High radiator fan speed and poor economy - E250 W212 - OM651, no codes

[ScootScoot]

First post - hello to all.

My car is a 2015 E250 CDI estate, OM651 engine. It's covered 118,000 faultless miles (50k with me) and up until recently my daily commute gives me 54mpg (+/- 2mpg), which is stunning for a big car. No faults, ever. Religiously changed the oil and filter every 8k, changed the transmission fluid last July.

About 3 weeks ago, I noticed that the radiator fan was reaching a high speed after approx 30 seconds after ignition. Even from cold (at 2C or 3C outside temp), to be honest I didn't immediately notice the drop in economy. The indicated engine temp reaching it's standard 90C after about 15 mins of driving. I'm also pretty sure that one day it didn't spin the fan up at all during my 40 mile commute home. If I turn the engine off back to position 2, the fan stops. So it's not the 'constantly on' problem.

I've read the existing threads about 'noisy fan' / 'radiator fan speed' and the AC Pressure Sensor fix. I'm hesitant to go there purely because of the drop in economy and wanted to ask opinion if you think that the overcooling from the fan is dropping the economy by not letting the engine reach it's economical temperature. Or that the fault is another sensor more related to fuelling, which in turn is signalling the ECU to a fail condition.

What I have noticed is that my MPG reading from cold is now starting at 14.1 MPG. I'm pretty anal when it comes to MPG, almost certain it never used to do this, leading me to think it's going into some limp mode condition although the engine power doesn't seem 'down'.

The relatively basic scanner I've got is a Launch CRP129 with up to date files, but I would have thought good enough to catch a code. There are no fault codes. The only 'live feed' mention of any error is in the EGR section. "Y85 EGR cooler bypass switchover valve Fault. 53336/6058" - this value changes to 'OFF' during driving. Other people have mentioned this but not in relation to a fan speed issue. I don't think this is anything.

So far, I've changed the coolant temperature sensor, original part from Merc. No change.

I took the car to my local diagnostic centre, in fact they passed the MOT the previous week when the car had the fault and we mentioned booking it in. The diagnosis was a faulty radiator fan controller, I've got this in writing on the bill. Purchased a Ridex fan shroud from Autodoc and fitted this myself, no change - just the same.

On the verge of calling the guy back and giving him a 2nd chance to diagnose, but hesitant due to his previous mis-diagnosis.

Thoughts? Apart from "god, that's a long post" ;-)

 

[MrGreedy]

Interrogate live data for all the aircon and temperature sensors on the car. If you find wildly low, high or implausible values, you have found the place to look.

I had the low aircon pressure (turned out to be condenser leak) that causes the fan to come on after 10s every time.

My mate with an E250 has just had a similar fault, likes yours, and that was a broken/rubbed wire for one of the templates sensors, but not right next to the sensor. But the fault was found in the same way by looking at live data, as no fault codes.

 

[mersum1es]

What if full fan speed is sign of voltage problem. Have you checked all fuses, and groundings. What is battery voltage and charging voltage?

 

[AndrewOl]

As you have found, this can be due to lots of things. I think you will save money and grief in the long run finding an Indy with Star who will no doubt be able to diagnose the fault. Parts are too expensive to hit and hope.

 

[ScootScoot]

Appreciate those pointers guys, car is booked back in this week for a 2nd diagnostic session. Someone summised today, "Could the additional load of the fan running effect the fuel consumption?" Of course the answer is yes, it must to some degree, but by 10%, is that beyond a realistic suggestion?

The main battery hasn't been changed in my ownership, its most likely the original. I replaced the boot mounted battery in Nov 22.

 

[ScootScoot]

Test: MPG drop caused by high fan speed. Before my commute this morning, disconnect radiator fan. Result: exactly the same MPG drop and odd behaviour with the MPG from start figures (starting at 14.1mpg and gradually indicating, 14.9 -> 16.x -> 18.x 0> 21.x over the next 3 to 4 miles. Previously, the car shows an initial 31.x MPG within a few hundred metres of leaving home, soon into mid 30's, 40's within 5 miles and into 50's for most trips. I can see the MPG drop big-style with any hint of a hill, levelling back out on a drop. Obviously that used to happen by 1 or 2 MPG, but the scale of drop is now dramatic, 5 or 6 MPG per hill.

Last week I did a full tank test, normally get 510 -> 530 miles (sometimes more), it's now doing 460.

Theory, whatever fault is causing the poor fuel consumption is also sending the ECU info suggesting that extra cooling is required.

 

[grober]

https://diysprinter.co.uk/reference/Introduction_OM_651.pdf

Look at the EGR Section- the bypass valve is pneumatically operated but electrically switched so a fault code is not always generated if theres a vacuum failure----- the normal function of the EGR is to reduce combustion chamber temperature to reduce NOx levels so there may be sort of a connection????

 

[ScootScoot]

Thank you grober, what an excellent document. If the orientation of the EGR unit is as per document, I may be able to partially inspect the vacuum section without taking too much off.