'96 850R - Where does the computer get its boost reading?

bmessina · Post by **bmessina** » 15 May 2013, 20:27

And does that impact anything in terms of running condition?

Bone stock 1996 850 R.

I've been doing some data logging via the OBD system to try to pindown a low power/very high fuel consumption issue. One of the things I noticed is that the boost/vacuum readings seem... wrong. At idle it was showing boost, without my foot on the gas. Peak boost it displayed on my way home was 44psi! That can't be right.

I also did a full throttle run (couldn't get any higher than 4.5k rpm) and the boost reading held at 24psi for several seconds.

The readings were disturbingly normal otherwise. Little high on fuel trim, +14% to start but ended the drive around +10%, so I'm checking for vacuum leaks but otherwise ok. Link below for others to check it out. Car has no codes.

https://docs.google.com/spreadsheet/ccc?key=0AiBsoOIbGADrdE03MnFHYUFDNXlVcTRub25kLXdnWHc&usp=sharing

Post by **JDS60R** » 15 May 2013, 22:39

I thought the computer saw it in kpa. Try treating your psi numbers as kpa numbers and see if it follows a more logical output

bmessina · Post by **bmessina** » 16 May 2013, 09:48

Torque (the app I'm using to monitor/log) is usually pretty good about knowing the unit of measurement, but if I use the .145 conversion factor the 44psi would turn into 6psi. The maintained 24psi I saw during a full throttle run turns into only 3.5psi, which seems very weird.

Regardless, I'm hooking up a vacuum/boost gauge tonight to verify what's actually happening at the manifold.

Post by **JDS60R** » 16 May 2013, 10:52

How have you verified its good? Have you measured it against other (non computer) sensors such as a quality boost gauge, calibrated temp gauge etc? The problem may also not be in the ap . It may be in your sensors. Keep in mind that your ca has no boost sensor and the pressure is determined using other sensors (rpm, MAF etc)

I would put a boost gauge on it so I knew what was happening.

Post by **JDS60R** » 16 May 2013, 11:45

Here is how boost is calculated in the OP's car

Post by **jimmy57** » 16 May 2013, 18:39

Mot 4.3 and 4.4 do not have a boost sensor and there is no actual boost number. Overboost is inferred by MAF signal vs. RPM. The engine displacement multiplied by engine speed effectively gives you the pumping capacity and the MAF signal will tell you pumping efficiency. Turbo engine will be more than 100% efficient as that is what the turbo is used for. The control unit map for safe operation watches for MAF exceeding the normal program map. On Mot 4.4 the atmospheric pressure sensor also adds another factor to the map to comp for less dense air at higher altitudes.

Mot 7x versions on later cars have boost pressure sensor and still employ atmospheric pressure sensor as a circuit board mounted unit inside ECM.

Post by **mercuric** » 16 May 2013, 18:48

The ECU has a "MAF limit" parameter. If too much air flows into the engine, it cuts boost and then fuel, if necessary.

More importantly, from a fail-safe standpoint, air flow directly correlates to fuel flow (to maintain proper air-fuel mixture) -- So it's not so much the pressure that needs to be limited, but the overall flow, as to maintain the ability to inject enough fuel to meet the demands of the air entering the motor.

However, because of this, it's not really possible to get an accurate reading of actual manifold pressure from the on-board sensors in an 850.

+14 long term fuel trim is quite abnormal. A vacuum leak could be part of it, but it'd be more than a single small hose -- Either one of the larger ones or a combination of multiple smaller hoses leaking.

Have a close look at the bellowed hose between the turbo and air cleaner -- Those like to leak at the bellows from flexing and the leak can be difficult to notice -- sometimes the fold-like structure that creates the bellows cracks, creating a crease that leaks air into the turbo after the MAF.

One useful parameter to add to your log is ignition advance. The computer sure thinks you're running lean, makes me wonder if ignition timing is getting pulled way back to avoid pinging -- significant ignition retard will indeed rob power, guzzle fuel and make the motor reluctant to rev.

However, running too rich will have a similar effect -- Do you get dark smoke (unburnt fuel) out the back when you mash the throttle under whatever boost you're getting?

Spark plug condition will also offer some clues as to how your fuel mix actually is.

bmessina · Post by **bmessina** » 16 May 2013, 21:49

Thanks for the reinforcement on boost readings - I haven't hooked a vac gauge up yet, doing so in the morning.

I do not get black smoke. Plugs are pretty dark, pic below. Also, here's a new log from this morning w/ timing. No full throttle runs though.

https://docs.google.com/spreadsheet/ccc?key=0AiBsoOIbGADrdGR0NU5wOWVwY2VHd0VYYzAtRmdEcmc&usp=sharing

Post by **JDS60R** » 16 May 2013, 22:23

Don't let the outer ring of the plug be your guide. Look for a light tan near the electrode. I am only seeing white on the isolator but sometimes the light tan is on the other side. I do see the black ring but couldn't tell if it was soot or oil deposits.

bmessina · Post by **bmessina** » 20 May 2013, 12:14

Ok - The boost reading I was looking at was calc'd not by the car's computer, but by the app I was using. Once I fixed my car parameters, it looks more in line.

Overall vacuum is low - about -10 at idle. I replaced the airbox to turbo inlet plastic hose as suggested (I already had the replacement on-hand), but that made no different.

Should I be getting vacuum at the first port after the MAF? I was noticing that I get... none. Seemed strange. The MAF is still giving out a reading via OBD, around 10-11g/s at idle.