what is the normal coolant temp for an 81 300D?
 

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

Should run 80 to 85 c.

Does sound like a partially open thermostat.

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.

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

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.

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

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.

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.

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,

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

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...

:D :D

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.

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.

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

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.

replace that t-stat!
 

rick, you have exactly the same symptoms i had - replaced t-stat with a bosch. no problem ever since. the temp should show around 80-85 deg c. even as it was 15deg f out this morning the gauge showed 80 deg c after about 15 mins. of driving. replacing the t-stat will make a world of difference to the amount of heat you get.

Rick,

A stuck-open t-stat will make the engine take longer to warm up, when ambient temps are fairly high. When it's cold out though, it will never let the engine reach operating temps. You almost definitely have a bad t-stat, replace it and your heater should work again. You didn't notice this in the summer because it just took longer to warm up, but did reach normal temps eventually...

:)

Well I guess that settles that, I'll replace the T-stat.. Now, when I went to change the thermostat of SD The head of one of the bolts broken clean off (galvanic corrosion). Is there any tricks to getting these bolts out without break them?

Yes there is a trick to getting the bolts out without breaking them!

hit them with a hammer firmly before attempting to break them loose. Naturally, if the cad plating is gone... new bolts.


??? So this jumping gauge question. Mine has good fuses(not so old) and the temp connection on the head is clean. It seems like the gauge jumps occasionally between 85-95 and then back down. I'm not sure how a linear resistance element could behave this way ?????? Is the sensor bad -$10 is cheap but I like to diagnois my problems and not throw money at them=)



Michael

The Haynes manual for the 123 series states that the thermostat for these cars should be fully open at about 94C. My car's operating temp is 90-100C, depending on the driving condition and it gets there within 5 minutes of driving, even in sub-freezing temperatures. This seems a little higher than what most of you here are getting, but it seems proper to me.

DieselAddict,

Yeah, that sounds a little high, but as long as it doesn't get over 100-105C in the summer it's perfectly normal. If you do see 105C+ temps in summer then something needs to be fixed. My cars also reach full operating temp in less than 5 minutes at 55mph, in sub-freezing temps; but stay in the 82-87C range when it's that cold out...


HTH,

Several folks mentioned their temps being 95C in the summer months ... while this is not "in the red" I believe it is higher than it should be.

I live in Northern Virginia - normal ops on mine is a quick warmup to 82-85C and then, if in very, very hot conditions in stop and go traffic, it may go to 90C ... as soon as moving above 25 MPG or so, it goes right back to 85 or so. It can be 100F plus and, while driving at 55 MPH, it stays rock solid at 82-85C.

Wasn't always this way ... after replacing the viscous fan clutch two summers ago, it started heating up past 95C and moving up and down ... replaced the thermostat and returned to normal ops running at 82-85 steady (literally no movement).

Bottom line - noticeable movement up and down and anything about 90C is likely abnormal ... take a look at your v clutch and thermostat.

I think some of the variation in coolant temp comes from people using varying thermostats. I believe these cars are supposed to have an 80C thermostat, meaning the thermostat starts to open at that temp and thus the operating temp would be in the 90's.

I too thought my operating temp is a little too high, but it's steady and only twice did it get a little above 100C on a hot summer day. Most of the time it's steady in the 90's. In the spring I plan to flush the entire cooling system and see if it makes a difference.

Even in fairly cool weather my engine runs a bit hotter when it is climbing steep grades at high speed. In the mountains in the summer, fully loaded and A/C on, I can get to 100c going up hills at speed. As soon as the load lessens the temp starts to drop. My car has been doing this for the 10+ years I have owned it without ill effect. As far as I know this is typical and won't hurt the engine provided it doesn't got into the red on the temp gauge.

May I be another to singthe praises ofthe Auto Zone Thermostat. Car came with NXXA knockoff was almost new. Following the advice of this forum changed to the AZ device and voila. Problem cured.

Thanks

Ditto on the 80 to 85 degrees C.

Folks, please don't get too caught up in expecting 80-85C temps as "normal". Believe me, I've been down this road ad nauseum in the past, both with my own cars and DOZENS of people on the MBZ.org email lists. The final consensus is ANYTHING above 80C and below 100C is perfectly normal. If your car runs 95C all the time with peaks at 100C in summer there is NOTHING to fix, don't bother with a different t-stat, or anything else. You only need to worry if under 80C ever, or consistently above 105C in summer. The "worry zone", IMO, is 110-120C. Sorry to beat this to death but not every MB diesel will stay parked at 82C (although SOME can and do!)

:cool: :cool: :cool:


Regards,