Manifold reading shows boost when accelerating

[Phillip]

I’ve been using the Torque ODB2 app to check engine data and identify possible causes for having reduced MPG.

When accelerating moderately fast from rest, I noticed that the manifold reading goes to +5 or +10Hg which is really boost pressure rather than vacuum. At idle the manifold reads around -22Hg. This is a 2007 Toyota Tarago (Previa) 2.4L, non-turbo. Is this normal for this engine? I’m wondering if this is due to the VVTi system or is there a problem with the manifold which I need to address.

Can someone tell me if this is normal engine behaviour?

Topic

[Bluesnut]

Is the gauge hose connected to manifold vacuum and not ported vacuum? Meaning below the throttle plate.

Does the car seem to run a bit sluggish? If so, I might suspect a partially clogged converter.

[Phillip]

It’s just the manifold readout from the ODB2 port. I don’t really know how it’s measured. Seems to be a built in pressure gauge.

The car doesn’t show any sluggishness. I tried ERTCG’s vacuum test for a clogged exhaust and the reading didn’t change for either 2000 or 3000 rpm, so exhaust is probably ok.

I’m just curious if other Toyota VVTi engines show boost pressure in the manifold when accelerating.

One thing that isn’t within Specifications is the MAF reading at 2,500rpm. Service manual says it should be between 2 to 6 g/sec but mine is 8g/sec at 2,500rpm. At idle it is still within spec at 2.8g/sec, reference range is 1 to 3 g/sec.

I’m suspecting that this higher MAF reading is related to the lower MPG I’m getting. Car reviews say it should get around 23mpg but I’m only getting 18mpg at the moment.

[Bluesnut]

I can’t speak for what’s going on with the scan tool but a vacuum gauge (be it the tool or a factory installed or aftermarket gauge) works like this.

At idle the gauge should show roughly 20″ of vacuum. Upon acceleration the needle will drop to zero or close to it depending upon how hard the acceleration is. If the throttle is released the needle will drop back to zero or a shade over before dropping to zero.

Even a turbocharged engine will work the same way. The only difference is that if the acceleration is hard the needle will climb into the positive at some point when the boost starts taking over.

The reason I mentioned a partially clogged converter is because that can cause the reading to go into the positive and it can also lower fuel mileage a bit.

[Phillip]

Thanks Bluesnut.

I’ll look more into the exhaust system. I guess it’s possible that there’s a partial clog that’s not bad enough to show a different vacuum reading with ETCG’s exhaust test, but would still cause a drop in MPG. It looks like I’ll need to stick a pressure gauge into the Air/Fuel sensor port to properly test for back up pressure.

I also just remembered that my mother drives a Rav4 with VVTi so I’ll test how the manifold readings go with that car for comparison.

[Steve Johnson]

Boost is measured by psi. Vacuum is Hg. At WOT pressure is almost the same as atmospheric ,so 5Hg sounds right.10Hg around normal load,and 17 to 21 at idle.

[Phillip]

I checked the manifold reading with another app (Dash Command) and it turns out that this engine doesn’t have a built in vacuum gauge. It estimates the vacuum from other sensor readings, so probably isn’t that accurate.

I tried the ODB2 adaptor on my mum’s 2000 RAV4 but strangely it couldn’t obtain data from the ECU.

Thanks for the advice guys. Looks like I’ll need a proper vacuum gauge to diagnose the engine.

[Author]

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