Lets Play Stump the Mechanic - Ignition

This problem has stumped all of us on the dock so I'm reaching out. My 1985 Volvo-Penta 225D (5.0 liter Chevy 305 CI) is having an ignition issue. The engine will not start, fire or run with the coil wire placed properly into the distributor cap. However, when the coil wire is held close to the distributor cap as when checking for spark, the resulting arc will cause the engine to start and even run. This arc occurs while holding the coil wire approximately 1/4 inch above the coil wire opening on top of the distributor cap.

Again, while arcing into the cap the engine will run. Putting the coil wire back into the cap will cause the engine to die and no longer fire. I have replaced the coil wire, coil and HEI retro fit for Prestolite distributor in an attempt to remedy this issue. The problem preceded the replacement parts.

Has anyone have this happen and how in the heck did you fix it. We're baffled. Thanks!!

Re: Lets Play Stump the Mechanic - Ignition

First guess is a rich fuel mix condition. Holding the coil wire just off the cap increases the gap the spark has to jump, and electronic ignitions provide more voltage as the dwell time builds until spark happens. When that extra spark gap is finally arced the resulting spark at the plugs is way hotter.

When it does start and run does the exhaust spit out black smoke for a bit? Also have you tried holding the throttle wide open while cranking to clear any flooded condition? What do the spark plugs look like after this?

Next guess is not the right spark plug gap, or wrong type of wires, but in addition to the too rich mix.

I had a car act like this, the fuel mix was way too rich due to early 1980's faulty anti-smog design and failing components. Holding the coil wire just off the cap alllowed it to work out the flooded condition. That was what the dealer did to get me out the door, they had to fix it all later.

Re: Lets Play Stump the Mechanic - Ignition

Maclin said:

First guess is a rich fuel mix condition. Holding the coil wire just off the cap increases the gap the spark has to jump, and electronic ignitions provide more voltage as the dwell time builds until spark happens. When that extra spark gap is finally arced the resulting spark at the plugs is way hotter.

When it does start and run does the exhaust spit out black smoke for a bit? Also have you tried holding the throttle wide open while cranking to clear any flooded condition? What do the spark plugs look like after this?

Next guess is not the right spark plug gap, or wrong type of wires, but in addition to the too rich mix.

I had a car act like this, the fuel mix was way too rich due to early 1980's faulty anti-smog design and failing components. Holding the coil wire just off the cap alllowed it to work out the flooded condition. That was what the dealer did to get me out the door, they had to fix it all later.

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That doesn't make any sense. No way the spark gets hotter at the plug because it jumped another gap. Jumping that first gap loses energy, diminishing what's available by the time it gets to the plug. If it were the case that that simple feature made it hotter, the OEM's would have done it as a cheap way of getting by with lesser components, and the aftermarket would have responded with wires that had a gap in them somewhere, because after all a more energetic spark is a good thing. As to the OP, did you get the wiring right? I'd start with checking the wiring and timing. I'd also check the cap for a crack at the coil wire tower.

Re: Lets Play Stump the Mechanic - Ignition

Look up booster gap plugs. The voltage builds until the gap is arced.

Re: Lets Play Stump the Mechanic - Ignition

Ok, I'll buy that it allows the spark to happen if the plug is fouled a bit thus shorting out the voltage path, but it's not making it more energetic or hotter.

Re: Lets Play Stump the Mechanic - Ignition

The gap is like a dam holding back the river storing more and more energy, when the dam breaks all heck breaks loose downstream.

To the OP, check the sparkplugs to see if they look like the mix is rich. Also check the dipstick to see if the oil has a gasoline smell.