Poor cold starting and HFMScan software reporting 34 degrees c coolant temperature at start

[Ianr8]

Our 1995 C280 has a bad cold starting problem but is fine once warm. Car starts and dies on first turn of cold start. Fires up well on second start. One warm engine starts perfectly. All service items inc spark plugs & fuel filter replaced. I’ve also removed and cleaned the injectors thinking they might be leaking - they are not.

I’ve just fitted a new engine wiring harness and throttle body (£1500 ouch!). The old harness was really bad and the car is running really smoothly now.

I cleared all the codes yesterday and went for a drive (36m). This morning the only code present was 004- Engine coolant temperature implausible (=P0116 OBD code).

I’ve attached a photo of the coolant temperature at start up 34 degrees c. Big difference from the air temperature. The car was left outside last night so was completely cold.

Again I am getting a very high coolant reading and yet this hardly changes as the car warms up. After almost 15 minutes of idling the coolant temperature was apparently only 40 degrees c. Yet my infrared thermometer said the water pump was 65 degrees c.

So the car now has a new genuine Mercedes Coolant Temperature Sensor, and new harness back to the ecu but I am still getting an incorrect coolant temperature reading.

I’ve put the CTS in a cup of hot water and the resistance (meter set at 20K Ohms) was

Pins 1 & 4 2.08 ohms at circa 20 degrees c
0.33 ohms at just under 100 degrees c

Pins 2 & 3 5.15 ohms at circa 20 degrees c
0.66 ohms at just under 100 degrees c

The old CTS was also giving 37 degrees c at start up.

I’m running out of ideas.

I can’t believe the ecu is faulty as everything else appears to be fine. No other codes and it runs better now than it has done for a long time.

Could a bad ground on the car be at fault - I’m running out of ideas. Or perhaps I'm going down a blind alley - may be I should change the fuel pump?

Don’t want to replace the ecu as its such an expensive part.

Does any one have any ideas or suggestions?

Many thanks and sorry its such a long post.

 

[mioba]

A decent indy buddy. You have shelled out a hell of alot there.

 

[Ianr8]

Too true - shelled a lot out but I think (hope) it will be worth it - for sentimental reasons.

I need to double check to see if thermostat has been replaced. Temp gauge in the dashboard works off a different sensor and is working as expected. Water pump was replaced two years ago (hopefully thermostat was changed at the same time). I've taken over looking after the car this year as it is so expensive taking it to a garage.

Vacuum lines. Nothing obvious as I checked them all after fitting the new Throttle body as the car then idled very high. I traced that back to needing to alter the throttle linkage ever so slightly that the ice micro switch engaged. Been perfect since (just this bad cold starting).

Just trying to understand why the ECU reports 34 degrees c when air temperature is 14 degrees c (having replaced the coolant temperature sensor).

May well have to resort to an indy - it is just trying to find one who genuinely knows about this era of cars.

 

[LostKiwi]

it is just trying to find one who genuinely knows about this era of cars.

Try Avantgarde Motors - not too far from you and Steve has an excellent reputation
He's also a member on here.
@Steve@Avantgarde

Just a thought...
Is the temperature sender a 4 pin one like this:


If so try fitting the connector 90° out. If it's the same as our 1993 129 the connector can go either way and is a useful means of testing.
Internally it's two independent sensors.

 

[Ianr8]

Thanks for the recommendation. I've just looked him up and he's around 90 mins away so definitely within reach if all else fails.

Yes the temperature sender is the 4 pin however the connector can only fit one way (Ours has straight thin pins rather than round ones). I think one goes to the ECU to record coolant temperature and the other goes to the ignition ( though not sure why).

 

[Ianr8]

Thought I should give an update to perhaps help others in future: short version - still have the problem and the car is booked into Cornerstone Garage in Tiverton next Thursday.


Long version - cleaned all the engine grounds and replaced the ground strap between the transmission and the chassis. No change to cold starting.

Replaced the thermostat - very easy job once I had sucked out the coolant in the reservoir. Didn't loose any. Did this as it was the original one and for £32 for the genuine MB part it seemed sensible.

Replaced the engine coolant temperature sensor (again). I compared the original one to this new one and got very similar resistances when both where put in the same jug of boiling water. Also noted recorded them as the temperature dropped. They were with 20ohms of one another so not enough to throw the temperature so far out.

I've also been speaking to Avilec on the Isle of Wight 9been very helpful) who repair ECUs. I've checked the new wiring resistance and that there is 5v across pins 1 & 4 when the ignition is in the 'on' position which there is.

I have made some progress though. Pins 2 & 3 of the ECT go to the Automatic Air Conditioning unit. Via this link

W202 1996 C280 A/C blowing hot - PeachParts Mercedes-Benz Forum

Got NO A/C and lucky me it's only been in the 90's with 90% humidity My next door neighbor is an Army mechanic and is the captain of his squad. He is

www.peachparts.com

I found you can test the ACC and one of the parameters is the engine coolant temperature. Well this read 15 degrees c, so confirms that the ECT is not faulty.

Unfortunately this now means it may be the ECU. The only thing that's not been replaced (only cleaned) is the MAF.

Still not had time to make a smoke machine to check the vacuum lines but it is on my list of things to do.

Hopefully I'll have some more info by the end of next week.

Will update then.

 

[alexanderfoti]

I agree on your fault finding, does look like ecu

 

[Ianr8]

Thank you alexanderfoti, it does indeed look as though the ECU is faulty.

I picked up the car up this afternoon from Cornerstone who couldn't have been more helpful (and good value for money). After having it for 48 hours (two cold starts) they say it looks as though it is the ecu as they too got incorrect coolant temperature readings. They also got a fault code saying the mixture was wrong which is more than likely linked to the incorrect temperature values.

Just deciding on to whether to go with an ECU repair or an exchange one with Mercedes. Suspect it will be the latter despite the huge cost difference.

Will keep the thread updated until a resolution is found.

 

[Ianr8]

Pleased to report the car is now starting perfectly. As Alexaderfoti above said yes it was the ECU.

Wasn't a straightforward swap as Mercedes Benz South West here in Exeter had to a raise a ticket with the factory to get the info off the old ecu and then onto the new one. Took ten days in all but at least its working. To be fair to MBSW they only charged the book time.

So a good if expensive result.