Anyone converted the 2 ignition coil system to 4 (rough idle solved)? 2000,V40

[pauldg123]

Hi all,

Since I bought my V40 ($3k AUD, 145k kms), I have been fighting the rough idle battle for months and spend lot's on replacement components (vacuum hoses, spark plugs, filters, cleaners etc). A car that doesn't idle properly is an annoying car and drives my wife nuts. Fortunately, I believe I have finally found the root cause issue of this idle misery.

First of all I read the Volvo forums about this rough idle and eliminated the usual suspects that were listed (wrong sparkplugs, bad coils, vacuum leaks, air filter, bad cylinder compression, etc).

The first time I opened up the hood (bonnet) of this car I was surprised to find only two ignition coils, each driving two cylinders. The maintenance manual that I managed to download online, mentioned the waste spark system. "Cool, well that should be fine then", I thought. Volvo probably wanted to skimp on the overall component cost of the car by doing something smart (or not so smart I guess).
The next clue I got was a post that mentioned that these cars must use the OEM Volvo sparkplugs and that they hate platinum plugs.

So my background as electronic engineer came in here handy. I concluded that the sparkplug resistance (the air gap) must be equal across a pair driving by one coil to get an even spark distribution. Despite having these sparkplugs setup with identical gaps myself, no luck... After swapping them around to pair them up differently, the misfiring/rough idle did magically disappear. I used the Champion RC8PYP sparkplugs (tried the NGK BKR6 and Autolite2322 but no luck).

Next conclusion: why not use 4 coils instead, issue solved? I still have the old ignition coils but driving two coils per ECU port might be a bit too much to ask of the old ECU and I definitely don't want it to break or repair it. An additional coil driver (MOSFET) is probably very easy to add to drive the additional coil. Before I jump into this diy project... has anyone converted this 2 ignition coil system to 4 coils?
Another thought is that this setup will continue with a waste spark... I guess that wouldn't be an issue. Adding a flipflop and additional logic to split the original signal into two separate signals would be a bit of engineering and adding complexity, cost and risks with it. Any thoughts or suggestions?

Finally for those with a slow hunting revs at idle: yes mine did that issue as well. A USA post mentioned a car recall for that. After setting the throttle cable tension correctly... issue solved. So I guess someone tried to raise the revs to get around the rough idle issues. My hypothesis is that the IAC valve is not enough to regulate the revs with an additional load e.g. Air-conditioning running. Volvo probably upgraded the firmware in the recall to use the cruise control servo in the throttle cable as an aid to smooth out the engine load changes at idle. Anyone who noticed this as well? Note to myself: It should be easy to confirm this by putting a piece of tape as a flag on the throttle cable and see if it moves when the car is idling with the air con running (I have not done it yet).

[matthew1]

I don't recall ever reading about a 2 -> 4 conversion...

The gap and brand of spark plugs in this application is probably of high importance, where Volvos in general already put spark plugs at high importance*. I'm not a 4-cyl guy, so these are just the thoughts off the top of my head.

* turbo Volvos like Volvo brand plugs, and are picky about the rest. The Champions you have make the 5-cyl turbo very happy.

[EddieRiff]

Regarding your rough idle, assuming your car is manual a faulty DMF (dual mass flywheel) can be at fault/failing.
You can sometimes feel it pulse when it begins to fail when you press and hold the clutch pedal down and give the throttle a blip/revs.

Obliteration of bell housing and gearbox avails when completely failed!