'm not so sure about the water pump. There is no seepage from the weep holes and it doesn't make any weird noises, beyond that, how would I diagnose further problems with that component?

As for the thermostat, I am going to test it tomorrow by boiling it alive on the kitchen stove. Is it true that I cannot reuse the thermostat rubber seal even though it was only in the car for four days?

As for blowing out the radiator, it is new, so I hope that you will forgive me if I do not follow through with this suggestion. I will get an infared thermometer to ensure that the temp gague is working properly, but what bothers me about this problem is that beore we replaced the radiator and thermostat and flushed and filled the system with the G-05 coolant whcih was pre-mixed 50/50, the gague didn't move a hair over 85C no matter how fast I drove. Also, if the new thermostat was bad, wouldn't the car overheat while driving around town?

This is very frustrating because out of all the parts we replaced, the cooling system was the only thing that was already working properly. I replaced the radiator and thermostat as preventative measures because the fins at the bottom of the radiator were all corroded and the entire metal core of the radiator had turned a nice Statue of Liberty green.

As for burping, when I initially refilled the system I did not take the percautions I should have because at that point (about a week ago) I was not aware there was a "technique". I simply used a funnel to fill the block placed directly into the thermostat housing before inserting the thermostat, I put everything together and then filled the radiator from the overflow tank.

So last night I attempted a better late than never burping. I put the front end up on ramps, put the CCU on defrost and idled the engine at about 1,700rpm util the temp gauge read 85C. The gague stayed there until I raised the rpm's to a higher, more highway like level...at which point the temp gague started to creep up again, plus the burping never occured, the level in the overflow tank didn't change at all. AAAAARG!

That's enough writing for now, thanks again to anyone who made it this far, I am very appreciative.

Cory

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Former owner, always looking.

1984 300D Silver/Blue (deceased 1997)
1983 300D White/Palimino, Miami, FL 230,000mi (deceased 2013)

Is it possible that the New Radiator does not have the same number of Core Tubes in it?
I mean is the New Radiator as thick in the Core Area?

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84 300D, 82 Volvo 244Gl Diesel

Is it possible that the clutch in the fan has failed?

You have not yet removed the WVO system, right? Doesn't that tie in to your cooling system to heat the WVO? I'd also be looking for a trapped air pocket in that system. You're going to remove it so you may have already taken in out of the cooling water system - sure you got every thing back to stock?

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Charles
1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/mercedes-benz-cars-sale/296386-fs-1-owner-83-mb-300d-turbo-rebuild-parts.html