Hi all, my NA 850 misses intermittently sometimes on light throttle, sometimes heavy throttle (but will sometimes clear itself on backing of, off course other times will go right on missing). I thought it would be my fuel filter ( due replacement especially after running with a low fuel level last week). I have scoured various posts here at the BB and other forums as well. WOW, it seems a lot of people with a similar problem, yet no one finger pointed at a particular cause . A lot of opinions expressed relate to turbo models only, yet the problem seems to be a common issue. Klaus mentions the O2 sensor a bit. Is there a test i can do for this? I dont have access to a code reader or any fancy equipment. Is there a logical sequence of checks and cleaning of parts to follow to isolate my problem? Also could the knock sensor cause an issue such as this? I realise this is a light post compared to some others, but i need to start somewhere and with a basic check list. Thanks for listening. David

Today i drove the car again and it now misses on idle as well as whilst driving. It gets worse the longer you drive. You would assume as it is intermittent that it is fuel, am i right or wrong. Have yet to check vac lines as other half has absconded with car. So if vac lines ok what is it that regulates the fuel? What do the various sensors determine? David

Engine temp sensor comes to mind. Does the temperature gauge stay cold for a long time? A broken thermostat will also keep the temp low, so the computer enriches the fuel. A broken vacuum line to the fuel pressure regulator will increase fuel pressure by at least 4psi.

A clogged PCV will just add to your problems.

A dirty MAF will give incorrect air readings.

Klaus
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Always willing to listen, just not able to take direction.

Hi Klaus, plugs changed last week showing too lean if anything..no change, changed the fuel filter today (due anyway) and went for a drive. Performs flawlessly when cold and drove beautifully when hot, no miss, thought yippee problem solved. Not so however as it now misses on idle, obviously more pronounced in gear. Yes i do run lower octane fuel with ethanol and currently with a bottle of injector cleaner. Will try a tank of higher octane next. Will check all you mentioned. Regards David

Because it clears when decellerating, I would suspect a bad vacuum line. Take off the cover on the throttle body and check all of the lines that connect to the vacuum tree. Check the lines that go to the air cleaner and towards the radiator.

If the PCV hasn't been cleaned in a while, take it apart and throw away the plastic screen. Clean the lines that go to the PCV.

The knock sensor will retard spark and cause a rich condition, but that usually only happens at throttle up with regular gasoline and high temperature. Switch to a tank of premium gasoline and then run with at least 89 octane to prevent knock.

If you haven't changed spark plugs in a million miles, pull them and inspect for wear, replace if needed.

I doubt it is your fuel filter, but that is a cheap item to replace and easily done. Book says every 90K miles. New Volvos do not have fuel filters.

Klaus
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Always willing to listen, just not able to take direction.