I am working on a 1990 240dl which overheats even on cool 60 degree days. I have replaced the fan clutch, water pump, thermostat, o-2 sensor, MAF and radiator. The temperature has been verified it is not a bad temperature sensor. The car is actually overheating I hear the fan locking up and blowing air but still overheating. I am at a loss what to look for next. Anyone have any ideas? No engine codes stored.

I had a similar problem with my '81DL. It had the thermostatic fan clutch. I had tried tying the fan blades to the engine pulley with nylon cable ties with no change in cooling. I verified the actual temperature by measuring the resistance of the temp gauge sensor with an Ohmmeter. The overheating was worst with the AC on.

I can't remember every thing I tested or tried, as this was 20 some years ago. One thing was to tie a strip of paper to a stick. While the engine was running I moved it from the back of the fan hub to the blade tips. I discovered an airflow reversal between the tips and the fan shroud. Hence, much of the air was being recycled and not being pulled through the radiator. I installed an OEM small electric fan from the PNP in front of the radiator and Wa-lah! Problem solved!

I have jumped pins 1 and 3 after removing the temperature compensation board a while back. Really confused at this point. I am going to put in another new thermostat but highly doubt it will change anything. Could a bad headgasket or intake gasket be causing this? Is there anyway to see the reading the ecm sees like an obdII car?

Make sure the hole in the thermostat housing is at the top and there is no air in the housing. Burp the radiator hose at the thermostat to displace any trapped air.
Dan

re: "The temperature has been verified..."

You've done everything that makes sense and it should be fixed, but because it isn't I'd like to know how the overheating temperature was verified.
Was it by a shop that was, by any chance, looking forward to your business? Or did you verify it yourself?
If so, how did you do it?

The reason for asking is that, 95 out of a hundred times (especially if you've done all that and have no improvement) you're actually chasing a phantom known as the Temperature Compensation Board. This is not a temperature sensor! It's a circuit board that overlays the temperature gauge behind the instrument cluster. There's a simple bypass, where you remove the board and insert the two ends of a wire (a jumper) to let the gauge become accurate, like in all pre'85 cars. In '85 they introduced the TCB, and that often fails with false and weird temperature indications -- I've had that happen in both of my '93s.

So you need to verify the coolant temperature -- get an infra-red (remote sensing) thermometer (they're are a lot out there) and just aim the laser aiming beam at the metal cover of the thermostat housing. That will be the true coolant temperature. Then you can go from that point.

Good luck.

I once had a problem like this and replaced a lot of parts with no success. Turned out that a/c condenser was full of crap and was blocking air to the radiator.
--
1966 122s, 1968 142s, 1969 144s, 1979 245dl, 1989 244gl

My first guess would be a blocked coolant passage. If you are 100% sure of the radiator and your water pump is doing its job, then I would fear corrosion/blockage in the head.

As long as coolant is circulating around the engine and through the radiator, you shouldn't have serious overheating issues, regardless of what else is going on.

I'm guessing you replaced the o2 sensor and AMM to rule out a lean running condition. Even with all electronic sensors failed, I do not think you would have to worry about lean fuel mixtures. You'd be more likey to get very rich mixtures. Perhaps seriously restricted injectors could cause such a lean burn as to overheat?

I guess I should have asked, how much is it overheating (since you say you have measured the temperature) and under what conditions? Just at idle? Or driving down the road as well?

Have u pulled the radiator yet to see if the fins are still there? have you modified the temp. compensation board? Had a cheap shit aftermarket radiator last 5 yrs all the fins just fell apart like burnt paper!!!!! would not overheat but would very close at times and i had to pull it out to see the damage.Did the temp. compenastion board modification first thinking it was the cause. Couldnt believe it was the radiator after only 5 yrs!!!!