In the past two weeks, my check engine light came on indicating a misfire on cylinder 3 (according to my ODBII) so I changed out all the plugs. this indeed resulted in a smoother idle. so I thought I'd change the cap and rotor today while I had some free time.......well, now the engine misses -- can't even drive it now because it misses so badly. OBDII code reader says P1310 (misfire on at least one cylinder - TWC damage) and P0301 (misfire on cylinder 1). So what gives! it was running fine and now after a simple cap and rotor change she misfires.

I did notice that the plate on which the rotor is bolted to is missing on retainer nut - so only two of three bolts are used to hold down the rotor. Could this cause an imbalance and misfire?? it wasn't happening before?

Of course I put the old parts back but she still misfires now. I guess the old saying 'if it ain't broke, don't fix it' applies here.

1. FIRING ORDER:
Note the firing order.
1-2-4-5-3
(or 1-3-5-4-2) depending counting clockwise or CCW.

http://volvospeed.com/Repair/sparkplugs.php

2. Check:
a. remove the plastic cover and check your BOOTS on the sp plugs; make sure they are tight.
b. check the firing order as above
c. check THE IGNITION wire, sometimes it is loosened up after manipulation at the CAP and at the Ignition Coil (mounted just in front of driver's side strut tower).

Thanks all for the input, the culprit was found to be a bad fuel injector at the number on cylinder. Not sure how changing the cap and rotor led to a bad injector that after changing it out she now runs like a top.

got good plugs? is firing order right (13542) use oem plugs or ngk or bosch check wires w/ ohm meter 5k ohms per foot of wire.wires should have some resistance.

Did you verify to be sure you did not mix them up???
Check the 5 wires to be sure they are in the right places.

Yup, tried checking the wires and thought I had mixed them up so I switched them around - she didn't even start.... so I switched them back and she runs but really rough. There's a gasoline smell now - probably from the misfiring and uncombusted fuel.

I order a new rotor flange from the Volvo dealer and should have it today - I'll let you all know if this fixes the problem.

Firing order is 1-2-5-4-3 and the number one plug goes in the cap at about 7 o'clock. Make sure the cap and rotor are clean and verify the gap on the plugs is about .028
Swap the plugs between 1 and 2 to verify that the #1 plug is OK.

Klaus
--
The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)

Replaced companion flange (as Volvo calls it) that was missing retainer nut to hold rotor. New cap and new rotor are installed along with it - it still misses badly. New plugs are gapped to 0.028 inch as specified but still no luck.

Bearing in mind that none of this was happening until I replaced the old cap and rotor (which was missing on retainer bolt on the cap and one retainer nut on the rotor) it really doesn't make sense. Unless a new problem was introduced to help and unbalance rotor and cap run smoothly.

If the cap is missing a screw, that could well be what is causing your misfire. Just a slight misalignment and the rotor will miss the #1 contact.

Probably, the person who lost the screw never tightened them down and you were lucky.

Klaus
--
The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)