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.

Alan Hamm · #14 09-01-2004, 12:14 PM

Quote:

Originally Posted by sarafin

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!

With these cars, is it not true that defrost only releases heat and does not engage the A/C compressor? I have not checked this lately, but this was a difficult adjustment compared to other species of cars. The Hi/Lo setting would be used to remove humidity (defrost) in the summer.

At least this is how my 3 work.

swogee · #15 09-01-2004, 06:04 PM

Thanks for all of the posts so far. I have read them, and I have a few responses.

1) The overheating problem was occuring prior to the replacement of the radiator. The old radiator was leaking out of the driver side tank, so I thought (and a mechanic that I took the 300TDT thought) that a new radiator would fix the problem. I guess we were wrong.

2) I didn't think about the fact that turning on the defroster would click the A/C on as well. On my Passat, the A/C compressor turns on when I turn on the defroster, however on the 300TDT it is a moot point since the A/C doesn't work due to a lack of R-12 in the system. I have turned the heater on as well to reduce the temperature of the engine and it pretty much has the same effect as using the defroster.

3) I have not had the thermostat replaced, and that will be done by tomorrow.

4) The radiator cap is a "Stant" brand 20 psi cap. The mechanic checked the cap and it held 24 psi no problems. I don't think it is the cap, but I will replace the cap when I order some more parts.

5) The fan is the original aluminum unit. It could be causing the problem, so I will have to check it out. The engine only heats up during the hill climb, which is actually about 2.5 miles long. As soon as I turn around and go down the hill the temp drops to normal.

6) The water pump is also a possibility. I am pondering if on changing that out next. I suspect it will cost a lot to have a mechanic do it so I will most likely do that myself along with the fan/clutch, belt tensioner, and belt. I am also planning on replacing the temp. sender when I pull the intake manifold off.

7) I may be overly concerned about a temperature of 110C when the 300TDT climbs hills, but after reading about all the cracked original heads with a "14" casting number I think I am justified since my head is original. The main thing that worries me is that if the car heats up with the outside temp only 85F, I wonder how hot it will get if the outside temp is 105F or if the A/C is actually turned on and functioning properly.

Side note: There is only anecodotal evidence that the Passat temp gauge is an "idiot" gauge. I have heard this story before, and I am not entirely convinced it is true. I have seen the gauge lower than 190 as the Passat was warming up. My Honda definitely has a regular temp. gauge since it moves up and down when I go over hills and into a hot areas, etc.

Thanks again for all of the responses. Keep the good ideas coming. I will keep everyone informed of what I find.

-Steve