MAP reading different to Boost gauge

[davidt]

Hi there,

Strange thing going on with my MS2. Fitted a different turbo to my 1972 Superbug and now the MAP sensor reading under boost is about 4 psi lower than my VDO boost gauge reading. Problem was not happening prior. Only replaced the turbo and exhaust piping.
When driving under boost, VDO will show 7psi (waste gate pressure) but Tunerstudio only shows 3psi.
Have updated to firmware release 3.4.4 and it made no difference. Turn ignition on and connected a pressure vessel so I could send a fixed amount of air pressure to the MS2 and VDO (both sense hoses are connected to the same fitting in the inlet manifold), provide pressure and I get the same boost reading. But when driving the car under boost, Tuner studio reads low. Barometric and MAP sensor read the same 101kPa when ignition is turned on. Tried different MAP sensor calibrations, no difference when driving. Tried older tunes, no difference. Retried the pressure vessel 3 times and no difference.
Acceleration feels like 7psi, not 3psi.
Using Tuner Studio release 3.1.05 just as before.
I have tried to attach my tune file and a data log but this forum says it is not valid.
I have been using Tunerstudion with this car for around 11 years now and have never had this issue.

Help!

David

[panel]

Can you try a simple solution like a bicycle pump with a separate (third) gauge ? Put pressure to the MS through that and then try to your VDO one maybe ? Or 'T' it into the line to simulate boost and watch the VDO gauge and Tuner studio ?

[davidt]

I have ordered a digital boost gauge to compare with my VDO and Tunerstudio, one of them has to be wrong.
What is baffling me is why I get the same boost reading with the ignition on and simulated boost from a pressure vessel, but when the motor is running and under boost, they differ by about 4psi.

[panel]

Well keep us updated as to what you find and how you fixed it etc.

[Schweg]

So it’s just boost that is different?

Did you change gauges?

I have the similar issue in that on TS it reads 2lbs high boost then before.

Though mine is consistent vacuum to boost


Sent from my iPhone using Tapatalk

[davidt]

Same gauges in Tunerstudio.
I even went back to a previous tune file where I had normal boost readings, but it made no difference. Tried different calibrations for the MAP sensor which gave different readings on my Tunerstudio dash but I still suffered the same problem of getting the same simulated boost readings but different actual driving boost readings.
Bloody strange. Hopefully the new electronic boost gauge will spread some light on the issue.

[davidt]

Hi everyone,

Problem fixed.
The new gauge arrived and was reading 1psi lower than the old VDO gauge. That was fair given its age.
But the original issue remained.
I removed the MS2 and pulled it apart. Nothing out of the ordinary here. So I was worried that the MAP sensor might be faulty. So while it was out I connect a new plastic vacuum line and ran it to the R/H/S inlet manifold. went for a drive and viola, Tunerstudio is now matching the gauge.

So it looks like there was a restriction to the vacuum signal to the MS2. Possibly a build up of oil or carbon in the hose.
When I was using the pressure vessel to compare readings, there was a consistent and long supply of pressure that was able to leak past the restriction and therefore gave the same readings. But during driving, the boost pressure was only for a few seconds before I had to change gears.

Sounds so simple now hey.

Thanks to everyone for your contributions and help with this confusing issue.

[panel]

Nice work !