Max engine temp under load - PeachParts Mercedes-Benz Forum

msyoder · #1 06-03-2002, 03:34 PM

This weekend I had my '87 300TD fully loaded with books luggage, picture frame on the roof and family. While traveling over the Cumberland gap on I68 west of Cumberland, as I usually do on that streach of hilly highway, I preformed an Italian tune-up.

For the first time ever (since I fixed the plugged ALDA line) I reached full load on this engine for an extended period of time. By this I mean that with the accelerator pressed to the floor the car stopped accelerating in third gear before reaching MAX RPMs.

I know, the exact required conditions for an Italian tune-up. We'll the good part was that my new (Jan 02) head was still clean as a wistle -- not a single burp of black soot.

The bad part was that the engine temperature never reached an equalibrium. It continued to rise slowly until, when it reached 120 deg C. (The temp line just below the red line on my guage.), I cried uncle and eased off. Temp stopped climbing and returned to 84 deg C shortly after cresting the hill and removing the load.

This hill is 4 mi or less in length so all this happend in a matter of minutes. I realize that engine load is the main factor in engine temperature, but I would have expected to reach an equalibrium temperature somewhere below the red-line. It didn't seem to be happening that way. With a recent head replacement in mind I wasn't going to see where the temp stoped rising!

In the list's experience what would I expect to be the fully loaded operating temperature of the '87 603 turbo engine?

Thanks,

#2 06-03-2002, 03:44 PM

I've climbed some very steep hills with my SDL, and I know that some of them, I had the car at full-throttle to get up the mountain at 40mph, and the temp guage was rapidly rising. This mountain trek was only about 1/2 mile, but at the top I was at just under 100c.

When you have the car at full load for 4 miles, I would expect you to get an overheat condition like you said.

engatwork · #3 06-03-2002, 05:47 PM

I just cruised up the largest hill on I-75 near here with the a/c on full and the temp got to right under 100 dC (in the black 300D). There was always some space between the needle and the 100 dC mark.
It was 100 dF outside and with the a/c on inside the car was probably 75-80 dF. At least the sweat evaporated pretty fast

BWatson · #4 06-03-2002, 05:55 PM

I have also noticed that my temperature goes up on hills. I guess this is normal for these cars. Has anyone ever taken one of these up a mountain road like Angeles Crest Highway, or route 70 through the rockies? Los Angeles gets pretty hot in the summer so I have my eye on the guage at all times.

gnolo · #5 06-03-2002, 06:16 PM

120C sounds me a litlle bit too hot !
With my 240D, I have never go over 90C.
The normal temperature when I drive calmly is about 82C (the needle is just above the 80C mark). When I 'm running on a hill, with the car loaded (with the foot on the floor, of course...), the temperature increase slowly, but so far it never go over 90C.

I should precise that on the 240D the radiator fan turn continuously contrary to the 300D which have a fan clutch. Maybe it can have an influence on the cooling.

HGV · #6 06-03-2002, 07:40 PM

I have taken my 87 300TD up a prolonged 18% grade with AC on and car under full throttle in second gear. The temp went to 110 deg C. This is after a recent valve job, new radiator, tpsta, fan clutch, oil-cooler and hoses. It did level off at about 110 but then I backed down. If you search for a thread I started a while ago, and read through it, you will get a good understanding of the operating temp on these engines. I did the search try this.
1987 300TD confusing running hot problem

JohnM · #7 06-04-2002, 02:20 AM

I’m not sure about other models, but the thermostat on my 83 240D is on the return water back to the engine. If you think about it, by placing the thermostat on the return to the engine and not on the water leaving the engine, you are always cooling your engine with a minimum of 80 degC water (176 degF), which certainly isn’t very cold. Even if the radiator could cool the engine more, the thermostat placement won’t let it. In engines where the thermostat is in the water leaving the engine (before the radiator), the radiator is allowed to cool the water farther than the 80 degC, maybe down to 70 degC?, since the Tstat has no idea and doesn’t care how cold of water it sends to the engine at the discharge of the radiator. We don’t have that ability on these engines, however.

Another consequence to the MB placement of the thermostat is that when it closes, the water in the radiator may cool to a point lower than 80 degC. Later, when the Tstat “sees” this cold water it shuts itself, which isn’t always what the engine needs. Also having the cooled water flowing over the Tstat (including the bypass water) makes it think the engine is cooler than it is. Basically, these engines have to heat up quite a bit to the point where the water going into the radiator is 100+ degC to raise the water temp at the Tstat to the 94? DegC fully open point. (depending on your actual outside temp, speed, etc. of course)

I’m sure being good diesel engine engineers MB wanted to always make sure they had a hot engine for good combustion, but it does make for a roller coaster of a ride for the temperature on these engines. There is something to be said for a Tstat on the engine discharge with hot water directly off of the engine pumped up against the Tstat. It knows exactly how hot your engine is.

JohnM 83 240D 4 sp 151K

#8 06-04-2002, 05:51 AM

The 602 engine is the same way, with the t-stat in the lower hose. I never understood why, as serviceability is much better when the t-stat is in the upper hose.

-Joe

JimSmith · #9 06-04-2002, 09:23 AM

The thermostat on all MB's is like this, I believe, for some very specific reasons. First, nearly all cars have some provision for at least low flow during warm up, so the cold coolant is not forced to go through the radiator, and you get the engine as well as yourself warm sooner. The main drawback of such systems is that when the thermostat first opens after the engine and head especially, are somewhat warm is that in very cold climates coolant at ambient (sub zero) temperatures is suddenly admitted and thermally exercises the head and block materials. Second, the water pump flow rate is controlled by the thermostat, meaning in part load conditions the water pump is likely throttled on either the input or the output side, depending on the arrangement of components. This can lead to cavitation damage in the pump housing and impeller.

MB's system involves a thermostat with two moving parts. One is used to close the radiator bypass line, while the other is used to control the flow through the radiator. MB does this to ensure the water entering the engine is within a temperature range they determine is healthy for the engine. The placement of the whole affair in front of the pump is intended to ensure the cold water from the radiator is mixed with the warm water from the engine as thoroughly as possible. Additionally, the flow rate through the water pump at a given rpm is near constant since the thermostat never throttles down on a flow path without simultaneously diverting that flow volume to another.

Overall the system is very technically elegant and robust, with the intent to protect the engine while maximizing the passenger comfort on a cold day by minimizing the warm up period. The weak link is the thermostat though, as both flow control plates need to move according to design to maintain the intended flow conditions. As a result the system is sensitive to the selection and installation of the thermostat. Any binding due to shipping or handling damage, or deposits from coolant chemicals on the alignment sensitive parts and the thermostat will fail. In my 190E 2.3-16 the failure mode I have seen twice is the engine gets warm, reaching normal operating temperature, the thermostat opens and begins to regulate temperature, then gets stuck in the "send the water to the radiator" position, and the engine would run cold. Not good for mileage or the cat. The car would run so cool the mileage would drop 4 or 5 mpg.

Hope this helps, Jim

msyoder · #10 06-04-2002, 11:58 AM

HGV,

Thanks for the pointer to your thread. I had read it prior to posting and was hoping that you would weigh in on this thread as well.

I'm still curious as to whether or not this engine (603) has an equalibrium under full load. My independant mechanic claims that the cooling system is only as big as it needs to be and these engines under full load will continue to increase until the engine over heats.

Keep in mind that full load is what you have under full acceleration. Like when your testing the max boost preasure. Most of the time this is only for a few secs at a time. But in a case where you have a long climb with a fully loaded vehicle you can get max boost going for an extended period.

The other question is whether or not the viscios (SP) clutch on the fan makes a difference at highway speeds? If my clutch isn't ingaging at 105 deg C as expected would that restrict the air flow enough to make a difference at highway speeds?

Thanks,

HGV · #11 06-04-2002, 12:28 PM

I have always felt that the fan clutch has minimial effect at highway speeds. It would just propeller itself with the blast of highway speed air being rammed through the radiator. I once had the unfortunate experience to have the crank shaft pulley shear off my old 63 190D. No place to hook the fan belt. I ran the car and it actually ran cooler at freeway speeds without the belt on. ALways thought that was interesting. Of course it would overheat at low speeds. The fan clutch is affected by radiator temp via latent heat from the radiator. Exit temperatures from the engine are indicated on your temp gauge, so I assume the fan clutch should be engaged at high temp but low speed and not at high speed. For the clutch to engage at high speed, the air temp would have to be in excess of 105 deg C. Unlikely when you have air approach speeds in excess of 100mph. To test this theory, attach a basic thermometer between the fan and radiator, run the car at speed, come to a rapid stop, run out of the car and check the temp. Might work. I am almost to the point to do a detailed engineering evalaution of the cooling system on the 603 engines. If it was not for the variable speed aspect of the pump, it would be easy. One day in my free time. One thing I will check on my car is exhaust back pressure. This has not been mentioned as a source of an engine running hot, but it deffinantly is a source of additonal; heat being be added to the cooling system. Also engine timing. Anyway, watch out fopr those speeding tickets. Also, does your temp run constant or does it move around in normal driving.

henry

Henry

msyoder · #12 06-04-2002, 02:41 PM

HGV,

My wagon runs steady between 80-90 which is to say somewhere less than half way between the 80 mark and 100 deg mark. It only fluctuates when explainable things happen, like sitting in traffice with the AC on or running hard uphill.

Marshall Booth had this to say on the diesel mailing list:

"As I have been told and proven many times, loss of temperature stability at high speeds is USUALLY a sign of restricted coolant low (could be radiator OR thermostat OR water pump - but that's rare) while loss of control at low speeds is USUALLY an air flow problem (but remember the fan(s) doesn't/don't come on before 95+ degrees C or higher)."

So I think there may be a flow issue. Of course before chasing down a ghost I'd like to know if the temp should reach a stead state under load or if it can be expected to overheat when run full out long enough.

Thanks,

JimSmith · #13 06-05-2002, 01:52 PM

When struggling to figure out why something is not working as it once did, and for all my Diesels the normal operating temperature, regardless of how hard it was working or not, was always withing a needle width of the 80 degree C or 175 degree F thick mark on the gage, it is best to have a good idea of how it was designed to work.

The thermostat does see hot water from the engine, and that is what initiates its movement. However, as the engine warms up and the thermostat moves around, it responds to water flowing at engine temperature and whatever the temperature of the water is coming out of the radiator. Its intended design is outlined in the manual I have for my 240D (as well as the 300D) from 1975, which applies to the W123 versions as well since the update to the manual identifies no changes for those models.

I scanned the flow diagram, some illustrations of the thermostat operations and some text. I will try to attach these here, in a sequence of posts unless the quality (black and white text and illustrations do not do well at the 60k size limits) is too poor.

I hope this helps get a solution for what is ailing these cars. Jim

JimSmith · #14 06-05-2002, 03:58 PM

Well, that was pretty poor but it still seems legible. I will rescan and see if there is a way to make the figures come through better. Jim

msyoder · #15 06-05-2002, 04:08 PM

Actually, Jim, it looks pretty good. Thanks, -Mike