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#42
08-31-2013, 11:31 AM
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Quote:
 Did you notice your fan clutch behaving any differently? Maybe a little more "active" than usual? Regarding cranking up the heater in order to pull more heat out of the coolant -- in many other cases this might help to some extent, but in the particular case of the D24/D24T cooling system, I think this tactic is unlikely to have any constructive effect and, in fact, might be *counterproductive* to keeping the engine cool. Times when I have tried it, it hasn't had any noticeable effect (other than making the driver very uncomfortable...), but the D24's cooling system layout makes a negative consequence possible. Since it's an unusual setup where the thermostat is located in the coolant *return* path from the external and internal circuits, on the suction side, rather than up high in the cylinder head outlet to the radiator as it is in almost everything else on the road, the thermostat is exposed to flow from many circuits -- not just what's inside the motor, but also the oil cooler return, IP cold start t-stat return, heater return, radiator return, etc. Some of these circuits, where heat exchanger matrices are involved, involve a temperature differential. Cool fluid from the radiator return doesn't flow past the thermostat's wax pellet since it enters from the front of the t-stat, but cool fluid from the heater return *does*. In a system like this, if you crank the heater up and start extracting a lot of heat out of the heater core, you start blowing a lot of coolant across the thermostat's wax pellet that is significantly cooler than engine (and especially cylinder head) temperature. Next thing that happens is that the thermostat begins to *close*, even in the case of an engine that is working hard to climb a grade and producing a large amount of heat, and begins recirculating hot coolant through the motor and restricting flow through the rad. Not a good thing... Much better approach IMO to keep the temp of the coolant that passes by the t-stat as close to engine (i.e. head) temp as possible, so that the thermostat is able to respond accurately to real temperature signals from the engine without being affected by "noise" created in the external circuits that it is also exposed to. The only other cooling system I know of that uses a similar design to this is the Subaru EJ motor's layout, which also has the heater return entering a thermostat housing located at the bottom of the engine at the inlet from the lower rad hose and blowing directly across the thermostat. This is the example case where the above theoretical scenario has proven to be a real-world problem... In the Subaru application, which has a fairly small heater core in a vehicle with a relatively small cabin, this setup doesn't seem to present any issues, but when those EJ engines are used in Vanagons, most of which have *two* large heater cores and a large cabin volume, in cold weather with the heater running, the temperature of the coolant in the heater return circuit can be pulled down low enough that the thermostat closes and is known to cause serious engine overheat. I helped a friend put a 2.2L into a Vanagon once and this was one of the issues we had to overcome. The work-around most folks dealing with that situation use is an adapter plate between the water pump and the t-stat housing that relocates the heater return to a location that isolates it from the thermostat. Despite having a large and efficient heater core the D24T doesn't seem to suffer this syndrome to the same extent as the Subaru does in the Vanagon, and I haven't succeeded yet in making one overheat when climbing by cranking the heater up with all else equal (have tried it), but I think it's important to be wary of the possibility due to the nature of the design. In any case I don't think running the heater can help, and it has the potential to cause unintended additional problems especially if there are marginal components in the system. Modifications to extract more heat out the backside of the head might be interesting as long as the method of cooling that fluid and where it was returned to were worked out carefully, but as far as the stock setup goes, IMHO it seems probably best to do climbs with the controls set to cool, or if in cold weather not any warmer than you would normally set it to maintain cabin temp where you want it. YMMV. 
Last edited by v8volvo; 08-31-2013 at 04:10 PM. 
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#43
08-31-2013, 02:16 PM
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I wold not describe the heater system as 'large' or 'efficient' in any 700 I've encountered. My Chevy pickup has = or larger heater core, shorter straighter ducting, and a better fan.
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#44
08-31-2013, 03:09 PM
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Yeah you're right, that probably is an exaggeration as far as the 700 series goes, though I think it is accurate for the 200 which is pretty large (assuming it's not half plugged). Maybe not surprising the truck has a bigger core than the 700 even with just a small cab since its probably the same system they used in the Suburban and presumably designed it to accommodate the demands of either application...
In any case as I see it the point is still the same -- to one degree or another, if you're running the heater and it's making the cabin warm then inevitably you're looking at cooler return temps from the heater core than what's in the supply side, and if the t-stat sees that then it will respond. It's mixed with hotter return from the oil cooler and the cold start t-stat circuit so the differential is probably buffered somewhat by those, but predicting the ratios would be pretty tough because of large number of variables, e.g. how plugged is the oil cooler, heater, how high do you have the fan turned up, etc. Regardless, given the system's layout I am skeptical that it could do any good, and with the right combination of circumstances I think it could cause harm, which is why to me it doesn't seem like a great idea to use as a rule of thumb unless you have very detailed information on what's going on under the hood. This is theoretical zone reasoning, but my experience has also been that when climbing in a situation that's making the gauge move up, cranking the heater has not helped. This has been in a few different 200 and 700 series cars but not a large sample size overall so maybe others have seen different results. 745, you've had lots of chance to experiment as well. What has your experience been?
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#45
08-31-2013, 09:04 PM
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I feel like I can run a little harder with my pathetic heat on.
My gut feeling is the biggest bottleneck is transferring heat from the head, and the radiator stuff isn't really that bad. (aside from the sheer number of hose connections).
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#46
09-01-2013, 07:54 PM
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topic: ``Trying to help cooling by having the heater on, to avoid overheating. ``
It became a very interesting thread. I`m happy it came up! Thanks to everyone for the detailed, awesome write-ups. Later on I`ll share my experiences/experiments with the heater. Rethinking it again, I found a dusty thread that mentions info that relates to this. Ngoma you wrote that ...``That heater coolant valve is normally open at rest. Vacuum closes it. System was designed to flow coolant always thru the heater core, except when A/C set to max. Also, the butterfly flap in the valve, when open, is really only open about 30%.``... May I ask what`s the case if my AC is not functional? I have no belt on+ compr frozen or clutch bad, idk. (I never struggled much to solve what`s wrong with it. I should have... but the killer Summer is over soon. ) Thanks. Last edited by RedArrow; 09-01-2013 at 07:57 PM.
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#47
09-02-2013, 05:58 AM
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Quote:
Those points about the thermostat design are interesting.... it makes me think that the D24 cooling system might be such that the thermostat tends to close when the fan clutch engages and coolant temps drop slightly. Might that start an opening/closing oscillation that leads to poor cooling efficiency? If so, that could explain why an 80C thermostat would seem to keep the engine cooler than an 87C thermostat despite an over 100C engine. I would expect to see coolant temp oscillation during steady state hill climbing with a logging temperature system similar to that described by 240diesel in this thread. However, I don't think the test conditions in his thread were severe enough to induce such behavior. I can now run the engine fairly hard uphill in hot weather with the A/C on when not towing. I sold the boat to my dad, but if I get another sailboat it will be a much lighter one- perhaps a 750 lb Montgomery 15.
__________________
'84 760 D24T/M46 '86 Isuzu Trooper Turbo Diesel '01 VW Golf TDI Last edited by casioqv; 09-02-2013 at 06:15 AM.
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#48
09-03-2013, 11:56 AM
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Quote:
I like having A/C operational during winter. It clears the fog off the glass within seconds!
__________________
1985 744 gle d24t 1985 745 gle d24t
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#49
09-04-2013, 05:02 AM
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Quote:
On a trip a few days ago I drove over Donner Summit (Labor Day trip with temps at 95F in Sacramento and about 75F at the 7,200 ft summit) in 5th gear (4th plus overdrive) pretty much running as hard as possible. I wasn't towing anything and I'm using an 80C thermostat and a new radiator. When I got to the summit I pulled over at the rest stop and as fast as I could, felt the radiator temperature on the inlet and outlet sides. It was very hot on the inlet side and almost cool on the outlet side. I could place my hand on the plastic side of the radiator so it was probably no more than 90F. My conclusion: the radiator can handle all the heat the engine puts out; increasing the hot water (coolant) flow out of the head (especially the back of the head) would help. I have to imagine the engineers that designed this engine had all the heat generation and cooling numbers at hand. They must have tested this thing under full load in hot ambient temperatures. There's a video somewhere on the internet showing a Pinzgauer running "wide open" in desert sand. If the cooling system can handle that type of heat generation......... What about using Evans coolant? I'm beginning to think overheating and warping of the head is only caused by coolant loss.
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J.D. in Reno 1958 Mercedes 180D (rebuilding now) 1985 VW Jetta 1.6TD 1985 Volvo 745 Wagon 2.4TD (sold but still maintain it) 1987 VW Quantum Syncro 2.2 (converting to 2.0TD) 1996 TDI Passat 1997 Chevy 3/4 ton 6.5TD 2006 V10 TDI Touareg
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#50
09-12-2013, 10:33 AM
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(I still haven`t forgotten the great thread about adding a sensor to the expansion tank with an audible alarm... that should be a must for all of us...) So...,... I was at Togwotee Pass, Wyoming (over 2900m elevation) when I decided not to risk unknown condition (to me) hoses with extreme pressures, so I twisted the exp tank`s cap a tiny bit to not have it so tight; so rather than exploding a hose up there in nowhere, I would lose some coolant instead... What a stupid theory it was (!) because it was Winter-time, a very strange snowstorm with shiny ice under the snow. The car was also loaded heavily (crosscountry drive). I started climbing but tires were spinning, car was sliding all over the place (worn Summer tires  ) so unfortunately I had no choice but to drive in higher gear like 3rd, 4th and occassionally 5th. At every slide I lost the speed I gained before, so there was again new struggle to cope with. 1st was impossible to use, 2nd was almost useless. This way RPMs were too low, consequently the coolant-circulation was not effective enough, so water temp went higher, at the same time OR before that, I lost a Gallon of coolant that I did not know about until this lazy-@SS temp gauge suddenly* jumped all the way up. What a feeling! There was a pilot car escorting every single vehicle up because of the crazy weather... I had to stop right there to wait, fill up 50-50 with G-05 coolant and distilled water &wait&wait. Grille was full of snow&ice, almost completely blocked. The usa-type small front lights fully covered too. Luckily it was subzero temperatures, -10-12`Celsius that time... My upper hose was very hot but my lower hose was somewhat much `cooler`....a good radiator should be able to handle it, I agree with you. Hood left open, I added fluids, kept idling, lots of air was burping up coming out of the system, bringing out coolant again as I was adding more and more to the correct level and started driving carefully to have all the air out. 2Gallons used up. Later, temperatures stabilized and I learned to rev it higher in order to get better flow with less heat; although that was not possible to do up there so my Volvo got angry Freaking coolant loss should be signaled immediately! Last edited by RedArrow; 09-12-2013 at 06:55 PM.
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