Tech article #1 - Swollen, but not broken glow plugs, yet!

So if you work on diesels long enough you will have to deal with frozen, swollen, or broken glow plugs.  You will find write ups on how to break free frozen glow plugs and special tools to go along with them.  You will also find lots of write ups on how to remove broken glow plugs and again the tools needed to accomplish that.  What you don't see is what to do when they are simply swollen but not 'YET' broken.  

I see this swollen glow plug scenario far more often then the other two.  The problem is that if you are not careful you will end up with broken glow plugs and that is a real pain.  To start things off, we treat this job like we are handling a delicate part, because we are.  The ends of glow plugs are extremely hard and because of this are also brittle.  If a glow plug hits the floor we discard it.  It is just not worth it being broken off the next time it is attempted to be removed. 

When it comes time to install or remove the glow plugs we use a rather special socket.  A standard short socket is not long enough to clear the stud on the end of the glow plug, a deep socket is often to long to fit in the limited space available.  Wrenches are usually not wide enough to get full engagement with the socket end of the glow plug.  What we use is a medium depth socket.  Yes they make them, but they are not common in the cheaper tool places like Harbor Freight or Northern Tool for example.  Ours is Snap-On but Matco and others make them too. You want full engagement so you don't strip the hex even slightly since you might really need all of it to get out a stubborn glow plug the next time you work on it.

So what do you do if you break free the glow plug with your socket and it unthreads nicely the rest the way by hand but then wont slide out?  Well this is what a swollen glow plug often does.  The end has started to fail and has swollen.  Sometimes the entire end is cracked and split open, others it is one small section.  You just don't know at this point other than it is not coming out.  The common next step I see is the brute force technique.  This might work, but having inherited the aftermath of this technique in broken glow plugs I can tell you that many likely would not have broken if they had used this simple technique.  

We have a few angle drills in the shop.  The smallest is pictured below, which is a low power battery unit that is compact and easy to fit around the engine.  We have larger units as well as 120V A/C powered and air versions as well.  The key here is you are going to need to grab onto the plug with the chuck.  I can tell how swollen a glow plug is by how much it moves in the bore.  This one pictured was stuck but not very swollen.  Had I forced this though the brittle end could have snapped off and caused a major amount of work.  Instead I grabbed the smallest angle drill in the tool boxes.  This one is so small that the chuck cant even grab the hex section of the glow plug.  No problem I just grabbed the end.  Now if you have a very tight glow plug then I dont suggest using a small drill, get a big enough chuck to get around the hex.  But really what we are doing here is not forcing the plug out but clearancing it in the bore.  By operating the drill and putting light outward pressure on the plug it often times will end up sliding out.  Again we are not putting high tourqe on the plug but just letting the rotation clearance the bore slowly.  This plug pictured came out in 20-30 seconds of turning.  

Now if you are binding in the bore, ease up and possibly re-tighten the threads down and then back out again.  Use some PB-Blaster or other lube as well.  You will be surprised how many glow plugs I have removed this way that otherwise would have become broken off.  

Hope this helps save a few headaches!  Happy HUMVEEing!

Swollen glow plug, bubble type.Drill grabbed onto the end of the glow plug.Smallest angle drill did the job this time!


  • Awesome advise, a very simple procedure but would not have been used by many if not shared on this website.

    Michael Tebaldi
  • Great suggestion. I’m getting ready to do a glow plug swap on my 6.5L soon and I’m sure this senario will probably surface. Now I have a good plan of attack.

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    Dary Khajavi
  • I had a old tech show me a trick years ago. You get a swollen glow plug that won’t come out. Pull the injector on the same hole. Use a zip tie and secure it on the loosened glow plug inside the cylinder. Use a hammer and knock off the outside part of the glow plug. Pull the swollen part out the injector hole. Shocked me, but it worked.

    David Haralson
  • Article doesn’t show up beyond 1st sentence on my Galaxy 7

    Mario Przepiora

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