Can someone explain to me, with some level of certainty, what can cause an intermittent spark? This "intermittent spark" is seen using an inductive timing light on a plug wire at idle.

Copy/pasted from some website:
"Fuel, compression and ignition can all cause what sounds like an engine misfire. To determine whether the misfire is ignition related, connect a timing light to the ignition system and view the light while the engine is running. It the light is consistent while the engine is misfiring, the problem is not spark related. If the light is intermittent while the engine is misfiring, [it is spark related]."

Could an intermittent spark, detected in the plug wire, be caused by:
1. Lack of fuel?
2. Low compression?
3. Wide plug gap?

My gut feeling is 'no' to all of the above. In my mind, shouldn't a spark be detected in the plug wire regardless of any of the above conditions?

 

Anyone? I'm trying to rule out possible causes. Throwing CEL 71-74, checked plug wires with timing light; the light is NOT consistent. Need to understand what would cause an intermittent spark...

 

Any drivability problem can be cause by either compression, fuel, or spark. This statement is true. now to anwser your question,"what would cause an intermittent spark..." An intermittent spark can be cause by a shorting out coil or ignitor, a ECM not controling the the ignitor intermittently, a weak or shorting out crank position sensor or cam position sensor,or a lose connection and grounds. But is some cases, it could be just a faulty ECM.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by civikdude &raquo;</TD></TR><TR><TD CLASS="quote">... checked plug wires with timing light; the light is NOT consistent.</TD></TR></TABLE>Since the timing light skips, it can't be fuel or compression. If it was, the charge may not ignite, but the SPARK would still be happenning.

If the ECU wasn't getting a good signal from the CYP, TDC, or CKP sensors in the distributor, it would throw codes for those sensors, right? So you've got ECM, ignitor, coil, wire, or sparkplug... or their wiring & ground connections.

Having a too-wide plug gap is a bit weird... Usually that's a problem at higher charge pressure. Like WOT or especially boost. You're watching the light at idle; I suppose even a wide gap would still spark. But it's easy to pull your plugs & look!

 

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If the ECU wasn't getting a good signal from the CYP, TDC, or CKP sensors in the distributor, it would throw codes for those sensors, right? So you've got ECM, ignitor, coil, wire, or sparkplug... or their wiring & ground connections.

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If the signal were weak from the cyp or ckp, it's not alway true that the ECM will detect a fault in the system. I have seen many weak cyp and ckp that does not set a DTC even with the obd2 system. However, if it was a CAN(communication area network) system it would more likely to set a code.

 

Thanks guys. Here's the deal: I already replaced wires (2x), cap (2x), rotor (2x), coil, ignitor, plugs. Checked grounds. Checked CKP, TDC and CYP sensor resistance according to Helms, reads at 370 ohms which falls within the specs of 350-700 ohms.

I can feel the hesitation when driving. I can see the misfire using a timing light. The PCM can detect it and throws P0300-304 (71-74). And those are the only codes it sets.

If I remove the spark plug wire from the plug and lay the end of the boot near a head bolt, the spark jumps across as it should and the timing light is steady! If I then set the wire away from a ground, the timing light skips just like it does if the plug wire were on the plug in the cylinder. What does all this mean? Are the spark plugs not grounded in the block? Is the battery or alternator at fault? (...really stretching here.) Do I blame the CKP/TDC/CYP sensors anyway? I just don't understand why the timing light skips when I seat the plug wire on the plug.

 

I am having this same issue as stated

"If I remove the spark plug wire from the plug and lay the end of the boot near a head bolt, the spark jumps across as it should and the timing light is steady! If I then set the wire away from a ground, the timing light skips just like it does if the plug wire were on the plug in the cylinder. What does all this mean? Are the spark plugs not grounded in the block? Is the battery or alternator at fault? (...really stretching here.) Do I blame the CKP/TDC/CYP sensors anyway? I just don't understand why the timing light skips when I seat the plug wire on the plug."


Please Help ASAP

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by civikdude &raquo;</TD></TR><TR><TD CLASS="quote">... Are the spark plugs not grounded in the block?</TD></TR></TABLE>This one's been bothering me for awhile. How can the plugs NOT be grounded into the head?? Did you figure anything out? I would have blamed bad spark plugs, but you replaced them. Do you have a good ground wire from the head to the chassis?

If the battery or alternator were at fault, you should be able to check by measuring battery voltage with different combinations of engine & loads turned on & off.