VY L67 Flat Spot/Hesitation ONLY when cold on acceleration.

[krusing]

Dear JC Members,

Vehicle -
Oct 2002, VY, L67, 188,000 k's

Last Sunday I finally did the LIM Gaskets,
as the loss of water, rusty milk shake in the radiator, and flat spot [when cold] was starting to bug me,
where the flat spot would almost caused the engine to stall sometimes while taking off from the traffic lights, but didn't, just a couple of coughs and splutters, and away it goes, after that, it ran ok.
LIM Gaskets replacement went well, and it all went back together [with no spare parts left over].
Used thread sealant on the LIM & SC Bolts, and made sure all LIM & SC bolts were torqued up correctly.
I have taken it for a couple of runs during the week,
But since it hadn't been for a drive yesterday, I took it for a spin today, and the damn flat spot is still there.

When the Temp is up, say 1/8 on the gauge, it runs perfectly,
idles at approx. 780 > 800 rpm hot and cold.
After resetting the computer, as I had the battery disconnected while working on it,
today I noticed its recorded 8.8L/P 100k's around the suburbs,
Its never been that low, its used to displays approx. 13 > 14 L/P 100k's,
Just goes to show how bad the LIM gaskets were I guess.

Items that have been either done, replaced/changed -
TPS - Replaced
IAC Valve - Replaced
Throttle Body Clean - Done
All the Vac Hose's - Replaced
LIM & SC Gaskets - Replaced
Thermostat 91c - Replaced - drilled a small 1/16 air bleed hole on the rim of it.
Leads - Replaced [28k's ago] @ 160,000k's
Plugs - Replaced [28k's ago] @ 160,000K's
I have scanned it numerous times and no hidden, or history error's in the ECU or BMC.

I do have another set of Plugs and Leads sitting on the shelf in the garage,
but I was keeping them as spares for a rainy day so to speak,
Where I may have to give them a try.

But after you change something and road test it,
you have to wait till its cold again to re-road test it, that's the most frustrating part.

So I am open for suggestions to what it could be, or what may need to be replaced.

 

[PappaYeet]

I have the exact same thing in my VT. I haven't replaced as much as you have, but I've been unable to find anything that could be causing it. I've cleaned IAC and throttle body, cleaned plugs and tested leads/coils. When running a scan it looks like the spark timing is pulled back quite a bit during the stutter before going back to normal. Not sure if that's a cause or symptom though. More annoying than an actual problem because, as you said, it goes away after 5 minutes of driving.

 

[UTE042_NZ]

Some Reset the ECU tips and shenanigans... https://forums.justcommodores.com.au/threads/ecu-reset.55163/

 

[PappaYeet]

Unfortunately I've also tried the ECU reset. Didn't change anything, even after giving it time to re-learn the fuel trims.

 

[TheIceman]

have you tried swapping out the MAF sensor?

 

[PappaYeet]

I haven’t, I’ve only cleaned it. Are there any simple ways to test it?

 

[krusing]

I have the exact same thing in my VT. I haven't replaced as much as you have, but I've been unable to find anything that could be causing it. I've cleaned IAC and throttle body, cleaned plugs and tested leads/coils. When running a scan it looks like the spark timing is pulled back quite a bit during the stutter before going back to normal. Not sure if that's a cause or symptom though. More annoying than an actual problem because, as you said, it goes away after 5 minutes of driving.

Thank goodness, I am not alone with this problem,
I even went and purchased a another S/Hand Set of a IAC Valve and TPS Sensor as a set of the same Car,
an the same thing happened, so I am none the wiser, very frustrating !

I have recently purchased some small black split PVC Conduit/Tubing that is used for cables,
So my next option is I am going to install the tubing on each of the existing Plug Leads in a separate PVC Conduit/Hose for each Plug Lead, just in case they are breaking down [under load when cold].
I did measure the resistance of the leads, and they range from 820ohm to 980ohms approx., so they in the specified resistance range.
And if that doesn't work, I guess I will have to get the gauges out do a compression Test,
Being my last resort, as I am NOT convinced its head gaskets,
But yet again, the faulty is only when its cold tho, and goes like a rocket when warm, and idles perfectly, and very responsive to the accelerator.
Otherwise I will be the trying the new Leads, and plugs, as the last resort,
Then again, wait for it to cool down overnight.

 

[Immortality]

What mods does it have (if any)?

 

[TheIceman]

I haven’t, I’ve only cleaned it. Are there any simple ways to test it?

how did you clean it? and what with? they are very sensitive things.
only real way to test it is to swap with known good one.

 

[UTE042_NZ]

But yet again, the faulty is only when its cold tho, and go like a rocket when warm, and idles perfectly, and very responsive to the accelerator.

Which makes me think fuel/air/mix delivery control issue.
Isn't that what changes most when ECU gets higher temperature sensor readings from cold?
And the occurrence utterly predictable with the only variable being engine temperature (therefore more likely to be choke/mix related than spark/timing)?