What kind of readings should I get if I shot the block at the sending unit and take readings at the top and bottom of radiator with a 80c stat installed? I've installed a new T-stat and radiator plus flushed the block. The only things I can think of left are the water pump or the temp gauge sending unit being bad. Gauge is reading between 100 & 120c.

I'd expect it to read a bit lower than the internal temp though not too much. I suggest you "borrow" some readings from a friend's car with decent aftermarket gages and compare the gage readings to the IR gun - say, the water temp vs the IR of the base of the gage sensor. That should give you a pretty good idea of the offset.

Dan

Did you burp all of the air from the system? A typical problem that many run across when doing a flush, etc.

The best way to fill is with the nose elevated on ramps or jack stands. Then fill with coolant through the top radiator hose before reattaching. Then run it with heater on.

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Sam

84 300SD 350K+ miles ( Blue Belle )

SD blue, did everything you mentioned. I'm thinking my temp sending unit or gauge may be bad. The radiator was leaking and I guess as old as it is (1982) the waterpump could be bad.

Don't expect to get accurate temperature readings with an IR gun. They are best for measuring relative temperatures.

The surface you aim at makes a big difference. For example, if you shoot at a shiny surface you will get a different reading than from a dull surface. I sometimes stick on pieces of masking tape just so surfaces would respond the same.

Sorry, can't help with what temperatures should be.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

+1 The temperature reading will depend on the emissivity of the surface as well as the calibration of the IR gun. Usually the reading is lower than true temperature. Rubber will read differently than bare aluminum, dull vs shiny matters, paint, oil, etc. I only use an IR gun as a rough approximation. I use inexpensive plastic thermostrips for accurate readings. I have them on the oil filter housing, alternator, thermo housing and engine outlet.

In general, you are looking for a 10-20C temp drop across the radiator core. But even that's not a constant, as water and airflow are affected by road speed, fan, RPM and thermostat.

Mercedes engines usually have sensors either at the water exit, the exhaust side water jacket or in the bypass passage. The idea is that head exit temperature is most important.

Hmm... at 100 to 120C, your fan clutch should be kicking in. Is it?

My experience with water pump problems usually was a bearing making noise and/or seal going out causing it to drip. Are you getting any kind of coolant loss now?

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Sam

84 300SD 350K+ miles ( Blue Belle )

I know the emissivity theory, but have been amazed how accurate my HF IR Gun is. I suspect it uses the "2-color method", so works well for any "grey body". But don't expect good results on shiny metal, though seems to measure well on the diffuse aluminum T-stat housing. I shot a M-B T-stat in a pot of water and it agreed well w/ a thermometer, and helped me identify a problem with the T-stat. Swapping it made the engine run normal 82 C on dash gage vs ~60 C before. When I shoot the sensor in the head (front driver's side), I measure +/-5 C w/ the dash gage. But, you must fill the sight of the IR gun w/ the surface, w/ no wires obscuring it, and the sensing area is likely much larger than the laser alignment spot.

Dealing w/ a similar issue in my 1985 car. It likes to run ~100 C on the dash gage, even creeping up to 110 C at stop lights on our 105 F summer days. Started Fall 2015 when I noticed it running ~90 C. Seems to have started after I added 1 tube of Aluma-Seal powder to seal a suspected heater core leak. Before, it always ran 82 C on the dash gage. The IR gun confirms the dash readings and suggests there is bypass flow past the T-stat even when >110 C on the gage (measured much cooler at lower radiator hose inlet than at bypass side of T-stat housing).

Things I have tried:

Drained Evans Waterless Coolant and ran straight water for a month, reflushed w/ citric acid, no change, back to Evans now. Run Evans in my 1984 and holds right at 82 C, so unlikely Evans is involved.

Changed water pump, removed T-stat - bypass housing from block searching for blockage (stuffed paper towel I forgot about, ...). Tested flow from radiator (excellent) and viewed top of core w/ endo-scope (clean, no debris).

Changed T-stat 3 times, even trying proven one from original 1985 engine, no difference. Some labeled 80 C, others 87 C. Swapped temp sensor w/ 1984 plus measured resistance and substituted resistors to verify dash gage. Insured hole in T-stat was up, plus purged air w/ front on ramps, etc.

Swapped fan-clutch at least once (forgot). Hard to tell if it is running optimal. Seems to move plenty of air when the engine rev's, but they can fool you. It is still my prime suspect. The electric fan would run when it went >100 C in the summer. It did seem to still run at 100 C on the interstate.

It appears the T-stat regulates at 100 C summer or winter (or dash gage is off), plus the fan-clutch is marginal and lets it creep up at long stop lights and slow speeds.

Any suggestions for other tests would be great. It appears the engine is truly running hotter than normal. Hasn't hit 120. I do measure slightly better mileage, so perhaps no problem to fix. It would probably be OK to run > 120 C w/ Evans since it won't boil over or get hot spots in the head. I can run w/ the reservoir cap loose so no pressure builds up. Could be a head crack started, coincidental w/ adding Alum-Seal, but I see no oil in the coolant, don't loose any coolant, and see no bubbles from combustion gases, so seems unlikely. One thought is to block off the bypass passage. My U.S. engines don't have that, so wonder if it is essential. Seems the bypass is to help the engine heat uniformly when cold. I wonder if the T-stat block-off plug is being forced open by w.p. pressure.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

US engines use a different system. The bypass is always open, but the thermostat is located so that the dynamic pressure of the flow forces the water into the radiator. There is always some bypass leakage with this system.

The bypass has to be open in a Merc, or the thermostat would never open. This is because the thermostat is located on the cool side of the system. Don't block it.

What may be happening is that the bypass entry has cavitation damage. This is actually pretty common, because pressures around the bypass lip are relatively low, and the fluid is very hot. It shouldn't happen with waterless coolant, but maybe it's old damage. Similarly, there may be some damage to the pump involute or impeller which is reducing flow.

You might try shorting the electric fan switch and see if the extra airflow makes a difference.