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I have a 2001 V70 T5 and it has suddenly developed an intermittent, extremely rough idle. When cold, the car always starts and runs fine, but after about 5 minutes of driving the car will hesitate for a second (usually at around 40 MPH, but occasionally at lower speed, never above 40 MPH) and then take off again smoothly. While moving, there are no further issues, but any time the car stops at idle it has an extremely rough idle like a cylinder isn't firing. The bahavior is the same in Drive, Neutral, or Park. Despite the rough idle, the car accelerates smoothly from a stop with no hesitation and plenty of power. Gas mileage hasn't dropped and we have only gotten a check engine light and "Engine System Service Required" message once which cleared the next time the car was started. Occasionally the problem will clear up on its own after driving longer. The dealer cleaned the throttle body for the known issue there but it had no effect. Would a bad ignition coil exhibit this behavior? If so, how can I verify a bad coil? If not, what else should I be looking at? The spark plugs have about 30k on them so I'm going to change them tomorrow, but I would think a bad plug would show up at other times than just at idle.
 

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Re: Ignition Coil Problem? (dhollow2)

Most likely culprit is the MAF. If the MAF does not throw a code yet, there might be long term fuel trim code which often seems to indicate a MAF problem (or air leak).
 

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I had a similar problem except no real rough idle when idling warm -- just skipping/missing on acceleration to highway speeds.

First noticed it when I took the car ('07 XC70) in for maintenance (my wife does not let me drive it much otherwise, it's her daily). Told the mechanic, who replaced the plugs --- and after a couple weeks the problem came back, albeit not as bad.

Took it in to the shop for a growling wheel bearing, and mentioned the problem. Mechanic test drove it, duplicated the problem but could not find it -- supposedly the plugs and coils checked out fine. I told the mechanic I'd have a look when I could free up some time. Which was a few weeks later.

So I had a look-see.

Ha ha ha.

I am pretty sure I found the problems.

One problem was, except for testing with a meter I have no way of checking coils, and Volvo does not publish specs. So although I was told that the coils all checked good, I replaced them with Delphi coils that I marked with the replacement date. As I replaced each coil, I cleaned the ground contact on the head with a cosmetic swab wet with MAF cleaner (cosmetic swabs are a little tougher than Qtips. I try not to use too many too often so my darling spouse does not bust me for whipping them...) I also cleaned the coil connectors with CRC QD on a swab, then wiped the orange boots with a small amount of sylglyde (tune up grease) so they would not bind. Before doing coil replacement, I inspected and checked wiring with a meter. As I mentioned already, I did not check the coils with a meter but I may get a junkyard coil and bust it apart to figure out how to electrically check it.

Anyway... what I found:

Plug and coil 1 (at timing belt end) seemed OK. I restored part of the wire loom that had embrittlement. No conductivity issues.

Plug and coil 2 had issues. I found that there was a white dust somewhat insulating the spring that contacts the plug, inside the boot. I used MAF sensor cleaner on a shop rag to clean off the plug. No conductivity issues.

Plug and coil 3 had issues. Coil 3 had a bit of oil inside the boot, and the boot slipped off the coil when I was pulling the coil. There was white dust insulating the spring contact inside the boot. Dirt trapped in the oil can present conductivity issues at ignition secondary voltages, primarily arcing. At primary voltages, it acts as a resistor would, bleeding current. Which explains why the car would drive OK under light to moderate loads and chugged only a little under heavy load. And also, this gave me insight as to why the problem would set no codes - it would not really be measurable by the power train computer. The connector at coil 3 had no wire conductivity issues, however the insulation had ablated from the conductors at the plug and the wires were oily. I could not get a replacement plug same-day, so I repaired the wiring using silicone amalgamating tape. This is a tape used by electric utility linemen to insulate high tension wire repairs. In that application it is a permanent, weatherproof repair. When you stretch it, you activate the amalgamation. Stretch it and wrap it around a wire and it fuses itself together to replace missing insulation. You can tell it's fusing because the finish changes from shiny to matte as amalgamation takes place. I cleaned the wires with QD then wrapped them with narrow strips of the tape. I then bound the wrapped wires with a full-width second layer of amalgamating tape to make a flexible boot on the connector plug.

Plugs 4 and 5 had no real issues. I made repairs to the wiring looms.

At some point I need to get some small (1/4") wire loom and re-cover the wiring in the ignition harness.

The coil harness is in a tough location, right on top of the engine. It's no surprise to me that on a car with almost 190,000 miles in 14 years of service, the original wire loom and tape have become brittle in places and that some of the cover screws have gone missing. What's more, the coils have to cook over time.

With the coils replaced and harness repaired, the car's running quite well.
 

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Bad coils will typically result in misfires and a check-engine light. Don’t think that’s the issue.


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Bad coils will typically result in misfires and a check-engine light. Don’t think that’s the issue.


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I agree.

The car would drive OK under light to moderate loads and chugged only a little under heavy load. It's my spouse's daily, I only get to drive it when we're taking it on highway trips or I need to diagnose/trooubleshoot something. She drives it fairly gently. Consequently, she set no CEL after the first round of repairs and only under heavy acceleration did it seem to have a problem.

Plug 3 got the oil from seepage past the oil cap. The seepage was fixed when I serviced the PCV but I didn't clean up old spillage that was under the beauty cover. I know better now.

I don't know why plug 2 had white dust in the coil boot.

(end)
 
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