VS V6 Auto with a flat spot

[Dannal]

I have a 96 VS Exec auto with around 200,000 k's on the clock. It runs perfectly except for that it has developed a flat spot or misfire from idle. It seems to misfire twice and then take off normally. I purchased it about 6 months ago and replaced the following parts:
-Spark plugs
-Ignition leads
-Air filter
-Fuel filter

I also had a mechanic look for fault codes but it was clear. The coil pack has been tested and is fine. The fuel pump has also tested fine. I really don't want to have to spend a fortune on this car at the mechanics so if anyone has any ideas I would be very happy to hear them.

Cheers!

[hako]

Does it hesitate without the A/C on? ....if not then it may be either a dirty throttle body or a sticky idle air control valve - there is a "how-to" on cleaning which is simple and something you can do yourself.
Another thing you can do is gap the plugs to 1.0mm instead of the recommended 1.5mm...this allows the plugs to fire at a lower voltage and sometimes will get rid of annoying misses. It cannot harm anything and if there is no improvement just regap the plugs.
Another question:- does it seem to be idling at a lower speed than normal?

[Dannal]

Thanks for your reply hako. It seems to misfire regardless of whether or not the A/C is on but is noticeably worse when the engine is cold. I might use the re-gapping of the spark plugs as a last resort if I can't get anything else to work. The car also seems to idle about where it should although it occasionally seems to have a bit of a stutter before coming good again. I also used some throttle body cleaner made by Subaru (all the mechanics here use it on every make of car) but I probably didn't do it right because it had no effect.

Cheers!

[WogBoyz]

Thanks for your reply hako. It seems to misfire regardless of whether or not the A/C is on but is noticeably worse when the engine is cold. I might use the re-gapping of the spark plugs as a last resort if I can't get anything else to work. The car also seems to idle about where it should although it occasionally seems to have a bit of a stutter before coming good again. I also used some throttle body cleaner made by Subaru (all the mechanics here use it on every make of car) but I probably didn't do it right because it had no effect.

Cheers!

you musnt have used it right man, i got about 120xxx on the clock and i used that upper engine clean by subaru it made a fairly noticeable differnce, have you taken your whole throttle body off and gave it a good clean?

[hako]

Maybe also put some injector cleaner thru it or a few tanks of the premium fuel which has an injector cleaner in it as you may also have dirty injectors. There is a "how-to" on cleaning the throttle body so maybe check that again (as WogBoys says).
These are all cheap options - gapping the plugs is nil, cleaning the throttle body is about $10 for some spray cleaner and the extra cost for premium fuel is only 10c a litre. Gapping the plugs down will not affect performance or economy from my experience. Good Luck.

[maldotcom2]

Hako is there any performance loss if the spark plugs are gapped to 1mm in an engine with no ignition problems? Im thinking in terms of spark strength etc.

[hako]

Hako is there any performance loss if the spark plugs are gapped to 1mm in an engine with no ignition problems? Im thinking in terms of spark strength etc.

I'd only be guessing here but back in the old days - like 40/50/60 years ago, plug gaps were around 25 thousandths of an inch or .635mm which is half the recommended Commodore plug gap. That's when most of the land speed records were set, and around that time muscle cars were born...with those small plug gaps.
Go forward 60 years to today and plug gaps are at least 1.5mm on most cars due to the improved electronics plus the need for that big gap to fire a very lean mixture...emissions etc.
So there are 2 sides to the argument.
Personally I've run 1.0mm and noticed no difference except that I got rid of a miss. Others will say 1.0mm will not even light a f**t.
Maybe an ignition system that normally is capable of firing a plug gap of 1.5mm will throw a fatter spark with the gap set to 1.0mm??.
Lots of things to think about and weigh up but in the end you have to go with what the factory recommends (but) with a 12 year old car maybe they need a bit of help which is why I'd say if the car is "as new" go for 1.5mm and if it's missing try 1.0mm.
The big plus is that it costs ZERO.

[delcowizzid]

the only thing you lose is economy when you shut the gaps 1mm uses way more fuel than 1.5

[hako]

the only thing you lose is economy when you shut the gaps 1mm uses way more fuel than 1.5

You could be right, however I ran mine at 1.0mm for 100,000km without removal - even posted about it before and after on this forum - and was always getting good economy. When I removed them they were at 1.8mm due to erosion but still not missing.
It is also of note that when a VS runs on gas and petrol, the recommendation by the experts is to gap the plugs to 1.0mm - this may be a compromise as a gas mixture takes a higher voltage to jump a spark gap...but seeing as changing to gas is about economy why would they also OK it for petrol.
So, I dunno...I'm open to listen. I've got my experience but it don't mean my feet are set in concrete. But anyway, the suggestion was to remove a miss.

[The1]

i would be checking iac or tps first, give them a clean and spray some contact cleaner in the tps, if it's not sensing your throttle properly it wont run right, some tps sensors get worn in most used spots which is down low and dont sense correctly but are ok at higher revs where it's not used all the time.

[Dannal]

Thanks people. I have been flat out lately and have decided (after reading these posts) to go and get some good injector cleaner. The throttle body cleaner probably didn't make much difference because it was probably not all that bad and the main problem was the flat spot. I will flock off to a shop in the next few days for some injector cleaner and then give you all an update on how it goes.

Cheers!