worn spark->start problem, why?

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worn spark->start problem, why?

Post by guest » Sun Mar 19, 2006 6:10 am

What is the reason a car will barely start with plugs that are a bit old. Espessially when the battery is not at its best. This was the jugdement of a peron that help starting this car and found the problem. The plug were not changed when they should have.
I would think that if there is a connection inside the isolater it will make a spark and make the engine run not considering the distance of the gap the spark has to jump.
Funny thing is when the engine is up to working temperature it runs fine.
And yes it is being seviced regularly at a "real" sevicepoint :evil: .


Post by jutz » Sun Mar 19, 2006 3:42 pm

I'm not sure what you mean by "isolator", however...

It takes a certain voltage to jump a certain gap under given atmospheric conditions. The ignition system is designed to provide slightly more voltage than the minimum for the worst case atmospheric conditions expected. If the plugs are worn, the gap is wider than it's supposed to be so more voltage is required. Cold air is harder to ionize than warm air so more voltage is required for a cold engine.

The ignition system uses a voltage multiplier to increase the voltage from the electrical system to the spark voltage. If the electrical system voltage is low, the spark system voltage will be low by a similar percentage. During start, all of the electrical system voltage comes from the battery so a low battery means a low spark voltage.

Add all this together and you get an engine that is difficult to start because it's cold, the plugs are worn and the battery is low.


Post by guest » Mon Mar 20, 2006 12:10 pm

Thank for the reply. From this it is very clear what the car is doing and not doing.
As for the "isolator" part i ment the ceramic piece that incapsulates the the rod. I understood that this rod can be damaged with model sparkplugs but then it will not run at all.
Looks like a stealership story.

J Tiers

Post by J Tiers » Tue Mar 21, 2006 12:51 pm

Another issue may be environment, either natural or due to engine conditions.

If the plug wires, connections, distributor or plugs are damp, especially dirty and damp, or distributor cap is cracked etc, there may be some leakage resistance. That can draw off some current from the spark, holding down spark voltage, and not letting the plug get all the spark power.

When the car warms up, the dampness is driven away by engine heat, and it may run fine. Also voltage is higher, giving more power to the spark.

Dampness may be due to rain, high humidity, etc, or there could be a coolant leak, etc.


Post by Preston » Wed Mar 22, 2006 10:55 pm

Engine condition should be determined first with a compression test. Low compression is a common cause of this type of problem and often overlooked by the most experianced Tech.

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Post by larry_g » Sat Aug 19, 2006 12:04 pm

Let me see if I can explain what little I know about the spark ignition system. The ignition coil builds a flux in the primary of the coil while the points are closed. When the points open the flux field colapses and induces a voltage in the secondary winding that is connected to the spark plug. This voltage will continue to increase until a current path drains off the voltage. Ideally this current path is across the gap in the spark plug and a spark occours and ignites the fuel air mixture. Now the failure modes. If the voltage in the secondary can find a path of lower resistance to ground than across the plug gap it will take it. A wet or dirty plug will allow this voltage to sneak across the insulator to ground without causing a spark. A rounded plug face will cause a high resistance to setting up a spark gap. There is a phenonom caused the edge or point effect that allows the energy to concentrate there and jump the gap easier. If the electrodes are all rounded then there is no point effect and the voltage/energy to jump the gap is much higher. The higher the energy to jump the gap, the easier for the secondary voltage to find a lower resistance route to ground. The voltage in the secondary will only build to the level that is necessary to jump the gap. So when you hear all this 40kv, 50kv, 60kv voltages the coil can do it may be true but most older engines with a .025 to .035" gap will run in the 10kv range and do quite well. Another item that plays in here is the condenser. Its job is twofold. It caused a sharp cutoff of the current flow in the primary so you have a fast colapse of the flux and it keeps the induced voltage in the primary from jumping the points gap and causing pitting and burning of the points contacts. As others have said many other things can cause problems but I just wanted to speak to how the spark is generated and what may lead it astray.
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Post by magic9r » Thu Sep 14, 2006 5:48 pm

Anyone missed out that pressure is a major factor?
The higher the compression the higher the voltage required to throw a spark across a given gap.
It's actually easier to throw a spark in an engine with low compression!

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