last week Glow plug oddity...
 

I got his car in, and tested the GP's with ohms... all but one read between 1.8 and 2 ohms... so I pulled them, to change them out. I replaced them with bosch and then took the plugs to some jumper cables, and they all heated up at the tip. orange, slowly, but they did all heat up at the tip...
the new plugs were all around .6 ohms...
why did the "bad" plugs heat up?
I was expecting them to heat wide, or not at all, but to see them heat up and at the tip surprised me.
they were slow to heat, around 6 seconds before glowing, and they never got yellow... only orange.
Hmmm
more fodder for the testing glow plug methods...

oh, they were all bocsh also... modern style tip.

When / where were these measurements made? Fitted to the engine or not?

Because if they were measuring bad when fitted the reason why they heat when they're out could be a problem with the insulation in the glow plug couldn't it?

ohm readings identical in the motor, measured from the harness socket, and from the GP itself, also the readings did not change when I had them in my hand...

The word "bad" is in quotes because you'd need to define "bad".

If you expect the plug to heat to a tip temperature that is nearly white, the plugs would be characterized as "bad". Roy would toss these plugs out because they don't generate the necessary tip temperature necessary to start at -40C.

Prior to testing them, the increased resistance told you that they wouldn't perform like a new plug and your testing confirmed it. Increased internal resistance translates into lower current and that translates to a lower tip temperature.

Said plugs would be fine for most folks down to an estimated 10F. or so, depending on engine condition, of course.

I've seen the same thing. Tested within acceptable parameters but did not glow very hot. I should have taken a picture of the old one and new one glowing. Big difference.

May be I'm mis-understanding this but aren't you proving that a human being is a better (thermal and electrical) insulator than a head fitted to an engine block?

Assuming you got them to glow out of the engine whilst being held in pliers...

Here is a pic of a bad one I tested. (This Monark Glow Plug was only in the Engine for about 1 Year; and that is in S CA Weather.)

In the drawing if the Glow Plug gets hottest in the center the closest thing to get hot is the Cylinder Head.

So the Ohm test seems to catch most of the bad Glow Plugs but is not the definitive test for Glow Plugs.

Best overall test with glow plug in engine but not totally conclusive either is the current draw test of a plug. You make a simple non calibrated meter shunt out of a piece of 12-14 gauge electrical wire. In effect you are putting another resistance in parellel with your meters circuit. Or dividing the current flow. Yet the meter can accuratly read the flow with a conversion factor. A factor or ratio of 10 to one or five to one is what I would shoot for. At ten to one each amp indicated on your meter becomes 10 amps.

This enables your meter to read much higher amperage on the maximum ten amp scale of the meter. You can calibrate it or not and just use it as a comparison refferance if not calabrited. Low draw on every plug would indicate the primary circuit feeding them is not up to scratch. Or all the plugs are getting tired.

Generally though if they are pulling the amps individually might be a much better test than just a resistance reading that may change faster or in the wrong way once power is applied to a glow plug.

Cheap to make as the shunt is just a piece of bare wire basically. And anyone interested can look up on how to make and calibrate a current meter shunt on the web.

This is a useful thing when examining any circuit that is over ten amps but under the new top end amp reading ability. It is basically something that you can do for free when you make a meter current shunt.

Did the engine exhibit actual symptoms related to starting?