Spark Plugs - A very odd problem

Below is a very odd problem that prevented engine starting as experienced by a customer with our system fitted to their Ford Escort BDA



When completed they drove the car for around 200 miles without issue.
After being parked over night it simply would not start the next morning.

Many days and weeks of fault finding and testing ensued without finding the root cause of the non starting
( it had sparks, fuel etc. etc., mechanically was fine without any issues )


Testing showed that when the spark plugs were removed and laid on the engine head ignition sparks were present when cranking.
However when the spark plugs were fitted there were no sparks present. ( measured with timing light flash and spark indicators )

Eventually it was found that whenever the spark plugs were fitted and torqued to correct values there were no sparks, remove plugs and tighten just finger tight and there were sparks.  ( seems when torqued to correct level there was some kind of external electrical short )

Spark plugs replaced with a known good set and the car started instantly and has been good ever since.


We can only assume they were a faulty batch of spark plugs that somehow made it through the production process or there are some very good looking but defective copy parts floating around in the market place.

If you have odd unexplained starting issues it maybe worth a change of plugs ?