Boost Gauge -20 at idle 0 at WOT?

[laniakea]

I recently upgraded the GT with the following:

FRPP Pulley
MAFIA Mass Air Flow Sensor
Performance Spark Plugs
Accufab Throttle Body
FRPP Muffler
Intake Boot Support Sleeve
Short Throw Shifter
91 Octane Tune (MMP)


When I picked the car up all seemed normal, boost was 0 at idle and about 13 pounds at or near WOT for the few miles I drove her home. A few days later I drove the car for about 20 miles at varying speeds and RPM’s , after stopping at a friend’s house for about 10 minutes, I started the car to leave and the boost gauge went immediately to -20 pounds and it continues to display those numbers. When the engine is off the boost gauge reads 0, but immediately goes to -20 when started. At WOT it reads 0, almost as if the needle slipped on its shaft? I’ve heard that the MAFIA may cause this problem and that I can re-program the car’s computer to reflect proper boost? A worse thought is that I may have joined the many others who’ve had gauge failures. Any help or information would be much appreciated. Short Throw Shifter is nice but short throw boost gauge is not so nice.

 

[Fubar]

Gauge failure usually involves the indicator needle sticking in one spot. This sounds like a bad ground in the gauge or maybe somebody installed the vacuum line incorrectly? If you have the means, I'd connect an OBDII reader and confirm that the ECU is seeing the correct boost levels.

Engine power is fine?
Is this the aftermarket 30lbs gauge or stock 15lbs?

 

[paul b]

The minus 20 is correct, that is read as 20 inches of vacuum. At idle I have a little better than 20 In. With the engine off it should read 0. As you drive the needle should move away from vacuum to pounds of boost. Depending on your pulley size and engine condition under heavy acceleration you should be reading at least 12 Lbs.

 

[laniakea]

Gauge failure usually involves the indicator needle sticking in one spot. This sounds like a bad ground in the gauge or maybe somebody installed the vacuum line incorrectly? If you have the means, I'd connect an OBDII reader and confirm that the ECU is seeing the correct boost levels.

Engine power is fine?
Is this the aftermarket 30lbs gauge or stock 15lbs?

Fubar,

Power is fine and consistent with the upgrade; the boost gauge never ran negative (that I remember) in the past and it seemed to act as normal until about 20 or 30 miles of driving after the upgrade. Did the computer reset itself and make any changes? Why doesn't the boost gauge go above 0 at WOT now yet it went to ~ 15 during the first 20-30 miles after the upgrade was completed? It is the stock 15 pound gauge.

 

[laniakea]

Corrections:

I have apparently supplied bad data in my original post. My son who normally drives the car reports the following:

Stock without any mods: -7 at idle 10 Pounds at WOT
With Mods: -23 at idle and 10 Pounds at WOT

 

[nota4re]

All bets are off when you use a MAFia and you still have the OEM boost gauge. The ECU computes boost - because it has everything it needs. It knows the engine displacement, temperature, RPM, etc. and also the Mass Air Flow (MAF) sensor is measuring the amount of air being ingested. When all is OEM, all is good. But, the function of the MAFia is to "lie" to the ECU about the amount of air that is being ingested. It does this in a scaled way and in conjuntion with a tune that accounts for this, it works well and prevents the MAF from sending a signal that the ECU believes is out of range. BUT, now the calculation that the ECU is doing for boost is wrong. AFAIK, there is no way to correct it. My recommendation: Don't be looking down when you're under boost!

 

[paul b]

If the readings really bother you, getting a mechanical vacuum/boost gauge should bypass the problem. If all your gauges are good maybe it's time to get the Speedhut set-up before any of your gauges go south. With the new gauges you could solve two problems at once.

 

[BlackICE]

AFAIK, there is no way to correct it. My recommendation: Don't be looking down when you're under boost!

There is a way to correct it, but the guage will still pin at 15 PSI. When building the tune don't scale the displacement or injector sizes, but instead just make corrections only in the MAF table corresponding to the setting in used in the MAFIA. This only works if you don't exceed the 64Kg/min max airflow which will be less than 900 HP. Won't work in a TT setup.

:agree: don't look at the boost gauge while under boost. :lol

 

[laniakea]

Thank you to everyone for the advice and information. The fact the gauge is reading much different than it did before along with the thought my first gauge failure may have happened are the only things that were bothering me? The best advice from this thread is “not to look at the boost gauge while under boost” great comment and magnificent advice. THANK YOU to the forum members for your detailed explanations and input.