Towing Mods: how hot can we safely get a D24T?

[745 TurboGreasel]

The 524TD will give you a run for the money, but no wagon option here anyway.

[Nevadan]

Here's some data from the "SAE paper 790206" - "New 5- and 6-Cylinder VW Diesel Engines for Passenger Cars and Light Duty Trucks" paper published in March 1979.

"The ratio between crankshaft and water-pump speed was raised to i = 1.1 (i = 0.85 in the gasoline engine). This results in a cooling water flow of between 140 and 170 liters per minute at rated speed, depending on the cooling-system flow resistance curve." (converts to 37 to 45 gallons per minute!)

That sounds like a lot of water moving through the engine.

My observation of the importance of a GOOD working thermostat is that when the engine is warmed up the back end of the thermostat must close the flow coming out of the block in the position right behind the thermostat, thus forcing all the fluid flow either out the back of the block or up through the water outlet on the head. The suggestion to use an 80C thermostat is probably a good idea since it opens sooner and thus closes that water flow coming out of the block.

It is probably a good idea when driving under a heavy load to turn on the heater to allow more water out the back of the head. My concern is there's not enough flow at that end of the engine, ie. in and around the head and cylinders at #'s 5 and 6. It may not make much of a difference but I like the idea!

[745 TurboGreasel]

So,me head gaskets are missing a row of 'steam holes', I wonder about adding one in the back(or more) .
Also a bypass heater valve(90s Ranger, Caprice)could give more outflow at the back, especially if you've removed the water-oil cooler.

Any idea what 'rated speed' is?

[RedArrow]

Cook YOUR head instead of warping aluminium! Ask me how I know... Many, many times I saved car(s) and myself too by turning the heat on, full blast; it definitely does help a lot!
Especially because you can never really truly know how close you already got to the edge of a later serious & irreversible overheating. Yesterday I PM-d him about it and I hope he got the message in time before his departure...
Let me off-topic a little: We used to have a unique Volga wagon pimped with the 6 cyl PRV engine that was one unreliable engine, needing so many things fixed all times, and that car was slightly overheating all the time, ate a couple of headgaskets but turning the heat on always worked for years to get around town... long story from around 1978-1989. RIP, he rusted away since then, I`m sure. My Dad might be driving it in Heaven...
Heat up, open the sunroof.

[ngoma]

Originally Posted by Nevadan

It is probably a good idea when driving under a heavy load to turn on the heater to allow more water out the back of the head.

Heater valve always open EXCEPT during max A/C. But arranging the controls for heat & max fan would maximize airflow thru the heater core, pulling more heat out of the engine and onto your feet!

[745 TurboGreasel]

Not sure if this one is the right way to 'hot' but you get the idea

[casioqv]

I made it no problem! I was so excited I yelled in joy when I hit the summit! Thanks for all the help and advice, I couldn't have done this without you guys. Now I get to spend a week sailing in the California Delta w/ my dad.

I climbed the grapevine at 3000-3500 rpm in 3rd gear most of the way, going down to second on the steepest bits. Peak temp was 102C when I floored it, but I could keep it under 100C. That's compared to 116C with no trailer last time I drove the same road. I think my cooling system can handle continuous full load at low speed without overheating.

After the grapevine, I was able to maintain 60mph in 4th gear, running the A/C and cruise control all the way to Sacramento and the engine stayed cool. I also never had to downshift once after getting past the grapevine. I was far from the slowest vehicle on the road. Once the weather got hot around noon power reduced a bit, and my speed dropped to 57mph.

Also, coming down the grapevine I didn't touch the brakes the whole way. With the A/C on at 3,000 rpm the D24T had enough engine braking to keep the speed constant. The brakes were cool to the touch at the bottom of the grade.

[RedArrow]

Great news! Enjoy every minute with your father and drive safe!

[v8volvo]

Originally Posted by casioqv

I made it no problem! I was so excited I yelled in joy when I hit the summit! Thanks for all the help and advice, I couldn't have done this without you guys. Now I get to spend a week sailing in the California Delta w/ my dad.

I climbed the grapevine at 3000-3500 rpm in 3rd gear most of the way, going down to second on the steepest bits. Peak temp was 102C when I floored it, but I could keep it under 100C. That's compared to 116C with no trailer last time I drove the same road. I think my cooling system can handle continuous full load at low speed without overheating.

Cool!!

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.

[745 TurboGreasel]

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.