About 10.5 volts at the Glow Plugs is normal. However, cold temps sap your Battery so if it was really cold you would expect the Voltage at the Plugs to be lower.


Try the same test after the Car has be running and it is nice and hot under the Hood and the Battery fully charged and see if the Voltage goes up.
If not it could be likely it is your Battery.


There is at least 2 Members who have said removing the Glow Plugs and reaming or otherwise cleaning the Carbon out of the Holes improved their starting.


As far as replacing all of the Plugs instead of only replacing the bad ones that seems to be a personal choice.


But, you can see the though going on with that. If one is bad speculation is the others could also be close to the end of their life.


New Plugs with the exception of occasional duds are though to be at the beginning of their life and the hope is it will be several years before any of them have issues.

Here in S CA where it would be a reall cold day if it was 48 degrees F I would have no issues only replacing the bad ones.

If I lived someplace where the temps are minus something (617.952 Engine) I would if one Glow Plug was bad and they all had been in there along time I would seriously consider replacing them all because I would hope to make it through the Winter without any further Glow Plug Issues.

On those Engines with Aluminu Heads that is a different story. In the Winter if a Plug is bad replace only the one Plug and hope it does not Break when you try to get it out.
If someone plans to remove the Glow Plugs on Engine with the Aluminum Heads that is better done when it is warm enough to remove the Cylinder head if you had to.

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84 300D, 82 Volvo 244Gl Diesel