Rough Idle / Misfire at Low Revs

[Haydo]

G'day All,
Apologies for starting a new thread on a well covered topic but I've done quite a bit of thread diggin' & had no joy as yet...
wanna see this one through to :spot on:

Got my ride serviced about a month ago, she was a little overdue & I haven't had time to start doin' it myself yet but after recent shenanigans I will be.
About two weeks ago, day or two after a trip to Melbourne (got some Vortex98 from Caltex on the way, normally just use premium) & back to Geelong Betty suddenly developed a pretty bad misfire. Only seems to happen at < 2000RPM, maybe a little power loss but she still gets up & boogies ;-)

/Rant
Took her to the local mechanic... waited a whole bloody week to be told the problem wasn't fixed & I'd be looking at min $1500 to fix a mechanical fault on cylinder 1 - suspect valve spring. :upyours:
I'm a professional computer geek, not a grease monkey but sure as hell keen to learn & I'd rather spend $$$ on tools & $h!t rather than handing over hard earned coin to some knob jockey.
/End Rant

Anyway, work done so far:

Mechanic
- #6 ignition lead found to be melted & earthing out to manifold. Replaced leads & plugs. Slight improvement.
- Compression test OK
- VACC test OK
- Scan tool used to monitor injectors, number 1 found faulty. Replaced injectors, Slight improvement. Replaced old injectors.

Me
- Cleaned MAF & tested.
- Cleaned IAT
- Cleaned Throttlebody.
- Inspected TPS. Seems OK but can't really be certian cos I'm a noob.
- Cleaned IAC. How easliy & how far should this move? It was pretty stiff & caked with crap, spent a bit of time on it.
- Checked fault codes: None, well I get a 12 (1 flash, 2 flashes) NFF.

Mechanic reckons they checked the coils, had to order one apparently but it's not on the job report & doesn't look like the coil packs have been disturbed so I'll have to check 'em myself I guess. While I grab the multimeter, has anyone got some suggestions to hook a brother up?

Cheers Fellas.

 

[Brett_jjj]

Its best to test these coils by replacing each coil, one at a time, with a new or known working one,and driving the car each time to see if it runs any better, because Ive seen these coils show up ok when tested with a multimeter, but still cause misfiring under load. Because you cant 100% rely on the multimeter readings when testing these coils, I now keep a brand new coil in my toolkit ,just for testing purposes. If you find one coil is stuffed,then its best to replace all 3 coils at the same time. These coils slowly degrade over time and slowly loose their "edge". Ive also found that most cheap aftermarket coils are mostly rubbish and the car wont run as good as it should with cheap coils fitted, and they dont last anywhere near as long as genuine ones either,so try to go with genuine holden coils if you find that they do need replacing.

 

[accentstencil]

I would start with a whole new set of leads. Bad leads are often responsible for a miss under load, I've had it happen a lot.

 

[Haydo]

Thanks for the tip Brett, thought I read there was some kind of spark test that was more reliable than multimeter!? Looks like I'm off to Smiths Holden for a new coil pack...

Already replaced the plugs & leads accentstencil, as above mate ;-)

Anymore info re: How to tell IAC & TPS are functioning normally?

 

[jas98]

Thanks for the tip Brett, thought I read there was some kind of spark test that was more reliable than multimeter!? Looks like I'm off to Smiths Holden for a new coil pack...

Already replaced the plugs & leads accentstencil, as above mate ;-)

Anymore info re: How to tell IAC & TPS are functioning normally?

how did you test the MAF?

 

[accentstencil]

Already replaced the plugs & leads accentstencil, as above mate ;-)

Sorry, I guess I missed that bit, I thought you only replaced No 6 lead.

 

[Haydo]

Hmmm... ordered a new coil pack from Holden, should turn up by tomorrow.
Got a fuel filter while I was there, replaced it & used some "Liqui Molly" (Zee Germans) fuel system conditioner/cleaner. Took Betty for a good burn to blow out any $h!t in the fuel system & bugger me sideways got an overheat warning - 1st time ever! Stopped, cooled then drove home nice and easy, no dramas.
Any ideas if misfire can be related to cooling?

Jas98 FYI:
Just to clarify, tested if MAF was cause of misfire by disconnecting it. Not sure it there is a MAF test as such ;-)
http://www.fullthrottlev6.com/forums/showthread.php?33209-got-a-misfire-read-all-about-it-here!

 

[Haydo]

Could a clogged CAT be the cause?

 

[Haydo]

OK, this is really pi$$ing me off now!

Tried a new coil pack which made no difference. Although I didn't reset the ECU each time I swapped each old coil pack over for the new one.
Anyone confirm it needs to be done?

Tried diagnosing by removing the IAC (increases idle rev) then removing each lead one by one to see which DID NOT drop idle.
Found #3 to be the culprit, so have tried both new lead & plug to no effect. Strange it was #3 as the mechanic said the fault was with #1 although they did say #6 lead was melted & earthing to manifold. With 3 & 6 sharing the same coil pack I had my money on that but still no joy.
While on the subject of leads, checked each & found the longer (5&6) to be slightly over spec ie. 18K on driver side, 11K on passenger side should be 17K & 10K max.

With the overheating shenanigans I double checked coolent & she took almost 2 litres which seems to have resolved the heat problem but only makes me wonder more so.
Plan to flush the system & check thermostat (maybe replace with a cooler one) but really need to sort the misfire first.

Reason I asked abot the CAT is my new mate at Holden reckons that might be the problem. When she's been running for a while I get a popping from the exhaust & it also seems to come from the mid section of the underbody. I've noticed what I think is water dripping from the same location too along with condensation from the exhaust.

This is doing my head in... can anyone throw me a bone here? PLEASE SAVE MY SANITY!

 

[Brett_jjj]

Seeing the new ignition parts havent made any difference, and the fact that theres water coming from the exhaust, It sounds like a blown/leaking inlet manifold gasket.This would also explain why its idling rough. A leaking/blown inlet manifold gasket is a very common problem on these engines once they get a few k's up. You can buy metal type inlet manifold gaskets instead of the plastic original ones. The metal type gaskets should last the life of the engine. When my inlet manifold gaskets went, im pretty sure it was cylinder number 3 that the coolant was leaking into, (second cylinder from the front on the passenger side).