MAP mystery

[Chas54]

I am still having issues keeping my car running reasonably for any period.
Yesterday it went OK .
Today , difficult to keep running on stat up .

I had investigated a small fluctuation in the TPS voltage and it began happening on 4th October. A swing now and again form 0.4 to 0.42v . It seems to have disappeared today although i have juggled cables about.

Now though i have a MAP sensor issue i dont understand.

The car will start and run rough with the vacuum at 0.45bar ... I would expect it to run rough at this . Normally i would see 0.34 bar and be okay.

So i disconnected the MAP line and fitted my old vacuum gauge there. The MAP side of the tube i attached to my hand vacuum pump .

I start the engine fine and then quickly pull a -200mm Hg vacuum , this shows as 0.73bar on the elanscan reading of the MAP.
The engine runs OK. The external vacuum gauge on the plenuum MAP point reads -500mm Hg .
My vacuum gauge measures 0 to 760 mm Hg but as the scale starts zero and perfect vacuum being -760 , -500mm equates pretty closely to 0.35bar.
The engine is now steady and idling evenly..
Pull a bit more vacuum to -300mm Hg and MAP reads 0.61 bar . Now pull 420mmHg and the MAP reads 0.45bar and the engine stops ?

So improving the vacuum stops the engine , why is that?

Ive taken the MAP off to do some on the bench resistance measuring using the manual vac pump although i cant find any figures other than the voltages to compare.

The only thing i can think is they may share a common earth somewhere ?

All advice welcome
Charlie

[Chas54]

MAP seems to check out OK. ie The better the vacuum the lower the voltage.
Ive been running with the MAP sensor on jumpers to prove it.

Checking the signal out put , the engine runs Ok until the voltage has dropped to 0.89v around 0.42bar. The plenuum actual pressure is steady at 0.35bar , but disconnected .

If i hand pump the MAP vacuum line further the engine stops. I can manually pull a vacuum down to 0.12bar at 0.19v .

The book states Vacuum at 60 kPa produces 0.7 to 0.9v So i take this as around 0.4bar.

The ECU must use this voltage to carry out some calculation but i'm not sure what.

If anyone has any advice i would be grateful ..

Charlie

[Simon_P]

The ECU must use this voltage to carry out some calculation but i'm not sure what.

from pressure and temperature you get the density of air and hence mass of the charge.

[Chas54]

Simon
That sound about right but i cant see why the engine should stop as the vacuum improves.

I cant see any fault with the MAP and the MAT looks good as well. Or at least it does on elanscan .

I can only think that the calculation is going wrong somewhere. I think the ECU uses the ignition on initially as the atmospheric pressure, the the other part of the calculation would be the manifold pressure at any given instance and the MAT. So you would think that a lower pressure in the manifold would mean a lower mass of oxygen and hence a reduction in fuel -- perhaps thats the point the engine stops as the fuel gets shut off by the injectors.
Whatever is happening just seems wrong to me.

Thanks for advice though.