missing at idle, with AC on

[exanVZ]

Hey guys,

Recently my VZ v6 has started missing at idle, it's 100% fine once you are moving. It's really really bad with the AC on, but barely noticeable without it. I've replaced the plugs already, with NKG platinums, but same problem still happening. I am beginning to suspect coils, as i have no idea when they were last changed, and car has 265,000k's on it. What is the most likely thing to check next, based on this info? Is the fact that it happens much more pronounced with the AC on a sign of anything in particular?

I have found I can get coils on ebay for $25 so not sure if i should just get 2 and try 2 cylinders at a time or just get 6 and replace em all, or if maybe something else is worth checking first?

Thanks guys.

 

[exanVZ]

found 6 coil packs from a wreck for $50 total, got em online so will report back once i've swapped em over as to if the issue was solved or not.
i put $20 worth of 98 in it earlier as it was really low on fuel and its actually running heaps better, but the issue has been ongoing for a few weeks, so i doubt its gone.

 

[Nut Kracker]

Is it actually missing, or does the idle drop lower (running rough,seeming like it misses) when the A/C is turned on.

 

[exanVZ]

the RPM/idle doesn't drop at all, it's just like a brief shake/bump coming from the enging, pretty sure it's missing.
i've checked battery voltage and stuff, it doesn't drop below 14v with lights and AC on so I think battery is fine, which makes me suspect coils.
cheers

 

[Nut Kracker]

Ignition parts, if faulty tend to break down with load on the engine, such as turning the A/C on at idle.
So things like spark plug leads, spark plugs, DFI module (part under the coils) and coils would be the things to check.

Sounds like the alternator is good.

Try this.
With a timing light and with the engine idling, hook it up to each lead one by one and see if you
get a constant pulse of the light.
The light should flash every time that cylinder fires.
If the light hesitates, doesn't pulse constantly or is erratic, that will tell you the faulty lead, plug or coil etc.

 

[diysv6]

Have you checked for vacuum leaks?
There is a small vacuum line at the rear left of the inlet manifold (whilst facing the motor) which feeds a one way valve then the AC vacuum switch then goes on to the heater tap.
If you nip it closed with a small clamp does it cause any changes to the idle quality? Contributors are now reporting problems with the vacuum hoses under the dash and the vacuum "switch" for the AC unit.

Are all of the other vacuum lines in good condition, power brake hose, purge line, loose upper inlet manifold, PCV hoses etc. OK?

Check for an excessively oiled up throttle body - the PVC/worn rings can cause excessive oil mist into the inlet tract.
If you have checked all of vacuum leak sources, then I would give the Throttle plate/body and the MAF/IAT sensor a good wash down with MAF cleaner.

This may give you a new check point for your poor idle investigation.

 

[exanVZ]

Thanks for the extra suggestions guys. Not sure if I can afford to buy a timing light, looks like about $50-70? Will see if my mate has one at his workshop, as that's a good idea. VZ has no leads it's just coil on plug, so i changed the plugs, didn't help now i've got some cheap coils coming too.

I haven't checked specifically for vacuum leaks, but I have checked that all the hoses are hooked up properly. I will need to google a bit more info on vacuum leaks, but will look into that.

I usually clean the MAF/throttle body every 6 months or so, but can't remember when I checked it last either so might give it a blast with MAF cleaner tonight.

Cheers again for useful suggestions.

 

[diysv6]

........a timing light, looks like about $50-70? Will see if my mate has one at his workshop, as that's a good idea.

When I had problems with my ECU, I pulled out each spark plug's ignition coil power connector in turn, to see what happened to idle quality.
It was a quick way to evaluate which plug was not firing regularly.

 

[exanVZ]

unfortunately it's not really pronounced enough for that to tell me if it's happening or not, it's a little bit intermittent so might be hard to test that way.
i would need to somehow make it much worse than it is, then i'd be able to tell if unplugging a coil makes a difference. Right now any coil i unplug will make it run like **** I think.

 

[ALitreBoy]

Gday exan,

my calais VZ do excatly same thing. dropping more when ac is on.

Did you found what is the problem?

Cheers mate