1987 300TDT Overheats on long hills - PeachParts Mercedes-Benz Forum

swogee · #1 08-31-2004, 01:01 AM

My 1987 300TDT overheats while climbging long hills. After reading various threads on this forum, I am beginning to wonder if there is some sort of head crack problem even though there are no symptoms.

There is no oil in the cooling system, and no water in the oil. The radiator is brand new. The upper radiator hose doesn't seem to be pressurized when the engine is cold as descibed on several threads. The engine temp. stays slightly over 80 degrees C while driving around town on flat roads, but when I drive up a long hill (over 1 mile at 6% grade) with the ambient temperature around 80 degrees F the engine temperature shoots up over 100 degrees C. It then becomes necessary to turn the defroster on to keep the engine temperature below 100 degreees C.

The mechanic I have looking at the problem hasn't found any hydrocarbons in the coolant yet, but he is planning on running some more tests tomorrow.

Does anyone have any ideas as to what might be causing the engine to overheat on hills?

Thanks in advance.

-Steve

jcyuhn · #2 08-31-2004, 10:06 AM

Coupla thoughts. First, that doesn't sound like overheating. 120C is overheating. So just how hot does it get? If you're only hitting 105 or so, I wouldn't worry about it. What is the speed up this hill? If it's low speed, then lack of airflow across the radiator is going to limit the amount of heat rejected. A 6% grade is a decently steep hill, I would expect a significant increase in coolant temperature while climbing it.

Have you ever tried leaving the defroster off to see where the temp settles? Is the a/c running while you do this test?

We don't have any 6% grades 'round here, so I can't tell you what my wagon does under the same conditions. However, I've never had any trouble driving the coolant temp up to 105C or so on long, gradual (2% maybe) grades taken at highway speeds in hot weather. Never thought much of it. Thermodynamics says the coolant temp is going to vary based on the amount of fuel burned (e.g. throttle position), coolant flow (engine speed), airflow across the radiator (car speed, wind direction, cooling fan operation), and temperature and humidity of the ambient air. There are a lot of variables, sometimes it's normal to get warm.

- JimY

Habanero · #3 08-31-2004, 10:49 AM

I have never seen my SDL above 90, but I also don't have any real grades to climb. Since I believe my head had been cracked since I owned the car, I don't know how to reconcile heating behavior with/without a cracked head. I did notice last Friday on my first interstate trip since putting the "17" head on, the temperature was much more stable as a general rule, running just a little above the middle of the 8.

Dana B. · #4 08-31-2004, 11:07 AM

My '91 2.5T does the same thing. On a recent 5,000-mile trip to the Midwest, my car ran hot on long steep grades. The worst was through the Mojave, where, with a passenger and heavy luggage, it hit 105C going uphill with the A/C on and the outside temp at 105F.

It's disconcerting to have to slow down and kill the A/C on a smoking 10-mile desert grade -- with minivans driving past with impunity.

The rest of the trip, the MB ran at a cool 85C.

I have to assume that my car is normal. But for peace of mind, I'd be better off with idiot lights.

Brian Carlton · #5 08-31-2004, 11:14 AM

I have the same engine in the SDL and have monitored the temp. quite closely on the one hill that I encounter routinely. I'm not sure of the grade, however, the vehicle requires maximum continuous power in fourth gear to maintain 70 mph. The hill requires this power for approximately 45 seconds.

The temp. always starts out at 90 and always winds up at 100. This change is independent of ambient temp. (same at 60 or at 90). The change also is the same even if the a/c is on or off. I have only watched it peak over the 100 mark on one occasion (90 degree ambient), but that was just briefly at the top of the grade.

Now, on an extended 6% grade, I might have additional temperature climb above 100.

The radiator is just about new by looking at it (installed by the PO).

I'm not going to get too concerned about it unless the temp climbs toward 110. If it were to do this on a long 6% grade, then I'd just slow it down to about 45 mph for a bit and allow it to cool.

dquance · #6 08-31-2004, 11:31 AM

You could try some water wetter, I believe there is now a diesel version available. Also, turning on the defroster is probably adding more load than just turning off the A/C. That is the purpose of the Economy button. ;-)

Good luck.

Doug Quance
87 300TDT
74 280C
69 230 (parts)

swogee · #7 08-31-2004, 12:33 PM

Thanks for all of the replies so far.

At this point I am inclined to believe that the overheating is not normal. I say this because all of my other vehicles don't even rise in temperature more than five degrees when climbing the same grade. Actually my VW Passat doesn't even move off the 190 degree F mark when climbing the hill with the A/C on at 70 mph.

My 1987 300TDT climbs to over 110 degrees C while climbing the same 1 mile long 6% grade and it would most likely hit 120 degrees C if I did not turn the heater on. The overheating occurs while traveling at freeway speeds of 65 to 70 mph with the A/C off. After reading many other posts on this forum, it looks like the only time the engine should heat up to 100 degree C mark is if the outside temperature is hot (over 100 degrees F), with the A/C on, on a long hill.

Keep the ideas coming. Thanks.

-Steve

tolive31 · #8 08-31-2004, 12:50 PM

It is my understanding that the temp gauge on your Passat actually functions more like an idiot light than a "real" gauge. I believe this is the case for all A4 and B5 VW's. The ECU keeps the gauge steady at 190F (or 90C, depending on which cluster you have) as long as the temperature is within an acceptable range; so your Passat could actually be running at 100C (or higher), but you'd never know it.

jcyuhn · #9 08-31-2004, 01:04 PM

I was going to make a similar comment with regard to temperature guages. I don't know about VWs, but many (most?) Asian cars have an "idiot guage." It points at the exact same spot on the scale for every temperature from slightly above stone cold to slightly below boiling over. If the guage never moves while sitting at a long red light in the summer heat, it's almost certainly lying to you.

- JimY

Mr. FancyPants · #10 08-31-2004, 01:13 PM

You said you have a new radiator. Do you also have a new thermostat?

Is it possible you have an air lock? Try by adding coolant if needed from the upper radiator hose.

I agree with the notion that your gain for more cooling by turning on the defrost, which also turns the A/C on, isn't likely going to change engine temps much at all since you are adding heat, and removing heat at the same time! With just the heat on, yes you will remove heat, but this doesn't solve your overheat situation if there is one!

I'd replace the thermostat anyway if it hasn't been replaced in a long while! Nothing to loose if you do and it might solve your possible problem!

goldenbear · #11 08-31-2004, 02:09 PM

Nothing about what you described is abnormal. Now if it does edge above 110C it may be, but from what you've stated you have an ideal hill for an italian tune-up

. Is your auxilliary cooling fan kicking on at 105C? If not, then that temp switch should be replaced.

dieseldiehard · #12 08-31-2004, 05:31 PM

I sense the water pump may be worn, with the impeller blades partially corroded off, like the one gsxr shows on his OM603.
Hopefully you (swogee) didn't already say that item has been replaced. If it hasn't and you do not know the hsitory on it, I would suspect it as a culprit.
My '87 300DT never overheats on a level road, with temps near 100 degrees and holding at a steady 75 MPH with the AC on. It will head for 100+ on long grades under the same conditions (and especially at slower speeds) but falls back to 85 - 90 when the terrain levels out. I also plan to add Redline water wetter on the next opportunity (already bought the stuff).
DDH

lrg · #13 08-31-2004, 05:37 PM

Maybe my car is unusual but I'll pull some pretty steep grades up route 80 in the Sierras in hot weather and never see the high side of 105. My 123 heats up easily under load but my 124 just doesn't. Without other symptoms of a cracked head I think you're OK there (for now). It's the new radiator that has me wondering. Whenever a new problem crops up after a recent repair suspect something related to the repair or because of damage done from the failure that necessitated the repair. In your case you may want to check the radiator for proper flow (new or not), the water pump and hoses. If the temp is normal except going up grades it's probably not the t-stat. You're problem sounds more like a cooling capacity problem. Also it's probably a good idea to check the fan clutch too, even though it ought not to matter at higher speeds it might be a contributing factor. With all due respect to others here who disagree I think you do have a problem, probably (hopefully) not a serious one. Good luck and keep us posted.

gsxr · #14 09-01-2004, 01:11 AM

Steve,

1) 110C is not dangerous but is hotter than normal, IMO, even on that kind of a hill. (Especially if ambient temps are below ~90F.)

2) If the thermostat and rad cap are not recent, as well as OE/OEM items, replace both with OE/OEM only (Wahler/Behr).

3) Make sure the AC condenser fins are clean! Especially in front of the fan clutch.

4) Time to check the fan clutch. Climb the hill, get the temp to 110C on the dash gauge. Pull over, pop the hood, and shut the engine off while watching the fan. It should stop almost immediately, or within 1-2 revolutions maximum. If it freewheels to a stop (5+ turns) the clutch is not engaging.

5) The electric cooling fan should trigger on high speed (very loud/noisy) at 105C. It will definitely be on by 110C. If not, the temp switch is probably bad - very common failure, it was bad on all four of the 87's in my family...! (This is NOT the cause of the high temps, just something to check while you can, as otherwise it's hard to get the car to 105C+ to test!!

)

HTH,

95300YDT-A · #15 09-01-2004, 02:09 AM

Hello Swogee,

After a lot of miles in the French mountains (very steep mostly) and with several 300D's, 603's AND 606's, turbo and no-turbo, in the W 124 AND in the 210, I can tell you : they all do the extra warming up. And all of these cars were/are in great shape and perfect working order.
Off coarse I talked this through with my indy, a very qualified man.
He agree with me : the cooling system of all these engines is to small under heavy load.
We have just to live with this item.
But maybe there is one solution. I remember a thread on this forum, a short while ago, about the same topic. There was someone suggesting to replace the original visco coupling by a "tropical" one that kicks in much earlier.
And that's exactly what I believe to be the cause of the "overheating". Don't forget that it is the ambient temp (under the hood) that makes the visco engage so it's allways to late!
I am also a strong believer of cleaning the radiator(s) with clean pressed air
from the rear to the front. You have to see once what will come out of there.
Last but not least : the driver's manual states that temps up to 120° C are normal. Anyway, by experience : these engines survive that temp without
trouble. Okay, I agree, it's a frightening experience to us drivers to see the gauge needle fooling around that figure!
Have a nice day,

Danny