1998 V70R occasional misfire (P0300)

[uyvolvo]

Greetings

I have a random misfiring at idle that is driving me and mechanic nuts. Details:

1998 V70 R, 165 K
Symptoms: car will start fine, run for a while and then occasionally misfire at idle or at stop-lights. Throws a P0300 code. Tachometer stays steady at 1000 RPM and car will usually run well after these misfirings It also runs well at higher speeds.
Did a full tune up (cap/rotor/spark-plugs/wires), changed crankshaft position sensor, knock sensor, coil. Nothing helps.
Mechanic thinks that it may be a weak valve spring and suggests doing the heads ($$) but I am not convinced, since I thought that a weak spring will lead to poor running at higher rpms, not idle.

Any suggestions / help greatly appreciated.

[WhatAmIDoing]

Grab a new Volvo fuel pump relay. I had a KAE relay cause random misfires at idle as it began to fail (didn't realize it was the relay until after I did new plugs, wires, cap, & rotor). Consider changing the fuel filter as well if you have no idea when it was changed.

You should look for vacuum leaks. Your idle should be closer to 800 RPM, not 1000.

[misha]

Check gap on those new spark plug(if they are not Volvo plugs),since it's critical for turbo engines.

[scot850]

We say it time and time again that the parts used can determine issues you may get. Can you state the brands of the parts you fitted as some brands are known to be incompatible with these turbo motors.

Neil.

[erikv11]

Agreed, super important: replaced ignition components with what brands of parts?

And I like the fuel pump relay idea as one possibility, and note that if the misfires are consistent you can test it with a jumper wire before you shell out the $45 for a genuine Volvo relay.