[mannasugar]

1981 BMW 320i, 3-series, 4 cyl, fuel infection,
car would not start, pulled plugs & gap was closed on spark plugs, re-gapped plugs car started right up, 4 weeks later car does not start any more, hard starting, takes 25 minutes to get car started, finally pulled plugs again, spark plug gaps are closed again, re-gapped plugs, car started right up...

What is closing the gap?

The plugs are black & wet,

There are no marks on the tip of the spark plugs ( as if the top of the piston were to be hitting the plug and closing them )

What could possibly be closing the gap?

[Jmark]

My first thought is there is a big buildup of carbon on the piston tops hitting the electrode? Since its "wet and black", I'd take a good look at the whole ignition system and ck for oil/gas fouling too.

Mark

[carsavvycook]

The factory plug calls for a standard tip, not the extended, or power tip spark plugs.

You may have too much carbon built up on the top of the pistons.

The timing belt may be just a little loose, or worn out.

JMO

[mannasugar]

Wouldn't there be some visibility of a mark on the electrode if the gap is being closed by carbon build up on the top of the pistons?

Would excessive heat cause them to close somehow? Not that the car is running hot or anything.

[carsavvycook]

The only thing that will close the gap, is close tolerance with the piston at TDC........ There should be a visible mark on the spark plug, or the piston. Look with a magnifying glass. You should see the markings.

I have never seen compression of oil, and or, coolant, or fuel close the gap on a spark plug.

[bobjob]

sounds like your hitting carbon!!it's just soft enough to stick on end of plug!! and it WILL be hard to see..
try a shorter plug,, if it's carbon,, something needs to be done about it!! may be your valve guides are worn,, and you are burning oil??
carb too rich??
ever change oil??
try resetting plugs,, and run some carb cleaner...
might burn off carbon..

[4 Jaw Chuck]

Detonation will close the spark plug gap.