Vr4 Legnum engine cut out at full boost.

[Guest]

Hi guys,
Ive been having a problem with my legnum. When i accelerate in either 3rd or 4th when it seems to hit full boost it feels like the entire motor shuts off for like a second. I was wondering if anyone else has had this problem or has any idea what it could be. If you watch the boost gauge as you accelerate flat foot it seems to happen when the boost peaks. The entire car shudders, engine power disappears and then all of a sudden it boots back in after a second or less. It can even happen twice in a row in third and has happened in fourth but i havent been able to test it too much in fourth as i would be exceeding the speed limit. It hasnt happened in first or second. As a mechanically minded person but also someone who hasnt had any experience with turbochargers my theory is that it could have something to do with when the turbo boost peaks and the wastegate opens up. I dont know if that makes sense but thats the only thing i can think of. At first i thought it could possibly be gearbox related as it was happening in third but i tried it on the freeway in fourth and it happened aswell. I know with the legnums when you dont shift smooth u feel a shudder and it feels similar to that shudder but is not related to gear change in any way as it happens mid revs. The boost also drops one or two psi when it happens and i dont know if thats related to the cut in engine power or if thats the problem itself with my wastegate theory i mentioned before. As i said i dont know anything about turbo systems so you'll have to excuse my ignorance. Any info would be much appreciated as it is quite a rough feeling when it happens and definately doesnt feel normal at all.

Cheers.

 

[TomTom]

Sounds like a boost cut to me.... wind back your boost controller!

 

[wendnarb]

I'd agree it's boost cut for sure! What psi does it happen??

I'm sure someone will correct me if I'm wrong but the factory ecu has a limiting feature of the ammout of boost that it will let the car run to protect the engine/turbos, if you exceed this ammount of boost, which is around 15(?) then it cuts the engine till it's running a more acceptable ammount of boost. That's my understanding I could be totally wrong though!!

 

[Guest]

yea mate your turbos gonna blow apart!

 

[Guest]

Aw man. You've rooted it now. But I'm a good bloke and would be happy to take it away at no charge to you. Leave the keys in the letterbox and I'll pick it up on the way home from work

Seriously though. As said above, turn your boost down... especially at this time of year. It's pretty normal when boosting it hard in the colder air.

For manuals, a decent EBC will help you to minimise the spiking. For autos, to be safe, aim for 12-13psi max as they always spike 2-3psi over on the gear changes.

 

[TME_Steve]

Is it really violent? If so, boost cut for sure at about 15psi usually. So yeah wind the boost down below that, usually more common in the higher gears as you have time to spool the turbos up, more common on cold days too as it is calculated by limiting the airflow for a certain rev range. You can go past this point at up to say 4000RPM with some simple mods so if you want to stop it happening and you are fortunate enough to have a 7202 or 7203 ecu (which you probably don't being a 97 model) then you can move the boost cut up the load range a bit, while you are at it, you can tune the ecu.... BUt if you have a 7201 ecu then you are going to have to get a burned rom soldered into your ecu which is harder and not really preferable as a change of tune would need a new burned rom soldered again.... You'd be best to see if you can find a 7202 or 7203 ECU (they're rare as everyone wants them). Doesn't seem to matter if they are an auto or manual ecu at least on the 7202's as NinjaDaniel has tested that.

 

[snickells]

I'm calling shagged spark plugs. Seriously.

 

[TME_Steve]

or coilpacks, depends what boost and how violent i guess....

 

[Peter_D]

15 psi is to much. Cut it back to about 12.5 and see if it still happens. If it does you might have another issue.

 

[fieldy107]

We don't even know what psi he is running, could be stock for all we know. If your running high boost, try turning it down a bit.
If its not boost cut then I would agree with Steve, sounds like plugs/leads/coil packs.

 

[Peter_D]

We don't even know what psi he is running, could be stock for all we know. If your running high boost, try turning it down a bit.
If its not boost cut then I would agree with Steve, sounds like plugs/leads/coil packs.

Correct. But turning the boost down is the quickest and simplest trouble shooting advice.
If it's still happening get some silicon spray and clean the connections on the coil packs. There is a whole thread about which sprays to use and the coil removal and cleaning method.
Next try new leads, if you still have the OEM ones there most likely a bit stuffed anyway. Finally pull your plugs clean and refit or replace. Again the forum write-up will guide you how to do it.

 

[Guest]

i have turned the boost down, seems to b ok, and i have new leads and plugs that need to go in, chris who i bought it off told me it was boosted to 12psi but last night it was running 15psi then going down to 14, it might have had something to do with the weather being so cold

 

[TME_Steve]

glad it's fixed but if you still want to know why....

A quick test to see if it's boost cut or ignition issues with zero tools and no familiarity with how boost cut or ignition problems feel (as they are quite different):

Go for a drive when the sun is low in the sky with the sun behind you. Nail it, when you hit the hesitation, if it's ignition you will see a big cloud of smoke behind you at the same time (unburnt fuel). If it's hitting the boost cut, the fuel is cut so no big cloud....

For those who want to know 80% 2 byte load is about atmospheric pressure in the manifold, 160% 2 byte load is the stock boost cut. If things flowed perfectly we could say 2 x atmospheric pressure in the manifold would be double airflow which would 14.7psi but the flow is not linear as pressure increases so in reality the real point of the boost cut will be higher (then again if air temp is lower and the air is therefore more dense the pressure will be lower and there is a delay on the activation of the cut but you get the idea). Point is, 15-16 psi is normally around the point of a boost cut assuming there are limited other mods... then again 15 psi could also be the point of ignition issues....

As for 15psi being to high there is another thread on here talking about max boost being suggested at 14psi which covers this pretty well (a few varying opinions so pick and choose what you like)

 

[bradc]

Yeah boost cut is normally around 13 to 16psi. If it is boost cut then it will be quite violent when it kicks in, feeling like you are hitting a brick wall. If it is plugs or coil packs it will just feel like stuttering and just feels like a bit of a loss of power.