Low engine vacuum at idle

[en]

I've been working through this problem but could use some advice.
I have a 3.5 Rover with hot wire efi, 14CUX.
I don't know the history of this engine.
Approx 150 psi on compression test on all cylinders on cold engine. 40 psi fuel pressure at idle.
Idle speed keeps going up and down, i suspect ECU is trying to adjust settings. Ignition timing
is in the ball park
I have only around 5"vac at idle. Around 2000 rpm, i have 20"vac.
I get some backfire in plenum at idle, backfire out exhaust at higher rpm.
I blocked off all vacuum ports on plenum.

I've read that wrong cam timing can cause low vacuum at idle, but would vacuum be okay at higher rpm?

Any suggestions on what i should check adjust?
Thank yoian

[tbush]

Ian, does the engine seem to run better and have more power when it is warm? If so I would check for a stuck or leaking EGR valve.

[DiDueColpi]

Possibly something with your firing order or ign system?
Heavily retarded timing will also do this and so will "sadly" a flat camshaft.

[en]

I'm afraid i committed a common mistake.
I looked for something high tech when problem turned out to be a vacuum leak from the plenum heater.
Idling good now.