P2 Volvo Multiple Misfire Diagnosis

[mlc611]

My 2006 XC70 was running really rough after a period of storage. Mechanic put in 5 new spark plugs, and new coils on cylinders 1 & 5.

That helped a lot, but it's still running rough. It throws a code for Cylinder 3 misfire. Later, it throws codes for Cylinder 1 Misfire and "random multiple" misfires.

When I unplug the wires to the coils one at a time, with the engine running, I notice a change in performance for each coil except for #3. That is, when I pull the wire on #3, the engine noise doesn't change at all whereas when I pull 1, 2, 4, and 5, it alters the tone or stumbles for a moment.

I'm going to start by replacing coil pack #3 (and maybe 2 and 4 while I'm at it).

But what I'm trying to understand is whether a faulty coil on #3 could cause the misfire code on Cylinder #1 (which already has a new coil pack). Or is there likely to be a secondary issue? The codes were cleared multiple times and problem reoccurs the same way each time.

Suspecting a vacuum leak, I tried pulling off the vacuum lines at each engine port I could find and plugging them with a thick rubber sheet. Plugging each one did not fix the rough idle. I don't think it's a vacuum leak though since the problem persists under hard acceleration, and I think vacuum leaks issues typically go away under high rpms....

Any other ideas?

Thanks!

[abscate]

Unlike GrandPas Volvo, unplugging coils to diagnose is not healthy for transistorized ECU fired coil packs.

Definitely replace coil pack 3, but first move a couple of packs around to see if the running fault follows the coil.

Plug it in, start it, shut it off, unplug, restart. Look up the cost of an ECU if you think that is too much work - don't forget to add a couple hours at the dealer for software



The cylinder misfire locator is not foolproof and the rougher the car runs, the worse it is.

[mlc611]

That's really good to know, thanks.

I'm afraid that if I looked up the price of an ECU replacement, I'd probably sell the car.

[abscate]

They almost never fail. There are a slew of them on eBay gathered from junkyards by people who sell them for Tunes.

From 1999- on, they are married to the car VIN so you can't just plug them in without software work