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Discussion Starter · #1 ·
Recently on my 1996 2.3 FPT Maptun Stg 3 I've hit overboost cutout a couple of times I think. The nearest I can describe the feeling is to compare it to a single backfire on other vehicles I've owned, i.e. big shudder then carries on as though nothing happened when on full throttle. Its happened when going WOT in 3rd and also in 4th. A quick glance at the boost gauge and it looked like it was off the end of the red when it happened. Conditions last week were pretty cool and I also wasn't running Optimax although I usually do. Do the symptoms sound like overboost?

So how is overboost detected and performed, and is it more likely to happen with lower octane fuel or higher? What about if you've been using higher octane for a while, then put lower in, does this make overboost more likely as the ECU wont have adapted down?

I've put a speedparts boost gauge in temporarily and applying the throttle relatively smoothly now rather than whacking it wide open, I get to 1.3 - 1.4 bar peak, base boost seems to be about 0.4 ish. Checked wastegate and it moves freely, no hoses split and solenoid resistances look about right.

Should I be unduly worried about the odd overboost under certain conditions?

Matt
 

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What you describe, sort of sounds like knock or predetonation to me..... Theoretically, running high boost, WOT, and lower octane fuel will promote knock. Stick with the higher grade fuels.

Others can probably add to this....
 

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No, it's definitely overboost. It's detected by the MAP sensor, and the Trionic ECU shuts off the fuel supply momentarily, resulting in the very sharp hiccup described. Theoritically it's more likely to happen with higher ocatane fuel.

Others may be able to comment as to whether it's happened with their stage 3's, but a short term fix may be to disconnect the battery for 20 minutes or so, then perform an adaptation run (procedure detailed in BillJ's 9000 site, link at the bottom of this page). It's probably worth contacting Maptun too to see what the maximum boost is meant to be- they're a helpful bunch
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I get it on my Aero Stage 3 (not quite the same, but...)

I do have base boost raised a bit because I like it that way. I think if you "nurse" it up rather than stomping on the pedal, the boost eventually takes off faster than Trionic can control it and that's when you get overboost.
 

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Discussion Starter · #5 ·
So basically, overboost is determined just by the MAP sensor then? Is it some predetermined fixed pressure set by the tuner, or is there adaptation of the overboost point?

I watched the boost gauge today and can see that the pressure does rise a bit quick and show a bit of overshoot in some situations so I guess that's what's happened, just have to be a bit more careful with the throttle I suppose... don't think I need to be too careful though, after all I've only hit overboost 3 or 4 times over the last year (and 2 of those were last week).
 
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