[nzenigma]

Hi fifthwheel,
My information about blowing fuses is not hypothesis / supposition.
 Fact: A fuse is a protective component in a power supply.
 In this case it is a 12volt supply and the 80A fuse is in the +ve line of the circuit, it protects other components in the circuit from damage due to high current flow.
  A calculated level of current must flow when the components in that circuit are connected to the -ve (ground) side of the supply.
An abnormally high current will result due to a full or partial short circuit to ground. At this point, the fuse will become an open circuit.
The relay is controlled by the ECU which is not in the 80amp circuit. The time on to off is correct. Forget it!
 END.

This is hypothesis.

From your information, you have excess current flowing intermittently in the 80 Amp circuit.

The fault lies in the glow plug circuitry beyond the hot side of the fuse.

When the buzz bar shorted, connecting wires could have also heated and could have destroyed some insulation. Movement can cause the fault to come and go.

I refrain from calling this a short circuit because the fuse is not blowing immediately, just overheating. I suspect a damaged insulator is 'carbon tracking'.

So trace back from the glow plugs to the relay and look for any carbon deposits, burns, soot etc.
If by chance a glow plug is at fault, you can isolate them by disconnecting the bar for a while. However, I favour the former due to the original short.

[Phil №❶]

Are the injectors still connected in the video 

If not, can you check to see if there is any current flowing through the fuse.

When you replace the covers there is no possibility of a short occurring, say under the fuse contacts or anywhere else.

[fifthwheel]

Hi nzenigma,

Thanks for all your info. I agree with everything you've said. I've done lots of tests to try and find where the glow plug circuit is grounding. And so far it appears it's doing so intermittently and always when I'm not testing  . All the evidence points to a ground that raises the current to above 80 amps. For me, this is a good hypothesis until I can actually find the degraded insulation and prove it. But maybe the fuses had 80 amp printed on them but at the first whiff of 5 amp they fry like an egg? Not probable, but possible.

tw2005 mentioned the relay, so my last post was to rule out that the ECM (as you say, controls the relay) is not responsible for not opening the contact if I had the key in the 'on' position for too long, or for that matter, the car starts and the contact remains closed! Let me add that I was/am skeptical that if the contacts remained closed, the fuse would blow/cook in any case. Just thought I should try it before I or the auto-elec start pulling out wiring.

My post should have been clearer about what I was trying to do.

[nzenigma]

In the electronic trade we used to call units with an intermittent problem, a dog. It may sit on a test bench with instruments hanging off it for days, until the fault appears.

Your circuit design is not based on rocket science. Bear in mind, most of the time that you are testing it, the relay is open, therefore no current is drawn (  assuming the relay is not damaged ).

A 12v bulb can be substituted for a fuse and/or open relay contacts. It will light if there is a short..  The glow plugs have such low resistance they are almost at short circuit; so disconnect the buzz bar and No current should flow. Therefore no light. With the circuit live, carefully work back to the fuse. Lift the relay box to see if anything is cooked underneath.

Also your digital meter can give a false reading because it has such high impedance. For this reason, I usually use an analogue meter ( with swinging needle) .