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

[PappaYeet]

Lately I’ve done inlet manifold + supercharger gaskets, 1.95:1 ratio rockers, new valve stem seals and springs, new lifters, replaced some old vacuum lines with new tubing and new leads + coils + plugs.

Tested everything spark related with timing light, no issue there. Still stutters when cold, sometimes almost stalling. Takes slightly longer to start sometimes now too. It occasionally coughs while sitting still while warm.

I’ll be replacing fuel injectors as soon as they arrive. Wouldn’t surprise me if it’s them given that they’ve done 286k.

 

[Lex]

New DFI?

 

[Lex]

double post sorry

 

[PappaYeet]

Still original DFI, I’ll see how the injectors go first but as I said, Timing light showed pretty good spark.

 

[Immortality]

A timing light only shows that a spark is occurring and not how strong it is.

 

[Lex]

Still original DFI, I’ll see how the injectors go first but as I said, Timing light showed pretty good spark.

Then there there is the dfi plug wiring?

 

[PappaYeet]

I have an old DFI lying around that I could try. Krusing replaced his with a new one without any improvement though.

The plug and wiring directly at the DFI is fine, can’t speak for the rest of it. I would assume a wiring issue would be there all the time though, not just when cold.

 

[PappaYeet]

Kind of forgot to reply here but new fuel injectors fixed my hesitation while cold.

I still have the occasional stutter though, and I''ve made a new post about it here: https://forums.justcommodores.com.a...cularly-when-changing-to-neutral-park.286353/

 

[krusing]

Kind of forgot to reply here but new fuel injectors fixed my hesitation while cold.

I still have the occasional stutter though, and I''ve made a new post about it here: https://forums.justcommodores.com.a...cularly-when-changing-to-neutral-park.286353/

That is interesting about the injectors,
I put service kits in mine, and still had the hesitation when cold.
I never though it being one of the actual injector, as they seem to work ok when warm, so I would of never thought it could fail when cold.
Thank you @PappaYeet for the reply, and the information.

 

[Immortality]

If an injector is leaking or partially blocked the symptoms would be most noticeable when cold unless it's severe. When you first start the car the PCM is running off programmed fuel tables (open loop). Once it reaches the correct minimum operating temperature the PCM goes into into closed loop mode and starts trimming the fuel and making the fuel on that bank to get the correct AFR's. This would give the false indication the car is running ok. With an injector problem you may also experience either an ignition miss under boost or spark knock which we probably mistake for a spark issue when it may well be due to one cylinder been lean or rich causing either a miss or knock.

I've been down that rabbit hole unfortunately.