Volvo 850 misfire under acceleration..

"tony sayer" wrote

HT leads?

Usually a sign the plugs and/or the leads need changing. Just because the electrode gap looks OK it doesn't mean that the plug is still servicable.

Firstly although plug interval is 30k on these engines, the plugs are past their best at 20k, so change them then.

Secondly, 120k on the original HT leads, and more-over rotor arm and cap is a fair innings, so you should be looking to replace these as a service item anyhow. Its testiment to Volvo quality that they last this long- if you had a Mondeo you'd have replaced the HT leads about 3 times by now!!

Change the plugs first, then the cap and rotor arm (using decent matched components) and finally the HT leads.

Ensure you bath the cap shroud in degreaser and make sure its scrupulously clean and grease free before applying it to the new dizzy cap.

Speak with GSF and ECP for best prices.

Tim..

In article , Tim.. writes

Seems like the leads and plugs it is then. Take the point of the age of it all;

Question, if the leads/plugs are duff and then presumably producing weak sparks, why doesn't it misfire when say ticking over or under acceleration when at higher revs when surely there is need for more sparks?, or have I got that reasoning a bit arse about face and more spark power is required at lower revs or is it still there, the problem, but more noticeable?..

FWIW the cap and rotor arm were quite grubby, but are now clinically clean!.

cheers

scrupulously

The most 'spark power' or voltage is needed when the cylinder filling is fairly high- this usually co-incides with the rpm that generates max torque- hence the cylinder filling is at its highest, and thus compression is at its highest. Richer cold start mixtures are much easier to ignite than running temperature lean ones, so the misfire usually comes on as the engine reaches running temp.

Therefore weak HT components almost ALWAYS show themselves at a wide throttle opening and low to medium rpm- somewhere under 3000rpm usually. (yes i know the Volvo produces max torque at 4200rpm but...)

Hope this helps.

Tim..

Ahhh - that explains why my chipped S60 needed plugs nearly every service...

Just to add to the knowledge dispensed hereon so far..

New set of leads fitted, slight doubt as old ones read betwixt 2 and 5 K ohms on test meter, but long 10 mile assorted drive under various load conditions and..

No misfire!

Job done

Thanks!!!!