what is the normal coolant temp for an 81 300D? - PeachParts Mercedes-Benz Forum

Erikir · #1 02-03-2003, 08:08 PM

...my gauge indicates around 60-70 celcius, which seems a little cold to me. Could it be that I need a new thermostat?

Mike123 · #2 02-03-2003, 08:16 PM

Should run 80 to 85 c.

Does sound like a partially open thermostat.

Lycoming-8 · #3 02-03-2003, 08:25 PM

Put a new t-stat in my '83 in the early fall. It was the Auto-Zone item recommended here, and it has yielded about 80 deg C on the dash inst. Believe that the unit said 87 on it, but either the gauge on the dash is off or the 87 was not the operating temp. of the t-stat.

rwthomas1 · #4 02-03-2003, 09:07 PM

My 84 runs at 90-95C, the needle is always at the 9 o'clock position, as in pointing straight to the left. It rises to there and never moves after that. New T-stat installed by seller when I purchased it. I assume this is normal. RT

1stimer · #5 02-04-2003, 02:55 AM

Mine has been running at no more than a couple degrees above 80. I was worried this is too low. Temperatures are around 80F right now.

Wallknight · #6 02-04-2003, 12:24 PM

Thanks for the weather report, 1stimer; all of us in areas where it hasnt been above freezing since Christmas appreciate it

lrg · #7 02-04-2003, 01:19 PM

Thermostats on the 123 and 124 usually fail in the open position. When this happens the engine doesn't get up to proper operating temp so you should replace it. There have been some discussions about people getting bad new thermostats so make sure you get a good one. You can always test it on the stove with a pot of water and a thermometer to make sure it opens at the right temp. Do a search and you should find all the information you need on this and then some.

BWatson · #8 02-04-2003, 01:25 PM

My 82 seems to go back and forth between 85c and 95c depending on the driving conditions. Start and stop traffic heats it up to the higher end. I live in Los Angeles so there is tons of traffic as well as some extreme hot spells in the summer. I did notice last summer that it was getting up closer to 100c a few times but I plan to go through the cooling system before the hot weather hits.

gsxr · #9 02-04-2003, 02:06 PM

Two things typically cause a low reading on the dash temp gauge. One is the thermostat, the other is the sender in the cylinder head. When I bought my '84 it barely got above 50C in the winter. Replacing the t-stat got it up to 60-70C or so on the gauge, although the heater worked great. I replaced the sender in the head ($10) and now it reads normally.

"Normal" engine temps are anything between 80-100C. Less than 80C and either the t-stat or temp sender is bad. Above 100C in less than brutal summer conditions (110F, AC on full) and it's too warm. FWIW, my 1984 now stays parked at 90C almost all the time. I've never seen it go above 95C, even with 110F ambient temps and AC on max. I've never seen it below 85C either, even in cold 20F weather.

HTH,

#10 02-04-2003, 05:01 PM

Silly question, but where is the temperature sending unit located on the head? I have looked and looked and havent got a clue. My gauge jumps from 85 to 90 sometimes and then jumps back. (step function) Thanks Lee

gsxr · #11 02-04-2003, 05:04 PM

It will be between the #2 and #3 glow plugs, I think (or is that 1 and 2?). If the gauge jumps, you have an intermittent electrical connection that really must be corrected. Clean the sender pin & socket and make sure it's tight. Also clean (and/or replace) the fuse connections in the fuse box. Doc Booth on the MBZ.org lists swears by replacing ALL the fuses every 5 years or so, and cleaning the tabs too. It's cheap and allegedly cures a myriad of electrical gremlins...

Erikir · #12 02-04-2003, 05:09 PM

Being that the heater doesn't function as well as I feel it should and that I've replaced the monovalve and auxulliary pump (both repairs have resulted in much more comfortable winter driving

), I figure it's gotta be the thermostat.

gsxr · #13 02-04-2003, 05:19 PM

Very likely, yes it does need a new t-stat. This would be a good time to inspect the short 2-inch length of hose between the water pump and t-stat housing, and replace it if it's swelled or leaking. This is a semi-big job but might as well do it while the system is drained for the s-stat replacement. Tip: Get a dealer, or OEM (Wahler/Behr), t-stat if possible... not a generic McParts unit.

#14 02-07-2003, 11:00 AM

Thanks gsxr, I found the sending unit. Boy that sure is hidden well. The gauge only jumps as the temp gets higher than 85 or so degrees so I think it probably is the sending unit. I'll try cleaning the connector first. Thanks again, Lee

rickjordan · #15 02-08-2003, 02:16 PM

Lately I have been having the same exact problem. However, I was attributing it to it not getting above 30 degs F. With the heat on max, the air coming out of the vents is luke warm at best. The temp gauge doesn't get above 60 even while driving on the highway, except once when the temps got briefly up to 35. Also, I thought that a stuck open thermostat would only make the engine take longer to get heat up, not prevent it from ever getting there.