Increasing Spark Plug Gap

I seem to remember volvos years ago getting long electrode plugs to make them run smoother (Renault engined ones)

Yeah, this is a Renault engined one. It's got the correct Volvo spark plugs fitted at the moment, and before that I had some Bosch Super 4's, which didn't make any difference (other than not being able to change the size of the gap easily).

Ben.

I think your symptoms changing with oil pressure/ temperature are probably just the petrol/air mixture changing a little with temperature; the oil pressure variation is also following the temperature change so can be ignored. Engines are often tuned to within a whisker of misfiring at the weak end of the usable mixture range at low throttle settings for economy and pollution reasons, so anything a little out at small throttle settings that makes it weaker or just less efficient at igniting it can cause minor misfires, stumbling etc.

If it was a Nissan I'd say get NGK plugs, it sounds just like the symptoms you get with Bosch 4 plugs when they've done a few thousand miles. The supposed reason is that the spark is in the wrong place on 'enhanced' plugs.

This sounds like a lean condition, and a wider plug gap is better at lighting a lean mix than a narrower one (if the ignition system is powerful enough to fire the wider plug gap).

You may have just a lean mixture (the fenix FI was never the most sophisticated) or it could be caused by carboned inlet valves (soak up some fuel) or poor spray pattern from the injectors- you dont say how many miles the car has...?

tim..

"Tim (remove obvious)" wrote in message news:d293r3$4dt$ snipped-for-privacy@hercules.btinternet.com...

The car's done 95,000 miles. I've tried swapping the injectors from another car at the scrapyard, and exactly the same slight miss occurred. It could be that they also didn't have a good spray pattern, but the problem was identical, so for the time being I've ruled them out. It's not a complete miss, like you'd get by completely removing a HT lead, it's less violent than that, but still noticeable.

Could it be that one of the gaskets on the inlet side is leaking? I thought that the vacuum gauge would have shown this, but I'd been considering replacing them anyway, in case they were the cause. Or.... what would happen if a gasket on the exhaust side was leaking? Would this confuse the ECU (via the lamba sensor) into thinking it was running richer, and then the ECU is making the mixture leaner to compensate?

Thanks for your advice!

Ben.

hydraulic -

in that case then, sounds like you have an inlet air leak on one intake runner- spray some soapy water or an unlit propane torch around the manifold to head joint and look for a change in engine note whilst at idle.

Tim..