Random stutter/misfire at idle - particularly when changing to neutral/park

[PappaYeet]

Hi everyone. I've tried searching but no one else seems to have quite the same issue as me. Car is a 1999 VT Berlina, series 2, auto, L67. 290000km on it.

Current mods are 10psi pulley, 1.95:1 roller rockers, W2A intercooler, Mace CAI, Mace memcal, ported supercharger and 82° thermostat.

--- The problem ---

When the car is at idle (in gear or park), it will occasionally stutter, as if it misfired just once. The revs will noticeably drop by a small amount. This is completely random, and will sometimes happen a few times in quick succession, or not do it at all. This is present even when moving, provided no throttle is being used (idling down driveway for example).

At times the idle just seems 'rough' and almost always has a rich fuel smell from the exhaust (although could be due to no cats).

When the car is cold, changing from gear to netral or park will cause a large stutter, and the engine will shake as if it is running on 4 or 5 cylinders. The revs drop considerably. The severity of this is random, and seems to be much less frequent when car is at operating temperature.

When driving normally (using throttle), the car runs perfectly. I've never had any misfire when accelerating, whether the supercharger is making boost or not.

--- What I've tried ---

• Clean IAC, throttle body, MAF, supercharger, intake manifold, intake ports, air filter.
• Checked all vacuum lines for leaks. Could still be a leak hidden somewhere but definitely not the tube from bottom of throttle body.
• O2 sensors function as expected, although issue mainly happens when cold so shouldn't matter.
• Replaced injectors with new.
• Tested fuel pressure and regulator, all within spec.
• Replaced fuel filter, engine oil and trans fluid recently.
• Replaced intake manifold gaskets.
• Replaced supercharger and throttle body gaskets.
• Replaced rocker cover gaskets.
• All intake related bolts are torqued correctly.
• Replaced spark plugs (NGK BPR6EFS-11), leads, coil packs (Mace high voltage) and DFI.
• Replaced valve stem seals, lifters and valve springs.
• Replaced battery and belts.

--- Some evidence ---

Video of stutter while changing gear
TunerPro log of random stutter while idling (definition file included)

--- Ideas ---

• The IAC valve steps value changes when the stutter occurs. Don't know if this is cause or effect.
• O2 sensors are old enough to replace. Will do this soon as I will be fitting extractors/new exhaust.
• Something trans related? I know it slips a bit from 3rd to 4th sometimes.
• Haven't done a compression test yet, will do one soon.
• Something tune related?

I know it's not really a big issue or anything as the car still drives fine, but it's definitely annoying and I'd love to figure it out. Thanks for any help or suggestions.

 

[Immortality]

You changed injectors, did you replace with the exact same ones that came out?

Did you gap those plugs? Stock plug gap is 1.3mm on the L67 and on 10 psi there isn't really a need to reduce the plug gap. If you had a miss before it might be injector related which you would have fixed with the new injectors you fitted.

 

[PappaYeet]

Injectors were replaced with the correct OEM ones. I switched to the 1.1mm plugs due to a recommendation in another thread, I’ll try setting them to 1.3mm and see what happens.

 

[Immortality]

That's where I would start.

 

[PappaYeet]

Well I gapped the plugs to 1.3mm, gave it a couple days but unfortunately it made no change.

 

[Immortality]

The IAC is supposed to adjust when going into/out of gear to compensate for load. Have you adjusted the throttle stop on the TB at all? Double check the IAC settings. With the engine up to temp, engine running but all other loads turned off (heater, aircon, fans etc) you should have between 20 and 30 steps on the stepper motor.

 

[PappaYeet]

I had a look at the IAC steps, it was 14 with the fans on, and about 7 with nothing on. Which direction should I adjust the throttle? Does a low IAC value mean open or closed? Thanks.

 

[Immortality]

Close the throttle a touch to open the IAC more. Try a 1/4 turn out.

 

[PappaYeet]

I got the IAC to sit at 25 while idling, but no improvement.

Is it normal for the IAC to make a loud hissing sound at a certain position? It used to do so with a tiny bit of throttle but now it happens during normal cruising. Pretty annoying.

 

[Immortality]

I don't think it is but mine does it too sometimes. Try adjusting the butterfly a touch more to get rid of the noise.

There is another possibility, one we have been discussing elsewhere but it's not an easy thing to check. These engines run a single row timing chain with tensioner, with age these wear and allow a fair bit of slop in the timing chain. Another possibility is tired valve springs .

edit: I see you already did the valve springs.