300SD running hot. What's next? - Page 2 - PeachParts Mercedes-Benz Forum

TonyFromWestOz · #16 07-22-2004, 11:21 AM

Quote:

Originally posted by Brian Carlton
As a consequence of this thread, I was paying attention to the temperature on the SDL yesterday.

Operating conditions: 85 degree ambient.

I ran the SDL with the a/c off and the windows down just to see what it does. Again, when boost is used, the temperature climbs up to 95 degrees immediately. This occurs when leaving a stop light and accelerating up to 60 mph using heavy boost in third gear between 25 mph and 50 mph.

Then, with the a/c on, the temp will go right up to 100 when using heavy boost for more than 30 seconds at 65 mph in climb.
In fact, on one hill in CT, it actually peaked at about 102 degrees.

As soon as the boost is released, the temp will gradually return to 90. If the engine is unloaded completely, the temp will return to about 85.

One possibility:

There is a differential between the gauge and the thermostat.
The water temp. that the thermostat sees is not exactly the same as the water temp. seen by the gauge. Under heavy load, the temperature at the gauge creeps up to near 100 degrees but the thermostat sees water at somewhere near 92-94 degrees.
This can be the only possible explanation because, as stated in previous posts, when the thermostat is removed, the temp. never gets above 80 or so.

This would explain why the 100 degree barrier is not easy to cross. The gauge will climb right up to 100 or so, but seems not to want to go too much beyond 100. This would imply that the thermostat is now seeing hotter water and is opening further.

However, the heat produced under full boost does appear to be more than the system can dissipate. I do not think that the 603 could run at full boost for more than about 90 seconds before getting into trouble.

To me it looks like you have coolant flow problems.

Driving away after idling at the lights your coolant temps peak. - When you have relatively high engine RPM, and you are moving. It seems that the coolant cannot get to the radiator, or the radiator cannot cool the coolant down properly.
It works fine W/O thermostat, so the problem could be due to low coolant flow.
Without the thermostat, your water pump moves the coolant around relatively easily. When you install the thermostat, the coolant flow, even with the thermostat open, is too low. Is there something wrong with my logic?

tenknots · #17 07-22-2004, 11:24 AM

It appears that the gauge sender is on the LH side of the block whereas the t-stat and sender for the aux fan are in front, just before the radiator. This, I would think, has to be the hottest water since it's the last place it goes until it gets cooled.

I may be resigned to having the car run hot. Up to 105c is only about 221f, which is acceptable. 110c = 230f which is getting to the nervous point as there's little room for an increase.

My feeling is the radiator is the problem even though it's not very old. I hate shotgunning problems anymore than I have to, so before that, I'd buy a laser temp gauge.

I have hot water going in and the lower return hose is much cooler, so that makes me believe the coolant is being cooled. It's almost as if the engine puts out more heat than the radiator can handle. But, without a t-stat, the engine is much cooler (I've tried 2 t-stats) so the radiator must be sufficient. AARRGG.

I've disconnected and plugged the EGR (from the vacuum source) since it was leaking vacuum and effecting trans shift. There is a temp/vacuum switch on top of the t-stat housing that no longer does anything, but (and I grasping for straws here) could that have any effect on temp? EGR and temp don't seem to be interrelated, but I've been wrong before.

Eric Eliel · #18 07-22-2004, 11:47 AM

When you use a laser guided thermometer, you'll see the temp difference at the thermostat housing vs. the temp sending unit. Its about a 5 to 10 degree difference as the coolant goes through the block and head.

Tenknots, you still need to check your thermostat on the stove first. With only 20K on a new radiator I doubt you have a problem there. You may in fact have a gauge system reading too low. It reads in the correct range w/o thermostat (which could be too low) AND then reads high w/the thermostat installed (which could be OK).

BTW without a thermostat in the engine, you should see more temperature fluctuation.

One member here went through 5 or 6 thermostats until he found a good one. All tested on the stove.

Brian Carlton · #19 07-22-2004, 01:50 PM

Quote:

Originally posted by TonyFromWestOz
To me it looks like you have coolant flow problems.

Driving away after idling at the lights your coolant temps peak. - When you have relatively high engine RPM, and you are moving. It seems that the coolant cannot get to the radiator, or the radiator cannot cool the coolant down properly.
It works fine W/O thermostat, so the problem could be due to low coolant flow.
Without the thermostat, your water pump moves the coolant around relatively easily. When you install the thermostat, the coolant flow, even with the thermostat open, is too low. Is there something wrong with my logic?

Well, I never had the thermostat out of the 603. My comments regarding the lack of thermostat were based on other posts concerning the 617. I have difficulty believing that there are flow problems with the 603 because the radiator looks to be brand new. However, I really cannot confirm the condition of the fins until I remove it from the vehicle.

I also need another 603 in an SDL for comparison purposes at similar ambient temperatures. Maybe Hatterasguy can make a judgment here.

tenknots · #20 07-22-2004, 02:33 PM

You're right Eric, the radiator is pretty new - 20k and 6 years old. However, I havae read about having the wrong radiator installed. I'll check teh P/N tonight.

Guess I'll have to pull the thermostat out again.

I do get quite a range without the thermostat. In heavy traffic, ac on 92c vs. 105c with. For the first 5 minutes, it never gets to 80c. Going up a mild hill with ac on seeing how the newly balanced tires did at 80 gave me 100c with no thermostat - seems a bit high.

With a temp sensor, is it aimed at the block by the sender? How can you know this is the same temperature as the coolant with all the iron absorbing heat nearby? Same with the thermostat housing, though it's much smaller and more conductive being made of aluminum.

No comment on the EGR?

Eric Eliel · #21 07-22-2004, 06:39 PM

You aim the laser light beam directly at the sending unit base. Or aim it at the thermostat housing. Its fast and has a digital readout.

If you could talk your wife into using it in the kitchen too, the cost might be justified! :-)

Did you check the thermostats that were currently out of the vehicle on your stove? You would use a standard meat thermometer for that. Put them in a pot and slowly bring it to boil. I hang the thermometer on a kitchen tool attached with a rubber band. This puts the end of the thermostat about in the middle of the water for an even measurement. As you hit 80c (180f) watch the amount of opening. Your thermostat may not be opening enough. My experience is that the Wahler opens more than the Behr.

Leave the coolant in the car, I don't want to be responsible for a disagreement between you and the misses. :-)

TonyFromWestOz · #22 07-23-2004, 08:28 AM

Oops, replied to a co-poster, not the original poster. My comments seem to be valid in either case though.
If the coolant flow thru the system is adequate to maintain engine normal temperatures without a T'stat, then insufficient with a T'stat, the only contender to accept the blame must be the Water Pump. That is what is failing to create sufficient flow thru the system.
The additional restriction caused by adding the T'stat should not be sufficient to reduce flow to the extent of causing temperatures to rise to 100C.
Please tell me I am

Brian Carlton · #23 07-23-2004, 09:41 AM

Quote:

Originally posted by TonyFromWestOz
....... the only contender to accept the blame must be the Water Pump. That is what is failing to create sufficient flow thru the system.

The logic makes sense, however, what component of the pump can deteriorate to the point where the flow is compromised? I don't see any parts to these pumps that would cause this.

It might be the case where the flow is insufficient with the themostat in place due to a faulty design. Or, in the case of M/B, they are not all that concerned with the engine operating at 110 degrees. We all seem to be concerned with the area between 90 and 110, however, I have not heard anbody, including myself, observe a temperature of greater than 110. Now I'm sure that it has happened to certain folks, but, usually the problem can be traced to a source.

In the case of many of us, the engine operates between 90 and 110 and no amount of parts replacement causes the engine to just sit there at 90 under all ambients and loadings.

tenknots · #24 07-23-2004, 10:25 AM

Good point Brian.
We use coolant to raise the boiling point of water and we use pressure caps to raise the boiling point of water so we won't have a boil over. However, the temps I was talking about (and most others) only exceeded the boiling point of water by 5 degrees c or so - not much of a concern since the water can't possibly boil away after what we've done to it. AND, MB says that up to 120c is OK. We just don't like to see our cars run hot, despite the fact that diesels seem to run better when hot!

After my second thermostat, yesterday I drove home in the same traffic as before, ac on as before with the outside temps even a bit warmer than before. I had got 105c with the old thermostat, about 92 with no thermostat. Yesterday I ran at no more than 98. I'm happy with that especially after some hard pulling after the traffic jam that showed the same 98, about what happened with no thermostat. It makes me think that the cooling system has the capacity. Before, the thermostat was choking it (and may be still a bit, but I'm tired of buying thermostats). All this was done after replacing the water pump.

After more consideration I decided to replace the pressure cap, despite the fact that I never got any pressure in the radiator since coolant won't boil at 105. I found this online regarding someone who spoke to an experienced MB mechanic about pressure caps:

He says that the pressure of the
system has a much greater effect on cooling than just outright flow. In other
words, removing your thermostat can actually lower the cooling systems ability
to carry away heat due to the reduced pressure and lowered boiling point.
Apparantly, on the engine side of the t-stat, the pressure can be quite high,
like 20 to 30 lbs due to the water pump building head pressure against the
t-stat. This helps keep the coolant from boiling and creating spots where it
can't carry heat away because it's now a vapor instead of a liquid. I had no
idea. Makes perfect sense, but I had never thought about it and just assumed
that flow was the most important thing.

WJJ · #25 07-23-2004, 12:08 PM

Hi guys, I just found this forum because my 1991 300D (175000 miles) is running hot, and this thread seems to be discussing my problems.

Most of the posters are discussing a 300SD, will most of the info be the same for the 300D??

While I ususally bring my car to a mechanic, this vehicle has been expensive to maintain and has had more than its share of problems. I have had to replace the tran, rear end, complete overhaul of the fuel system, new A/C (with more problems relating to the switches and the dampers, no air from the center vents, lots of air out of the defrost vents), new water pump, fuel guage, etc., etc., so I am going to start working on the car myself. This is my second 300D, I had a 1982 which I sold with 200K miles when I bought this one.

I have plenty of questions, but will start with the most pressing matter, the overheating engine. It's normal range is about 100 I think, about halfway between the 80 mark and the one immediately before red. Whenever the engine has a load, such as going up a bridge, the engine runs hotter. Now that I have recently repaired the A/C, it runs even hotter, with A/C off, it cools down slightly. What do I check first? And to ask a stupis question, where is the thermostat located?

Thanks for any info/direction.

Wayne

Brian Carlton · #26 07-23-2004, 12:40 PM

I've said this before but I'll say it again.

The most important, and easiest, thing you can do to reduce temperatures is to remove the radiator from the vehicle and spend about two hours trying to get every little piece of crap out from between the fins. You cannot stick anything in there, but you can use compressed air and some chemical cleaner. The fins on these radiators seem to trap all kinds of little particles of organic matter and sand over the years and it sure cuts down on their airflow.

Hold the radiator up to the sun and look into the fins. See all that crap? It is restricting airflow. Guaranteed. When you look at the radiator from the outside, it looks fine.

I still need to find some type of industrial cleaner for condensers. I won't do the job again without it.

BTW, the 300D has the same engine and the same issues as the 300SD

tjohn · #27 07-23-2004, 01:24 PM

Quote:

Originally posted by Brian Carlton
BTW, the 300D has the same engine and the same issues as the 300SD

Actually, the year 300D in question (91) has a different engine. However, the general principles do remain the same. Please note everyone, that diesels do generate more heat when under load. Its perfectly normal for our temperatures to increase when running flat out, or up long hill, or when towing a trailer . Boost is not really the limiting issue so much, because after about 2000 rpm the wastegate dumps excess boost so that no further power is made. Once the load goes away, the coolant temperature should drop right back down. Using the A/C should cause the temperature to be higher, because there's a higher load, and of course because heat is being pumped out of the cabin.

Cooler is not always better - Mercedes doesn't want our engines running under 82 - that's why the thermostats restrict cooling at temperatures below that. (I think its 82, maybe its 85, you get the idea). Once the thermostat is fully open (94) the cooling system's capacity comes into play. Anything under 105 during the summer w/ A/C on really doesn't worry me. And as has been pointed out, we aren't hurting the engine unless you get to the point of overheating. Having said that, I do understand the concern people have, especially those who own the crack prone 300 SDL heads.

Finally, Brian's point is soooo important. There's 20 years of grime in most of our radiators. Obviously, their ability to transfer heat is somewhat compromised by the grit on the cooling surfaces.

Brian Carlton · #28 07-23-2004, 01:39 PM

Thanks for the correction Tjohn. I missed the model year (1991).