Annoying Intermitent Misfire

[melonfarmer]

My beloved 1996 440 2.0i has an intermitent misfire,the symptoms are as follows:-

at about 30mph very jumpy ride
at about 70mph heavy cough and sometimes judders
BUT No MISFIRE UNDER FULL ACCELERATION!

i have changed plugs,dist cap,rotor arm,air filter,cleaned idle valve,etc

Please can anyone help me?

[B20F]

Try cleaning the flywheel sensor. Take it out and clean the tip with some contact spray and a toothbrush, clean the connector of it too. It's located nearby the seperation of gearbox and engine right under the throttle housing. It's attached by a C-formed bracket and holded down with 2 11mm bolts. While doing this job, disconnect the minus lead from the battery so the ecu can reset itself. 10 minutes should be enough.

[GTMaster]

Could also be the throttle plate sensor. If it goes fine at full acceleration, then it might be clogged up and needs cleaning, or needs to be changed. The pot. meter wears away after time, and if the car was used mainly in city driving, then the car goes shaky at low speeds and at times when the throttle plate isn't opened too much.

[Paul240480]

Going on from B20Fs great advice, check the resistance of the sensor, before & after cleaning. Should be 220 Ohms+/- 60 (IIRC). If you cannot get it within these parameters, bin it & source a replacement.:thumbs_up:

[bkf_uk]

Check also that your lambda sensor is ok, mine was dead and caused this kind of problem, also change HT leads they have a habit of breaking down after a while (mine certainly did!)

[Paul240480]

Check also that your lambda sensor is ok, mine was dead and caused this kind of problem, also change HT leads they have a habit of breaking down after a while (mine certainly did!)


Funnily enough, I've just replaced my Lamda Sensor. To test yours use this:

http://volvo480.dragons.org.uk/forum/viewtopic.php?t=14683

If you find that yours is 'switching' but not to the measurements given, it MAY be working but has gone 'sluggish' with age so best to change it. It my be sooted up too as mine was.

If you replace it, go for the Bosch Direct fit (£52 @ GSF) as universal ones can cause all sorts of snags.

[Old Volvo Lover]

It my be sooted up too as mine was.

Question on this - would a heavy misfire clog up the sensor more quickly? Had a really bad one a couple of weeks back with all sorts of fun and games ensuing (see my other post on those!) One thing I did notice was a lot of rubbish coming out of an exhaust that was otherwise clean before the worst of the misfire was corrected.

[Paul240480]

Possibly. One thing to try if you repaired the fault. The fault 'code' may still be in the system, so the info going to the ecu is still telling the ecu to account for the fault. One way to clear codes is to disconnect the battery for a good few hours (overnight to be sure). Then reconnect & see how it goes.

Good luck