Random misfire, maybe once a month, sometimes every 3 days
 

I've had this really random misfire with my '98 S70 for a few months. I've replaced the plugs and wires with OEM from FCP thinking it would clear it up. Sometimes it may not happen for a month, this week it happened 3 time one day, and fine since. Will run great but when it does happen it's after I've shut it off at the store. Come back out and it will misfire, sometimes clear up and I'm on my way. Others it's quit and had to restart with a continued misfire, but it always clears up and I can drive away. I took it to Advanced Auto today for them to read it and he said it showed misfire on 4 and 5, he didn't mention a code. I also have Evap small leak, but I highly doubt that has anything to do with it. He suggested maybe cleaning the MAF sensor or it could be injectors, or cam position sensor. Is there a way I can rule out what the issue is exactly without having to replace parts one by one ? I've seen some suggestions here in reading that the fuel pump relay could be at fault, would that actually cause the random miss on 4 and 5 ?

I doubt it would be fuel pump related. I'd replace the cap and rotor if they have not been replaced.

Make sure they give you all of the codes next time.

Fuel pump relay usually causes the car not to run. The cam position sensor won't cause this. I agree with the cap and rotor. Then you'll need to check cylinder leakage if it persists in the same cylinders.

I will pick up the cap and rotor tomorrow then, thanks. I was going to pull the cap off to take a look at it before I ordered the plugs and wires but the ears of the cap started to twist some when I turned the bolts and I didn't want to risk braking one of those small bolts...so I got cheap and didn't order them.
I suppose cylinder leakage could be remotely possible, but I personally doubt it. The car runs phenomenal, other than this little glitch. Lots of power and no hesitation. I do give it a good run every so often to get my acceleration fix lol. If it is a leakage problem, I would think it would be a couple hanging valves, and from my experience with old tractors MMO and Seafoam work wonders for that.

Thanks guys, I'll post an update as to how it worked out.
rspi....Thanks for all the videos you've made ! I've watched quite a few of them, great resources for Volvo newbies.

I did replace the cap and rotor, the definitely needed it. Been fine till today, acted up on my wife something fierce. I'll have the codes read again by someone else and see what comes up.

When you replaced the cap and rotor, did you inspect the cam seal?
If its cracked or partially dislodged you will get blowby in the distributer cap causing moisture - which in turn causes misfires.

I did look at the seal while I was in there, looked in good shape, no signs of any blowby or seepage. My wife decided she wanted to have a local shop we use check it out better so off we went this morning, didn't act up for me at all this morning. When the tech pulled it in the shop it acted up so he went right for the hood and heard something hissing, a hose on the intake manifold by cylinders 4 and 5 was split. He plugged it with his thumb and it cleared up, took his thumb off and acted up again. He went ahead with the whole diagnostic routine and seen a couple plugs looked a touch burnt but not bad. So that hose making a lean condition could burn the plugs. He thinks that was the problem the whole time, causing 4 and 5 to misfire seems it is right between the two. I had them do a once over of everything else too while it was on the lift and the rack I just put in last year is leaking already ! Good thing for lifetime warranty, will only cost me labor because they didn't put it in last time, I did. But I'm just not into doing it again.

Try cleaning your throttle body with throttle body cleaner. Remove hose open butterfly and spray liberally with car not running. Let sit for ten minutes and start. Will be rough to start but will smooth out.

Ok, another update. After the shop did all their work on the car and reset after reading the codes all was well for a couple days. Wife hit a bump and the check engine light came back on. Runs really smooth still until it decides to miss. It quit at the gas station again yesterday but started right back up for me. Had her take it to the shop today to have the codes read again and it only shows PO442, which is just a small EVAP leak. This doesn't sound to me like anything that will make these symptoms happen, or is it ? I'm starting to wonder if there's a bad electrical connection on something. Any thoughts ?

Check the gas cap.... Consider putting in some octane boost.

I am dealing with an Acura that has all 6 cylinders randomly misfiring - all after filling the tank up. You mentioned the gas station so i am thinking it may be related

@ Frank, what vacuum between #4 & #5?

Usually a random stalling issue is caused by a failing fuel pump relay.

There is a vacuum hose on the intake manifold, pretty much where the intake splits between 4 and 5. It was in bad shape and has mad a huge improvement in idle and drive-ability.
Every so often we will feel a skip, only at times it will stall after startup. My wife says it sometimes won't start right back up. When it happens to me, I shut the key off then restart and it's usually fine after that. Occasionally I have to do it again. Two months ago when it would stall after startup, it would sometimes run really rough and quit if you tried to put it in gear. I was thinking part of that may have been that big vacuum hose because it hasn't acted like that since.

The skip we feel now isn't like a number of misfires at once, just a simple skip then clears up. Usually when sitting at a red light or in park. I am considering that maybe there is a bad connection somewhere, ignition related maybe, so the vibration could be causing a short and temporary loss of connection.

I may try jumping the relay this week when it warms up. Have no garage and I'm beyond tired of working on our vehicles in these -10F temps we've had this winter.