My ancient 2003 Zetec has a small but annoying problem.  When coming to a junction (or after blipping the throttle), the revs drop to about 500 and it only just seems to recover.  I have no codes shown, so I am looking for air leaks and taking readings in Forscan, and I have included the MAP value in these readings.  The base reading is about 102 kPa, which is about right for atmospheric pressure (I assume a baseline bias is simply added to the reading), and when the engine is running it is usually showing about 22 kPa, with little variation.  So, I know that it is at least responding basically as expected (pressure drops when the engine is running), but are these values in the correct range?  22 kPa is about 3 PSI, which seems low.  Is an engine really capable of that much suction when the throttle is slightly open and air is available to it via the air filter?  Added to which, I would expect an underperforming sensor to show a higher pressure value, not a lower one.  I have no idea, and I'm hoping that someone can provide a typical value.

1 hour ago, leederbyshire said:

My ancient 2003 Zetec has a small but annoying problem.  When coming to a junction (or after blipping the throttle), the revs drop to about 500 and it only just seems to recover.  I have no codes shown, so I am looking for air leaks and taking readings in Forscan, and I have included the MAP value in these readings.  The base reading is about 102 kPa, which is about right for atmospheric pressure (I assume a baseline bias is simply added to the reading), and when the engine is running it is usually showing about 22 kPa, with little variation.  So, I know that it is at least responding basically as expected (pressure drops when the engine is running), but are these values in the correct range?  22 kPa is about 3 PSI, which seems low.  Is an engine really capable of that much suction when the throttle is slightly open and air is available to it via the air filter?  Added to which, I would expect an underperforming sensor to show a higher pressure value, not a lower one.  I have no idea, and I'm hoping that someone can provide a typical value.

I would have expected nearer 32kPa at Idle although I don't have anything definitive to back that up, maybe the Throttle Body/Plate is a bit gummed up so not open as much as it should be.

Engine off should be Atmospheric Pressure, so 102 kPa is about right, maybe slightly high but there is a High Pressure System over the country at the moment, not sure if it is that high though.

I presume that the Pressure goes up to near Atmospheric at full Throttle.

Yes, it goes to around 80~90 at wide throttle, then comes down again as the engine sucks more air in.  The throttle plate is actually very clean - I don't let the deposits build up.  I've tried spraying Easy Start around the inlet manifold area around where the various pipes are connected, and around the base of the fuel injectors (which I've also had in an ultrasonic bath), and nothing makes any difference.  If I spray it into the throttle, it almost stalls; which is, I suppose, not much of an indicator either way, but could mean it's running rich.  Long-term fuel trim moves around a bit more than I'd expect (it looks more like a short-term reading, really, not gradual at all), but settles around 4%.  Perhaps this ties in with a too-low value for the MAP?  But since the MAP has to be pulled low, I don't see how it would produce a lower number if it was failing.