Do both the upper and lower radiator hoses get hot? If not, that points to a water flow problem. (Bad t-stat, bad water pump, internally-collapsed hose.)

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1981 Mercedes 300TD, 1994 Honda Civic Del Sol
http://mefi.us/images/fuelly/smallsig-us/67195.pnghttp://mefi.us/images/fuelly/smallsig-us/103885.png

For kicks, remove the plug (17mm wrench) on the #1 exhaust runner and go for a drive from a cold start. Does coolant temp rise as quickly?

Sixto
87 300D

I replaced the radiator cap and noticed a mild improvement. I started taking off the cap every time I stopped today, and heard no pressure escaping when removed slowly. Temp was 90 C. Is that normal?

90*F is good. That's what I get with AC compressor engaged this time if year.

Sixto
87 300D

Like Sixto suggested. You may want to try running with the exhaust open to see if exhaust back pressure is a problem. Has to be open before (upstream) the Oxidation Cat. Some will refer to it as the catalytic converter as I did for a while.

My SDL ran warm and I did all the cooling system stuff. And I do mean all.
Finally had the Oxidation CAT replaced with straight pipe to test the theory....what a huge difference...runs so much cooler, spools faster, and on and on. Also may want to make sure the gauge is reading true.

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1986 300SDL 440,xxx

OK. I"ll test the exhaust line. I meant 100*C, the average running temp.
I laser thermometered the sending unit and it read really close to the cockpit gauge at a variety of temps.

We're getting there! Thanks everyone for the support.

Good job listening and applying what we have said...cooling and 603 is a common theme. In Florida with AC running and while cruising im right around 90, which is dead centered/horizontal. And that is with NO auxiliary fan right now, plus a working but otherwise unknown fan clutch off of a parts 87 300d. Some people say just above 80 but they are in different climates I think.

I'd recommend making sure that you have a sealed coolant system. I had a small leak for the longest time that caused me a lot of grief!

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000

I have an '87 300SDL 308,000 miles. I had a gradual rise in operating temperatures until it operated at 100 degrees C after a mile, down the road. Was not the thermostat, waterpump, leak, lack of coolant, clogged catalytic converter. It was simply a clogged radiator. Even if the coolant is clean and clear it doesn't mean that your radiator is not clogged Try the citric acid flush of your radiator. You should find that the temperature going up a steep hill, 90 degree outside temperature and the a/c on shouldn't bring you anywhere the 100 degree mark after the flush. I've had two other 300SDL's and two 350SD's all have suffered from the same overheating malady and all needed the same thing, the citric acid flush of their radiators.

'87 300SDL 308,000
'89 300TE 176,000
'94 SL500 55,000

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jcciem
'94 SL5000 60,000
'91 350SDL 545,000
'91 350SD 197,000
'00 E55 AMG 77,000

This morning's startup: 68 dF ambient. Start car, drive away, no hills. 0.25 miles of steady driving at 20 and then 1 mile at 30 MPH got me just up to 82 dC thermostat temp. About four minutes.

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James Marriott
2003 Buick Regal
1983 300D (228k, frau Auto)
1996 Suburban K2500 (192k, 6.5 turbo diesel/4WD towmaster 10,000)
www.engineeringworks.biz
1987 300SDL junker 170k
1982 300SD junker, 265k

I followed these instructions and didn't notice any difference. The only thing that happened was the ABS lite came on

Go around and check your coolant hose fittings. Have you been losing coolant? Even in small amounts?

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000

I'm thinking to focus on refining the cooling systems first. Need to get the system pressure tested. A citrus flush is cheaper than a new radiator. I can spray the fins of the ac cooler down with aluminum wheel cleaner. Also, changing the coolant out for zerex.
I've heard it could be due to "running lean" from under combustion of fuel. I do hear some injector nailing and will get those done in time.
Thanks again for all of your help.

OK. I don't see anything on the ground, but I will check the hoses and connections. One weird thing is that the part between the cap and the reservoir turns with force. Perhaps I'm losing some pressure there.

Absolutely go for the citrus flush and new coolant. And I imagine if the expansion tank cap fittings isn't tight, it must not be able to make a complete seal.

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000

Pressure tested system and the side of the reservoir tank is pissing coolant under 15psi