Misfire on deceleration + idle hunt

[02gzm]

I had the head gasket replaced a few weeks ago and used a slightly thinner gasket to raise the compression a little bit. After i had the work done i noticed a slight misfire on decel. It was accompanied by a little backfire. The car started to misfire on acceleration shortly after and we found out the TPS was buggered. Now thats been replaced I've noticed the car still has it's little misfire on decel and on a hot day like today it idle hunts while sitting at the lights (Drops down to about 500rpm then goes up to about 750 then down to a little under 500 and repeats).

Ok my question is, because there is a backfire (therefore possibly ignition related), is it possible that i need a different heat range plug due to compression change? Or the other thing i'm thinking is since it only happens with the throttle closed it may be the O2 sensor?

I dont think it is to do with the replaced TPS as it was doing it before the original one died. Oh and the dizzy and leads were replaced about a year ago now with new ones so i doubt it's them either.

Anyone got any ideas?

[aaronng]

What heat range are you using now? Did you replace your plugs when you replaced the head gasket?

[dsp26]

pretty sure its just back fire... a misfire will cause your engine to surge and that will almost never happen off-load/decel...
exhaust pop would probably be caused by your raised compression and additional heat produced.. your cat is just doing its job burning rich mixture in the exhaust and 'popping'

backfire won't happen to your engine.. backfire is when ignition/combution blows back out the intake system.. this is one of the primary reasons oiled pods are illegal.. could set them on fire.

[02gzm]

The plugs are the standard heat range for the engine (b16a). They were iridiums so i take it they were just cleaned and reused. I'll have to go and ask the guys at hipower racing if they changed them.

The odd thing is i can feel the car jerking when it backfires so thats why i think that it's a misfire. But yea it only does it on decel with the throttle closed.

[aaronng]

Change to a cheap set of copper plugs and see if it helps. If it doesn't, you're out of pocket $16 only.

[02gzm]

Put in some injector cleaner + octane booster last night when i filled up. It doesnt seem as bad now so it is probably just a byproduct of the upped compression. It is a lot cooler today though with the rain so that could have something to do with it. And the idle issue is still there. Will post back in a few days after the weather changes back hot again and see if theres still a problem. Oh and with new plugs too

[dsp26]

^^interesting... dizzy timing is set at what degree?

and how high a compression are you running exactly? coz aaroonng maybe right... it was really hot one day last week and mine misfired on load once....

[aaronng]

Octane booster is not good for long term use. Add in 98 RON instead next time.

I'd say check the plugs first, also "roughly" check your dizzy timing (take a pic of the locking bolt on top of the dizzy) so that it is not set either ends of the timing range and check when your fuel filter was last changed as well. Do the cheapest things first to diagnose I reckon.

[02gzm]

^^interesting... dizzy timing is set at what degree?

and how high a compression are you running exactly? coz aaroonng maybe right... it was really hot one day last week and mine misfired on load once....

Not sure on the exact compression ratio so i'll find that and the dizzy timing out. It never misfires under load with acceleration. It just pops a bit with a few little jerks while slowing down coming to a stop at the lights as an example. Nothing violent or anything. I can just feel it and it's annoying me.

Oh and i hardly ever put octane booster in. And it's always 98 ron (BP Ultimate to be exact)

[dsp26]

oh...

thats normal.. sounds like your just feeling decel exhaust pop on the pedal coz misfire or ping is VERY noticable.. not just to the pedal but the whole car jerks

[02gzm]

It's the whole car thats jerking. I have my foot off the pedal (throttle closed).