Intermittent Misfire
 

Hi

Over the last few months my 960 has developed an occasional misfire; suspect it's running on 5. The misfire is really sporadic and doesn't seem to be associated with any specific conditions.

I have had a look at the coil packs and think one of them may be the problem; they're all looking their age (heat tarnished), and a couple have hairline cracks in their exterior. Unfortunately though, as I never know when it's going to happen, I don't get chance to pop the bonnet and unplug individual packs to check them.

I was therefore wondering if anyone has any suggestions on how I could confirm a failure before replacing any and, also, dose anyone know if coil packs can fail intermittently or do they normally just stop working.

Any thoughts would be really appreciated.

Thanks

I don't know if Volvo specify renewing the coil packs at certain service intervals, i know some manufacturers do. Chances are with an older car like yours will be, the coil packs went past their last replacement that should have been done at a service interval.

If one or two of them have started playing up, you can bet your next hot dinner that the rest won't be far behind.

I'd check the service book firstly to see if they should be replaced, if so how often and when the last dealer service was that would have replaced them.
I suspect if they are a service item the last service that a dealer would have replaced them either wasn't done or if it was then a back street garage/independent did it without renewing the coil packs.

Just for reference, i know some cars go through them every 50k miles.

Other than that, it could be your cat breaking down internally and causing a temporary blockage which could also cause a misfire. ;) :D

On my 1993 960, the coil packs are original and have done about 380,000 miles and all of them have wires attached that have seen better days. Just make sure on yours there are no wires touching anything nearby. I would pay more attention to the relays supplying power to the coils and injectors; they are the two grey relay packs near to the n/s front suspension tower. They take main battery power to these units and are prone to burnt contacts. The other relay for your attention is the green relay, along with the others, center console near to the radio. That relay supplies battery power to the two fuel pumps and the O2 sensor. The in-tank pump supplies fuel to the second pump underneath the front seats, underneath the car. If this pump "sees" an intermittent supply due to bad contacts, that will give you a missfire. I would go for the fuel relay first and you can take off the green cover and inspect the contact points and you can be sure they will be burnt. About £20 for a new set. (Volvo part 1362914)