Is this the underlying problem or a symptom of something else?

I have a 1999 Ford Expedition that has a knocking and pinging problem. Two different mechanics have narrowed it down to the ignition cable (spark plug wire), but I have replaced the cable three times now over four years and I need to do it again. The replacement normally works for a few months to a year or so depending on the quality of the cable, but then slowly the knocking comes back and gets worse. It also in the early stages only happens on up hill grades and in higher gears, over time the problem becomes more constant but always above the 2nd gear. The vehicle is an automatic. I also have a very slow oil leak I have not fixed on the engine. I mention them in case they help pin point something.

Does anyone have an idea regarding what the root cause of this might be?

  • I think I'd give it a Seafoam treatment first and see if this doesn't clear things up. Jan 9, 2015 at 12:28
  • 1
    Some questions: What's happening to the cable? Are they burning by chance? Do you have any cb equipment or an outlandish stereo installed? Have the mechanics checked the current going to and coming out of the distributor? Are the correct plugs installed?????(this is a big one). Could you be getting bad gas frequently? Are other fuses out in the car? I would do this one as a last resort but putting a new comp in it may fix it, and a junkyard comp can run pretty cheap. If your cylinder is gooked the seafoam is a good easy option to try first, I would do it a couple times in a 99 suv
    – rhill45
    Jan 11, 2015 at 1:04
  • I will look into the cables to see if they look burned and try double checking the plugs this next weekend too, I iwll also try a foam too. No CB or radio, still factory install. I get gas at random city locations so I would think it would not be such a constant. I will have to ask the mechanic to check the current next ime I am in with him. I will update and let you know what I find from this good list of thoughts. thanks.
    – Brian H
    Jan 12, 2015 at 20:52
  • what octane fuel are you using? Jun 9, 2018 at 8:43

1 Answer 1


When you say "knocking" and "ignition" you send up some red flags. True knocking is death on a motor It'll crack pistons, pound bearings and could cause bearing failure (spun bearings).

I assume you have either the 4.6 or 5.4. The earliest of those had distributers, but at some point they were coil near spark plug and of course now (since about 2003) were coil on spark (no wires).

If you have a distributor, have it timed. There is a little plug that needs to be removed to set the timing (force the distributor to base timing). One of the last cars/trucks to need a timing light.

If you are burning oil. It is possible that the oil affects the octane rating. Oil past the rings or past the guides can lower the octane and induce pre-ignition. Not to mention miss-fires. Check the plugs. (Plugs will also tell you what kind on knock).

Its rare to have a spark plug wire fail if not from heat. The fact that you said it gets better after a change (and slowly goes bad) is a bit baffling.

Maybe a lean mixture (caused by an air leak / vacuum leak) watch your O2 sensor and make sure its working (primary O2 - with a scan tool)

  • Three years to late, but I hope maybe this answer helps someone else here, thanks
    – Brian H
    Jul 29, 2020 at 15:20

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