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Negative boost (vacum)
Posted: Sat Sep 26, 2020 6:07 pm
by tobreathe71
Noticed my Ultragauge will read negative boost (vacum) value when off the throttle every now then. Its like the 'zero' shifts a couple of psi back. Stop, restart and all is good. Am i not understanding something or.......?
PX3 xlt auto
Re: Negative boost (vacum)
Posted: Sat Sep 26, 2020 7:38 pm
by saeb
Will be to do with EGR.
Block EGR and you will see vacuum on throttle off everytime. Unless you are seeing a high figure vacuum then I would say everything is working as it should.
Re: Negative boost (vacum)
Posted: Sat Sep 26, 2020 8:25 pm
by tobreathe71
Generally no more than -5psi ish.. So this is normal then? Not real keen on doing significant mods yet - only 4mths/ 8000klm new
Re: Negative boost (vacum)
Posted: Sat Sep 26, 2020 8:41 pm
by twodogs
Mine does that while it's doing a burn..
Re: Negative boost (vacum)
Posted: Sun Sep 27, 2020 6:57 pm
by saeb
Being EGR is closed from memory when a burn takes place it would make sense.
Re: Negative boost (vacum)
Posted: Mon Sep 28, 2020 12:27 am
by easyflow
have seen this. Also cmd throttle position position 50-60% during a burn and reduced boost pressure when driving. Assume it is to reduce airflow thus causing rich mixture to increase ext temps for regen
Re: Negative boost (vacum)
Posted: Wed Apr 20, 2022 5:18 pm
by Descott
Did this get confirmed as occuring during a burn? I've started to have a similar issue at the same time as my VGT actuator failing. Has happened 3 times in last ~800-1000km where if I take my foot off the accelerator, boost will drop to -3 to -6psi and engine revs will bounce around, drives really rough.
I've set up the scangauge to monitor boost, soot %, EGR position and EGT13. Will monitor to see when it happens next, but I've also just gone into COVID iso
