OM642 glow plug relay failed but no code?

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Anything101

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Jul 19, 2023
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CLK 320 cdi
Hi, I've got a problem with smoke/rich running under relatively specific circumstances but have no error codes to show for it on STAR.

Under 2 scenarios, my car will produce a steady plume of diesel smoke. As you can imagine, it's something I want sorted but every garage/specialist has said a different thing (turbo, injectors, oxygen sensor etc). No one has mentioned the glow plug relay, but my reading seems to indicate this is worth trying.

Scenario 1:
Cold start and then sitting in slow traffic - if the coolant temperature is between 40 and 80 celsius, the car will reliably start to smoke. If the smoke isn't visible, there's an acrid smell which is noticeable. Also, there's a very slight misfire at low revs under low load which occurs with the rich running.

Scenario 2:
I'm unsure of the exact circumstances but after a reasonable motorway journey where the DPF has got hot enough to begin a regen, the car will produce noticeable amounts of smoke until the process is complete if I am interrupted by stationary traffic. I can't be certain that a DPF regen is occurring at that point, but it is the only circumstance which I can think of where the car would be running rich while warm.

Things to note:
  • The GP light never stays on after start. I'm pretty sure it never comes on during the start process, only flashing for a second or so when the key is turned to position 2. This is the case no matter the ambient temperature. I know the bulb works, but I believe the plugs never actually fire?
  • I believe the O642 uses the GPs for 2 purposes. The first is to start the engine from cold and the second is to increase the temperature when the engine is running. This would make sense for when a DPF regen is needed.
  • Any smoke/smell can be stopped by driving and getting the car warm (reliably sits at 85 celsius). Once warm, it does not return even if left to idle for 30+ mins except in scenario 2 which is very rare.
  • The car is hard to start in the cold. Turning the key between stage 1 and 2 an extra time makes a notable difference in cranking duration under these conditions.
  • Other issues have been investigated. Injector correction values were all fine, turbo was inspected and oxygen sensor was changed. The car drives brilliantly.
So, is it possible that the relay fails without leaving any codes? Is this a good place to start?
 
A common symptom of a failed relay (apart from poor cold starting) is the glowplug light coming on occasionally when driving, staying on for a few mins, then going off. AFAIK the plugs are only used at startup ... I think the amount of heat they produce would be fairly trivial compared to combustion temperature in the cylinders. Apart from the relay there's also a 225A 'mega fuse' which can also fail. FWIW I've driven mine for some time with the glowplugs out (due to both a failed relay and failed fuse), and never had any smoke whatsoever or DPF issues. The glowplug light staying on after start indicates a failed plug (or plugs) btw.
 
I didn't get any codes when mine failed. Just a flashing coil icon on the dash. When I say flashing, I mean you'd be driving along and you'd catch something flash up for a fraction of a second until you catch it.

But I only got problems starting. No smoke. I don't think the glow plugs not the relay would be responsible for that, I'm afraid.
 
Thanks for the replies so far. Teddy, I've seen the flashing light you describe on a previous car (w220 OM648) but on this one I never see anything beyond the bulb test sequence.

When yours failed, did it smell/feel completely normal once the engine was started? The only other thing I would be tempted to check are the injectors but if the engine is already warm or is just left to idle from cold, I do not get any kind of smoke at all. It only happens if the car is in gear and coolant is above the minimum 40 celsius marker. However, at c.£100 and a DIY fit, I was minded to just try a new unit compared to sending the injectors for testing at a minimum of £300 + some hours of labour to remove/refit.

I get normal readings from the oxygen sensor and MAF, so it seems that the injectors are not supplying excessive fuel but the combustion chamber is just not as hot as the car believes.

Could anyone confirm the parameters for the 2 stage glow plug operation on this engine?
 
When yours failed, did it smell/feel completely normal once the engine was started?

Mine certainly did, over the course of several months.

However, at c.£100 and a DIY fit, I was minded to just try a new unit

As an aside MB ones have been hard to get for some time - backordered with no ETA when I needed one last year. My local garage managed to get one via a contact at a dealership (they'd ordered one in previously and not needed to use it).
 
My car was normal once started, too

You may have multiple issues, but I don't think the problems are running are related to your glow plugs or relay
 
Cold start and then sitting in slow traffic - if the coolant temperature is between 40 and 80 celsius, the car will reliably start to smoke.
I wouldn't expect glow to be coming on under this scenario.

You mention rich running. Rich means low air/fuel ratios. That's nothing to do with glow plugs. Diesels don't throttle, so operate with variable A/F ratios. Rich, too much fuel for the available air, gives black smoke. A DPF will mean you wont see that at the tailpipe though.

Glows aid combustion under low cylinder temp scenarios. If you have incomplete combustion (regardless of the air fuel ratio) you'll have white smoke.
 
I've read a few times that a DPF means no visible smoke,
but I have seen it even on a new DPF.

Before spending I would get your compression checked.
I chased my tail with oodles of dosh on a 642 some time back.
I gave up when I concluded that the engine was passing oil.
 
A DPF will trap soot particulates. Smaller particles will still pass, so you will still have smoke. Just not soot.
 
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The only symptoms on my om642 car when the glow plug relay failed was longer cranking time to start when cold and white smoke when cold. No warning light. I do not think your problems will be fixed with new glow plugs or relay.
 
Thanks for the help so far. The smoke isn't black. It could almost be mistaken for condensation (i.e. what you see on a cold day), except that it clearly contains unburned diesel. The problem is only present when the coolant temperature is cold or what appears to be an interrupted DPF cycle.

Wouldn't issues with compression show all the time? The engine uses no oil/coolant. It drives perfectly fine. The sole problem is the rich running when cold. Seems to agree with Smiley?


The graph on this site explains that the glow plugs are used after the engine has started. This led me to believe the engine has calculated the 'correct' quantity of fuel to inject, but the cylinder temps are do not actually match the feedback it has been given.
 
I wouldn't expect glow to be coming on under this scenario.

Glows aid combustion under low cylinder temp scenarios. If you have incomplete combustion (regardless of the air fuel ratio) you'll have white smoke.
If I start the car and am immediately in traffic (2 mins or so from starting), the problem is likely to manifest. If I am able to drive even 500m from cold without encountering traffic, the car is completely fine.
 
The graph on this site explains that the glow plugs are used after the engine has started.

That's a rather simplified overview. Glowplugs are used for post glow, but when and for how long will depend on things like air and coolant temps.
That doc implies that the Beru controller decides when the glows are needed. That might have been the case before people cared about emissions, but don't know of any vehicle where the glow controller is anything other than a slave device to the ECU. So MB decide, not Beru.


I think you need to read this thread

'Diesel smoke issue' Diesel smoke issue | Engine
 
I think your 'relay' is fine.

As above Glow plugs are used for emission control when engine is cold and runing, no light indicates when.

I would measure injector tank line leaks and check electric correlation values (all in cold, warm, and hot). To me it sound like you have bad injector which p*ss in the cylinder at idle. That is dangerous for piston. Problem is not rare.
 
Thanks. Injector was my first thought many months ago. I think I have come across that thread previously. So my question becomes why I don’t see symptoms when the engine is hot and left idling. Nor have I seen any increase in oil level.

A secondary point is that I’ve never had to wait for the glow plugs when I turn to start. Even when very cold. That seems unusual from my experience with other diesel cars. So given that the relay seems to fail without codes (one member above reported as such) is it worth changing it anyway?
 
My both OM642 are heating plugs very fast (~1-2sec), especially my W212 does that. Only below -10C it begin take longer. What is your 'very cold'?

Injector tolerances are very tight, that's why cold start problem should be investigated at engine cold. Leak can be VERY different if engine/injectors temperature is 30-50 degree higher or lower. However if injector is not closed properly, should be seen in all conditions.
 
Very cold would have been between -5 and 0C. I am making a direct comparison between my bmw which required 2-3 seconds of glow time before trying to crank and the Mercedes which cranked immediately but spun for 5+ seconds before firing.

Are you suggesting it’s a minor injector drip which is reduced/stopped as the injector gets warm?
 
I can pinpoint the smell/rich running to when the coolant hits 55C and it stops when the coolant hits 75C. This coincides with a slight misfire between 1100-1300 rpm under no load. If I drive through this (the car is moving when it hits this temp window), I get no such symptom.

When the car is hot (85C), it can idle for extended periods (30 mins today) without any diesel smell etc.

Would injectors be that inconsistent?
 
I am making a direct comparison between my bmw which required 2-3 seconds of glow time before trying to crank and the Mercedes which cranked immediately but spun for 5+ seconds before firing.

If your OM642 installation is the same as mine you have to manually pause the key (until the glowplug light goes out) before moving it to the start position. If you turn the key straight to the start position it will spin the engine immediately without any heating from the plugs (e.g. for a hot start).

??
 
Yeah, when it's cold I manually pause for a couple of seconds until the glow plug light goes off. I'm sure most people don't do that, though - I didn't before I knew what they were and before my glow plug relay failed.

What year is your car by the way? Which OM642 do you have?
 

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