hi
how much in Hg should the gauge reading show at idle in a healthy engine in a GT 94/95 5.0?
On my car it shows 17 in Hg, with slight fluctuations (1 in Hg).
Is that a indicator for possible ignition problems?
I believe 17 to be a little low but not outside the realm of 'normal,' especially if you have some miles on your engine. My boost gauge typically shows about 20 at idle with very little fluctuation, though my intake is through an intercooler so mine might be higher than normal. Handy chart below.
You're welcomed! That's largely true, though if you're hunting / surging obviously it's going to move around a bit. It may not be a bad idea to run the cylinder balance test if you haven't already. Bad plug / wire, weak injector, manifold leak, stuff like that would probably show up with that test.