hi. i have a 2006 s40 that sets a check eng light when rev over 3200. it always sets the same code misfire # 3 cyl. have replaced plugs twice have replaced all 5 ign units and fuel injectors. have also replaced wiring to #3 ing module.tested fuel pressure it is to spec.ran leakdown test, all cyls at 3%. smoke test showed nothing. eng has had recent cam belt and water pump, is absolutely in time. eng starts easily, runs smooth, looses no coolant, uses no oil. i have driven it like this for @ 6 mo.i have been told the misfire sensing is normally an idle or low speed function, is this true? in scanning the computer i saw a notice that the misfire was turned on, does this mean it can be turned off? any help appreciated.

 

well, no replies yet. i was hoping that a volvo computer guy would have some knowledge to advise me if misfire feature could be turned off. recently i reved it to 4000. the engine was pulling strong with no misfire, but it set the light as always. right now i do not believe it actually misfires and think it could be a computer issue, although am uncertain how it could be confined to #3 cyl. also any information about how the computer reads misfire might help. any help would be appreciated.

 

have you looked at run time data for fuel trim to and O2 sensor readings? I suppose you could try reseating the ECU and clean the connectors to the front O2 sensor etc but I'd probably look at whatever run time data you can get - could be something tangential like an ECT sensor, fuel pressure etc that may cause a lean condition. That's where seeing what the O2 sensor is trying to do may help. Only other thing may be to test out the knock sensor(s) and cleaning those connectors.

 

thank you for responding. not included in my original post was that i had replaced both 0xy sensors. i had also replaced the knock sensor and have cleaned all engine connections. i may have given the impression that i am computer savy, i am not. i am an older driveway auto enthusiast more suited to carburetors and distributor ignition. please keep the suggestions coming, i still have hope of hitting on something.

 

yep I started working on cars about 55 years ago helping my dad rebuild a carb on the kitchen table :-) As to computerized fuel management / emissions systems, its pretty rare for the computer to go bad. When it comes to the newer cars, they can be quite sensitive to things like aftermarket vs Genuine Volvo parts etc. Misfires are typically related to fuel trim (ie lean/rich mix) and the ECU is taking data from multiple sources and applies a map to determine fuel vs air. So thinking aloud about what's in play: Air metering is done by the MAF sensor. Its usually a 3" short tube in the intake air path with a connector going into it. You can pop it out and inspect/clean. If you've ever had an issue with your PCV, its possible some oil got into the intake and can dirty up the MAF's filaments. Clean using a MAF specific product like CRC MAF cleaner. Carb cleaners may be too harsh for the wires. Temperature is measured by the engine coolant temp sensor (ECT) and fed into the ECU. Its typically found right under the thermostat - you can unplug the connector and measure resistance on a cold engine and a warm engine (google for resistance curves). There's also the MAP sensor (some engines have two) that measure air pressure (ie its an altimeter) that feeds the ECU. You said you did a leak down test plus I assume there's no issue with oil or coolant consumption which would suggest a head gasket issue (have you had the coolant system pressure measured?). When these sensors die they usually set a fault code but not always so you may want to test the ECT on your own or just consider replacing for $50 along with refreshing the thermostat (particularly if you have the original). I've had both fail on my 850 and a 2000 S40 so it may be good preventive maintenance. Last thing is did this happen right after doing the timing belt? I'm wondering if your cam timing might be one notch off on one of the cams - not sure if that would set a code. I'm thinking after doing a few of these simple self tests you may want to find a Volvo shop or a dealer with a VIDA system to do a full hour's worth of diagnostics including collecting run time data during a driving cycle to see if they can catch some measurements when the fault code is being set.

 

hey.thanks again for your second reply. you have given me several things to look at. what concerns me is that i dont see why any of these would be confined to always setting the same code, cyl #3 misfire. the engine has been out of the car at some time, so almost any thing is in play. one thing i am sure of is that the cams and com belt are in time correctly. having checked repeatedly myself, i also had two different pro mechs check. driving the car, the engine starts and runs smoothly, used no oil between changes, comes up to temp midgage and sits rock solid even at 100 degree days with a/c blowing cold. when accelerating past 3200, even while setting light,engine pulls strong w/no noticeable miss untill computer cuts in, cutting out #3 and taking control of idle speed. reset w/scanner and everything returns to normal until next time. it is a very nice driving car otherwise. i have had a recurring nightmare that the ign timing ring on flywheel might have been damaged when eng was taken out or reinstalled. this thing has stymied me and several mechs i respect, again not volvo specialist. again all suggestions are welcomed and appreciated.

 

I suppose you could try reseating the ECU and clean the connectors to the front O2 sensor etc but I'd probably look at whatever run time data you can get - could be something tangential like an ECT sensor, fuel pressure etc that may cause a lean conditi