Screwed up boost gauge or screwed up pressure sensor?

DeusExMachina · Post by **DeusExMachina** » Sun Dec 19, 2004 7:13 am

When I first installed my Saab IC and MBC, the gauge was reading correctly (as far as I know, it is Autometer) but lately something's been acting up. As soon as it got cold I was hitting boost cut but the gauge was only displaying 6 psi. I backed off the MBC a tiny bit and the gauge reads 5-6 psi but no fuel cut.

Any ideas? has anyone had the pressure sensor go bad?

douglas vincent · Post by **douglas vincent** » Sun Dec 19, 2004 8:58 am

Check for leaks in the tubing to the boost guage. I had a cracked line myself and couldnt figure out why the boost was fluttering so bad!

vrg3 · Post by **vrg3** » Sun Dec 19, 2004 8:27 pm

Can you try hooking up my scan tool? See what the injector pulse width and boost/vacuum readings look like when the problem happens?

It might not be fuel cut. Maybe you're having some kind of ignition problem.

DeusExMachina · Post by **DeusExMachina** » Sun Dec 19, 2004 8:44 pm

No, it feels like boost cut. I haven't made your scan tool yet so I can't connect it.

In 4th or 5th gear when its cold, I can count 1, 2, 3 then it cuts. When its warmer it wont cut. All signs point to boost cut, but I'll see. I'm gonna put some teflon tape on the gauge fittings soon and it should narrow it down. Everything on my ignition is new except for the coil pack, but no matter where I'm giving it full throttle (in the RPM band) it will cut after 3 seconds (only when its cold out)

vrg3 · Post by **vrg3** » Sun Dec 19, 2004 9:07 pm

Okay, yeah, the 3-seconds thing clinches it. It's almost certainly fuel cut.

It might help diagnosis to know what the ECU's seeing as far as boost pressure.

Many MBCs do raise boost when its gets cold, so you may in fact just be overboosting.

Did you originally set your boost pressure to be above 6 psi?

DeusExMachina · Post by **DeusExMachina** » Sun Dec 19, 2004 11:00 pm

Yeah. I have no doubt from air density I'm overboosting. Its just that my boost gauge still says 6 psi. I'm wondering if anyone's ever had a pressure solenoid malfunction so it read higher than it was.

-K- · Post by **-K-** » Mon Dec 20, 2004 5:39 pm

Sounds like FCD time to me.....

BAC5.2 · Post by **BAC5.2** » Mon Dec 20, 2004 8:04 pm

If the boost controller is a mechanical unit (i.e. run a hose from one of the vacuum runners), then the pressure solenoid would not effect it at all.

It reads "real" boost as seen by the manifold. It's not an electronic boost gauge that reads from the ECU, or directly from the boost control solenoid on the strut tower.

It's most certaintly fuel cut. My car started doing that, then started idling really high, then would stall constantly while in neutral. That was a fault of my IAC valve though. I was also stuck in 'limp mode' where the stock boost control would only keep the car at 5.5-6psi (VF11 wastegate boost). Good lord, the car was slow then, lol.

DeusExMachina · Post by **DeusExMachina** » Mon Dec 20, 2004 8:06 pm

My stock boost control is completely looped off to itself. The pressure solenoid is still connected, and my gauge is T'd off the intake there.

BAC5.2 · Post by **BAC5.2** » Mon Dec 20, 2004 8:25 pm

Right, but the boost gauge isn't reading from that, and the solenoid isn't touching the turbo, therefore it's not controlling boost. So a problem with that would not manifest itself with lowered boost, or fuel cut.

DeusExMachina · Post by **DeusExMachina** » Mon Dec 20, 2004 8:31 pm

I'm not sure what you're trying to say. The pressure solenoid can't control boost (since its not connected to anything) but if the pressure sensor is detecting overboost it can apply fuel cut.

I'm just wondering if its possible for the pressure sensor to be malfunctioning, or if my boost gauge is malfunctioning (by giving me a low reading).

BAC5.2 · Post by **BAC5.2** » Tue Dec 21, 2004 1:15 am

Oh, I thought you were saying that you thought your boost gauge was reading incorrectly because of the control solenoid.

Overboost fuel cut hits at ~14psi at sea level. Hook up the scan tool and see what the ECU sees.

If it is seeing under 14psi, it could be detonating and just not throwing a code, or it could be IAC related or something like that.

georryan · Post by **georryan** » Tue Dec 21, 2004 6:41 pm

I thought the ecu cuts fuel when it sees a sustained combination of atmospheric pressure combined with boost giving a certain reading. If that was the case, a well working pressure sensor would have fuel cut at around 13 psi or so around sea level, but if the preassure sensor was off, and giving more pressure, it would cut fuel earlier, right?

I'm not sure how that comes into play with a manual boost controller, though.

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Screwed up boost gauge or screwed up pressure sensor?

Screwed up boost gauge or screwed up pressure sensor?