OM642 Rough running when cold. Glow plug change as a starting point?

[DSM10000]

My E Class 3.0 with the OM642 V6 has developed a trait of rough running when started from cold and whilst warming up.

My Indie though the rear engine/transmission mount may have been responsible because the vibrations are worse when cold and in reverse gear. This mount has been changed and although there is some improvement the main problem remains.

The degree of rough running does seem to vary with outside temperature with it being worse on days when it is zero degrees C or below. It also gets worse when under load such as low-speed cornering when the engine falls to idle and the power steering is adding additional load to a cold engine. When warmed up it is smooth with a stable idle and performance is unaffected.

I have/not scanned it for any fault codes yet ( I have Icarsoft MBii) and am unsure what to look for in terms of aberrant values. I know from its history that 3 Glow plugs have been replaced and am wondering if it would be a good starting point (no pun intended!) to replace all of them as I understand they carry on working for some time after the initial light goes out and, if faulty this may be the problem. At140,000 miles it certainly would not hurt.

Any suggestions of what to look for on a scan welcomed as indeed are any other suggestions, also does anyone have a part number for the OE Glow plugs? I assume MB does not make them so a reputable make such as Beru or NGK etc is perfectly acceptable?

Many thanks in advance

 

[TeddyRuxpin]

The glow plug relay is actually a more likely culprit. It’s also easier to change, and cheaper. You can also test the glow plugs in situ with a multimeter - you just need to remove the grey caps

 

[DSM10000]

The glow plug relay is actually a more likely culprit. It’s also easier to change, and cheaper. You can also test the glow plugs in situ with a multimeter - you just need to remove the grey caps

Would this flag a fault code if I scanned the car?

 

[BTB 500]

The glowplugs started failing on my OM642 at 36k miles and I've had at least one out for much of the time since then (dealer is always reluctant to replace them due to the risk of shearing them off). I've never had any rough running when cold - my understanding is that the plugs are solely there to help each cylinder fire when turned over relatively slowly by the starter. Once the engine is running all the cylinders will fire on compression alone, unless something else is wrong?

 

[W1ghty]

The glow plugs are used when the car is running , not just when the car starts . If I were you I would replace all the glow plugs and the gp module at the same time , because if a glow plugs fails it “can sometimes” cause the gp module to go (£100+) .
If it’s the original module it’s well overdue anyway.
What amount engine mounts for the vibration ?
I used beru plugs (4.4v ) when I did my E320cdi the other year

 

[DSM10000]

The glowplugs started failing on my OM642 at 36k miles and I've had at least one out for much of the time since then (dealer is always reluctant to replace them due to the risk of shearing them off). I've never had any rough running when cold - my understanding is that the plugs are solely there to help each cylinder fire when turned over relatively slowly by the starter. Once the engine is running all the cylinders will fire on compression alone, unless something else is wrong?

They are also designed to stay on after starting to aid combustion during warm-up as well.

 

[BTB 500]

I can only say that I've had the glowplug light on for at least 17 months now (it was last serviced in July 2019, and they didn't want to replace the failed plug at that time) without any trace of rough running, even in the coldest weather. So I would still suspect there's some other issue as well. Mine's on just over 120k miles at the moment.

 

[W1ghty]

I can only say that I've had the glowplug light on for at least 17 months now (it was last serviced in July 2019, and they didn't want to replace the failed plug at that time) without any trace of rough running, even in the coldest weather. It's on just over 120k miles at the moment.

The engine won’t run rough because of your failed plug , it just won’t run as efficiently as it was designed to

 

[BTB 500]

The engine won’t run rough because of your failed plug , it just won’t run as efficiently as it was designed to

Precisely. The OP is reporting rough running and vibration, which is why I think there's something else involved other than the failed plug(s).

 

[DSM10000]

Precisely. The OP is reporting rough running and vibration, which is why I think there's something else involved other than the failed plug(s).

But when warmed up it is running normslly.

 

[Jobsworth]

Do just check that your coolant temperature sensor is reporting plausible temperatures when cold.

 

[DSM10000]

Do just check that your coolant temperature sensor is reporting plausible temperatures when cold.

Does this also supply the signal for the temperature reading in the instrument cluster? That seems to be reading correctly.

 

[Jobsworth]

It may do, but it’s worth checking that at, say 3 degees C it’s not stuck at 20 degrees. That’ll make it run rough until 20 is reached.

 

[DSM10000]

It may do, but it’s worth checking that at, say 3 degees C it’s not stuck at 20 degrees. That’ll make it run rough until 20 is reached.

That's a good call, will check and report back.

 

[TeddyRuxpin]

This is exactly what my dying relay did: it was the occasional coil light, normal running

Coil light would flash on (sometimes so quickly and just once so if you’re looking at the road, you can only catch it in your peripheral vision and then it’s gone)

It would flash more and more regularly until it became harder and harder to start the car, but the car would run perfectly after it was up to temp.

I managed to change the relay just before I was left stranded as the car wouldn’t start, but some of the last few starts were really laboured.

I managed to get a BERU one on eBay which is OEM, pretty cheaply

 

[DSM10000]

This is exactly what my dying relay did: it was the occasional coil light, normal running

Coil light would flash on (sometimes so quickly and just once so if you’re looking at the road, you can only catch it in your peripheral vision and then it’s gone)

It would flash more and more regularly until it became harder and harder to start the car, but the car would run perfectly after it was up to temp.

I managed to change the relay just before I was left stranded as the car wouldn’t start, but some of the last few starts were really laboured.

I managed to get a BERU one on eBay which is OEM, pretty cheaply

I have had no such symptoms so far, the coil light illuminates for under 2 seconds and stays off until the next start. Where is the coil relay situated?

In terms of the temperature sensor,

I have checked it with Icarsoft and the outside air temperature is reading 0.5 C and the coolant temperature (car has not been driven for over 24 hours) was fractionally under 10 Degrees C. I have no idea of the insulating effect of the engine etc but would expect the coolant to show a lower temperature?

 

[Smiley]

I have had GP and GP relay issues on the 642 engine. When a GP was out then it just showed as the GP light coming on again after starting; no other symptoms. When the GP relay was faulty the symptom was delayed starting - took several seconds of turning over before it started and then white smoke; but no poor running symptoms. As others have said I don't think replacing the GPs will help. Could be the relay - if it is the original then it really is on borrowed time so I would replace it anyway - it is very easy sits at the front of the engine just under the top cover. The new one is an improved design apparently. But more likely something else imo

 

[W1ghty]

I have had no such symptoms so far, the coil light illuminates for under 2 seconds and stays off until the next start. Where is the coil relay situated?

In terms of the temperature sensor,

I have checked it with Icarsoft and the outside air temperature is reading 0.5 C and the coolant temperature (car has not been driven for over 24 hours) was fractionally under 10 Degrees C. I have no idea of the insulating effect of the engine etc but would expect the coolant to show a lower temperature?

Relay

 

[TeddyRuxpin]

It’s an easy diy and I think the part cost me about £50

 

[DSM10000]

It’s an easy diy and I think the part cost me about £50

That's interesting . On eBay a genuine MB part was £180.

ECP had them around £130.