The day I bought the car was about 30ºC and it started beautifully. But the car would have a hard time starting if the outside temperature was below about 15ºC.

I knew from my 617.95 service manual, that my car had the later glowplug system where the glowplugs are wired in parallel with each other, this means that if one glowplug fails that you do not lose all of them. The older system did wire them in series and one failure would lead to the glowplug system being rendered useless.

From the service manual I knew, that I could measure the resistances from the plug going to the glowplug relay and the each numbered terminal corresponded to the cyclinder that one would expect and that my glowplugs should have <1Ω of resistance at 20ºC. So I pulled out my multimeter and was able to check the resistance of each of the 5 plugs.

As you can see from the results I only had a single plug that was in tolerance. I bought 5 new Bosch plugs and swapped them out with relative ease, the weirdest part about the removal was that the threads would come out but the plug tip would be trapped by the carbon build up in the pre-chamber.

So I used a drill bit and a reamer to clean out the carbon deposits from the pre-chambers. In order to aid break up the deposits I sprayed in some PB Blaster (IIRC). This is an important step because years carbon build up will cause the glowplug to first become insulated from the chamber and second fail prematurely because it gets too hot from being insulated.

I had thought that the no. 5 glowplug would be the least accessible but it transpired that no 2. held that title. I wasn’t bothered removing the fuel hardlines so I decided to leave no. 2 for another time (probably when I rebuild my leaky injectors). Four out of five glowplugs means no problems starting.