2002 Volkswagen Passat GLX 2.8L misfiring with low vacuum

Car problem question:

We have testing a 2002 VW Passat 2.8L engine that has a slight misfire. Called it slight because it won't register on my scanner which showed no codes. My generic scanner is verified good because I tried it on another car and it worked. What bothers me though is the low vacuum reading at idle which is only 16 inches. Normal range should be 18-20 inches at engine idle and I double checked the engine compression which showed 160 psi for all the cylinders. This tells me the engine is fine. Any suggestion?

ATS Adviser:

I agree with you that mechanically speaking, if the engine compression is about 160 on all the cylinders, it should be fine. The issue here is your vacuum at engine idle. Have you checked for vacuum leaks? The engine might be putting out a good suction pressure but if you have a vacuum leak, low vacuum will result which can cause the slight misfire.

The reason for this is the fuel mixture becomes lean (more air than fuel) whenever the vacuum is lower than required. Another way to test vacuum leak is when at engine idle, cover the air entrance to the throttle body where the air filter sits. When you do this, engine should have a hard time running or it might quit. If it keeps running, you know air is still coming in the intake and that means vacuum leak.


You pointed me to the right direction! While road testing at moderate speed, I put my brakes gradually and noticed the engine misfired and stalled. Stopped and parked the car and sprayed mist of water around the fuel pressure regulator and the engine quit. A new part from the dealer fixed the problem. The brake booster must have put additional vacuum load on the engine. Thanks for your wonderful suggestion!

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