Hi Guys,

Have the following issue on my 1995 W124 E500 Limited with 74.000 Km. When driving slow below 90 km/h or in traffic and in idle the temp. stays normal. When driving above 90/100 km/h and the engine is under more load the temp. rises above the 100 Deg. C mark/line and climbs to 105-110 Deg. C. Once I slow down or am in traffic the temp. gradually drops down again. This with the help of the electric cooling fans at high speed. What has been done so far:

New radiator OEM
New fan clutch aftermarket ACM
New thermostat OEM
OEM water pump removed and inspected. Re-installed as found to be ok
Replaced MAF
Replaced Lambda sensor
Replaced all temp. sensors
New ETA/Throttle Body
New upper and lower engine wiring harness
Citric Acid flush
No faults with the OBD1 blink code reader

Several tests for head gasket failure turned out to be negative. EZL and control units still original.
From the emissions test we are reading a lean mixture on idle with practically nil CO and HC ppm after a hot run. Lambda around 1.1.
The on-off ratio using a duty cycle measurement at pin 14 on the x11/4 socket is averaging about 18%. Does that tell you guys anything?

In addition, the E500 was parked for about 6 weeks with coolant level in the reservoir full and without being started. After starting, the warning light for low coolant came on. Checking the reservoir there was no coolant at all left in the reservoir. Empty. No leaks detected anywhere and pressure tested the cooling system. No leaks. So am wondering if there was an air pocket in the system which after extensive parking disappeared causing all the coolant to flow from the reservoir into the radiator?
Today we noticed that the heating is also not working. So could there be an air pocket in the cooling/heating system causing all this?
Is there any method we can use to get rid of the air lock? Could the mono valve or aux. pump be causing an issue?

Appreciate any inputs to solve this issue. Thanks.

I will be the first to admit I know very little about E500 and am surprised Mercedes was still fitting the E500 with K-jetronic in 1995. I thought that was stopped in 1993 for all other models.
But since you have confirmed that yours has K-jet, let's first make sure that your duty cycle is indeed 18% and not 82%. That matters in the diagnosis. Also, is it cycling like it should around the percentage, about +/- 5%. I'm sure there are some error codes in ECU if it is so far off.
No matter the answers there is something terribly wrong there but perhaps you can help us with the above questions.
Also, the engine lambda running lean will certainly overstress the cooling but that is some serious side effects you are seeing.
- Cheers

Hi Dolucasi,

Thank you very much for your response. The W124 E500 has the LH-Jetronic. I will check with the workshop again to make sure and get back to you. Have replaced the MAF a while ago. Symptoms were very high CO readings, cutting out and hesitation. Diagnosis was faulty MAF PCB. While the MAF issue was present, engine was running mostly at normal temp. My explanation is that probably it was due to running very rich. I had the MAF replaced with a reconditioned unit and all issues of bad emissions, running poorly and cutting out are now solved.
Since the MAF was replaced I am however facing these temp. issues. So not sure if the faulty MAF was masking the temp. issue or the replacement MAF is causing higher temp. and lean running. The OBD 1 blink code reader is not
showing any faults. I wonder if the ECU is shot but again not showing any faults.

In post #1, you indicate oem replacements;
What about the MAF?

Other areas:
Fan shroud in place? with all the flaps present?
When you replaced radiator, was the condenser cleaned to blow out debris from the fins? (You need to blow from back to front)
Dash gauge is accurate?
Compared this reading to the other 2 temperature sensors at the front of the manifold.
Possibly requires re-calibration? if the readings are not similar.

You probably have compounding issues here but I would first start with the emissions part of it and get that fixed. When manufacturers jam a large power plant in a smaller engine bay sometimes cooling becomes more challenging and compromises have to be made and there is less margin for cooling. Not saying this is the case for the E500 but your emissions issue is not helping and needs to be fixed anyway.

Now it is surprising to see a MAF fail at 75K miles. I am more familiar with the pre 91 ECU's instead which hardly ever fail for the life of the cars. Possible that when Benz split the controller into two after '90, that brought in a weakness. I do not know. But then again it is also strange to have cooling problems on such a "young" car.

If you would like help online I will first need to know if your duty cycle is truly 87% or 13% and if it is static or not. Probably best if you report the average DC voltage on the diagnostic port where you read the duty cycle just to make sure.

If you are experienced in emissions for these cars my apologies up front, it would be good to know what you find out in your diagnostic work.


- Cheers!

Well, my apologies, you have the EFI system not the old Mechanical fuel injection it seems. Even though I had a '95 E320 for 20+ years, it never developed any emissions or cooling issues before I sold it at 145K miles. Since I never had to fix anything, I will not be of any use here diagnosing your emissions. I will be reading your post with interest though .....

- Cheers!

@khomer2

The replaced MAF is a reconditioned original unit from a renowned company. We can definitely rule out the cooling system. Have been through everything.
Yes fan shroud in place and all flaps present. Condenser was cleaned out. Verified with live data that dash gauge matches sensor readings.

@dolucasi

We hooked up Star to get some live data. Ignition on the on/off ratio is 30%. Warming up at idle the on/off ratio is 50%. These are as per spec. as far as we know.
Once engine is at operating temp. around 95 deg. C the on/off ratio hovers between a low of 5% and a peak of 15%. Should that not be at around 50% (+/- 10%) ?

Hi E500LTD, like I said I am not familiar with diagnosing the HFM Jetronic at all, and am surprised that the diagnostic signals are the same as the old KE-Jetronic system.
This is what threw me of with your earlier post because the signals were the same as the KE-Jetronic.
So based on that, you definitely have a closed loop emissions control issue.

Now on your production year on top of the list is always the 2 harnesses that are biodegradable however you mentioned in your first post that you replaced those already.
Hopefully the shop used new harnesses and not old recycled wiring that may also be biodegradable. It is also even possible something went wrong with the installation, and went un noticed.

Other than that, I believe the emissions control of these models (at least the E320 version) are rock solid, but I am no use to you on how to diagnose your system.

- Cheers!