I had a similar issue in my '83 300D. It was running at around 90-95C per the gauge so I replaced all the stuff you mentioned (radiator, hoses, water pump, etc.) and then when that didn't work I *finally* decided to shoot the block with an IR thermometer, after spending hundreds of dollars on replacement parts. Turns out that the block was right around 80C, despite the high reading on my temp gauge. I concluded from this that the gauge itself was faulty.

I ran an extra ground wire from the gauge and that immediately "fixed" the problem for less than $2. Now the gauge reads around 80-85 all the time, even in stop and go traffic with the AC running here in Houston during the summer.

Elaborate on that extra Ground wire. My gauge reads hot too but I shot the block with IR and it's at least 15-20 degrees cooler than it says...

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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FACEBOOK: PANZER450

On the back of the temp/fuel/oil pressure gauge cluster there are some small nuts. Loosen the nut behind the temp gauge and wrap one end of a length of wire (small gauge is sufficient) around the now exposed threads and then tighten the nut back down. I actually used wire with small "u" shaped terminals so that I didn't have to mess with wrapping the wire around the threads in such a confined space (I didn't want to pull the cluster completely), but that's optional.

Then attach the other end of the wire to some metal part of the body. There are plenty of options here. I loosened a bolt on the left side of the driver footwell and attached it to that. Problem solved.

I certainly am not the first one on here to do this, and in fact I seem to recall following a pretty nice write-up with photos. A search would likely locate that useful guide.

I'd say you made too many changes at once! I'd suggest putting the old temp sensor back in and see if the temp drops. If not, put the old thermostat back and see what that does. If neither one lowers the temps, maybe the citrus flush loosened enough crap to cause a blockage somewhere?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The purpose of a citric acid flush is to remove the thin layer of scale which acts as an insulator so the coolant can now contact the engine block allowing it to properly cool it. Since he performed a hose water flush, degreaser flush and a citrus acid flush I can't believe there is still a blockage. IMO the new thermostat is the probable cause for the high temp reading. Also +1 to using an IR heat gun.

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1984 Mercedes Euro 300D NA
1997 Ford F-350 Powerstroke Turbo Diesel

You likely didn't "fix" anything. You just "fooled" the gauge. Sort of like wiring your fuel gauge so it always reads "Full."

I have read of many cases where problems develop after a citric acid flush, where there were no problems before the flush. Some of these problems include leaks, normal running engine becomes a hot running engine etc., never ending debri coming out. Ever heard of a log jam?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I'm no expert in electrical matters, but I don't see how adding an additional ground could "fool" the gauge. It's not as though I'm injecting more or less current; I'm just giving what the temp sender does transmit someplace to go. Other members have solved the same problem through the same method or by cleaning the points at which the grounds attach to the car.

I was imprecise when I wrote that the gauge always reads 80-85. The point I meant to convey is that the gauge no longer indicates that the car is running at a higher temperature than it actually is.

In any case, the gauge now accurately reflects what my IR thermometer says through the range of temperatures from 60 up through around 85 and no longer bounces around. Call it what you like, but the problem that existed before no longer existed after I added the extra ground.

Then you might want to reconsider the wisdom of giving advice on gauge calibration.

The sender itself serves as a variable resistance ground path. Anything that interferes with the integrity of that circuit affects the gauge.


By providing an additional ground path, you are allowing more current to flow through the gauge, which is essentially an ammeter calibrated to indicate temperature.

Thanks for the help, guys. Just to clear one or two things up...

Reads hot with the OLD and the NEW temp sensor.

Reads hot with the OLD and 2 different NEW thermostats

Read hot BEFORE the citrus flush

I replaced things almost one-at-a-time. Radiator and thermo, then hoses, then water pump, etc.

This happened before the citrus flush, and sort of suddenly. I was hunting down a leak for weeks. It ran fine and cool, even when leaking. Then, one time after re-assembling and refilling the cooling system, the temp gauge soared up to 100º+ during a test drive. Since then I fixed the leak - reads hot. Flushed and filled - reads hot. Hose flush, degrease, hose flush, hose flush, citrus flush, hose flush, hose flush, hose flush, fill - reads hot.

There was no one change that I can pinpoint as coinciding with the temp issue. I'm a computer technician so I am very familiar with and particular about troubleshooting, but as I think about it, I can't figure out what the THING was that did this. I'm hoping it's a gauge/electrical issue, but have no idea.

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Cam
1982 240d

Is this 100F+ temp confirmed with a IR thermometer? At this point I would test each component for blockage. start with the easiest to access component and run a flow test through each with water. Are you sure your hose routing is correct and nothing has been changed (accidentally)

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

You're right. Please ignore my response to the OP. It is a complete coincidence that the extra ground wire made the gauge read the same as the IR thermometer. Everyone else on this forum that has managed to achieve the same result by adjusting/cleaning the ground strap or adding an extra ground has only benefitted from the exact same coincidence without fixing the problem. Since these outcomes are all the results of dumb luck and nothing more, I suggest that all posts advocating the addition of an extra ground or adjustment of the ground strap be permanently deleted.

Also, everyone who does not consider themselves an "expert" on a particular topic shall henceforth cease to post any advice whatsoever on that topic. Those with merely basic knowledge of a particular subject are banished from giving advice on that subject. Never mind if you have benefitted from the advice of actual experts on this forum and wish to pass along to others the knowledge that so benefitted you in the past.

I still think the system needs to be burped.
If the upper radiator hose and the rear block to firewall hose trick still doesn't fix it, try this:

With the engine not running, remove the expansion tank cap then, unplug the electric aux. water pump and hook the pump up to the battery using jumper wires, making sure the polarity is correct, so it spins on its normal direction of rotation.

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'85 300D Turbo - CA Version

I am not up on the 240D but does it have a clutch fan? and also does it have an auxillary electric fan? if so are they both operating properly? My 300sd was running warm today, normally runs at 80C but after sitting 15 minutes with the air on BS'n with an old buddy it was about 100C it cooled down once it got on the road but was still about 85C. also is your AC condenser clean? just some things i thought of that were not related to what you have already done. Stay Cool

No fan clutch. No aux. water pump. What's the "rear block to firewall trick?" I think I did it, but want to know what you mean in case I didn't do it or did it wrong.

Thanks again for all the help, guys!

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Cam
1982 240d