W202 220 CDI glow plug change

[David Pemberton]

Part 1 of 2

Tools, parts & consumables needed:

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Multi-meter (not shown)
Torx 30 bit and driver
Pliers
Torque wrench
10 mm deep socket
Extension bar for socket
4 x glow plugs (Mercedes part A001 159 49 01)
Penetrant spray (3-in1 Professional from Halfords)
Elephant bog roll

Time to do the job: 1 hour (I spread this over a week, see below as to why this makes the job easier)

Expertise: moderate (about the same as changing spark plugs or changing engine oil)

Car on which plugs changed: C220 CDI, 2000 W-plate / 202 193

[NB the use of left and right relate the car as viewed from the driver’s seat looking forward.]

The first symptom that something is wrong with your glow plugs may be that the glow plug warning light comes back on immediately after the engine has started, and remains on for up to three minutes before going out for the rest of the journey. The reason that the light comes back on (after going out to indicate that pre-heat has finished) is that the glow plugs continue to work for up to three minutes after the engine has started to improve cold running. The system detects that there is a problem during this after-heat phase and lights the warning lamp.

The diagnosis method is to test the electrical attributes of the glow plug system (it is an electrical problem that has tripped the warning light) and to do this you’ll need a multi-meter that can measure resistance (ohms).

Open the bonnet to its fully upright service position by releasing the hinge locks (left side first) and raising it to the vertical, position whereupon the hinge locks will re-catch to hold it securely (to return it to its normal position release the left side first, the right side doesn’t need releasing on the way down). Remove the engine cover by unscrewing four Torx screws using a Torx 30 bit. You may also need to remove the oil filler cap to make it easier to slide the cover off. Remember to replace the oil filler cap immediately so that debris can’t enter the engine.

The glow plugs are located on the upper left side of the engine (see picture).

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Each plug has a push-on black electrical connector cap.

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Remove the cap by gripping it with pliers and gently but firmly pulling upwards. Test each plug with the multi-meter set to measure resistance. Touch one of the multi-meter probes to the engine (for a good earth) and the other to the silver tip of the glow plug. The three working glow plugs on my car gave a reading of 0.6 ohms, the defective glow plug read zero. If all the plugs have a similar reading (and I’ve seen elsewhere the acceptable range being 0.5 to 1.5 ohms) then the plugs are probably OK and this DIY isn’t for you. You could continue your investigation by checking the operating voltage for each plug of 11.5v, and, if your meter can manage it, the current draw of ~15 amps. That way you can see if there is a break in the individual feed to a plug and perhaps trace the problem back to the glow plug control unit.

Continued in Part 2

 

[David Pemberton]

220 CDI glow plug change Part 2

Part 2 of 2

But this article is about changing glow plugs, so assuming you have diagnosed a faulty plug, here’s what to do next.

I read a lot of posts on this forum that made me very wary of taking the plugs out myself: tales of sheared and broken plugs, reaming out threads, and blowing debris out of cylinders by running the engine didn’t make me feel comfortable. But I did read a very good post about taking plenty of time to let penetrating oil do its work. So I decided that this would be the route for me, and I allowed myself a week from diagnosing the problem to fixing it, during which time I applied 3-in-1 professional penetrant spray to soak in overnight every night. Once you’ve taken the engine cover off for the diagnosis, you can leave it off for the rest of the week and still use the car; applying the spray just takes a minute each evening.

After a week of applying penetrating oil I gave a final spray, and drove the car briefly to get the engine up to a cool operating temperature (~60C). To remove the glow plugs, unclip the cap that supplies the power to the plug (as described in the diagnosis section above), and use a 10 mm deep-reach socket fixed to an extension and torque wrench. I set my torque wrench to 20 Nm which is what the Haynes manual says is the tightening torque (albeit for the glow plugs in the older 2.2 and 2.5 diesel engines). My logic being that I’d be very unlucky if they sheared at this low setting, and if I couldn’t get them to unscrew I could review my options. After a week of penetrant spray each glow plug came out very easily, the torque needed didn’t seem near to 20 Nm. The plugs needed the wrench all the way and never got to the point where I could spin them out with my fingers. Once the threads were clear I pulled them out using pliers.

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This is what the plugs look like when removed. The new one is at the top of the picture - and no, I couldn't see much difference either...

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I changed all four plugs because I reasoned that at 80,000 miles if one had failed the others would be near the end of their life too. I used Mercedes original parts because after half-an-hour search online for glow plugs I wasn’t much the wiser as to which Bosch plugs were right and who would sell them to me, e.g. the Eurocarparts website simply doesn’t list anything for my last-of-the-202s but CDI powered car. The Mercedes items are made by Beru and cost £13.77 + VAT each. My franchised dealer is friendly and helpful, recognises that I’m never going to pay his service rates on a car worth about only £6k, and probably hopes (rightly) that one day I might buy a second hand car from him. So I buy most of my parts from him and enjoy a look around his gleaming showrooms at the same time – how the other half live!

The glow plugs screw back in to 20 Nm torque (again they need to be gently driven all the way, always needing more torque than my fingers would provide), then re-fit the push on caps that supply the power. A quick check round the engine bay to make sure no tools have been left lying about, then start the car as normal. On the first start the glow plug light came back on again after the engine had fired, so I cursed a bit and doubted my diagnosis. On subsequent starts the light has behaved normally, i.e. going out to indicate the end of pre-heat, and staying out when the engine has fired. I can only assume that during first run of the engine after the plug change, the glow plug circuitry was still running a fault mode that didn’t clear until one complete correctly functioning start & run cycle had been completed.

After a test run the only thing left to do is replace the engine cover. Before you do this it’s worth having a wipe round with rag, especially by the oil filler if past spills have occurred. The oil filler cap needs to come off so that the cover can be slid back into place, make sure that you mate the engine cover and the silver manifold cover by sliding the lugs together (see picture). A final check round for discarded tools and the job is done.

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[Blobcat]

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