MB 300 cooling system question

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MB 300 cooling system question

Postby othello » Mon Apr 23, 2012 5:41 am

I have a 1985 MB 300 series and have trouble achieving proper oil temperatures at the injectors, a separate thread deals with that.
But one item that I have so far taken for granted probably deserves more scrutiny and possibly more engineering, the cooling system which I expect to heat my oil.

My car runs cool.

Everything works fine, the heater works and the temperature eventually settles at 85 C, if I am driving.
But I could start the car and let it idle forever and it would reach around 60C, never 85C.

The big rubber hose on the top of the radiator is nearly never "full" or firm to the touch, even though the overflow tank has the right amount of water in it.
I recently installed a FPHE which works a bit erratically and often tops out at 60 C or cooler.
The only reason I can see is poor flow thru the FPHE, if I do drive in low gear to rev up the engine for a while the temperature goes up, I think because at that point the water pump turn faster and flow improves.

The 30+ ft. of tubing to the rear tank, half of it of reduced size due to the inserted pex tube, certainly does not help flow and also acts as an additional radiator.

So I question the ability of my stock cooing system to drive the coolant thru all the extra hardware reliably and would like to hear about the experience of others regarding this problem.
othello
 
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Re: MB 300 cooling system question

Postby John Galt » Mon Apr 23, 2012 1:34 pm

The most reliable systems have the engine coolant circuit connected like this:

╔=coolant pump====╗
╠=cab heater======╣
╠=FPHE ==========╣
╚=HOH+filter+etc==╝
John Galt
 
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Re: MB 300 cooling system question

Postby othello » Mon Apr 23, 2012 2:37 pm

[quote="John Galt"]The most reliable systems have the engine coolant circuit connected like this:

╔=coolant pump====╗
╠=cab heater======╣
╠=FPHE ==========╣
╚=HOH+filter+etc==╝[/quote]


John, I've seen this diagram, but I have to admit, I have no good idea what it means.
Maybe the problem is in the formatting because in some versions of these diagrams things don't even connect.

And even if it shows what I think and hope it shows (several "circuits" in parallel) it does not address my question regarding the different resistance of the loops.
If one of those loops had a much higher flow resistance than the others wouldn't that means that coolant would flow much slower or possibly not at all??
The analogy in electronics would be that the current in each loop or branch depends on the resistance.
othello
 
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Re: MB 300 cooling system question

Postby John Galt » Mon Apr 23, 2012 2:44 pm

The FPHE should be on it's own parallel loop with no other appliances so that the FPHE gets full coolant flow.

The other parallel loop with the HOH+filter+etc in series will get less flow; it only needs to keep the VO warm.

The diagram looks OK to me.
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Re: MB 300 cooling system question

Postby othello » Mon Apr 23, 2012 3:29 pm

Ok, to accomplish this physically I would love to hear from another MB 300 owner who has routed the FPHE in this way.
I don't really see the space to do this.

Is there anybody out there?
othello
 
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Re: MB 300 cooling system question

Postby dragonfly » Thu May 10, 2012 12:45 am

othello:
I have an '80 MB 300D on VO since 2005. First, I always recommend, to MB owners, changing the thermostat to an 82C (179.6F) or higher temp*. The higher temp thermostat will get the overall temps of the VO higher. I usually see a 10F difference between the engine coolant and HPHE VO temp. I have a MB aux water pump plumbed into a "Tee" right off the engine block (hot water hose to the cab heater) and wired to the ignition. The pump will start pumping coolant water right away to the FPHE ; then into a 30 plate HE; then to a small heated fuel filter; then flows through a 3/4" HIH to a heated in-tank HE. The switch over time is about 4 miles from a cold start up on diesel. Remember the IP is a big heat sink and will be approximately the same temp as the engine block when the coolant is up to operating temp.....

* Thermostats: 84C = 183.2F; 86C = 186.8; 88C = 190.4F
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Re: MB 300 cooling system question

Postby othello » Sat May 12, 2012 5:14 am

[quote="dragonfly"]othello:
I have an '80 MB 300D on VO since 2005. First, I always recommend, to MB owners, changing the thermostat to an 82C (179.6F) or higher temp*. The higher temp thermostat will get the overall temps of the VO higher. I usually see a 10F difference between the engine coolant and HPHE VO temp. I have a MB aux water pump plumbed into a "Tee" right off the engine block (hot water hose to the cab heater) and wired to the ignition. The pump will start pumping coolant water right away to the FPHE ; then into a 30 plate HE; then to a small heated fuel filter; then flows through a 3/4" HIH to a heated in-tank HE. The switch over time is about 4 miles from a cold start up on diesel. Remember the IP is a big heat sink and will be approximately the same temp as the engine block when the coolant is up to operating temp.....

* Thermostats: 84C = 183.2F; 86C = 186.8; 88C = 190.4F[/quote]

dragonfly:

Thank you for your comments. First of all in spite of my search I could not locate an MB thermostat with the higher temp rating, my dealer says there is only one model available. My injector temperature with WVO is lower than your and only reaches 65 to 70 C after some time. But you are cheating by living where you do, up here in Maine it takes probably extra effort to get your results :D .
I wonder about the importance of the extra circulator pump you installed, did that bring a notable difference in oil temperature?
othello
 
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