Funny sparks in a 740

Someone I know has a volvo 740 turbo (LH 2.4) with an
interesting problem: two of his spark plugs give out weak sparks and two
give nice and bright ones. He did check that by connecting one spark
plug to each wire and seeing the spark. He has swapped distributor cap,
rotor, and wires, and tested the spark at the coil (nice and uniform).
What else should he check?

 

Sparkplugs. Thy have internal resistor the frequently goes from few Kilo
ohms to much higher value. Very high value of internal resistance will limit
the current. If you have a bucket full of old sparkplugs check them with
ohmmeter between the tip and the cable end. Some will read open circuit and
still fire (sort of)



Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

void _-void-_ in the obvious place

 

But why if he connected the same spark plug to the 4 different spark
plug wires he would get two wires producing bright sparks and two
producing wimpy ones?

 

Is the distributor rotor arm bent or loose? Check inside the cap for
damage after the bent arm has been going round. The 2 good sparks
contact points maybe bent away from the centre increasing the distance
from the bad ones.

Never heard of it before, but its the only thing I can think of if
everything else is ok.

Could be something to do with Engine management, crank sensor maybe, are
you sure a good engine does not do this?

Is there a more direct symptom? Is the engine actually running badly?

 

Also is the cap seated properly? ie nothing trapped between the cap and
the distributor base.

Does the cap wobble when pushed on?

 

It also occurred to me it might be a damaged interrupter plate or
rotting engine wiring harness depending on the year.

 

Good point. It is not clear the he actually moved same plug from wire to
wire.