Problems with my first Truck

To test them,unplug all the wires from all 8 of them first...
Then,take a 12V test lamp,hook the lead to the positive battery terminal,then touch the probe to the glow plug's terminal--if it is good,the lamp will light...if not,its dead,and will need to be replaced..

A multimeter can be used too,to measure the resistance of a glow plug...but I've never used that method...some claim a glow plug can be "dead",yet still light a test lamp up...I haven't had that happen though...

Removing them can be very difficult--not only can the small 3/8" hex on them rust away and make getting a socket to fit well enough so as not to strip them is a pain,I've had to use vise grips or "gator" sockets to unscrew them--they only have 10mm threads too,so go easy,or them may snap off in the head!...some of the original glow plugs Gm used would swell up and make removal difficult or impossible--usually they will unscrew OK,but refuse to come out of the small hole in the head and let the tip come out--using a lot of penetrant and working them back and forth sometimes frees them up enough to get them out--in other cases,you'll need a special puller,or end up removing the fuel injector in that cylinder to retrive the broken tips if it does break off...(a rather involved job that requires intake manifold and fuel line removal!).

..some guys have started the engine up with the glow plug unscrewed and let compression blow them out,but I have been hesitant to try that method!...(I have one glow plug that unscrews,but refuses to come out on my 6.2 diesel,I just let it be,rather than cause myself a lot of greif--it has always started fine with only 7 good ones in it so far!)...

I switched my glow plugs to some AC # 60G ones,and wired my glow plugs up to a manual push button--these plugs can be left on without fear of them overheating and failing or coming apart in the engine,they usually must be energized for about 10 seconds to start easily,and most stock glow plug controllers wont stay on that long without some modification to get them hot enough...but they wont swell up and cause troubles like the stock 9G ones will...

I find often the stock wiring harness to the glow plugs and the fusible links in them are corroded badly,and dont let enough current get to the glow plugs,making you think they are "bad" when in fact they are just not getting enough voltage to heat up good enough...I've had a few glow plug relays get corroded internally that did the same thing too..

The loader we had at the junkyard had a 4 cyl Detroit in it--I am not sure if it used glow plugs,all I know is that thing would start within a crank or two,no matter how cold it was,and we often used our cars to jump its dead battery!..I was told it had some kind of magneto setup that fired it off,but I never really looked at the engine ,all I ever did to it was check the oil and sometimes had to jump the starter to get it to crank over--..

I wish I could screw 8 10MM spark plugs in my 6.2 and get rid of the glow plugs!!--they are an expensive pain in the butt to replace and dont seem to last long,as I use my truck only once in a while and in short trip use..I've have brand new AC 60G ones rot away so bad in 5 years they looked worse than the original 1982 ones I replaced,I had to use vise grips,smashed on metric sockets to get the darn things out!..all the connectors were litteraly fused to the glow plugs too,with rust...