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Discussion Starter #1
First off I am grateful that I found this website. There appears to be a lot of experts that are kind enough to share their knowledge with others. I recently bought a 1990 mercedes 300sl. It is cherry red and I love it. However, it is sitting in my garage rn because there appears to be a cooling problem with my car.

Problem: After starting the car for a few minutes, the check engine light goes on and the temperature begins to rise. The check engine light is never on when I first start the car, only after it has been running for a while.

Other information to help guide diagnosis:

1. I pulled out the thermostat and boiled it in a pot, and it opened. So I crossed that off the list as possible problems.

2. I tried draining the radiator fluid. A little over a gallon came out. I tried filling it with new radiator fluid (via the reserve tank) and it only took 1.2 gallons or so. My car is supposed to take in 12L so about 3 gallons. My first thought was perhaps the radiator is clogged somewhere, but when draining the original fluid, it was very clean with very little residue. So I found it to be highly unlikely that there is a clog somewhere.

Could anyone give me guidance?
 

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I assume your radiator is about the same size as the one in my '98 SL500 which holds about 1.5-Gal.
But there are sill coolants in the engine and there are drain plugs in the engine block for that.

I would do a scan to see what causes that Check engine light.
 

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Yes.
So if your fans do not come on at all when your engine heat up that could be your problem.

Scanning will help to verify that.
 

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what reading is on the temperature gauge when the check engine light comes on?

It's possible the check engine light has nothing to do with the coolant temperature, but is throwing a code for something else. Since it is a 1990, it's pre OBDII, look in the stickies and you'll probably find how to read the codes.
 

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Discussion Starter #8
First of all, I am so impressed at how quickly you guys responded to me. I really appreciate it!

The check engine light goes on when the engine temperature is about half way, which leads me to think that the cooling system is failing to start. At lower temps its not supposed to run, hence no check engine signal. Once its supposed to kick in, it doesnt, and hence the signal.

As for reading the check engine codes, how do you do that? I'm a newbie when it comes to the r129. All I see is a red check engine sign.

I believe the fans start. I can check again tomorrow. I need to reinstall the thermostat housing.
 

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I am not familiar with the 6 cyl engine, but I think there is a diagnostic 'blinker' in the engine bay.
Just wait and others might chime in to help.
 

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Discussion Starter #10
It is about half way. The temperature goes above that afterwards. But the check engine light goes on about half way. I'll check and get back to you tomorrow.
 

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It is about half way. The temperature goes above that afterwards. But the check engine light goes on about half way. I'll check and get back to you tomorrow.
You need to be a bit more specific than that. What temperature is "about half way"? I think I have the same engine as you and I would expect the temperature to remain around the 90C mark. If you leave the engine running, does the temperature rise beyond 100C?

And if it does, do the auxiliary fans start running at 105C?

It is necessary to know these things if you are to determine if there is a problem and where the problem may lie.

Your check engine light may not be related to temperature issues.

RayH
 

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Discussion Starter #13
Update with engine codes read

Hey guys, sorry for replying so late. My sister is getting married and I've been busy :laugh:laugh.


I purchased a check engine code reader and I got the following codes (I didn't reset the codes so idk what is relevant):

3: 22 17
6: 3 8 9
7: 3 4 5 6 7 8
8: 17
10: 20 23
11: 3 14 12 6 5
13: 1
14: 2 3 13

As for the engine temperature, it rises to the 75% mark. I don't drive the car, I just rev it in my garage. When t reaches that level I just stop reving. I know this is high because I drove it for over an hour when I first purchased it and it never rose abovethe half way mark. Now if I just rev it the garage the temperature rises.
 

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I purchased a check engine code reader and I got the following codes...
Some of those codes may have been set during the Clinton Administration and subsequently the underlying cause was corrected without the code being erased. So you need to erase all of those codes.

A manual with definitions for all your codes is found in post #3 of this thread: https://www.benzworld.org/forums/r129-sl-class/1266535-r129-cel-diagnostic-trouble-codes-procedures.html.

Your fuel injection codes indicate a faulty oxygen sensor signal. Perhaps a wire is broken or the sensor is unplugged. The check engine light comes on when the temperature gauge is "half way" (c. 40 degrees) because that is the temperature at which mixture control switches from "open loop" (no feedback from the oxygen sensor to correct the mixture) to "closed loop".

My advice is to quit revving the engine with the car sitting in the garage and to drive and enjoy it instead. Presuming it runs well, at your leisure you can look for a wiring problem with the oxygen sensor.
 

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Discussion Starter #15
It is about half way. The temperature goes above that afterwards. But the check engine light goes on about half way. I'll check and get back to you tomorrow.
You need to be a bit more specific than that. What temperature is "about half way"? I think I have the same engine as you and I would expect the temperature to remain around the 90C mark. If you leave the engine running, does the temperature rise beyond 100C?

And if it does, do the auxiliary fans start running at 105C?

It is necessary to know these things if you are to determine if there is a problem and where the problem may lie.

Your check engine light may not be related to temperature issues.

RayH
Hey Ray. Thanks for the input. I can run the engine tomorrow and check the temp. The temperature got past the 100c mark. However I am not sure it went past 105. The 75% mark on my gauge is 100c. It went just past that and I turned it off.
 
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