ejenner

This is a good news story, I've already sorted the problem and just putting this up in case anyone else can benefit from the techniques I used successfully to remove 2 stuck glow plugs from my OM648 320CDI.

When I decided to change the glow plugs (glow light always showing on the dashboard) I hoped they would all come out easy but the car itself always fights me so I half-suspected there would be trouble.

Sure enough, first 3 came out lovely, 4rd and 5th both snapped off and 6 was okay. At this point I wasn't aware of how bad this could be. I did it with a warm engine as I thought that would be sensible precaution against the possibility of seized plugs.

What happened in both cases... with original Mercedes / Beru branded plugs was that the exterior housing broke away from the tip and internal electrode leaving both inside the cylinder head.

So we know what engine we are talking about:




This is what you get when the outer body of the glow plug detaches: the dot in the middle is the electrode sticking up.



This is what the broken plug looks like if you can extract it:



First of all I wanted to minimise expenditure and use existing tools as it's an old workhorse... not my pride and joy. The ideal level of expenditure is zero!

I managed to extract plug number 5 by attaching a pair of needle nose vice grip pliers to the end of the plug. Then levering against the bottom end of the pliers jaws where they were gripped against the electrode and the opening on the cylinder head I pulled it out. I used angled needle nose pliers to sharply yank the plug out of the head. If you could grip the end of the electrode with a slide hammer that would also work... but how would you do that? The message here is that yanking it out can be successful and is the first thing you should try. These easily available needle nose vice grips can very effectively clamp to the tip of the plug that's sticking out. The angled needle nose pliers are the perfect shape to get under the jaws of the vice grips and provide even pressure to both sides of the vice grip and force the plug upward out of the hole.

ejenner

Part 2: the more difficult plug!

I tried yanking this out the same way but the tip was lower in the head as I had already tried welding the electrode to the outer casing and broke off the top 1/4 inch of the electrode. That was unsuccessful and to be honest, welding in the top of the engine like that is not a comfortable experience for many reasons.

So I bought a generic glow plug extraction kit from eBay for £39.00.

The glow plugs on the OM648 are deep in the head and this kit really only works for plugs that are much higher up and easier to access.

The only bits which were of any use were the two metal sleeves which have an internal diameter of around 4mm.

The basic principle of extracting a plug using one of these kits is to drill out the long electrode so only the tip is left. Then they provide a stepped drill and a tap to drill into the back of the plug, tap a thread into it, screw a threaded drift into the plug tip and then yank it out with a slide hammer or the extractor device supplied in the kit. I'm not sure what cars this universal kit is designed for but it isn't that much use on the Mercedes OM648.

The useful parts of the kit were the two little metal sleeves. I also used both the 3.5mm drill bits but these are just generic drill bits you can get from anywhere.



You put both the sleeves onto the electrode which is sticking up out of the head and then drill into the hole with a 3.5mm drill.



The sleeves act like a guide to keep the drill bit centred on the tip of the electrode and as you drill deeper the electrode is drilled away and gets shorter. When you have drilled deep enough you can take out one of the sleeves and drill just into the second one. It might be useful to tape the two sleeves together when moving from the top sleeve to the second sleeve.

Lots of metal swarf will be generated when you're drilling all the way down the whole length of the electrode and turning it all into dust. You will have to find a way to get the metal swarf out of the hole as you don't want it dumped into the cylinder and you require clear space around the plug to keep working on it. I used my air compressor to blow the swarf out of the hole and I also used my magnetic pick-up tool to attract bits of metal shavings out of the hole like that.

I snapped both drill bits. Don't use your best ones for this job as the chances of them breaking are quite high.

When I got to the bottom I used a section of the outer casing of the glow plug and put a 5mm drill into that to remove the final little bit of electrode.

Essentially what you're trying to do is remove all of the glow plug electrode so only the tip of the plug is left.

At this point I assumed I was going to be using the drill and tap which was supplied in the extraction kit. But none of that stuff was of any use due to the depth of the plugs on this engine. All the stuff in the kit was too short.

I couldn't have done this without the kit as the sleeves were vital to be able to drill out the electrode. But you could get sleeves cheaper from elsewhere if you knew in advance.

At this point I was feeling some despair as I had pinned my hopes on using this kit to 'simply extract the plug'

After going on YouTube I saw some people on other engines were using the technique of punching the plug tip all the way through into the combustion chamber. When the tip is in the combustion chamber you can pull it out through the injector hole.

For this I was going to require a new magnetic pick-up tool with a small tip on it. The tip of the injector is around 7mm so anything which goes into the top of the combustion chamber would have to be quite small.

This magnetic pick-up tool has a diameter of 6mm and it fits into the combustion chamber. £7.99 off eBay.



IMPORTANT: I had to take off the radiator fan so I could access the crankshaft pulley and turn the engine a few degrees until this piston was at bottom dead centre. I imagine in a worst case scenario the piston could hit the tip of the glow plug... I'm not sure if that is possible but I didn't want to test it so I put the piston all the way to the bottom of the cylinder. I used my new magnetic pick-up tool inserted into the injector hole to watch when the piston was at the bottom.

I punched the remains of the plug tip through into the combustion chamber quite easily with a long punch tool.

NOW... this is where it got even worse. On the first couple of attempts at fishing the tip out of the combustion chamber I heard a 'clink' as the tip caught on the inside of the injector hole and fell off back into the combustion chamber. After this I tried to fish it out around 20 times on two different days but never heard the same clink. I tried to bend the magnetic pickup tool to get to the side of the combustion chamber as I guessed the tip was hiding at the edge of the cylinder. I still couldn't get it out.

I was getting desperate so I decided to try more random purchasing of tools off eBay. The alternative was to take off the cylinder head. If you go down that rabbit hole you're getting into the hundreds to pay for gaskets, head bolts, manifold studs and reconditioning work for cleaning and flattening the head.

I bought a bore scope with a small tip. The commonly available ones in stock at DIY shops were 8mm, my father-in-law has one and I tried that first of all... didn't fit and not bright enough to illuminate through the injector hole.

This one is brilliant value for money, very good quality for £60




The camera tip is 5.5mm




Upon inspection I could see (like a stricken submarine) the tip of the glow plug sitting off to the side of the combustion chamber.




I used a long copper rod (a piece of power line copper, I reckon you could use thick welding wire just as effectively) which I put a very slight twist in to reach into the combustion chamber through the glow plug hole. At the same time I had the camera stuck through the injector hole and could see the copper rod moving in the combustion chamber. I used the copper rod to knock the glow plug tip into the centre of the piston.

With the glow plug tip central in the combustion chamber I was then able to put the magnetic pick-up tool into the injector hole, feel a little click as it connected with the glow plug tip and then really carefully lift the plug tip out through the injector hole.




Needless to say... this took far longer than I had hoped as each time I got stuck I had to order more tools. It also cost more than I had hoped. But now I have some extra tools and it wasn't so expensive that removing the head would be cheaper.

Not a bad result overall and about as far from an ideal situation as it gets.

ptkacik

Wow! What a story. My heart sunk as you described the steps.

A buddy was trying to clean up a valve by spinning it when it dropped into the cylinder. My heart sunk as he described hours of working through the spark plug hole with a magnet trying to feed it backwards up through the valve guide. Like you, he was ultimately successful.

BTW, how many miles did your glow plugs have on them? I’m wondering if there is a safe time to remove them before they get too old a frail.

Peter

ejenner

I didn't fit the previous set of plugs so hard to say. They were MB branded though... so probably been on there since it was last dealer serviced... could've been 10 years or more.

When I was doing my research one guy suggested removing them annually just to guard against the possibility of them seizing.

shertex

Interesting write-up. On one of mine, #2 and #3 are original at 170k miles and 17 years! Haven't had one break yet on either of my CDI's, but I wonder what will happen with those two.

ejenner

People talk about doing this on a hot engine to take advantage of the thermal expansion situation.

I wonder though whether or not that is exactly the wrong advice and maybe it is best to do it when it's below freezing on a cold engine instead?

It's not sticky threads which causes the problem, it's the tip that gets stuck.