What would cause this?
My shop hooked up a mechanical boost gauge to verify actual boost being produced, but the in-dash gauge is showing about .2 bar lower.

 

Could be your location. Boost reading can vary with altitude, depending on how the baseline pressure is defined. OTOH, most people find that their boost reads higher at higher altitude.

 

In this case, that's not a factor. I've had the same problem in Denver and the shop I had this checked out at is at sea level.

 

Alot of factors in the way it was tested,,, what gear/load,what RPM and how fast the engine accelerated to keep up with the gauge.... .2 bars seems to be more then normal in my experiences on the dyno.....IMO..

 

Testing was done on long 3rd and 4th gear pulls, so there was plenty of time for the in-dash gauge to "catch up."

 

Thats good testing then...I know that the factory gauge rounds up or down but not sure were the common denominator is for that to do it???

 

One thing that you could do is to use a Durametric to log data, and compare the reported boost to the gauge reading. To compute boost from Durametric output, which is absolute pressure in mm Hg, subtract the baseline pressure at idle and divide the result by 760.

 

I'd like to keep my original question in mind, which is "what would cause this?"

It's already been established that the ECU is not seeing correct, actualy boost levels.

In other words, what would need to be replaced, or fixed typically to take care of this?

 

where did he hook it up...
is the car stock? flashed?
Durametric will tell you the truth...

 

Not sure....I'll ask the shop tomorrow. The car is flashed. Had a turbo upgrade installed a few months ago.

I wasted several months, and lots of money, trying to figure out why the car wasn't producing commanded boost.
Turns out it was all along- the DME just can't see the correct amount.

 

Well then who's flash is it? If u know the issue ( flash) then I dont understand your thread.

 

Maybe the sensors need recalibrating?

 

I know the issue but I don't know what's causing it. Which particular sensor reads boost? (and how do you calibrate it/ replace it) if necessary?

 

People are trying to help you here, but you're not being very forthcoming with information. Here are some questions whose answers would help pinpoint the issue, but nobody could answer based on the information you have provided so far:

How do you know that the ecu is not seeing the correct boost? Do you know for sure that the boost readout on the dash is the same as what the ecu is seeing?

One of your posts suggests that maybe you are seeing a difference in "set point boost" vs "actual boost of sensor" both of which are reported by Durametric. Is that what you trying to diagnose?

How do you know that the mechanical boost gauge is correct? You said that the car is producing the correct boost, but the ecu isn't seeing it. This doesn't seem to make sense, because the ecu controls the boost based on what it sees.

If you want to get meaningful answers, then here is what I would suggest that you do:

Hook up a Durametric and do 4th gear pulls. Include rpm, engine load, O2 sensor lambdas, ignition angle, set point boost, actual boost. Post the log, along with information about your fuel and your car setup, including the tune.

If you're not willing to do that, I don't see how anyone is going to help you.

 

I don't have a durametric, in fact I don't even know what that is.

I thought I had provided enough info to get a basic question answered, but here are some more details.

I have aftermarket turbos and a flash that is supposed to be commanding 1.3 bars of boost. My in-dash boost gauge never shows more than 1.1, so the car's been in and out of two different indy shops over the course of several months to try to figure out why it's not producing 1.3 bars of boost.

Well, the last shop I took the car to, Autoscope in Plano, TX, hooked up a mechanical boost gauge and found that the car is producing 1.3 bars of boost.
Still though, the in-dash gauge shows 1.1.

So back to my original post- what, specifically, would cause the actual boost and displayed boost to be different? I'm trying to figure this stuff out on my own so I can take the car back and say "replace this" or "fix this." I'm tired of getting charged for B/S troubleshooting that has, so far, led know where.