When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Misfire at all RPMs but really rattles the steering wheel around 3000RM.
Took someone's advice and started the engine in total darkness and found the #5 spark plug wire shorting to the radiator hose that goes to the lower DS opening. Spark was visible between the spark plug sheath and the radiator hose. I have never seen this before.
What needs to be replaced? Electricity likes to go to the closest ground. So, I guess there is excessive resistance between the distributor and the spark plug. I'm going to guess I need to replace the wire.
If one spark plug wire is arching & sparking, I'd replace all 8 and the coil wires without hesitation.
My 81 was not visually sparking (even in a dark garage while spritzing water over the engine) but you would get one hell of a shock reaching for the throttle linkage.
If one spark plug wire is arching & sparking, I'd replace all 8 and the coil wires without hesitation.
My 81 was not visually sparking (even in a dark garage while spritzing water over the engine) but you would get one hell of a shock reaching for the throttle linkage.
Ohm'd that wire - zero ohms... I'll ohm them all later today.
So that tells you they have no resistance. What does this have to do with the wires shorting out and arcing to the nearest ground? This means the external housing of the wire has failed, has nothing to do with the resistance of the wire.
Are you removing the plug end from the wire and checking only the wire and also checking the resistance of the plug end?
To be sure we are talking about the correct measurements, and for those playing along at home:
-- A reading of zero Ohms means there is perfect continuity (no resistance) between the points measured. A short solid piece of wire would give you this reading, as would touching your meter probes together. As far as conductivity goes, your wire with zero Ohms is perfect.
-- A reading of infinite Ohms is an indication of no conductivity. A broken wire would give this reading. Your meter reading with nothing connected will give you this reading.
-- The second critical reading on a plug wire relates to the integrity of the insulation. There are testers for this, but they cost a lot more than a new set of wires or ten. The insulation on the wires develops small cracks over time and heat exposure, reducing the dielectric strength of the insulation when moisture and other conductive contaminants start to fill those cracks. The water-mist lightning demonstration is a decent "test" for insulation breakdown so long as there is no other path to ground available. ie: If a wire is already faulting to ground, you'll see no arc or corona flash from that wire with the water mist. Where you already have a miss from another path to ground, including a fouled spark plug, there won't be enough voltage to cause the external arc or corona flash to ground.
-- Remember too that the S4+ wires anyway have resistors in the plug end connector of each wire. Because the wires have different lengths, using a resistance wire would offer different current available and therefore different timing of the sparks among cylinders. Putting the same fixed resistance in the end connectors only solves that issue nicely. You don't share in your initial post what year car you are working on, unfortunately. (See the "New Visitors" sticky at the top for renewed guidance on this...)
An inexpensive oscilloscope can be used to look at relative firing voltages. It can be as easy as one of those add-on kits that plugs into a USB or even the microphone port on a PC or tablet. Regardless, a few wraps of copper wire around the coil wire, with a similar trigger from a plug wire, will net you a sequential display of firing voltages at the individual plugs. Got one peak of the first eight that's noticeably higher or lower than the rest? High generally means a wire break, low generally means a fault to or towards ground. That fault might be a cracked distributor cap, a fouled plug, or insulation breakdown in a wire.
My very expensive at the time 4-channel digital storage 'scope is desk art at this point, except that it can still play the role of a diagnostic tool for coil-and-plug ignition systems. You can often find used but serviceable analog 'scopes in the used market for a tiny fraction of what they originally cost. Or buy the PC add-on module and software if space is more precious than $.
Kevin --
-- A reading of infinite Ohms is an indication of no conductivity. A broken wire would give this reading. Your meter reading with nothing connected will give you this reading.
That is what I am getting. My meter stays at zero when I do the test.
Kevin -- confirm that your meter is set to read resistance in Ohms. Touch the two probes together, and the meter should be 0 Ohms or a very low number (less than 3 Ohms). Do your test. Do you see 0 Ohms, or the same reading you saw when the probes weren't touching anything?
My cheap H-F meter shows a "1" on the left of the display when there is no continuity, which can be a little confusing. The meter will display something when there's nothing connected, and that's the same reading you see when testing your broken wire, right?
Separated the wire from the leads. The spark plug wire tip at the distributor side connector was green with corrosion. Once I cleaned the connector and the spark plug wire connector I got 4000 Ohms from end to end. #s 4 and 8 ohm's @ 4000ish. #1 also showed zero as in no connectivity. It too had corrosion at the distributor end. Cleaned and it ohmd at 4000ish. I am going to check all the distributor ends. The wires without ends were purchased from Roger. The ends came from 928Intl and I may not have looked at them closely before I used them.
08-21-2017, 10:03 PM
Chasing a misfire
