overheating 240d about to lose my mind
 

I know there have been multiple post on this issue but none really have answered my problem, I have a 81 240d manual transmission, and she has ran a lil hot since I got her, only boiled over once and that was last week after sitting in a drive threw with the ac on, now she has always ran hot while going down the freeway, usually between 90-100C but if I turn on the ac the temp gauge will quickly go above 100, I've replaced the thermostat, water pump and radiator, and I also flushed the block out and removed a lot of rust, nothing big though, so after replacing the radiator today I took it for a spin, she is running a lil cooler around 90 at 70-75mph but as soon as I turn on the ac the temp starts to rise till its just above 100C, I have checked with a inferred thermometer and the gauge is a lil off maybe around 10-15C to high, but the engine still hits 90-95C, what could cause this? Thanks in advance

90-95C is totally acceptable for a 616 in summer with AC on.

Boiling over is a big problem. Be sure you're using 50/50 mix and have a good radiator pressure cap that's holding a bar of pressure.

Only other things are an air pocket or a thermostat installed incorrectly or defective from the factory.

The 240D had a rigid mechanical fan, right? Aux fan is operating as it should with the AC?

Yes I'm running 50/50 zerex go-5, plus a brand new cap I'm going to test it at work tomorrow, but it seems to be holding pressure just fine, it only boiled over that one time, but it wasn't until I got home and shut it off, I was running cheap green stuff coolant while I was waiting for the new radiator, and yes the aux fan kicks on normally, I'm gonna go to the junk yard and get a gauge set and hope that will take care of that, I also just replaced the temp sending unit

Well update I drove to work thus morning mainly freeway driving, and the gauge read 90 and didn't fluctuate so I'm hoping I fixed it, I will find out more once it gets to over 100 degrees, when I go to lunch, I know everybody says 100C isn't too bad but I really want thus thing to stay at 80-85C like I've read about in other posts and forums, is that to lofty of a goal

It boils over due to ' heat soak' ... the lack of circulation and radiator and fan once you turn off the key.... so as of this time you may can avoid that simply by letting it idle for a couple of minutes when you are about to turn it off....
but Manny hit all the current first important things to start with....

Ok.... you posted while I was composing.... yes, you are being overly optimistic ... perhaps even delusional due to the physics of the situation.. the design of the cooling system, the degree thermostats available, etc....

What's the temperature on the thermostat that you got? I remember replacing the thermostat in my diesel and the temperature went way up. Ran OK, but just a bit on the hot side. Turned out it was a 90 C instead of the 75 C that is specified.

Also any thermostat which has been overheated is now suspect and should be replaced on principal.

The new thermostat should be tested in a pot of water on a stove to measure the temperature at which it begins to open, and the temperature at which is has reached full stroke (wide open).

I'll dig into my Technical Data Book, April 1985 edition, to get those numbers...

From "The Book", Mercedes-Benz Technical Data Passenger Cars, April 1985 edition.

For 615, 616, and 617 engines:
Thermostat begins opening: 80+/- 2 deg C.
Minimum stroke: 8 mm at 94 deg C.

I like to heat up the t-stat a pan of water with a digital thermometer probe inserted to monitor temperature, and run it up to full open, then add ice to bring the temp back down and make sure it closes (usually they close at less than 80 deg C), run it back up again to re-check both parameters again. I use a digital calipers from Harbor Freight which cost about $15, digital grilling thermometer from just about anywhere for about $25 (probably can be found cheaper on Amazon).

Note that "Minimum stroke" is at 94 deg C, so observing engine temperatures between 80 and 94 is within the range of operation of the thermostat. I.e. those temperatures are "normal" in the sense they are within the design parameters the MB engineers decided on. Sure, it is nice to have rock steady temperatures right in the mid 80's, but if you live in a hot climate and drive hard with AC blasting, don't be surprised to see temperatures at the high end or higher. Note the red-zone on your dash temperature gauge (I suspect it is at or above 120 deg C): that is the temperature you need to be afraid of. Personally when I see my temperature gauge climb over 100, I start to really pay attention and change the load demands I'm placing on the car to bring it back down (slow down or maybe shut off the AC), but if I'm cruising on the highway hauling a heavy load, I let the temperature stabilize where it is happy, which may be at 105 or 110, and be happy. When the engine is that hot, I always give it a few minutes of idling once I pull off and stop so that the engine temperature can get back down to normal.

2x that last post

Ok.... will mention something a little deeper and harder to find and deal with...
In the Old Days.... when engines got rebuilt..... the block had everything unbolted....and it was taken to a VAT... a literal bath of corrosive liquid..... the ' core plugs' which we usually refer to as freeze plugs.... were out and all other plugs removed...
Then it was cleaned up and the rebuild process started with a nice clean water cooling cavity....
But these days and particularly on OUR engines.... I never hear of that being done in rebuilding no matter how many ' millions ' of miles are on the engine...
Rust and corrosion can and does build up over time where the coolant flow slows down and changes direction..... this will usually be at the lower rear of the engine block...
If you clean the block and use a non contact thermometer... you might find that when you turn off the engine without cool down idling... that that part gets really hot.. or it may not show up due to the convection of the fluid..
If you were to take out your core plugs....and use a mechanical device ( think drain uncloging.....and there is a cheap Harbor Freight tool which might help )to reach into that section of the coolant cavity... which doing Copious Flushing and I think vacuum with a shop vac would also be necessary..... you might help out your situation..

OK well I'm staring to be less worried with all the info yall have given me, this is my first benz and I love it, I'm a mechanic by trade but this thing has a lot of differences from classic American muscle, or even the military diesels I work on every day.

I had that same shock when I got my first MB... an 81 turbo wagon..
the temperature would rise going up long but slight grades...
but turns out... very sensitive temp gauge....and very common...
I got 27 mpg out on the road going more than 70 mph...
find a paper shop manual if you can... and just read it front to rear... these Germans thought differently than old American engineers...

So I have another crazy idea I haven't heard to much about it anywhere, so my lift pump is failing, it won't even pull enough fuel to keep the car running with the primary filter on, which explains why there was just a hose from the metal line, and when I was priming the system fuel was leaking really bad around the old plunger, it was the unscrew type. I've heard that a failing lift pump can cause the injection pump to not get enough fuel and cause uneven fueling could that cause certain cylinders to run hotter then others?

A leaking old screw type Primer Pump will suck air. Replace it with the Bosch type you just push w/o unscrewing.
000-090-88-50

1982 Mercedes-Benz 240D Base Sedan - Fuel Injection - Page 1

As I understand it, you don`t have the primary Filter? The clear plastic one?
It may have been plugged up and was replaced by a piece of hose if the PO or/ didn`t have a replacement.
It really just catches the big stuff mostly, and will let you see if you have bugs growing in the fuel.
001-477-66-01
1982 Mercedes-Benz 240D Base Sedan - Filters & Belts - Page 2

I was having a problem with our 240 surging a bit when starting out from a stop. Finally replaced the 2 plastic valves in the Lift Pump and that really made a difference. I had a spare LP off a 84 300Sd that I went through and then swapped it for the )M616 one. the one from a OM617 supposedly has a stronger large spring inside. 000-090-02-10
1982 Mercedes-Benz 240D Base Sedan - Fuel Injection - Page 1

You will need the Lift Pump Gasket and the metal crush washers.

Charlie


sent from my pos computer