'78 280ce hot weather temperature question

[Ten13]

Hey there,

I've continued to shake-out the bugs from my recently acquired '78 280ce, and have a question regarding operating temperatures. I've searched the forums but haven't found anything conclusive.

Today, in the inland East Bay San Francisco Area it reached about 103º degrees F. I had my car out on the highway, plus some suburban driving for about an hour, on and off. I decided to run the A/C on my way home; about a 20min highway run with slight inclines. The temperature gauge (see photo) was higher than I've seen it before, but didn't climb higher than you see here.

Would you consider this to be "normal" operating temperature given the conditions and A/C use? I was just cruising, at around 65-70mph with the flow of traffic. It was only me in the car. Aux fan works fine, as does the engine-driven fan.

Also, does that mark between 175 and 250 approximate 200º?

Thanks, all!

[Mike D]

Normal and in fact exceptional considering the conditions you describe. You can check/verify the gauge's accuracy using a thermometer immersed in the coolant via the radiator fill throat. Don't remove the radiator cap when it is hot. Start the test with the coolant cold and idle it up to temp.

[Ten13]

Thanks, Mike -

The car didn't strike me as overly hot when I got home, so I thought I'd ask those in-the-know! Next, I need to track down the hot start issue, and will begin by replacing the accumulator (and while at it, the screen and filter and hoses that feed them), before moving on to the cold start injector gasket. It's only after a period of about 15-20 minutes that the car is hard to start (I suspect starvation, as there's no smoke from a flooded engine).

Also, with A/C engaged, when hot like it was yesterday, the idle is very, very low - enough for the alternator light to flicker and there to be 0psi of oil pressure showing. I just put it in neutral at stop lights and provide some throttle to keep everything happy... but something I want to sort out, for sure.

This car is so much fun to work on - everything comes apart and goes right back together as intended!

[rbtoj]

You should have good oil pressure in those conditions.
I had low oil pressure on hot days with my w123 300td and it turned out to be the oil thermostat stuck closed. Yes, your car has an oil thermostat that regulates the temperature at witch the oil circulates to the oil radiator.

To test this thermostat, just take the car to a good long drive (let's say more than 10 miles), stop by the road and touch the oil radiator. It should be about as hot as the water radiator (maybe even hotter). If it is only slightly warm, then you probably need to replace the oil thermostat. (Obviously take care not to burn yourself )

Also, the low idle speed with the A/C on is probably a vacuum line issue. The M110 engine manual could be very helpful in this regard (unless you happen to own an Euro version of the car).

[280EZRider]

Originally Posted by rbtoj

You should have good oil pressure in those conditions.
I had low oil pressure on hot days with my w123 300td and it turned out to be the oil thermostat stuck closed. Yes, your car has an oil thermostat that regulates the temperature at witch the oil circulates to the oil radiator.

To test this thermostat, just take the car to a good long drive (let's say more than 10 miles), stop by the road and touch the oil radiator. It should be about as hot as the water radiator (maybe even hotter). If it is only slightly warm, then you probably need to replace the oil thermostat. (Obviously take care not to burn yourself )

Also, the low idle speed with the A/C on is probably a vacuum line issue. The M110 engine manual could be very helpful in this regard (unless you happen to own an Euro version of the car).

I have seen euro W123s with the M110 that have an oil cooler, but USA imports with the M110 generally don´t because in its space is the charcoal cannister for fuel emissions.

At idle with the AC compressor on there is current sent to an ignition change-over switch (inside left fender behind head-light) that has vacuum lines from the intake manifold and the vacuum retard on the distributor. When activated the idle will increase. My switch is bad and it´s impossible to find a new one - even from MBZ, so if the idle ever gets too low I do the same - go into neutral at long stop lights.

I have my idle set for between 600 and 800 rpm with AC on while in gear, so generally long stops are not a problem unless it´s super hot outside. It sounds like your idle may be set quite low. Oil psi should never go below 15 psi.