albert_k

Hello,

I recently bought a 1994 Urs4 2.2T and I'm currently sorting out a couple of issues with it.

One of the things that concerns me is the manifold pressure shown on the digital boost display on the dash during idling and decelerating. The absolute pressure value shown is 0.5-0.6 bar at idle (regardless of engine temp., AC off) and around 0.3-0.4 bar when decelerating (rarely, when the revs are high it drops to 0). In my previous turbo car (a 1.8T) the values were 0.4 at cold idle, 0.2 at warm idle and 0 when decelerating (regardless of revs) - so I'm wondering that maybe there's a leak in the manifold and/or the idle stabilization valve. Idle is a bit rough. Peak boost is fine.

Since I suspected the ISV I did a small test and disconnected it when the engine was running. I expected the engine to die after doing that - but it didn't. Revs just dropped a bit and idle became more rough. I always though that the throttle valve should stay shut on idle and air should flow only through the ISV. Either I was wrong or the ISV has some leak.

Could someone shed some light on it?

Thanks.

UrS4boy

N71 ISV info here:

CLICK

albert_k

Thanks a lot for the link, I'll take a look into it.

One thing I've already determined is that the throttle is aligned correctly (i.e. it's closed shut by default), but the TB was very oily and dirty. Perhaps it's the same with the ISV and it will need cleaning as well.

UrS4boy

You have to remember that the digital boost/vacuum gauge is absolute pressure, not relative. With just the key on, but the engine not running, the display would be around 1.01 Bar (nominal atmospheric pressure = 14.7 psi absolute, 1 Bar = 14.5 psi).

When you first start the engine and let it idle, there will be some vacuum and the reading will be less than 1 Bar. As the engine warms and the piston rings expand a bit and seal tighter to the cylinder walls, there is more vacuum and the reading on the display will go down further.

I don't have a digital boost gauge on my 93 UrS4. I have an analogue mechanical gauge. On first start, my engine draws a vacuum of about 10 inches of mercury (10" Hg). As it warms up, the vacuum increases to around 15" Hg. Later, with a engine at full temperature, coasting in gear with the foot off the throttle (throttle plate closed), the engine will pull about 22" Hg vacuum. Now I will translate these readings into Bars absolute:

Key on, engine off = 0 psig (no boost, no vacuum), 1.01 Bar Absolute
Engine started but cold = 10" Hg vacuum = 0.338 Bar vacuum = 0.675 Bar Absolute
Engine running and warm = 15" Hg vacuum = 0.508 Bar vacuum = 0.502 Bar Absolute
Engine hot, coasting in gear = 22" Hg vacuum = 0.745 Bar vacuum = 0.265 Absolute

Hope that helps.