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Car 'jerks' on very light / no throttle

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Hello all,

I have a car that basically shakes forward and back around 3 times a second with very low / no throttle, mostly on lower gears.

Timing is stable and so is AFR (which wouldnt cause this I believe). No apparent vaccum leaks. Also dont think it is the IAC valve.

Car is a Holden V8 355 stroker with cam and extractors, Haltech Plug in ECU.

I want to know if there is something in the tune that may cause this? Running out of ideas here.

Thanks,

Paulo

How low in the revs are we talking? A few things that can cause that, and one of the simplest is if while "stable" timing and fuel numbers don't suit what the engine actually needs or wants to drive nicely. A big cam can make smooth driving in low rpm light load situations quite tricky, you may need to run richer AFRs than you'd expect or ideally like to... at least that's my current experience.

Can you log data, and look for sensor values that are changing when the car misbehaves? Is the TPS and/or MAP signal noisey? Perhaps the problem is a compensation, or a too sensitive transient fueling.

There probably isn't enough information in your first post to thoroughly diagnose the issue you're experiencing. One thing I will mention is that depending how aggressive your cam is, you may not be able to achieve perfectly smooth driveability. A cam with a decent amount of overlap can make the car a little jerky at light throttle cruise.

While you say that the timing and AFR are stable, another aspect I'd check is to make sure that the ECU isn't moving in and out of over run fuel cut, or that it's trying to apply an idle ignition compensation. I've seen both cause jerky light throttle performance many times.

Hello and thanks for the input.

I found out that the problem is likely to be attributed to the cam not liking that rpm and load range, making the car very unstable and 'jerky'. The way around the issue was to lower the timing considerably in that map area to lower the torque and smooth out that roughness.

With the lower timing there is only a hint of the problem.

Thanks again for your helpful tips. Happy tuning

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