CEL on Turbo's

[LegacyT]

Does the ECU on turbo cars cut boost at 0 psi whenever it throws a CEL? Or is it just for certain codes?

Mark,

[ciper]

There is no such thing as boost cut, its purely mechanical.

[boostjunkie]

The ecu will cut FUEL if it sees boost with certain CELs. The only one I've encountered that has caused fuel cut is the MAP sensor.

FWIW, I've had TPS, Crank, Pressure Exchange, Knock, and MAF. Actually, I think the knock and MAF sensor codes threw the ecu in no boost mode (via fuel cut). I can't remember. You should search for my posts about electronic problems for more info.

[LegacyT]

Ok so Its selective on which CEL's it cuts boost, via fuel to make ciper happy

Thanks,
Mark

[ciper]

It doesnt cut boost

[LegacyT]

I see 0 psi with a cel, therefore I think it cuts boost alright, a whole 8.9 psi. your right it does it by cutting the fuel. But I called it boost cut, is that a problem for you?

Mark,

[vrg3]

It's not really cutting on boost though... It's cutting on load. Without the pressure sensor, for example, it can't know whether or not manifold pressure exceeds barometric pressure. It just cuts fuel if you ever try to put the engine under heavy enough load.

[LegacyT]

how does it know to cut it at exactly 0 psi then? So your saying it monitors engine load??

Mark,

[vrg3]

It's not cutting fuel precisely at 0 psig manifold pressure. It's close, though.

The ECU does compute load at all times. It uses all its inputs and comes up with an 8-bit load value that indexes into some of its maps and tables.

[boostjunkie]

It's damn close in calculating it!! Funny thing is that I knew I had a MAP sensor problem even when the car wasn't moving. I could floor the gas at idle and since the vaccum in the manifold would get to be close to 0.00 the engine would bog.

[LegacyT]

See when I had a CEL for the IAC I watched my boost guage, which is very accurate. Every time at exactly 0 psi It would quit, felt just like fuel cut at 13 psi. I could floor the car in any gear above 1st (Ie high load), or roll into boost ever so slightly, and every time at exactly 0 psi it would quit, which made me think it wasn't load dependant. S I thought it simply monitors boost and with a cel it cuts any building of boost above 0 psi.

Mark,

[vrg3]

Well, manifold pressure is closely correlated to load. I imagine the design of the fuel cut algorithm was such that under standard conditions (RTP and whatnot) the load threshold is just about where you'd start developing boost.

But maybe you're right that it cuts on boost if the IAC valve code is active then (and that the IAC-related fuel cut is different from the MAP-related fuel cut). I may have only actually examined its behavior with other codes active, and I can't say I fully understand the ECU's programming that I've been poring through.